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ORIGINAL ARTICLES
Institute of Pharmacy and Molecular Biotechnology1, Department of Biology, University of Heidelberg; Department of
Biopharmaceutics and Pharmaceutical Technology2, Saarland University, Saarbrücken, Germany
Comparative study on the in vitro human skin permeation of monoterpenes
and phenylpropanoids applied in rose oil and in form of neat single
compounds
S. Schmitt 1, U. Schaefer2, F. Sporer1, J. Reichling1
Received July 27, 2009, accepted August 21, 2009
Prof. Dr. Jürgen Reichling, Institute of Pharmacy and Molecular Biotechnology, Department of Biology, University
of Heidelberg, Im Neuenheimer Feld 364, 69120 Heidelberg, Germany
juergen.reichling@urz.uni-heidelberg.de
Pharmazie 65: 102–105 (2010) doi: 10.1691/ph.2010.9716
Essential oils are ingredients of cosmetic and health care products as well as massage oil used in aro-
matherapy. There is no doubt that essential oils and their components are able to permeate human skin.
But information is rare dealing with percutanous absorption of essential oils in more detail. In this paper we
investigated the in vitro skin permeation of monoterpenes and phenylpropanoids applied in pure rose oil
and in form of neat single substances. We found that the application form had an exceeding influence on
the skin permeation behaviour of the compounds. For substances applied in rose oil a clear relationship
between their lipophilic character, chemical structure, and skin permeation could be confirmed. Regard-
ing the Papp-values the substances are ranked in the order: monoterpene hydrocarbons <monoterpene
alcohols < monoterpene ketons < phenylpropanoids. In contrast, for neat single substances there were no
relationships between their lipophilic characters, structures and skin permeation. Furthermore, except for
␣-pinene and isomenthone, the Papp-values of all other substances were several times higher when applied
in pure native rose oil than in their neat form. This suggests that co-operative interactions between essential
oil components may promote skin permeation behaviour of essential oil and its components.
1. Introduction
Rose oil (rose absolute) is the essential oil extracted from the
petals of damask rose, Rosa damascena (Rosaceae), which is
widely grown in Bulgaria, Turkey, Russia, India, Iran, and China.
Rose absolute is obtained through solvent extraction or super-
critical carbon dioxide extraction. The oil is a yellow or colorless
liquid with a deep-sweet, rich and tenacious floral rose-like
odour (Arctander 1960). The complex mixture of more than
300 compounds contains mainly monoterpenes and phenyl-
propanoids. Besides phenylethanol, citronellol and geraniol
are the major components of the blossom (Arctander 1960;
Guenther 1952; Lawrence 1991).
Rose absolute is known as the queen of oils and its feminine
properties make it emotionally smoothing. It is used in high-
price perfumes, especially in floral and oriental bases and in
small amounts to round off synthetic compositions. Rose oil is
also an excellent skincare oil; it is perfect for dry and mature, but
also for aged skin. It is used for palpitations, poor circulation,
relieving cardiac congestion, digestive problems due to emo-
tional upset, inflamed gallbladder and liver, jaundice, but also
asthma, cough, hay fever, and sore throats (Lawless 1992; Price
1993; Rose 1992a, b; Ryman 1991; Sheppard-Hanger 1995).
External application of rose oil is useful in smoothing irritated
skin. Rose oil is utilized to counter depression, anxiety, grief,
and negative feelings (Grieve 1971). It offers a pleasant, usually
relaxing ambiance due to its very pleasant nature.
The ingredients of essential oils have lipophilic properties, and
therefore essential oils are thought to be absorbed through the
skin. In the middle of the last century, Strähli et al. (1940)
demonstrated that after dermal application of essential oils, the
essential oil compounds appeared in respired air after a certain
time. Despite the frequent dermal use of rose oil the knowledge
of skin permeation of the oil and its different compounds is rare.
The percutaneous permeation of some terpenes has been proven
in some studies (Cal 2006; Jaeger et al. 1992; Fuchs et al. 1997;
Lademann et al. 2006; Schuster et al. 1986). Considering the
phenylpropanoids, there is hardly any study available analyz-
ing skin permeation of these compounds. Furthermore, question
poses whether the application form has an influence on skin
permeation behaviour of monoterpenes and phenylpropanoids,
the bioactive compounds of rose oil. The knowledge on skin
permeation of rose oil components is an important prerequisite
to assess their skin and systemic bioavailability, respectively.
