Molecules 2001, 6, 736-769
Tyrian Purple: 6,6’-Dibromoindigo and Related Compounds
Christopher J. Cooksey
59 Swiss Avenue Watford WD18 7LL, UK; tel +44 1923 241 688; fax +44 870 054 7454
Received: 15 August 2001 / Accepted: 20 August 2001 / Published: 31 August 2001
Abstract: The genesis of the purple dye from shellfish, its composition, origin,
intermediates, analysis and synthesis of the components, 6,6’-dibromoindigo, 6-
bromoindigo and 6,6’-dibromoindirubin are reviewed
Keywords: 6,6’-dibromoindigo, 6-bromoindigo, 6,6’-dibromoindirubin, tyrindoxyl,
tyriverdin, tyrindoleninone, tyrindolinone, synthesis, structure, properties
6,6’-Dibromoindigo is a major component of the historic pigment Tyrian purple, also known as
Royal purple, shellfish purple and Purple of the Ancients. Arguably, it is the oldest known pigment,
the longest lasting, the subject of the first chemical industry, the most expensive and the best known.
The colour is derived exclusively from marine shellfish of the Muricidae and Thaisidae families. The
long history, stretching back well into the pre-chemical era, and embracing chemistry, biology and
sociology, contains not a few misconceptions and erroneous conclusions. This review attempts to set
the record straight.
This molluscan dye has been known since pre-Roman times and in the Mediterranean region there
is evidence for the industry around the 13th century B.C.  at Sarepta, now Sarafand, Lebanon. The
ancient industry was distributed world-wide . Surviving details of the ancient process are
insufficient to explain the chemistry involved and this is the subject of continuing speculation. But it is
clear that the dye does not exist in the mollusc and is generated from precursors, sometimes termed
chromogens, contained in the hypobranchial gland.
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The process is described by Pliny , writing in the 1st century AD:
The vein [hypobranchial gland] already mentioned is then extracted and about a
sextarius [ca. 7 lb] of salt added to each hundred pounds of material. It should be
soaked for three days, for the fresher the extract, the more powerful the dye, then boiled
in a leaden vessel. Next, five hundred pounds of dye-stuff, diluted with an amphora
[about 8 gallons] of water, are subjected to an even and moderate heat by placing the
vessels in a flue communicating with a distant furnace.
Meanwhile, the flesh which necessarily adheres to the veins is skimmed off and a
test is made about the tenth day by steeping a well-washed fleece in the liquefied
contents of one of the vessels. The liquid is then heated till the colour answers to
expectations. A frankly red colour is inferior to one with a tinge of black. The wool
drinks in the dye for five hours and after carding is dipped again and again until all the
colour is absorbed.
In more recent times, a distinctly different process for obtaining the purple has been described, first
by Cole in 1685 , in which the contents of the hypobranchial gland are spread on to cloth and the
colour develops in response to air and light.
The Ancient process
Meyer Reinhold  has reviewed in detail the importance of the purple in ancient Greek and Roman
times and a large body of literature has accumulated which has been summarised by Dedekind ,
Becker  and in less detail by Born [8-9]. It is generally believed that the dyeing process involved
generation of the purple dye from the precursor(s), followed by reduction to a leuco compound and
subsequent oxidation on the cloth to give the colour in the same way as modern vat dyes. Much effort
has been devoted to the discovery of potential reagents which could have been used at that time to
reduce the dibromoindigo to the leuco form. In Pliny’s description, the word plumbum has no adjective
and depending on whether nigrum (“black”) or album (“white”) is added, could be translated as either
lead or tin. Experiments with lead and tin usually in strongly alkaline solution [10-12] have met with
variable success but tin is the stronger reductant . Other suggestions for reducing agents are
mercaptans ; methane thiol is a potential byproduct but the amount present is small and would be
insufficient to convert all the dye into the leuco form. Experiments with dodecanethiol in 1M NaOH at
78-88 ºC were successful . Honey has been suggested, although the reducing properties of glucose
are not very powerful. Experiments with iron reducing systems, mentioned by the ancients, did not
succeed, but may have referred to other dyes which were cheap alternatives to the purple.
The alternative suggestion is of a biochemical reduction, analogous to that for indigo in the woad
vat, which has recently been shown [15-17] to utilise Isatis clostridium, but was reportedly
unsuccessful for dibromoindigo. However, based on the successful woad vat parameters (pH 7.8;
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50 ºC), and the long (10 day) fermentation period, this process has been successful  in producing a
dye bath from Murex trunculus and reducing synthetic dibromoindigo using cockles .
The direct process
William Cole  clearly described this process in which the contents of the hypobranchial gland of
Nucella lapillus (at Minehead in the UK) are spread on to linen...
he "found this species on the shores of the Bristol Channel, which on cracking and
picking off the shell, exhibited a white vein lying transversely in a little furrow or cleft
next the head of the fish; which must be digged out with the stiff point of a horse hair
pencil being made short and tapering; which must be so formed by reason of the
viscous claminess of that white liquor in the vein so that by its stiffness it may drive in
the matter into the fine linnen or white silk ....... if placed in the Sun will change into
the following colours, i.e., if in the winter about noon, if in the summer an hour or two
after sunrise and so much before setting (for in the heat of the day the colours will come
on so fast, that the succession of each colour will scarce be distinguishable) next to the
first light green will appear a deep green; and in a few minutes this will change into a
dull sea green; after which, in a few minutes more, it will alter into a watchet blue; from
that in a little time more it will be purplish red; after which, lying an hour or two
(supposing the Sun still shining) it will be of a very deep purple red; beyond which the
Sun can do no more."
