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cis-Urocanic acid, a derivative of histidine, is one of the essential components of human skin. We found that it can bind nickel(ii) ions in a pH-dependent manner, with the dissociation constant in the low millimolar range, as revealed by potentiometry, and confirmed by isothermal titration calorimetry and UV-vis spectroscopy. The binding occurs within the physiological skin pH range. Considering the fact that cis-urocanic acid is present in the human skin in concentrations as high as millimolar, this molecule may be a physiologically important player in nickel trafficking in the human organism.
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Dalton
Transactions
PAPER
Cite this: Dalton Trans., 2014, 43,
3196
Received 12th November 2013,
Accepted 2nd December 2013
DOI: 10.1039/c3dt53194e
www.rsc.org/dalton
cis-Urocanic acid as a potential nickel(II) binding
molecule in the human skin
Nina Ewa Wezynfeld, Wojciech Goch, Wojciech Bal and Tomasz Frączyk*
cis-Urocanic acid, a derivative of histidine, is one of the essential components of human skin. We found
that it can bind nickel(II) ions in a pH-dependent manner, with the dissociation constant in the low milli-
molar range, as revealed by potentiometry, and conrmed by isothermal titration calorimetry and UV-vis
spectroscopy. The binding occurs within the physiological skin pH range. Considering the fact that cis-
urocanic acid is present in the human skin in concentrations as high as millimolar, this molecule may be a
physiologically important player in nickel tracking in the human organism.
Introduction
Urocanic acid (UCA, 3-(1H-imidazol-4-yl)propenoic acid) is
present in the uppermost layer of the skin (stratum corneum). It
is produced by histidine ammonia-lyase (histidase) from histi-
dine as a trans isomer (trans-UCA, Scheme 1). As urocanate
hydratase (urocanase), the enzyme which catabolizes trans-
UCA, is not present in the skin, trans-UCA accumulates in the
epidermis to 0.7% dry weight, which means that in live tissue
its concentration is in the millimolar range.
1
trans-UCA iso-
merizes to cis-urocanic acid (cis-UCA) upon exposure to ultra-
violet radiation (Scheme 1). The equilibrium concentration for
cis-UCA reaches 6070% of total epidermal UCA.
2
Both UCA
isomers are removed from the skin in sweat and by the desqua-
mation process; however, some UCA is transported to blood
and consequently to urine.
3,4
UCA, as a component of Natural
Moisturizing Factor (NMF), is involved in the regulation of pH
of the skin and skin hydration.
5,6
On the other hand, cis-UCA
has been shown to play a role in photoimmunosuppression
79
and photocarcinogenesis.
10
The mechanisms of these actions
are yet to be elucidated, however.
Allergy to nickel is one of the most common causes of aller-
gic contact dermatitis. It is estimated that 1015% of women
in the general population are sensitive to nickel.
11
The mech-
anism of this condition has not yet been fully elucidated. One
of the important unexplored factors is the first step of deliver-
ing Ni(II) to the organism. It is possible that a role in it is
played by metal complexes with biomolecules present in the
uppermost layers of the skin.
Considering the similarity of cis-UCA and histidine we
assumed that this molecule can bind Ni(II), as the ability of
histidine to form strong Ni(II) complexes is well documen-
ted.
12,13
Using UV-vis spectrophotometry, potentiometry and
isothermal titration calorimetry (ITC) we proved that the cis-
UCA molecule interacts with Ni(II) ions. We measured the stoi-
chiometry, stability constants, enthalpy and entropy of these
interactions. Our results suggest a possibility of interaction of
cis-UCA with Ni(II) in human skin.
Experimental section
Chemicals and reagents
cis-UCA and trans-UCA (purity 98%) were obtained from
Sigma-Aldrich. Nickel(II) nitrate hexahydrate, 99.999% trace
metal basis, was obtained from Sigma-Aldrich. HEPES was
obtained from Carl-Roth. Potassium nitrate(V) was purchased
from Merck. Deionized, ultra-pure Milli-Q water was used for
sample preparation. In order to prevent UCA isomerisation, all
experiments were performed with special care to protect
samples from ambient light.
Scheme 1 Structures of trans-UCA and cis-UCA.
Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Pawińskiego
5a, 02-106 Warsaw, Poland. E-mail: tfraczyk@ibb.waw.pl; Fax: +48 22 659 4636;
Tel: +48 22 592 5766
3196 |Dalton Trans.,2014,43,31963201 This journal is © The Royal Society of Chemistry 2014
UV-vis titrations
Two types of UV-vis pH titrations were performed. In the first
one, samples containing 0.1 mM cis-UCA were titrated with
NaOH in the pH range of 4.79.8. In the second type, samples
containing 4 mM cis-UCA or trans-UCA, 2 mM Ni(NO
3
)
2
and
100 mM KNO
3
were titrated with NaOH in the pH range of
2.68.0. The UV-vis spectra were obtained using a Lambda 950
UV/Vis/NIR spectrophotometer (Perkin Elmer) at 25 °C over
the spectral range 200850 nm.
Potentiometric titrations
Potentiometric titrations were performed on a 907 Titrando
automatic titrator (Metrohm), using a Biotrode combined glass
electrode (Metrohm). The electrode was calibrated daily by
nitric acid titrations,
14
and 100 mM NaOH (carbon dioxide
free) was used as a titrant. Sample volumes of 1.5 mL were
used. Samples were prepared by dissolving solid cis-ortrans-
UCA in 4 mM HNO
3
/96 mM KNO
3
to obtain a 4 mM UCA con-
centration. The Ni(II) and cis-UCA/trans-UCA complex for-
mation was studied using samples in which the molar ratios
of cis-UCA/trans-UCA to Ni(II) were 2.2 : 1, 3.3 : 1, 4.4 : 1 and
6.7 : 1. All experiments were performed under argon at 25 °C,
in the pH range of 2.711.0 for titration of cis- and trans-UCA,
2.78.0 for Ni(II) and cis-UCA, and 2.76.5 for Ni(II) and trans-
UCA. The collected data were analysed using the SUPERQUAD
and HYPERQUAD programs.
15,16
Five titrations were included
simultaneously into calculations, separately for protonation
and Ni(II) complexation.
ITC titrations
ITC experiments were carried out on the Nano ITC Standard
Volume calorimeter (TA Instrument). The sample cell (950 µL)
was filled with UCA solution and the reference cell was filled
with the Milli-Q water. The syringe (250 µL) was loaded with a
Ni(II) solution. The Milli-Q water was degassed under vacuum
for 15 min before sample preparation. Furthermore, sample
solutions were degassed for 5 min before loading into the cell
and the syringe. The cell solutions contained 5 mM cis-UCA/
trans-UCA, 100 mM HEPES and 100 mM KNO
3
at pH 7.4. The
syringe solutions contained 25 mM or 50 mM Ni(NO
3
)
2
and
100 mM KNO
3
. The pH values of the cell solution did not
change during titrations. The solutions in the cell were stirred
at 170 rpm using the syringe. Volumes of titrant injections
were 2.57 or 4.00 µL. Intervals between injections were 1000 s
to allow complete equilibration of the system. The measure-
ments were performed at 15, 25 or 37 °C for cis-UCA and at
25 °C for trans-UCA. Background titrations, consisting of the
identical syringe solutions and the cell solutions except for cis-
UCA/trans-UCA were subtracted from each experimental titra-
tion. The obtained data were analyzed with the SEDPHAT
program (version 10.58d) utilizing the global fitting feature.
17
The conditional complex formation constants were calculated
for the following equilibria:
KNiL:Ni
2þþUCA NiðUCAÞ
KNiL2:Ni
2þþ2UCA NiðUCAÞ2
Results and discussion
The pH dependence of UV absorption of cis-UCA is presented
in Fig. 1. The most significant changes, observed at 250 and
290 nm, allowed us to determine the pK
a
values for its carboxy-
late and imidazole groups, given in Table 1. The maximum of
absorption blueshifted from 269 nm to 263 nm for pH 1.74.7
and back, from 263 nm to 282 nm for pH 4.79.8. This shift
was not significantly aected for 0.1 mM cis-UCA by equimolar
Ni(II).However, for higher concentrations (4 mM cis-UCA and
2mMNi(
II)), the pH dependent changes were observed in the
regions specific for Ni(II) octahedral complexes (390 nm and
650 nm) (Fig. 2). Such eects were seen neither for cis-UCA
Fig. 1 The pH dependence of UV spectra of 0.1 mM cis-UCA in pH
ranges of 1.664.73 (A) and 4.739.81 (B), and absorbance values of
0.1 mM cis-UCA at 250 and 290 nm (C).