This information is of common interest because some rose oil
ingredients are suspected of genotoxic side effect.
In this study, we comparatively investigated the permeation of
the major rose oil components applied in pure rose oil and in
form of isolated single substances through excised abdominal
human skin.
2. Investigations and results
Monoterpenes such as -myrcene, limonene, ␣-pinene,
-pinene, linalool, geraniol, citronellol, isomenthone are typical
components of rose oil. Furthermore, rose oil comprises also the
phenylpropanoids eugenol, methyleugenol, and phenylethanol.
102 Pharmazie 65 (2010) 2
ORIGINAL ARTICLES
Table 1: Concentration of the main constituents of the investi-
gated rose oil (means ±SD; n = 3)
Substances Concentration (mg/ml)
-Myrcene 4.584 ±0.173
Limonene 11.724 ±0.486
␣-Pinene 7.188 ±0.363
-Pinene 1.067 ±0.046
Linalool 15.742 ±1.178
Geraniol 106.847 ±0.696
-Citronellol 458.838 ±4.495
Isomenthon 1.132 ±0.033
Eugenol 3.350 ±0.154
Methyleugenol 2.054 ±0.064
Phenylethanol 15.607 ±1.993
The quantitative composition of rose oil with respect to the
investigated substances is shown in Table 1.
H3C
H3C
CH3
H3C
H3C
CH2
Limoneneβ-Myrcene α-Pinene β-Pinene
OHH3C
CH2OH
CH2OH
β-CitronellolGeraniolLinalool
O
OH
OCH3
OCH3
OCH3
OH
PhenylethanolMethyleugenolEugenolIsomenthone
We investigated the permeation behaviour of these compounds
through heat separated human epidermis (HSHE) applied in
form of pure native rose oil and neat single substances. The
Papp-values of the investigated substances are shown in Table 2.
The steady-state flux and the Papp-value were assessed from
in vitro experiments in which the donor concentration of the
penetrants was maintained more or less constant (infinite dose
conditions) while the receptor phase provided sink-conditions.
Over the time the flux increased to a steady-state value. The
Papp-value was simply calculated from the slope of the linear
portion of the graph of the cumulative amount penetrated as a
function of time (Geinoz et al. 2004). The apparent permeability
coefficient is described by the quotient of the flux (J) and the
donor concentration (CDonor). Concerning rose oil, the concen-
tration of the main constituents in mg/ml were used as CDonor
values (see Table 1):
Papp =J
CDonor
(1)
2.1. Skin permeation from native pure rose oil
Considering the ranking of the Papp-values of the ingredi-
ents of rose oil, the monoterpene hydrocarbons ␣-pinene,
limonene, and -myrcene showed only small apparent per-
meability coefficients (1.43 ×10−5– 1.57 ×10−5cm/s). The
skin permeation of the monoterpene alcohols -citronellol,
geraniol, and linalool was about twice as high (2.74 ×10−5
–3.87 ×10−5cm/s) compared to the skin permeation behaviour
of the hydrocarbons. It was remarkable that the monoterpene
hydrocarbon -pinene behaved different from the other hydro-
carbons tested. The Papp-value of -pinene was about four
times higher (5.76 ×10−5cm/s) compared to that of -myrcene,
limonene, and ␣-pinene. The skin permeation behaviour of the
monoterpene keton isomenthone (Papp-value: 5.26 ×10−5cm/s)
was comparable to that of -pinene. Considering the skin per-
meation behaviour of the phenylpropanoids, methyleugenol
showed the smallest Papp-value (5.23 ×10−5cm/s), comparable
to the Papp-values of -pinene and isomenthone. The Papp -value
of phenylethanol was about twice as high (1.63 ×10−4cm/s)
as the apparent permeability coefficient of methyleugenol. The
Papp-value of eugenol (9.14 ×10−5cm/s) lied in between.
2.2. Skin permeation of neat single substances
Considering the ranking of the neat single substances according
to their Papp-values it becomes evident that the skin permeation
behaviour of the neat single substances was different from their
behaviour as ingredients of rose oil (Table 2).