Every author who describes this process remarks on the distinctive colour changes that take place in
the sunlight. In Mediterranean sunshine, the process is complete in less than ten minutes [20, 21]. The
second notable characteristic is the powerful odour which accompanies the colour development: it is
likened to garlic or assafoetidea and is quite distinct from the odour of decomposing shell-fish. Other
examples are given by Letellier  and Schunck . Réaumur  confirmed the need for oxygen
in the purple forming process, Duhamel  confirmed that light was necessary, but it was left to
Lacaze-Duthiers  to demonstrate the potential use in photography.
In South America, the direct application route has a long history. Here the species used is Purpura
pansa and in contrast to the Mediterranean molluscs, it does not need to be sacrificed to yield the dye.
The elements of this dyeing process were described by Juan and de Ulloa  in 1744:
On the coasts belonging to the province of Guayaquil the finest purple is found. The
animals from which it is derived are contained in shells, about the size of walnuts, and
live on rocks washed by the sea. They contain a juice or humour, which is taken out,
and yields the true purple. ... Cotton, thread, and other delicate materials are dyed with
it. It gives a lively and durable colour, which does not lose its lustre by frequent
washings, but is rather improved thereby, and does not fade through long-continued use
and exposure. Near the port of Nicoya in the province of Guatemala [now Costa Rica]
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the same kind of shellfish is found and is used for dyeing cotton ... Various processes
are employed for extracting the juice or humour . Some kill the animal. They take it out
of its shell, and, having laid it on the back of the hand, press and squeeze it with a knife
from the head to the tail, and then separate the expressed juice, the rest of the animal
matter being thrown away. They treat in this way a number of animals until they have
sufficient quantity of juice. They then draw through the thread which they wish to dye,
and no more is required ... Others express the juice without killing the animal. They do
not take it entirely out of the shell, but only press it so as to cause a certain quantity to
be ejected, with which the threads are dyed. The shells are then laid again on the stones
from which they were taken. They recover, and after some time give a fresh quantity of
juice, but not so much as the first time. If the operation is repeated three or four times,
the quantity is very small and the animal dies of exhaustion.
The features which distinguish this process from others are that cotton threads are dyed separately
and subsequently woven, and that the mollusc is apparently not harmed by periodic milking. A century
later, Squier  describes the process on the Pacific coast of Nicaragua:
The process of dyeing the thread illustrates the patient assiduity of the Indians. It is
taken to the seaside, when a sufficient number of shells are collected, which being dried
from the sea water, the work is commenced. Each shell is taken up singly, and a slight
pressure upon the valve which closes its mouth forces out a few drops of the colouring
fluid, which is then almost destitute of colour. In this each thread is dipped singly, and
after absorbing enough of the precious liquid, is carefully drawn out between the thumb
and finger, and laid aside to dry. Whole days and nights are spent in this tedious
process, until the work is completed. At first the thread is of a dull blue colour, but
upon exposure to the atmosphere acquires the desired tint. The fish is not destroyed by
the operation, but is returned to the sea, where it lays in a new stock of colouring matter
for a future occasion.
This operation has more recently been described by Nuttall  in 1909, Thomson  in 1995,
Rios-Jara, et al.  in 1994 and Michel-Morfin in 2000 [32, 33].
The identity of the purple
In the nineteenth century, there was gradual progress towards identifying the purple pigment as
dibromoindigo. Bizio  showed that the pigment from Murex brandaris and Murex trunculus had
the characteristics of indigoid pigments; Schunck  isolated 7 mg of the dye which he called
punicin from 400 Purpura capillus (after which his patience was exhausted). In 1880, Schunck 
obtained a sample of cotton dyed in this way at Realejo on the West Coast of Nicaragua, said to be
made with the extract of Purpura patula. It was a dull purple, harsh to the touch and emitted a peculiar
smell. Extraction with warm dilute hydrochloric acid followed by boiling ether brightened the colour.
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He then treated the cloth with boiling aniline, which after a second extraction removed all the colour.
After cooling the aniline solution, all the coloured material crystallised. In this way, he obtained 0.099
grams from 24 grams of cloth. The colouring matter had all the properties of punicin, which he had
earlier obtained from Purpura capillus.
In 1909 Friedlander  solved the identity problem by processing the hypobranchial glands of
12,000 Murex brandaris and obtained 1.4 grams of pigment. The hypobranchial glands were removed
from the animals, spread on to filter paper and the colour allowed to develop in sunlight. The filter
paper was macerated and heated for 0.5 hour with dilute sulfuric acid (1:2) and after filtration washed
with hot water. The residue was Soxhlet extracted with ethanol to remove impurities and then the
pigment extracted with ethyl benzoate from which it separated in shining crystals. A second
crystallisation from ethyl benzoate and finally from quinoline completed the purification. Elemental
analysis showed that it contained bromine, which was unexpected, with an empirical formula
C16H8Br2N2O2. An unsymmetrically substituted indigo is unlikely since the precursor would be a C16
compound, but the precursor is not coloured, and of the four symmetrical isomers of dibromoindigo,
two of which were then known, 5,5’- and 6,6’-, the latter was the most similar. Two syntheses
confirmed the conclusion. Confirmation that dibromoindigo is the major coloured component of
Tyrian purple has been repeated for many mollusc species over the years. The rapid advance in
analytical techniques in the second half of the 20th century has greatly reduced the effort required. A
summary of the techniques used is shown in Table 1.