Table 1 Logarithmic protonation constants (log βand pK
a
) for UCA
isomers
Species
cis-UCA trans-UCA
Log β
a
pK
aa
pK
ab
Log β
a
pK
aa
H
2
L 9.532(4) 2.79 2.86(4) 9.342(2) 3.51
HL 6.744(2) 6.74 6.76(4) 5.830(1) 5.83
a
Values determined by potentiometry at 25 °C and I= 0.1 M (KNO
3
).
Standard deviations on the least significant digits, provided by
HYPERQUAD
16
are given in parentheses.
b
Values determined by
revealing the pH dependence of absorbance at 250 and 290 nm at
25 °C for the experiment shown in Fig. 1. Standard deviations on the
least significant digits are given in parentheses.
Such low concentrations were chosen due to the high absorption coecients of
UCA chromophores.
Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2014 Dalton Trans.,2014,43,31963201 | 3197
nor Ni(II) alone. For the cis-UCA/Ni(II) system the absorbance
maxima blue-shifted from 391 nm to 383 nm and from
658 nm to 648 nm. The molar absorption for those bands
increased along with the increasing pH. This eect is typical
for substitutions of oxygen donors in the first coordination
sphere of high-spin Ni(II) complexes with nitrogens.
18
Above
pH 8.0 a green precipitate appeared in the sample. Dissolution
of this precipitate at pH 5 demonstrated that it was composed
predominantly of Ni(OH)
2
, with traces of cis-UCA. Only minor
changes in the spectra were observed for a sample containing
4mMtrans-UCA and 2 mM Ni(II), compared to free trans-UCA,
and the precipitation occurred at pH 6.5. This whitish precipi-
tate was partially re-dissolved at pH 5. The UV-vis spectra
demonstrated that it was composed largely of trans-UCA, but
also contained significant amount of Ni(II). There was no pre-
cipitation of trans-UCA seen in the absence of Ni(II). Therefore,
most likely the precipitate was composed of Ni(trans-UCA)
complex neutralized electrostatically by one OH
ion per Ni(II).
Potentiometric titrations of cis-UCA and trans-UCA alone
and with Ni(II) ions yielded protonation and Ni(II) complexa-
tion constants for both UCA isomers. They are presented in
Tables 1 and 2 in the logarithmic form. Protonation constants
for both ligands are in a satisfactory accordance with the
literature data, obtained solely by UV-vis (lit.,
19
3.43 and 5.80
for trans-UCA, and 2.7 and 6.65 for cis-UCA). The pK
a
values
determined by potentiometry are also close to those obtained
in our UV-vis titrations. The slight dierence (Δ= 0.07) in pK
a
values for the carboxyl group is probably the result of dier-
ence in the ionic strength between these experiments.
Both cis-UCA and trans-UCA bind Ni(II) in the 1 : 1 and
1 : 2 metal to UCA ratios. The distributions of species for cis-
UCA/trans-UCA and Ni(II), calculated on the basis of potentio-
metric constants, are shown in Fig. 3. It is worth noting that
the complex formation occurs in a pH range specific for
stratum corneum (pH 56), with the predominance of 1 : 1
complex, and is even more pronounced for deeper layers of the
skin (pH 67.4), with higher amounts of 1 : 2 complex.
On the basis of potentiometric and UV-vis titration data,
using the least square method, the spectra of Ni(cis-UCA) and
Ni(cis-UCA)
2
complexes were calculated and are shown in
Fig. 4. The absorbance maxima for these complexes are 387
(ε/dm
3
mol
1
cm
1
; 10.5) and 658 nm (2.9), and, 380 (15.3)
and 640 nm (5.3), respectively. The blueshift of the bands for
Fig. 2 Selected spectra of Ni(II) (2 mM Ni (NO
3
)
2
, dashed line) and its
complexes with cis-UCA (4 mM). The pH values are indicated in the
graph.
Table 2 Logarithmic Ni(II) complex formation constants for UCA
isomers
Species
cis-UCA trans-UCA
15 °C 25 °C 37 °C 25 °C
Log K
a
Log K
a
Log β
b
Log K
a
Log K
a
Log β
b
NiL 3.09(4) 3.09(2) 3.406(4) 3.13(3) 2.53(3) 2.599(7)
NiL
2
5.72(8) 5.61(3) 6.239(4) 5.61(8) 4.04(7) 5.272(7)
a
Conditional association constant values determined by ITC at 15, 25
and 37 °C, in 0.1 M KNO
3
, 0.1 M HEPES and pH 7.4. Standard
deviations on the least significant digits, provided by SEDPHAT
17
are
given in parentheses.
b
Values determined by potentiometry at 25 °C
and I= 0.1 M (KNO
3
). Standard deviations on the least significant
digits, provided by HYPERQUAD
16
are given in parentheses.