Of all substances tested, the monoterpene hydrocarbon limonene
and the monoterpene alcohol geraniol showed by far the lowest
permeation behaviour with Papp-values of 2.00 ×10−7cm/s and
6.78 ×10−6cm/s, respectively. In contrast, the monoterpene
hydrocarbon -myrcene (1.26 ×10−5cm/s), the monoter-
pene alcohols -citronellol (1.61 ×10−5cm/s), linalool
(2.03 ×10−5cm/s), and the phenylpropanoids methyleugenol
(1.70 ×10−5cm/s) and eugenol (2.61 ×10−5cm/s) clearly
revealed a better skin permeation. The Papp-values of -pinene
(4.84 ×10−5cm/s), phenylethanol (4.71 ×10−5cm/s), and
␣-pinene (6.49 ×10−5cm/s) were about twice and three times
as high, respectively. The Papp -value of isomenthone was by far
the highest (2.88 ×10−4cm/s).
3. Discussion
The purpose of this study was to get more information about
the in vitro percutaneous permeation of rose oil and its
major components. Therefore, skin permeation behaviour of
eight monoterpenes, namely ␣-pinene, limonene, -myrcene,
-citronellol, geraniol, linalool, isomenthone and three phenyl-
propanoids, namely methyleugenol, eugenol, phenylethanol,
applied in pure native rose oil or in form of neat single sub-
stances, were explored using heat separated human epidermis in
static Franz diffusion cells. The Papp-values of all investigated
substances are summarized in Table 2 and ranked accord-
ing ascending values. Regarding the Papp-value ranking of the
investigated substances it becomes obvious that there are some
important differences in skin permeation behaviour of monoter-
penes and phenylpropanoids when applied in form of a complex
mixture like rose oil and neat single components, respectively.
Pharmazie 65 (2010) 2 103
ORIGINAL ARTICLES
Table 2: Papp-values (apparent permeability coefficients) [cm/s] of monoterpenes as ingredients of pure rose oil and as neat single
substances (means ±SD;n=12and3,respectively), ranked according ascending values; octanol-water-partition coefficient
(log P)
Substances Papp (cm/s) Pure native rose oil Log P (25 ◦C) Substances Papp (cm/s) Neat single substances Log P (25 ◦C)
␣-Pinene 1.43 ×10−5±8.10 ×10−64.37 Limonene∗2.00 ×10−7±3.93 ×10−84.45
Limonene∗1.54 ×10−5±7.16 ×10−64.45 Geraniol*6.78 ×10−6±1.27 ×10−73.28
-Myrcene 1.57 ×10−5±7.37 ×10−64.58 -Myrcene 1.26 ×10−5±5.57 ×10−64.58
-Citronellol 2.74 ×10−5±1.40 ×10−53.38 -Citronellol 1.61 ×10−5±1.10 ×10−53.38
Geraniol*3.22 ×10−5±1.53 ×10−53.28 Methyleugenol*1.70 ×10−5±5.43 ×10−62.97
Linalool*3.87 ×10−5±1.48 ×10−53.28 Linalool*2.03 ×10−5±3.95 ×10−63.28
Isomenthone 5.26 ×10−5±1,93 ×10−52.63 Eugenol*2.61 ×10−5±2.23 ×10−62.20
Methyleugenol*5.23 ×10−5±2.11 ×10−52.97 -Pinene 4.48 ×10−5±7.26 ×10−64.37
-Pinene 5.76 ×10−5±5.17 ×10−54.37 Phenylethanol*4.71 ×10−5±4.44 ×10−62.63
Eugenol*9.14 ×10−5±4.67 ×10−52.20 ␣-Pinene 6.49 ×10−5±9.39 ×10−64.37
Phenylethanol*1.63 ×10−4±5.80 ×10−51.36 Isomenthone 2.88 ×10−4±1.71 ×10−52.63
*p <0.01, high significant differences of Papp -values regarding substances as ingredient of pure rose oil and as pure single substances
In case of rose oil, one can consider a clear relationship
between the skin permeation behaviour, the chemical struc-
tures and the lipophilic properties of the compounds in
question. Except for -pinene, all investigated monoterpene
hydrocarbons displayed the smallest permeation coeffi-
cients (1.43 ×10−5to 1.57 ×10−5cm/s), followed by the
alcohols (2.74 ×10−5to 3.87 ×10−5cm/s), the keton iso-
menthone (5.26 ×10−5cm/s), and the phenylpropanoids
methyleugenol (5.23 ×10−5cm/s), eugenol (9.14 ×10−5cm/s),
and phenylethanol (1.63 ×10−4cm/s). In addition the skin
permeation behaviour of the investigated substances corre-
late not only with molecular features but also with the
octanol-water-partition coefficient (log P) (Table 2). Higher log
P-values resulted in lower Papp-values. This means that moder-
ately polar components, such as isomenthone, methyleugenol,
eugenol and phenylethanol, had a higher permeation rate than
lipophilic compounds (Roberts and Walter 1998).