Table 1. Methods used to identify dibromoindigo in molluscs and on artefacts.
Species / artefact
Driessen L A 1944photodebromination of
elemental analysis; MS
van Alpen 1944 textile
Baker & Sutherland 
Sasaki K 
Gibaja Oviedo & Salazar
de Cavero 
Taylor G W 
McGovern & Michel 
Sarepta pot shard
Daniels [46,47]1985textile from Enkomi
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McGovern, Lazar, Michel
Nazca (200BC – 600AD)
Purpura patula pansa
Kosugi Y & Matsumoto
K 1994 
Shimoyama S & Noda Y
Clark & Cooksey 
Cooksey & Withnall 
Shimoyama S 
Cooksey, Withnall, Patel,
1994Rapana venosa MS
1994Rapana thomasiana 3D fluorescence spectra
coloured textile remnant
3D fluorescence spectra
In 1922, Friedlander  speculated that there was a second blue more soluble component in
Tyrian purple, but at that time, it could not be identified. The HPLC of indigoid dyes, pioneered by
Wouters and Verhecken , revealed that the more soluble blue component is 6-bromoindigo and
that there is another minor component, 6,6’-dibromoindirubin. Pigments derived from Murex
trunculus are more complex and variable, containing also indigo and indirubin.
The synthesis of 6,6’-dibromoindigo
The first synthesis was reported by Sachs and Kempf (Figure 1a)  in 1903. They obtained 4-
bromo-2-nitrobenzaldehyde from 2-nitro-4-aminobenzaldoxime by diazotisation in HBr and reacted it
with acetone and alkali in the classic Bayer-Drewson  indigo synthesis manner to give
dibromoindigo. The product dissolved in hot aniline to give a blue solution and precipitated as an
amorphous solid on cooling. A year later Sachs and Sechel (Figure 1b)  published another
synthesis from 2-nitro-4-bromophenyl lactic acid ketone. Friedlander (1909) used two methods:
starting with 4-bromo-2-nitrobenzaldehyde as above and also from 4-bromo-2-methylaniline via N-
acetyl-6-bromoindoxyl (Figure 1c).
Figure 1(a). Sachs and Kempf (1903) .
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Figure 1(b). Sachs and Sichel (1904) .
Figure 1(c). Friedlander (1909) .
Figure 1(d). Grandmougin and Seyder (1914) .
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CrO3 / Ac2O
acetone / NaOH
Grandmougin and Seyder (1914) (Figure 1d)  diazotised 6,6’-diaminoindigo in HBr to obtain
dibromoindigo. Subsequently, all syntheses rely on 4-bromo-2-nitrobenzaldehyde as the key
intermediate, followed by the Bayer-Drewson indigo synthesis, or the Harley-Mason  modification
thereof, which appears to give higher yields. In this modification , a solution of CH3ONa/CH3OH
is added to a mixture of 4-bromo-2-nitrobenzaldehyde and nitromethane to give yellow crystalline
sodium 1-(4-bromo-2-nitrophenyl)-2-nitro ethanolate. This product is dissolved in cold aqueous
NaOH solution and solid Na2S2O4 added. Aerial oxidation completes the reaction.
4-Bromo-2-nitrobenzaldehyde can be prepared (Barber and Stickings, Figure 1e ) from 4-
bromo-2-nitrotoluene by CrO3/acetic anhydride oxidation to give 4-bromo-2-nitrobenzylidene
diacetate which was then hydrolysed by boiling in aqueous-ethanolic sulfuric acid. The aldehyde was
purified via the bisulfite compound and was obtained in an overall yield of 17.5%. Using the same
process, Pinkney and Chambers (Figure 1e)  obtained 14.1% yield and Torimoto, Morimoto and
Shingaki (Figure 1d)  obtained 13.1%.
Figure 1(e). Barber and Stickings (1945) ; Pinkney and Chambers (1979) 
Torimoto, Morimoto and Tadao (1991) .
An entirely different route to 4-bromo-2-nitrobenzaldehyde has been disclosed by Voss and
Gerlach (Figure 1f)  which entails selective lithiation of 1,4-dibromo-2-nitrobenzene followed by
reaction with dimethylformamide to give the product in 92% yield. This is attractive since 1,4-
dibromobenzene can be nitrated in nearly quantitative yield under mild conditions using nitric acid in
trifluoroacetic anhydride , but the low temperature and complete absence of oxygen required for
the lithiation present some problems. An alternative route from 4-bromo-2-nitrotoluene to 4-bromo-2-
nitrobenzaldehyde involves benzylic bromination followed by the Kröhnke reaction (Figure 1g) .
Although multi-step, each reaction proceeds in high yield and purification at each stage is not
necessary . These syntheses are summarised in Figure 1.
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1) CH3NO2, CH3ONa
1) BuLi 2) DMF
1) CH3NO2, CH3ONa
Figure 1(f). Voss and Gerlach (1989) .
Figure 1(g). Cooksey (1994) .
How ever it is produced, dibromoindigo is easily purified, being only slightly soluble in most
organic solvents at room temperature. The crude material can be washed with copious amounts of
water and ethanol without loss. But, when first formed, sufficient time must be allowed for the finely
divided material to aggregate (a day or two or even a week), otherwise it will pass through a filter.
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Recrystallisation can be achieved using a variety of high boiling solvents (methyl benzoate, ethyl
benzoate or quinoline, 80 cm3 / gram).