Fig. 3 Species distributions for Ni(II) complexes in a solution of 2 mM
Ni(II) and 4 mM cis-UCA (A), 1 µM Ni (II) and 14 mM cis-UCA (B), 2 mM
Ni(II) and 4 mM trans-UCA (C) and 1 µM Ni(II) and 6 mM trans-UCA (D),
based on potentiometric data shown in Tables 1 and 2.
Fig. 4 Spectra of Ni(cis-UCA) and Ni(cis-UCA)
2
complexes, calculated
on the basis of potentiometric data shown in Tables 1 and 2, and UV-vis
titration data shown in Fig. 2.
Paper Dalton Transactions
3198 |Dalton Trans.,2014,43,31963201 This journal is © The Royal Society of Chemistry 2014
the latter complex is in agreement with the stronger ligand
field eect exerted by the imidazole nitrogen of UCA vs. water
molecules.
18
The analysis of relation between the complex stability and
the ligand basicity for imidazole-type ligands can help reveal
whether the chelate eect occurs or the ligand binds the metal
ion monodentately by its imidazole nitrogen.
20,21
Specifically,
such dependence is linear for ligands with no chelating
groups, and for Ni(II) and the ionic strength of 0.1 M is
described by the following equation:
log KNi
NiL ¼0:225pKH
HL þ1:380
Such a relation is shown in Fig. 5 for 1 : 1 Ni(II) complexes
taken from the literature,
20,2226
and compared to our data for
cis-UCA and trans-UCA. The stability constant for trans-UCA is
within the linear correlation observed for ligands interacting
only by the imidazole part of the molecule (e.g. imidazole,
4-methylimidazole). It means that only the imidazole nitrogen
binds Ni(II)intrans-UCA. For cis-UCA the stability enhance-
ment is observed, although it is not as large as e.g. histidine or
histamine. It shows that also the carboxylic moiety participates
in the interaction of cis-UCA with Ni(II). The carboxylate is not
so eective in the stabilization of complex as the primary
amine in histamine or the secondary amine in 4-(2-methylamino-
ethyl)imidazole. The Ni(cis-UCA) complex is also not so
stable as complexes with imidazole-4-acetic acid, which corre-
lates with the higher stability of 6-membered chelates over the
7-membered one which we propose here (Fig. 6).
The binding of the second molecule of cis-UCA compared
to the first one is weaker by 0.58 log units (log β
NiL2
log β
NiL
).
The nature of this eect is largely statistical, because this
dierence is only slightly dierent from the statistical factor of
0.38, resulting from the decreased number of bidentate
binding modes for the second ligand molecule (10) vs. the first
one (24). The bigger dierence is probably due to the repulsion
between the carboxyl groups of the two cis-UCA molecules.
The analogous statistical factor for the monodentate
binding of the second trans-UCA molecule is much lower and
equal to 0.08 (log(6/5)). This explains almost the same binding
stability of the first and the second trans-UCA molecule.
ITC experiments were conducted at 15, 25 and 37 °C for cis-
UCA and at 25 °C for trans-UCA, at pH 7.4. The results are pre-
sented in Tables 2 and 3. Temperature influences only slightly
the logarithmic stability constants for Ni(cis-UCA) and Ni(cis-
UCA)
2
complexes and the binding enthalpy and entropic con-
tribution (TΔS) for the 1 : 1 complex. The binding enthalpy
for the Ni(cis-UCA)
2
complex is higher at lower temperatures.
On the other hand, the entropic contribution is higher at
higher temperatures for the Ni(cis-UCA)
2
complex. In the
result, the decrease of enthalpy at higher temperatures is com-
pensated by the increase of the entropic contribution, which is
reflected in Gibbs energy values. The formation of Ni(II) com-
plexes with cis-UCA is energetically more favoured compared to
trans-UCA in the same conditions.
Ni(II) complex formation constants are higher for cis-UCA
than for trans-UCA, as revealed by both potentiometry and ITC.