Furthermore, focusing phenylpropanoids differences between
chemical structures and skin permeation become more evi-
dent. Phenylpropanoids are aromatic compounds with C6-C3
skeleton. Regarding the different chemical structures of the
investigated phenylpropanoids, it is likely that due to the differ-
ent substituents at the aromatic ring system a steric hindrance
appeared. Phenylethanol with no additional substituent pene-
trated skin the best. Methyleugenol with two methoxy groups
at the aromatic ring system revealed the smallest Papp-value.
This effect could also be observed in connection with the dif-
fusion rates of various -lactam antibiotics through the porin
channels of Escherichia coli (Yoshimura and Nikaido 1985).
The introduction of different side chains amongst methoxy
groups led to a retardation of penetration through the porin
channels.
Unexpectedly, the skin permeation behaviour of -pinene was
much higher than of the monoterpene hydrocarbons and alco-
hols. Its Papp-value (5.76 ×10−5cm/s) was in the range of
isomenthone and methyleugenol, both showing small log P-
values. So, due to the log P-value, one could expect that the
Papp-value of -pinene would be similar to that of ␣-pinene (1.43
×10−5cm/s) or the other monoterpene hydrocarbons. Although
differences in the skin permeation behaviour of ␣- and -pinene
have already been reported previously (Cal 2007; Mackay et al.
2001) the obvious discrepancy can not only be addressed to dif-
ferent melting points, solubility and structural differences of the
compounds under consideration.
Focusing the skin permeation behaviour of the investigated
neat single substances, we found evident differences in com-
parison to the same substances applied in rose oil. In contrast
to rose oil, the ranking of the substances in question accord-
ing to their Papp-values did not display a clear correlation
between chemical structures, log P-values and skin permeation
behaviour.For instance, the monoterpene hydrocarbon limonene
and the monoterpene alcohol geraniol permeated the heat sep-
arated human epidermis the worst, followed by -myrcene,
-citronellol, methyleugenol, linalool, and eugenol with Papp-
values of 1.26 ×10−5to 2.61 ×10−5cm/s. In contrast to rose oil,
the monoterpene hydrocarbons -pinene and ␣-pinene perme-
ated heat separated human epidermis as well as phenylethanol,
the best penetrant when applied in rose oil.
Conclusion: In our study, apart from ␣-pinene and isomen-
thone, it is evident that the skin permeation of all monoterpenes
and phenylpropanoids tested were several times higher when
applied in pure native rose oil than in form of neat single sub-
stances; the differences were highly significant for geraniol,
limonene, linalool, methyleugenol, eugenol, and phenylethanol.
These findings correspond very well with data of a former
investigation done by Cal and co-workers (Cal 2003; Cal and
Sznitowska 2006). They found that selected monoterpenes per-
meated human skin barrier more effectively when applied in
pure lavender oil. In a further investigation, Schmitt et al. (2009)
explored the skin permeation of an artificial mixture of selected
monoterpenes and phenylpropanoids. It was found that limonene
revealed an enhancing effect on the skin permeation of cit-
ronellol and eugenol while -pinene increased the Papp-value of
methyleugenol but not of geraniol (Schmitt et al. 2009). Based on
these findings, there is every reason to believe that co-operative
interactions between rose oil components may enhance their
skin permeation behaviour. Those interactions may also explain
the differences in skin permeation behaviour of ␣-pinene and -
pinene when applied in rose oil. In addition, skin permeation of
eugenol and methyleugenol were promoted when applied in rose
oil. This result may be of common interest for further studies on
the systemic bioavailability of both substances.
4. Experimental
4.1. Materials
Rose oil (Oleum Rosea verum, Rosa ×damascena) was obtained from
Caelo, Hilden, Germany. The ingredients of the essential oil were identified
via GC/GC-MS.