The properties of 6,6’-dibromoindigo
The principal physical characteristics of the compound are the insolubility in solvents, high
melting temperature and intense colour.
(a) Visible spectrum
The colour of the compound is various shades of purple depending on the particle size and, when
crystalline, it is black with a copper lustre as are many compounds with a high light absorption: the
distance the light travels through a crystal is less than the wavelength of the light.
The indigo molecule has long been subjected to theoretical investigations into the reason for the
blue colour. A successful outcome is described by Lüttke and Klessinger in 1964  and the
chromophore turns out to be the indigo molecule but without the benzene rings as shown in Figure 2
. The topic is still being investigated .
Figure 2. The chromophore of indigo.
There is a quite extraordinary difference between the colour of dyed cloth (purple) and the
colour of the dye in solution (blue). Since the visible absorption spectrum of indigo dyes is readily
measured, the variation of the wavelength of maximum absorption (λmax) with solvent, concentration,
temperature and chemical substituent on the indigo ring has been extensively investigated.
Bromination of the indigo ring actually has quite a small effect on λmax as shown in Table 2.
Table 2. Visible absorption spectrum maxima (λmax) of various brominated indigo dyes.
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Solvents: tce = 1,1,2,2-tetrachloroethane; xyl = xylene; chl =chloroform.
With solutions of indigo there is some evidence of an absorption at higher wavelength which has
been ascribed to a dimer in solution  since the intensity increases with concentration and decreases
with temperature. The effect is shown for indigo solutions in 1,2-dichloroethane in Figure 3. This
phenomenon has been observed for some of the monobromoindigo compounds, but not for dibromo
compounds which have a lower solubility. A similar situation may exist in aqueous solutions of indigo
carmine (indigo disulfonic acid) . This could be important for those analytical chemists who
measure indigo concentrations spectrophotometrically.
Figure 3. The variation of absorption spectrum of indigo in
1,2-dichloroethane with concentration (a) and temperature (b) .
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In contrast to the blue colour in solution, the colour of dibromoindigo-dyed cloth is purple. The
visible reflectance spectra of indigo and dibromoindigo dyed cloth are shown in Figure 4 . The
absorption maximum of indigo shifts from 605 nm in solution to about 680 nm as a dye, whereas that
of dibromoindigo shifts from 590 nm to 520 nm. The reasons for this difference are considered in
more detail in the section on structure, but essentially result from the interaction of two dibromoindigo
molecules. Maya blue [80-82] is an interesting pigment of antiquity in which indigo molecules are
inserted into a clay layer matrix. In this situation interaction between two rings in dibromoindigo
molecules is not possible and the colour of the dibromoindigo analogue of Maya blue is blue . The
blue colour of dibromoindigo in the solid state has been observed over the centuries in the colour
changes during the generation of the pigment from shellfish. Here the molecules are generated with a
random orientation in a solid matrix and only after re-orientation does the blue colour change to
purple. In the same way, it has been observed that dyeing silk  at a low concentration gives rise to
a blue colour and this may be caused by the way the dibromoindigo molecules are bound to the silk
which precludes an easy re-orientation to give molecules in which the rings are closely associated.
Figure 4. The visible reflectance spectra of indigo and dibromoindigo dyed wool .
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(b) IR and Raman spectra
The IR spectrum of dibromoindigo shows the expected characteristic peaks at 3300 cm-1 (NH
stretch) and 1630 cm-1 (C=O stretch) [66, 75, 84] and has been used for characterisation purposes,
although it is reported that results depend on the method of preparation . A complete analysis of
the IR and NIR-FT Raman spectra for indigo has been published ; comparable results for
dibromoindigo are not yet available. The near-infrared FT Raman spectrum of dibromoindigo, in
contrast to the IR spectrum, shows the strongest peak at 1582 cm-1 (C=C stretch). These spectra are
illustrated in Figure 5.
Figure 5. IR (above) and FT-Raman (below) spectra of 6,6’-dibromoindigo .
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R = COCF3
(c) NMR spectra
It is just possible to directly observe the NMR spectrum of indigo , but the insolubility in the
usual solvents at normal temperatures precludes this for dibromoindigo. The observation of the solid
state NMR spectrum should be possible, but has not been reported. The solubility problem has been
circumvented in two ways. The first is to derivatise the dibromoindigo by reaction with trifluoroacetic
anhydride to give the N,N’-bis(trifluoroacetyl)-6,6’-dibromoindigo  which then shows the three
proton multiplets in the 1H-NMR spectrum at δH 8.28d (1.3), 7.75d (8.3), 7.56dd (8.1, 1.5). The
second route is to reduce the dibromoindigo in D2O with Na2S2O4  to give leuco-dibromoindigo
which then shows δH 7.50d (1.8), 7.48d (8.4), 7.09dd (8.4, 1.6). These structures are shown in Figure
Figure 6. Derivitisation of dibromoindigo for NMR determination.