All values obtained for 25 °C by ITC are lower when compared
with those obtained by potentiometry at the same temperature.
The probable reason for this is the dierent ionic strength
used in ITC (0.15 M) and the presence of HEPES buer in
these samples.
Fig. 5 The relation between the Ni(II) complex stability and the ligand
basicity for imidazole-type ligands, shown for 1 : 1 Ni (II) complexes taken
from the literature,
21
and compared to our data for cis-UCA and trans-
UCA. The blue line represents linear dependence for ligands binding
Ni(II) monodentately by the imidazole nitrogen (blue circles) described
by the equation: log K
Ni
NiL
= 0.225pK
H
HL
+ 1.380, valid for the 0.1 M ionic
strength.
19,20
Ligands with chelating groups (red squares) show stability
enhancement. Data shown are for histidine
22
(1), 4-(2-methylamino-
ethyl)imidazole
23
(2), histamine
24
(3), 2,2-biimidazole
25
(4), imida-
zole-4-acetate
25
(5), 4(5)-aminoimidazole-5(4)-carboxamide
25
(6),
5-chloro-1-methylimidazole
20
(7), trans-UCA (8), cis-UCA (9), imida-
zole
20,24
(10), N-(2,3,5,6-tetrauorophenyl)imidazole
20
(11), 4-(imida-
zol-1-yl)acetophenone
20
(12), acetylhistidine
26
(13) and 1-methyl-
imidazole
20
(14).
Fig. 6 The proposed structure of the Ni(cis-UCA) complex. Shown are
Ni(II), cis-UCA and four water molecules in an octahedral complex.
Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2014 Dalton Trans.,2014,43,31963201 | 3199
Of note are higher Gibbs energy values for NiL
2
complexes
compared with NiL, which is in agreement with absorbance
maxima being shifted to shorter wavelengths for Ni(cis-UCA)
2
complex, reflecting the presence of two nitrogen ligands in
NiL
2
complexes.
We showed that both UCA isomers can bind Ni(II) ions with
moderate anity, with a stronger eect visible for cis-UCA. Of
note is a correlation between concentration values in the
stratum corneum of cis-UCA and Ni(II) ions upon the contact
with the nickel-releasing material, which are the highest in
this part of the skin. Considering these facts we propose that
urocanic acid, especially the cis isomer, can be Ni(II) binding
molecule in the human skin. Based on the literature
27,28
we
estimated concentrations of main components of NMF as
43 mM serine, 30 mM glycine, 23 mM pyroglutamic acid,
18 mM alanine, 14 mM lactic acid, 14 mM cis-UCA and 6 mM
trans-UCA. Concentrations of nickel can be estimated taking as
the reference the maximum value, allowed by the European
Union Nickel Directive (Directive 94/27/EC) for the release of
this metal from products intended to come into direct, pro-
longed contact with skin, being equal to 0.5 µg cm
2
per
week.
11
Locally this concentration can probably easily reach a
micromolar level in the stratum corneum, especially because
some kinds of jewelry and coins release 100 times more nickel
than allowed by the Directive.
29,30
Taking the protonation con-
stants and stability constants of Ni(II) complexes from the lit-
erature
26,31,32
and from this paper we calculated the
distribution of Ni(II) species for above concentrations of
ligands and of 1 µM nickel ions (Fig. 7). At pH 5.6, 22% of Ni
(II) is complexed with cis-UCA, which places this molecule as
the second binding compound, after serine. 10% of Ni(II)isin
complex with trans-UCA. At pH 6.5, still 22% of Ni(II) is bound
with cis-UCA, after serine. The formation of Ni(II)-UCA com-
plexes can be even more pronounced following the intended
use of cis-UCA as a component of pharmaceuticals. Topical
application of cis-UCA has been proposed as a safe and
eective way to treat inflammatory skin disorders.
8,9
Deeper
investigation is needed as it was shown that cis-UCA can form
a complex with human serum albumin (HSA),
33
another Ni(II)-
binding biomolecule.
34
The general important issue which
remains to be resolved is whether cis-UCA plays a role of a
transporting or rather a buering agent for Ni(II) ions in the
human organism.
Conclusions
We proved that cis-UCA molecule interacts with Ni(II) ions with
moderate stability, the complexes being strong enough to exist
in physiological conditions in the outermost layers of the
human skin, with consequences for nickel toxicity.