The standard substances tridecane, myrcene, limonene, linalool, geraniol,
-citronellol, isomenthone, eugenol, methyleugenol, and phenylethanol
were purchased from Fluka, München, Germany; ␣-pinene and -pinene
were obtained from Sigma-Aldrich Laborchemikalien GmbH, Steinheim,
Germany and ethanol was purchased from Mallinckrodt Baker B.V.,
104 Pharmazie 65 (2010) 2
ORIGINAL ARTICLES
Deventer, Netherlands; n-hexane was obtained from Merck KG, Darmstadt,
Germany, tert-butyl methyl ether from Carl Roth GmbH, Karlsruhe,
Germany.
4.2. Skin samples
Abdominal skin from Caucasian female patients who had undergone plastic
surgery was used. Approval from the Ethical Committee of the Ruprecht-
Karls-University, Heidelberg, Germany was available. Immediately after
excision the subcutaneous fatty tissue was removed using a scalpel, the skin
was wrapped in aluminum foil and stored in polyethylene bags at –26 ◦C
until use. Previous experiments had shown that neither the penetration char-
acteristics nor the thickness of the stratum corneum (SC) were diminished
after a freezing period of 3 and 6 months, respectively (Bronaugh et al.
1985; Harrison et al. 1984; Schaefer and Loth 1996). Heat-separated human
epidermis (HSHE) was prepared according to Kligmann et al. (1963) for per-
meation experiments. Skin discs with a diameter of 30 mm were punched out
of the frozen skin. After thawing, the skin pieces were cleaned with Ringer
solution and put in a water bath of 60 ◦C for 90 s. Then the stratum corneum
(SC) and the viable epidermis were carefully peeled off using forceps.
4.3. Permeation experiments
In the Franz-Diffusion Cell (FD-C) the HSHE and a cellulose membrane
(MC 10000; Medicell, London, UK) were positioned between the donor
compartment and the acceptor compartment containing 50% (V/V) ethanol
as receptor medium. 50% ethanol was used according to the OECD Guide-
lines TG 428 (2004a) and No. 28 (2004b) to provide sink conditions over the
whole experimental time. After 30 min of hydration, 1000l of native rose
oil or 1000 l of the single substances were added to the donor compart-
ment. During the whole experimental time, the acceptor fluid was kept at
32 ±1◦C using an incubator. The acceptor medium was permanently mixed
with a magnetic stirrer (400 rpm, 7 mm). After 0, 3, 6, 9, 12, 24, and 27h
samples were withdrawn and immediately replaced by preheated acceptor
fluid. The samples were analyzed via gas chromatography.
4.4. GC-Method
The samples were extracted with n-hexane/tert-butyl methyl ether (1:1) con-
taining tridecane as internal standard. A gas chromatograph (Varian 3400)
equipped with a flame ionization detector (FID) and a capillary column was
used. The supplier of the column (OV-1 bonded 0.25 m, 30m ×0.25 mm
ID) was Ohio Valley Specialty Company, Marietta, Ohio, USA. The tem-
perature of the injector was 250 ◦C, the split 1:30. Helium (15 psi column
head pressure) was the carrier gas and the detector was heated to 300 ◦C.
Temperature program: the initial column temperature was 40◦C (holding
time 2 min), up to 105 ◦C the temperature was rising at 3◦C/min, up to
140 ◦C it was rising at 6 ◦C/min and up to 300 ◦C the temperature was ris-
ing at 10 ◦C/min (holding time 10 min). The data were analyzed with the
software Peak Simple, Version 2.91 (SRI Instruments, Torrance, CA, USA).
4.5. GC/MS-Method
A gas chromatograph (1090 Series II, Hewlett-Packard, Bad Homburg) was
directly coupled to a quadrupole mass spectrometer (SSQ 7000, Thermo-
Finnigan, Bremen).
The temperature of the injector was 250 ◦C, the split 1:10. Helium was used
as the carrier gas (15 psi column head pressure). Dimensions and phase
of the used capillary column were the same like in GC (see 4.4). Mass
spectra (70 eV) were recorded in MID Mode (m/z: 40–400) and analyzed
with the software Xcalibur, Version 1.2, Thermo-Finnigan, Bremen). Char-
acterization of the individual constituents of the samples was achieved by
comparison of their mass spectra and GC retention indices (RIs) with that
of authentic compounds.
Acknowledgements: We thank Mrs. Melanie Krieg (Wala, Bad Boll) for
helping in mass spectrometry analysis and Prof. Dr. Leonhard Doebler for
providing skin samples.