The mass spectrum of dibromoindigo shows the characteristic signature of a compound with two
bromine atoms in the molecule: a 1:2:1 triple peak with 2 mass unit spacing centred at m/z 420. First
reported by Baker , the technique was widely used by McGovern et al  to detect
dibromoindigo in dyed materials. Because of the involatile nature of the compound, high injection
temperatures need to be used and if the substrate is mixed with other organic matter such as textile
fragments, reduction can occur and the resulting MS shows the presence of 6-bromoindigo . This
problem is circumvented by extraction of the dye with a hot solvent such as DMSO or DMF and using
that solution to determine the MS. The spectrum shows little fragmentation, reflecting the stability of
the molecule, and does not distinguish between dibromoindigo and dibromoindirubin which have the
Molecules 2001, 6
same molecular formula. The spectrum is illustrated in Figure 7  and has been reported elsewhere
[48,89]. The N,N’-bis-trifluoroacetyl-indigo derivative has been used to detect indigo on paper
samples via CI-MS  and this technique is expected to be equally successful with the dibromo
Figure 7. The mass spectrum of 6,6’-dibromoindigo .
(e) The structure
In 1880, Bayer drew the structure of indigo as the cis isomer . This was the convention until the
1920s when spectroscopic and other evidence suggested otherwise . It was Hélène von Eller’s
crystallographic studies  of indigo in 1954 that finally confirmed the trans structure. The X-ray
structure of dibromoindigo was has been determined by Süsse and Krampe  and Larsen and
Watjen . The parameters are monoclinic, space group P21/c, a=12.609, b=4.842, c=11.611,
β=104.42 and Z=2 , and the molecule is practically planar with deviations of less than 0.05 Ǻ.
Each molecule is hydrogen bonded to four others. The crucial difference between the structures of
indigo and dibromoindigo is the distance apart of the two parallel molecules: in indigo it is 5.77 Å
, but in dibromoindigo it is 4.82 Å, caused by van der Waals attraction between the bromine atoms
. Quantum mechanical calculations confirm that this aggregation causes a shift in absorption
maximum to lower wavelength in the case of dibromoindigo.
The analysis of Tyrian purple
In 1880, Edward Schunck  suggested that indigo, indirubin and dibromoindigo could be
distinguished by an observation of their sublimation temperatures: 140, 170 and 190 ºC. Other
analytical techniques of the day were the measurement of the absorption spectrum (recorded
photographically on a plate) and an examination of the morphology under a microscope.
There is a need for the detection and quantification of purple pigments on artefacts where often the
amount of material is small. As seen in Table 1, MS is a convenient technique for qualitative analysis
of the purple, IR has been used and in recent years HPLC is the method of choice. Single wavelength
detection is useful, but diode-array detection has the advantage of providing both a retention time
parameter and an absorption spectrum. An example is shown in Figure 8. In the future hyphenated
techniques such as HPLC-MS may provide more sophistication.
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Figure 8. HPLC of (A) a mixture of (a) indigo, (b) indirubin,
(c) 6-bromoindigo, (d) 6-bromoindirubin, (e) 6,6'-dibromoindigo
and (f) 6,6'-dibromoindirubin, with retention times (minutes)
and (B) the dye from Nucella lapillus .
Typical results of the HPLC analysis of Tyrian purple from different mollusc sources is shown in
Table 3. The composition of the indigoid constituents of
the purple dye from various molluscs.
Thais haemastoma 0
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Excluding Murex trunculus which contains non-brominated pigments, the major component of the
purple, 77-91%, is 6,6’-dibromoindigo with smaller amounts of 6-bromoindigo (1-16%) and 6,6’-
dibromoindirubin (1-14%). The origin of these minor components will be considered later.
For those with less resources, TLC is attractive. The intense colour should be easily detected. But at
normal temperatures dibromoindigo is not soluble in the solvents which are required for TLC, leading
to the incorrect conclusion that it is not mobile on TLC. Because of intermolecular hydrogen bonding,
the molecule as a whole is not polar and requires a non-polar solvent for a substrate like silica gel. The
solubility problem can be overcome by using high boiling solvents and 100 – 120 ºC as was
demonstrated by Bide and Choi in 1992  for 19 vat dyes including indigo. Although attractive,
there are technical problems with a simple beaker on a hot plate apparatus.
Table 4. Rfs of N-monoacetylindigos in various solvent mixtures.
3:2 ethyl acetate – cyclohexane
1:9 methanol – chloroform
1:1 benzene - chloroform
In the solvents tried, the separation of the monoacetylated brominated indigos was too small to be
useful for identification purposes. Replacing acetic anhydride with trifluoroacetic anhydride and
warming to 30 ºC gives N,N’-bis(trifluoroacetyl) derivatives. The reaction is self-indicating: the initial
violet colour, due to the monoacylindigo, changes to orange when the reaction is complete. TLC then
gives two spots on development, a more mobile orange spot of the N,N’-bis(trifluoroacetyl)-indigo
and a violet spot for N-trifluoroacetyl-indigo which results from hydrolysis. After standing in the air
for 0.5 hr both spots changed colour to blue (indigo) or purple (dibromoindigo) indicating complete
hydrolysis. This hydrolytic instability leading to two spots suggests that this method would be of
limited utility for mixtures of brominated indigos. Typical Rfs are shown in Table 5.
Table 5. Rfs of N-monotrifluoroacetylindigos and
N,N’-bis(trifluoroacetyl)indigos in various solvent mixtures.
3:2 toluene – ethyl acetate
3:2 heptane – ethyl acetate
3:1 hexane – ethyl acetate
1:1 dichloromethane – ethyl acetate
1:1 chloroform – ethyl acetate
3:2 heptane - dichloromethane
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Other derivatives need to be investigated before derivatisation can be considered as a viable
technique for identification.
The precursors of the purple.