Acknowledgements
This work was supported in part by the project Metal depen-
dent peptide hydrolysis. Tools and mechanisms for biotech-
nology, toxicology and supramolecular chemistrycarried out
as part of the Foundation for Polish Science TEAM/2009-4/1
program, co-financed from European Regional Development
Fund resources within the framework of Operational Program
Innovative Economy. The equipment used was sponsored in
part by the Centre for Preclinical Research and Technology
(CePT), a project co-sponsored by European Regional Develop-
ment Fund and Innovative Economy, The National Cohesion
Strategy of Poland.
Notes and references
1 T. Mohammad, H. Morrison and H. HogenEsch, Photo-
chem. Photobiol., 1999, 69, 115.
2 N. K. Gibbs, J. Tye and M. Norval, Photochem. Photobiol.
Sci., 2008, 7, 655.
3 A. M. Moodyclie, M. Norval, I. Kimber and T. J. Simpson,
Immunology, 1993, 79, 667.
Fig. 7 Ni(II) species distribution simulated for NMF and 1 µM Ni
2+
ions.
The protonation constants and stability constants of Ni(II) complexes
were taken from the literature
26,31,32
and from this paper. Concen-
trations taken for calculations are 43 mM serine, 30 mM glycine, 23 mM
pyroglutamic acid (PCA), 18 mM alanine, 14 mM lactic acid (Lac), 14 mM
cis-UCA and 6 mM trans-UCA.
27,28
The binding of Ni(II) by alanine in
these conditions is negligible and omitted for clarity.
Table 3 Thermodynamic parameters for Ni(II) complexation deter-
mined at pH 7.4 by ITC
a
cis-UCA
trans-UCA
15 °C 25 °C 37 °C 25 °C
ΔH
NiL
18.6(0.2) 18.2(0.1) 18.4(0.4) 20.0(0.3)
ΔH
NiL2
26.7(1.6) 22.7(0.7) 18.5(2.4) 20.5(0.1)
TΔS
NiL
1.5 0.6 0.2 5.6
TΔS
NiL2
4.8 9.4 15.7 2.5
ΔG
NiL
17.1 17.6 18.6 14.4
ΔG
NiL2
31.5 32.1 34.2 23
a
The values are presented in kJ mol
1
. Standard deviations on the
least significant digits, provided by SEDPHAT
17
are given in
parentheses.
Paper Dalton Transactions
3200 |Dalton Trans.,2014,43,31963201 This journal is © The Royal Society of Chemistry 2014
4 A. Kammeyer, S. Pavel, S. S. Asghar, J. D. Bos and
M. B. M. Teunissen, Photochem. Photobiol., 1997, 65, 593.
5 P. M. Krien and M. Kermici, J. Invest. Dermatol., 2000, 115,
414.
6 S. Kezic, A. Kammeyer, F. Calkoen, J. W. Fluhr and
J. D. Bos, Br. J. Dermatol., 2009, 161, 1098.
7 M. Norval and A. A. El-Ghorr, Methods, 2002, 28, 63.
8 M. V. Dahl, G. N. McEwen Jr. and H. I. Katz, Photodermatol.
Photoimmunol. Photomed., 2010, 26, 303.
9 J. K. Laihia, P. Taimen, H. Kujari and L. Leino,
Br. J. Dermatol., 2012, 167, 506.
10 S. Beissert, D. Ruhlemann, T. Mohammad, S. Grabbe,
A. El-Ghorr, M. Norval, H. Morrison, R. D. Granstein and
T. Schwarz, J. Immunol., 2001, 167, 6232.
11 S. Garg, J. P. Thyssen, W. Uter, A. Schnuch, J. D. Johansen,
T. Menne, A. Belloni Fortina, B. Statham and
D. J. Gawkrodger, Br. J. Dermatol., 2013, 169, 854.
12 Y. Zhang, S. Akilesh and D. E. Wilcox, Inorg. Chem., 2000,
39, 3057.
13 G. Garrido, C. Rafols and E. Bosch, Talanta, 2011, 84, 347.
14 H. Irving, M. G. Miles and L. D. Pettit, Anal. Chim. Acta,
1967, 38, 475.
15 P. Gans, A. Sabatini and A. Vacca, J. Chem. Soc., Dalton
Trans., 1985, 1195.
16 P. Gans, A. Sabatini and A. Vacca, Talanta, 1996, 43,
1739.