References
Arctander S (1960) Perfume and flavor materials of nature origin. Allured
Pub Corp, Elisabeth, New York, pp. 551–561.
Bronaugh R, Stewart R, Simon M (1985) Methods for in-vitro percutaneous
absorption studies. VII: Use of excised human skin. J Pharm Sci 75:
1094–1097.
Cal K, Sznitowska M (2003) Cutaneous absorption and elimination of three
acyclic terpenes in vitro studies. J Control Release 93: 369–376.
Cal K (2006) Skin penetration of terpenes from essential oils and topical
vehicles. Planta Med 72: 311–316.
Cal K, Kupiec K, Sznitowska M (2007) Effect of physicochemical proper-
ties of cyclic terpenes on their ex-vivo skin absorption and elimination
kinetics. J Dermatol Sci 41: 137–142.
Fuchs N, Jaeger W, Lenhardt A, Böhm L, Buchbauer I, Buchbauer G (1997)
Systemic absorption of topically applied carvone: Influence of massage
technique. J Cosm Sci 48: 277–282.
Geinoz S, Guy RH, Testa B, Carruptand PA (2004) Quantitative structure-
permeation relationships (QSPeRs) to predict skin permeation: a critical
evaluation. Pharm Res 21: 83–92.
Grieve M (1971) A modern herbal, 2nd ed., Dover Pubns, New York,
pp. 1–443.
Guenther E (1952) The Essential Oils, D van Nostrand, Inc., Princeton,
New Jersey, p. 30.
Harrison S, Barry B, Dugard P (1984) Effects of freezing on human skin
permeability. J Pharm Pharmacol 36: 161–262.
Jaeger W, Buchbauer G, Jirovetz L, Fritzer M (1992) Percutaneous absorp-
tion of lavender oil from a massage oil. J Soc Cosm Chem 43: 49–54.
Kligman A, Christophers E (1963) Preparation of isolated sheets of stratum
corneum. Arch Dermatol 88: 702–705.
Lademann J, Richter H, Schaefer UF, Blume-Peytavi U, Teichmann A,
Otberg N, Sterry W (2006) Hair follicles-Along-term reservoir for
drug delivery. Skin Pharmacol Physiol 19: 232–236.
Lawless J (1992) The Encyclopedia of Essential oils, Shaftesebury.
Lawrence B (1991) Perfum Flavor 16: 43–77.
Mackay K, Williams A, Barry B (2001) Effect of melting point of chi-
ral terpenes on human stratum corneum uptake. Int J Pharm 228:
89–97.
OECD (2004a) Test Guideline 428, Skin Absorption, in-vitro Method. In:
OECD Publication Office, Paris.
OECD (2004b) Series on the testing and assessment No.28, Guidance
Document for the conduct of skin absorptin studies.
Price S (1993) Aromatherapy Workbook, London.
Roberts MS, Walter KA (1998) The relationship between structure and bar-
rier function of skin. In: Roberts MS, WalterKS (eds.), Dermal absorption
and Toxicity Assessment, Marcel Dekker Inc., New York, pp. 1–42.
Rose J (1992a) The Aromatherapy Book, Berkeley, CA.
Rose J (1992b) Herbal Studies Course, San Francisco, CA.
Ryman D (1991) Aromatherapy. The encyclopedia of plants and oils and
how they help you, London.
Schaefer U, Loth H (1996) An ex-vivo model for the study of drug penetra-
tion into human skin. Pharm Res 13: S366.
Schmitt S, Schaefer U, Doebler L, Reichling J (2009) Co-operative interac-
tion of monoterpenes and phenylpropanoids on the in-vitro human skin
permeation of complex composed essential oils, Planta Med, in press.
Schuster O, Haag F, Priester H (1986) Transdermal absorption of ter-
penes from essential oils of Pinimenthol-S ointment. Med Welt 37:
100–102.
SciFinder®ScholarTM (1997–2007) http://www.cas.org/SCIFINDER/.
Sheppard-Hanger S (1995) The aromatherapy practitioner reference manual,
Florida.
Strähli W (1940) Dissertation Bern.
Yoshimura F, Nikaido H (1985) Diffusion of beta-lactam antibiotics
through the porin channels of Escherichia coli K-12. Antimicrob Agents
Chemother 27: 84–92.
Pharmazie 65 (2010) 2 105