In the 19th century, Schunck  observed that a yellow crystalline material accompanied the
generation of the purple from Purpura capillus, but was unable to identify it. Letellier  made
progress with the same species, isolating the same yellow light insensitive material (xantho-lapilline)
and two others which were colourless and water soluble (leuco-lapilline) and green and ether soluble
(chloro-lapilline) and which on exposure to light gave the purple. In 1954 Bouchilloux and Roche
[102,103] isolated two prochromogens from Murex trunculus which were colourless and water
soluble. This investigation was taken further by Malaszkiewicz  in 1967 and Fouquet [105, 106]
in 1970 who elucidated the structures of four prochromogens which were brominated and non-
brominated indoxyl sulfates. These are shown in Figure 9 compounds 1 – 4. 4 was also detected as the
single precursor in Murex brandaris, Murex erinaceus and Purpura haemastoma, but this observation
has not been confirmed.
Figure 9. Precursors to the purple colour in muricids.
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Meanwhile Baker and Sutherland  isolated a single different brominated indoxyl sulfate
precursor, Figure 9 compound 5, which they named tyrindoxyl sulfate from Dicathais orbita.
The complexity of the purple precursors from different species is well illustrated by paper
chromatography of an ethanol extract of the hypobranchial glands as shown by Baker  and
reproduced in Figure 10.
Figure 10. Paper chromatography of ethanol extracts of purple molluscs,
Dicathais orbita, Murex brandaris, Murex trunculus, Purpura haemastoma,
Murex erinaceus and Rapana bezoar compared with potassium
indoxyl sulfate (K.I.S) .
The precursors in Dicathais orbita and Murex brandaris are shown to be identical. For all these
precursors, the first step in the reaction towards the purple is hydrolysis of the sulfate group with an
enzyme (purpurase, arylsulfatase) leading to indoxyls. Indoxyls which lack a substituent in the 2-
position are then readily oxidised to give indigos. Those indoxyls which have substituents in the 2-
position are oxidised to indoleninones which dimerise to give the green photolabile tyriverdin.
Tyriverdin was initially thought to be a quinhydrone type of compound, but later was shown to be a
mixture of dimers. Photolysis of tyriverdin gives dibromoindigo and the long observed odorous
dimethyl disulfide. The products from Murex trunculus are complex by reason of cross coupling of
different indoxyls or indoleninones, eventually leading to indigo, indirubin, monobromoindigo and
Molecules 2001, 6
dibromoindigo and because there may be a variation in the composition depending on the gender of
the animal or on the season [107, 108]. The entire process of purple generation is shown in Figure 11.
The recent suggestion  that leuco-dibromoindigo is the major precursor in the mollusc cannot be
Figure 11. The generation of the purple.
Molecules 2001, 6
(a) Tyrindoxyl sulfate
Tyrindoxyl sulfate is the original source of the purple colour derived from molluscs. It is contained
in the hypobranchial gland. The anion could be associated with one or more quaternary ammonium
cations, e.g. β,β-dimethylacrylcholine [110, 111]. When the gland is excised, an enzyme (purpurase) is
released which removes the sulfate group to give tyrindoxyl. Tyrindoxyl sulfate can be isolated from
the glands by extraction with ethanol if this is done before the enzyme has had time to act. Baker and
Sutherland  describe how the live hypobranchial glands were extracted with ethanol and after
evaporation the residue was dissolved in water.
Ion exchange chromatography using Amberlite CG-50 (H form) resin gave a fraction of pH 3-4
from which a silver salt could be isolated. The silver salt was converted using KCl into the potassium
salt. Sodium tyrindoxyl sulfate could be chromatographed on Whatman No 1 paper using butanol –
acetic acid – water (78:5:17) giving a single spot at Rf 0.56 which could be visualised by UV light,
spraying with the purpurase (sulfatase) enzyme or with 1M hydrochloric acid followed by heating to
100 ºC. The latter two processes eventually give after exposure to air and light purple spots.
NMR δH 9.76s (1H), 7.14m (3H), 2.42s (3H) (from Dicathais orbita) 
(DMSO) 7.57d (8), 7.44d (2), 7.08dd (8,2), 2.44s (from Thais clavigera) 
This is a transient compound formed from tyrindoxyl sulfate by enzymatic removal of the sulfate
group. It is very readily oxidised by air but was probably observed by chromatography of the ether
extracts of hypobranchial glands of several species.
Baker and Duke  examined Mancinella bufa, Mancinella keineri and Mancinella
distinguenda in this way and showed that the products were identical to those of the previously
investigated Dicathais orbita. Thus alumina chromatography with benzene showed, red and yellow
spots at Rf 0.8 and 0.5 which were oxidation products of tyrindoxyl, with tyrindoxyl itself giving a
yellow spot at Rf 0.3.
(c) Tyrindoleninone and Tyrindolinone
These intermediates are formed from tyrindoxyl by reaction with oxygen in the air.
Tyrindoleninone is the primary product and some of it reacts with methane thiol to give tyrindolinone.
Tyrindolinone readily loses methane thiol during chromatography and on heating to give
tyrindoleninone. They are both readily detected by chromatography in a variety of species.