17 J. C. D. Houtman, P. H. Brown, B. Bowden, H. Yamaguchi,
E. Appella, L. E. Samelson and P. Schuck, Protein Sci., 2007,
16, 30.
18 H. Sigel and R. B. Martin, Chem. Rev., 1982, 82, 385.
19 P. Juusola, P. Minkkinen, L. Leino and J. K. Laihia,
Monatsh. Chem., 2007, 138, 951.
20 L. E. Kapinos, B. Song and H. Sigel, Inorg. Chim. Acta, 1998,
280, 50.
21 H. Sigel, A. Saha, N. Saha, P. Carloni, L. E. Kapinos and
R. Griesser, J. Inorg. Biochem., 2000, 78, 129.
22 A. Krężel and W. Bal, Chem. Res. Toxicol., 2004, 17, 392.
23 A. Braibanti, F. Dallavalle, E. Leporati and G. Mori, J. Chem.
Soc., Dalton Trans., 1973, 2539.
24 S. Sjoberg, Pure Appl. Chem., 1997, 69, 1549.
25 I. Torok, P. Surdy, A. Rockenbauer, L. Korecz Jr., G. J. A.
A. Koolhaas and T. Gajda, J. Inorg. Biochem., 1998, 71,7.
26 Y. Yamada, N. Nakasuka and M. Tanaka, Inorg. Chim. Acta,
1991, 185, 49.
27 M. O. Visscher, G. T. Tolia, R. R. Wicket and S. B. Hoath,
J. Cosmet. Sci., 2003, 54, 289.
28 A. V. Rawlings and C. R. Harding, Dermatol. Ther., 2004, 17,
43.
29 F. O. Nestle, H. Speidel and M. O. Speidel, Nature, 2002,
419, 132.
30 A. Schunch, J. Wolter, J. Geier and W. Uter, Contact Derma-
titis, 2011, 64, 142.
31 A. E. Martell and R. M. Smith, in Critical stability constants,
Plenum Press, New York, 1982, vol. 5.
32 R. Portanova, L. H. J. Lajunen, M. Tolazzi and J. Piispanen,
Pure Appl. Chem., 2003, 75, 495.
33 B. Schwarzinger and H. Falk, Monatsh. Chem., 2004, 135,
1297.
34 W. Bal, J. Christodoulou, P. J. Sadler and A. Tucker, J. Inorg.
Biochem., 1998, 70, 33.
Dalton Transactions Paper
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... We have previously shown that UCA can bind Ni(II) ions [17]. Bearing in mind the similarity of Ni(II) and Cu(II) complex formation with many low molecular weight compounds, copper binding by UCA is conceivable. ...
... A series of control experiments was performed for both cis-and trans-UCA in presence of Cu(II). We have previously characterised the interaction of cis-and trans-UCA with Ni(II) ions and found out that the complexes were sufficiently stable to be of potential biological relevance [17]. Of the two isomers, cis-UCA binds Ni(II) ions more strongly owing to the chelate effect of its imidazole nitrogen and carboxyl oxygen. ...
... A series of control experiments was performed for both cisand trans-UCA in presence of Cu(II). We have previously characterised the interaction of cisand trans-UCA with Ni(II) ions and found out that the complexes were sufficiently stable to be of potential biological relevance [17]. Of the two isomers, cis-UCA binds Ni(II) ions more strongly owing to the chelate effect of its imidazole nitrogen and carboxyl oxygen. ...
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The tripeptide NH2–Gly–His–Lys–COOH (GHK), cis-urocanic acid (cis-UCA) and Cu(II) ions are physiological constituents of the human body and they co-occur (e.g., in the skin and the plasma). While GHK is known as Cu(II)-binding molecule, we found that urocanic acid also coordinates Cu(II) ions. Furthermore, both ligands create ternary Cu(II) complex being probably physiologically functional species. Regarding the natural concentrations of the studied molecules in some human tissues, together with the affinities reported here, we conclude that the ternary complex [GHK][Cu(II)][cis-urocanic acid] may be partly responsible for biological effects of GHK and urocanic acid described in the literature.
... Metal presence in Vycor and silica gel composition can be verified by their spectroscopic properties [23,24]. The UV-Vis absorption spectrum of the Ni(II) is stable [25] and does not change in silica gel [26]. The UV-Vis spectrum of 0.14 M NiCl 2 ·6H 2 O solution and 10% Ni(II)@ORMOSIL show similar absorption peaks at about 400 nm ( Figure S2). ...