Tyrindoleninone has been identified as the pungent component of the edible Japanese Thais
clavigera  and has been identified by GC in the egg masses of Australian and Mediterranean
Molecules 2001, 6
Table 6. Rf data for TLC of tyrindoleninone and tyrindolinone.
a Silicagel60, 9:1 toluene-methanol 
b Silicagel60, 9:1 toluene-methanol 
c Alumina, benzene 
d Silicagel 60, 1:1 petrol – dichloromethane 
red needles, mp 109.5 °C
NMR δH (CCl4) 7.3m (3H), 2.60s (3H); NMR δH (CDCl3) 7.3-7.4m (3H), 2.65s (3H) 
λmax (hexane) (log10ε) 217 (3.94), 242 (4.40), 247 (4.40), 278 (4.04), 318 (3.72), 333 (3.70), 355
(3.43), 426 (3.23), 447 (3.23)
MS m/z 257, 255, 242, 240, 133 [112, 114]
yellow needles, mp 117 °C
NMR δH (CDCl3) 7.4m (3H), 5.45brs (1H), 2.20s (6H)
λmax (EtOH) (log10ε) 225sh (4.32), 248 (4.40), 270sh (3.90), 355sh (3.20), 402 (3.43)
This pale green solid is derived from the reaction of tyrindoxyl with tyrindoleninone. The most
characteristic property is sensitivity to light when it gives the purple 6,6’-dibromoindigo and the
highly odorous dimethyl disulfide. Being only slightly soluble in solvents, it is easily isolated and was
observed in 1890 by Letellier . Tyriverdin was initially thought  to have a quinhydrone
structure. The solid darkens above 60 ºC and gives a blue sublimate at 220 ºC without melting .
The dimeric structure, confirmed by synthesis (Figure 12a), was proposed by Christopherson 
and later Fujise, Miwa and Ito  showed using 1H-NMR that two stereoisomers exist, one much
more stable than the other. When a chloroform solution of tyriverdin is exposed to sunlight, dimethyl
disulfide is released and 6,6’-dibromoindigo formed  along with 6-bromoisatin, depending on the
amount of air present. Quantitative photochemical conversion to dibromoindigo has been observed
 by IR in KBr. Although always described as green, the chemical structure would suggest it is
yellow when pure. The absorption spectrum in methanol (λmax 350, 402 nm) also shows a maximum
absorption at 598 nm which can be assigned to a trace of photodecomposition to give blue (in
Molecules 2001, 6
solution) 6,6’-dibromoindigo. The combination of yellow and blue colours gives green. Tyriverdin is
highly bacteriostatic at 0.5 ppm concentrations .
UV (MeOH) λmax/nm (log10ε) 237(4.37), 252(4.51), 275(4.08), 350(3.41), 402(3.46), 598(2.85) 
UV (CHCl3) λmax/nm 255, 280, 366, 400, 595 
IR (KBr) νmax/cm-1 3380, 1680, 1600, 1450, 1310 ; IR (KBr) νmax/cm-1 3387, 1680, 1448, 1315
1H-NMR (DMSO-d6) A δH 8.22brs, 7.48d (8), 7.31d (1.8), 6.97dd (8, 1.8), 1.88s ; 1H-NMR
(DMSO-d6) B δH 8.03brs, 7.33d (8), 7.08d (1.7), 6.89dd (8, 1.7), 1.92s ; 1H-NMR (DMSO-d6), δH
7.46 (8.4), 7.27(1.5), 6.96dd (8.4, 1.5), 1.90s ; 1H-NMR (DMSO-d6, 270 MHz), δH 8.20s, 7.47d
(8), 7.28d (1.5), 6.96dd (8, 1.5), 1.88s 
13C NMR (DMSO-d6) δC 195.5, 159.9, 125.6, 121.5, 118.0, 114.5, 72.7, 11.6 
Figure 12. The synthesis of the intermediates of the purple and the minor components.
(a) Tyriverdin 
(b) 6-Bromoisatin 
Molecules 2001, 6
(c) 6-Bromoindigo 
(d) 6,6’-Dibromoindirubin 
The minor components in the purple
This yellow material, unconnected with the purple colour, has been reported as one of the products
found from the autolysed hypobranchial glands of many molluscs and has been known for a long time.
It was mentioned by Schunck  in 1879 and Letellier  in 1890. It could originate from the
oxidative photolysis of tyriverdin , tyrindoxyl sulfate [104, 105] or hydrolysis of tyrindolinone
. 6-Bromoisatin may be the precursor of 6,6’-dibromoindirubin which is nearly always associated
with dibromoindigo of marine origin. It is readily synthesised (see Figure 12b) from 3-bromoaniline
by the Marvel and Hiers (1941) modification of the Sandmeyer reaction (1919) followed by a pH-
based separation procedure from the 4-isomer .
Molecules 2001, 6
Mp 270 °C ; 273-274 °C 
1H-NMR (DMSO-d6), δH 11.15brs, 7.42d (7.9), 7.24dd (8.0, 1.6), 7.06d (1.6) 
This compound is found to some extent in all the Tyrian purple samples. The origin is uncertain,
but it seems likely that it comes from oxidation of leuco-bromoindigo which is derived from leuco-
dibromoindigo by photodebromination. The synthesis and properties have been described . 6-
Bromoisatin is reacted with PCl5 in chlorobenzene and then 3-acetoxyindole added. The use of
chlorobenzene as a solvent is dictated by the need for a higher reaction temperature than can be
achieved in benzene  and for an inert solvent. The product is filtered and recrystallised from ethyl
benzoate to give black crystals with a copper lustre in moderate (20%) yield. This reaction seems
generally applicable to monobromoindigos. The synthesis is illustrated in Figure 12c. When used to
dye wool, the colour is blue at low concentration and purple at high concentration, which appears to
mirror the behaviour of dibromoindigo on silk.