... Metal presence in Vycor and silica gel composition can be verified by their spectroscopic properties [23,24]. The UV-Vis absorption spectrum of the Ni(II) is stable [25] and does not change in silica gel [26]. The UV-Vis spectrum of 0.14 M NiCl2•6H2O solution and 10% Ni(II)@ORMOSIL show similar absorption peaks at about 400 nm (Figure S2). ...
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... On the other hand, the binding of Ni(II) to Aβ 5−9 was strong enough to prevent the Ni(OH) 2 precipitation noticed for other biomolecules, such as isomers of urocanic acid or peptide models of the prion protein. 53,54 Thus, the stability of the Ni(II)−Aβ 5−9 complex should be sufficient for its employment as a receptor for molecules of low Ni(II) affinity (such as phosphates). ...
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... Based on the spectra obtained for systems with nickel ions, the largest changes were observed in wavelength range 600-800 nm. Overall, these results suggest a change in the internal coordination sphere of metal ion with an increase in pH [43]. ...
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... To improve drug stability, we rationally selected urocanic acid (UA), a molecule endogenously abundant in the human skin (Safer et al., 2007), as an excipient for TAF fb in LA platforms. UA has pH buffering properties (Wezynfeld et al., 2014) and unlike typical pH buffers, presents low solubility comparable to TAF fb . This allows UA to be released from the implant with similar release kinetics as TAF fb , while maintaining stable pH conditions within the range of 5.2 to 5.4 in the implant for a long duration. ...
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... In girls, urocanic acid, a breakdown (deamination) product of histidine, showed the highest level. Urocanic acid is one of the essential components of human skin [26]. It could accumulate in the epidermis and may be both a UV protectant and an immunoregulator. ...
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... 5-7 Although the mechanisms mediating the association between nickel allergy and FLG mutations are unknown, increased epidermal penetration of nickel because of a reduction in chelating histidine is a possibility. 8,9 Epidermal deficiency of filaggrin is also associated with steadystate skin inflammation. 10-14 For example, increased levels of interleukin (IL)-1β are found in the SC of uninvolved skin and in blister fluid of ...
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... UA has recently been the focus of study in a multitude of scientific fields such as macromolecular chemistry, transitionmetal chemistry, genetics, medicine and dermatology. [5,6,7,8,9,10,11,12] Spectroscopic and kinetic studies on UA in aqueous solution are available in abundance. [13,14,15,16,17,18,19,20,21,22,23,24,25] Probably the most important result of these studies is the identification of a wavelengthdependent quantum yield for E→Z photoisomerization. ...
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A new computer program has been developed in which formation constants are determined by minimisation of an error-square sum based on measured electrode potentials. The program also permits refinement of any reactant concentration or standard electrode potential. The refinement is incorporated into a new procedure which can be used for model selection.
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Stability constants for different aliphatic 2-hydroxycarboxylic acid complexes in aqueous solutions with protons and metal ions published between 1960 and the end of 1994 have been critically evaluated.
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Nickel(II)-, copper(II)- and zinc(II)-complexes of four imidazole derivatives (4(5)-aminoimidazole-5(4)-carboxamide (aic), 2,2′-biimidazole (biim), bis(1,1′-imidazol-2-yl)(4-imidazol-4(5)-yl)-2-aza-butane (biib) and imidazole-4-acetic acid (iaa)), having different coordination environments around the imidazole ring(s), have been studied by potentiometric, UV–VIS and EPR spectroscopic methods. The data revealed very strong bidentate coordination of biim, in spite of the low basicity of the donor sites. Three dominant species (CuLH, Cu2L2, Cu2LH−1) are formed in the Cu(II)-biib system. Since the biib offers extremely stable tri- and tetradentate coordination to copper(II) (in CuLH and Cu2L2, respectively), the bis-imidazolyl like coordination, which is predominant in metal complexes of any other bis-imidazolyl like ligand, does not appear in its original form in the copper(II)-biib system. The EPR spectra of above dimer species show coupling between the copper(II) centers. In the M(II)-iaa systems only parent complexes are formed up to pH 9, while at higher pH further deprotonations were observed, which resulted in a formation of dimer complexes in case of copper(II), having antiferromagnetically coupled metal centers.