λmax (tetrachloroethane)/nm 601 ; λmax (reflectance)/nm 521
MS m/z 342 (100), 340 (100), 314 (14), 312 (14), 262 (5), 233 (9), 205 (28)
1H-NMR (N,N’-bistrifluoroacetyl derivative, CDCl3), δH 8.30d (1.2), 8.06d (8.4) 7.95d (7.7), 7.79td
(7.4, 1.3), 7.75d (8.1), 7.56dd (8.1, 1.4), 7.42t (7.4)
IR (KBr) νmax/cm-1 3271, 1610, 1609, 1608
Indirubins are invariably prepared by reacting isatins with indoxyls. 6,6’-Dibromoindirubin (0-14%
in Tyrian purple, Table 3) is probably derived from 6-bromoisatin (Figure 11). The suggestion that the
origin is isomerisation of dibromoindigo  in unlikely. 6,6’-Dibromoindirubin was first
synthesised by Ettinger and Friedlander  in 1912 by reacting 6-bromoisatin with 6-bromo-1-
acetyl-3-acetoxyindole in acetic acid containing 10% HCl. Fouquet  achieved 34% yield by using
the same route, and Clark and Cooksey  obtained 76% yield from 6-bromoisatin and 6-bromo-3-
acetoxyindole in methanol with Na2CO3. The product was obtained as fine brown needles which gave
a purple colour in solution.
λmax (CHCl3) (log10ε) 552 (4.07), 525, 387sh, 367 (3.94), 350sh, 305 (4.43) ; λmax (xylene) 567,
520sh ; (tetrachloroethane) (log10ε) 552 (3.89), 371 (3.73), 305 (4.28) 
1H-NMR (DMSO-d6), δH 11.2brs, 11.1brs, 8.67d (8.5), 7.68d (1.6), 7.62d (8.1), 7.22dd (8.5, 1.7),
7.20dd (7.9, 1.6), 7.05d (1.8) 
MS m/z 422 (47), 420 (100), 418 (43), 394 (4), 392 (8), 390 (5), 314 (14), 312 (10), 285 (16), 283 (13)
IR (KBr) νmax/cm-1 3325, 3164, 1672, 1586, 963, 909, 824, 815, 770 
Molecules 2001, 6
Chemical properties of 6,6’-Dibromoindigo
In the solid state the purple dye is remarkably stable. Ozone is known  to cleave it to give 6-
bromoisatin and 6-bromoisatoic anhydride. In solution, indigo is quite reactive to oxygen, giving
isatin, but 6,6’-dibromoindigo is less soluble and this oxidation reaction is less important. More
powerful chemical oxidants (e.g., dichromate) will cleave 6,6’-dibromoindigo to give 6-bromoisatin
Figure 13. The reduction of indigos to leucoindigos and the photodebromination of leucoindigos.
In the same way as indigo, 6,6’-dibromoindigo can be reduced to a water soluble yellow leuco-
compound either chemically (e.g., with sodium dithionite) or biochemically . Leuco-6,6’-
dibromoindigo is unstable in light, UV or sunlight, readily losing one or two bromine atoms to give
leuco-bromoindigo or leuco-indigo. Oxidation of these leuco compounds in the air restores the
corresponding indigo compound. The reactions are shown in Figure 13. This photodebromination has
long been observed [38,39], is the basis for a qualitative identification of dibromoindigo , and has
been used to synthesise deuterated indigos from the corresponding halo-derivatives .
Molecules 2001, 6
The literature about dibromoindigo is extensive and wide-ranging. Several bibliographies have
been published, notably by Bruin (1966, ), Schweppe (1993, ), Spanier (1982, ) and
Cooksey (1993, ). Most writers on the purple give a resumé of past publications and useful
summaries are given by Baker (1974, ), Cardon (1990, ), Michel and McGovern (1987,
1990, [10,11]), Clark (1993, ), and for Japanese language readers Fujisi (1999, ). For the
Jewish connection, consult Spanier (1987, ).
In 1859, Henri Lacaze-Duthiers wrote [26, 132]
A quel usage pourrait-on employer la pourpre? Aujourd’hui que les manufactures de
produits chimiques versent à torrent dans l’industrie des matières qui, avec la plus grande
facilité et la plus grandes perfection, peuvent servir aux teintures les plus delicates et les
plus riches, comment pourrait-on espérer de voir ce peu de matières animale donnant du
violet, quoique fort beau et fort tenace, être employé par l’industrie? Il n’est guère
probable que la pourpre revienne en honneur.
But there is considerable interest. In Mexico in 1909 , a traditional purple skirt de caracolillo
would cost 10 dollars gold. Recently  the Tyrian purple pigment (from Nucella lapillus) become
available at 58.50 GBP for 25 mg, rather more than the 10 – 20 times the price of gold it was in
Roman times . Chemical synthesis should be able to afford a viable alternative to harvesting
The author investigates Murex trunculus,
Photo: Rolf Haubrichs, Geneva.
Molecules 2001, 6
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132. ‘How could the purple be used? Today when chemical manufacturers produce a torrent of
materials for industry, which, with great ease and perfection, provide fine and strong colours,
how can we expect to see this small animal which provides the purple, though beautiful and
long-lasting, being used by industry? It is hardly likely that the purple will return to favour.’
Sample Availability: Samples of 6,6'-dibromoindigo, 6-bromoindigo and indirubin are available from
© 2001 by MDPI (http://www.mdpi.org). Reproduction is permitted for noncommercial purposes.