Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

10B / 2003G286

Complex Formation of Mercury(II) with Cysteine in Aqueous Solution
Bonnie O. LEUNG, Farideh JALILEHVAND*
University of Calgary, Calgary, Alberta, 2500 University Dr. N.W., T2N 1N4, Canada
Introduction
The toxic properties of inorganic mercury have been
attributed to the high affinity of Hg(II) ions to thiol (-SH)
groups of cysteine residues in proteins and peptides,
damaging enzymes and disrupting membrane transport.
To obtain further insight into the mechanism of thistoxicity, the structure and speciation of mercury(II)complexes in aqueous solutions with the amino acid L+
cysteine (HSCH2CH(NH3 )COO or H2Cys) was studied.
6-Experimental
2+
Under argon, five solutions with [Hg ] ~ 0.09 mol/Land H2Cys/Hg(II) molar ratios of 2.2 (A), 3.3 (B), 4.3 (C),
5.3 (D), and 10.1 (E) were prepared at pH = 11 and their199
Hg NMR and Raman spectra measured. Hg LIII-edge
EXAFS spectra were collected at BL 10-B of the Photon
Factory (PF) of the High Energy Accelerator ResearchOrganization. During the least-squares curve-fitting
analysis, which provides Hg-S bond distances (R) and2
Debye-Waller parameters (σ ), the amplitude reduction
2factor (S0 ) was fixed to 0.9. Principle Component
Analysis (PCA) applied on this series of EXAFS spectra
revealed three different Hg(II)-species. A characteristic
Raman band was used to obtain the concentration of the2[Hg(Cys)2] complex [1].
Results and Discussion
3
Fourier transforms of k -weighted EXAFS oscillations
for solutions A – E show only one peak, corresponding tothe Hg-S path. Least-squares curve- fitting analysis of theEXAFS spectra for these solutions resulted in a roughly
estimated (± 20%) coordination number varying between
2.1 (A) to 3.5 (E), and Hg-S distances of 2.36 Å (A), 2.39
Å (B), 2.44 Å (C), 2.50 Å (D) and 2.50 Å (E), with anaccuracy of ± 0.02 Å. A literature survey of Hg(II)thiolate crystals indicated that the mean Hg-S bonddistance falls in the range 2.32–2.36 Å for linear[Hg(SR)2] complexes, 2.42–2.44 Å for trigonal [Hg(SR)3]
2and 2.52-2.54 Å for distorted tetrahedral [Hg(SR)4]
complexes. By comparing the mean Hg-S distances fromthe EXAFS of solutions A – E with those from crystal
structures, it may be concluded that in solution A, a linear
2[Hg(Cys)2] complex and in solution C, a three4coordinated [Hg(Cys)3] complex is dominating. However,other solutions contain mixture of three Hg(Cys)n (n= 2, 3,4) species, as indicated by the PCA analysis. To determine
the amount of each complex in solutions A – E, theEXAFS oscillations for Hg(Cys)n (n= 2, 3, 4) weregenerated theoretically by means of FEFF8.1, and a linear
combination was fitted to the experimental data for eachsolution (Table 1, Figure 2).

10Users' Report

Figure 1. The proposed structure of the novel [Hg(Cys)4]
species, from a simulation. The presence of this species in199
solutions with H2Cys/Hg(II) ≥ 5 was confirmed by Hg
NMR [1].
The best fits were obtained by assuming average Hg-S
distances of 2.35(2), 2.44(2) and 2.52(2) Å for Hg(Cys)2,Hg(Cys)3 and Hg(Cys)4 species, respectively, with thecorresponding Debye-Waller parameters of 0.003, 0.006
2and 0.008 Å , calculated by the FEFF 8.1 program.

3Figure 2. Hg LIII-edge k - EXAFS oscillation of solution
C fitted to linear combination of Hg-S interaction inHg(Cys)2 (12%), Hg(Cys)3 (62%) and Hg(Cys)4 (26%).
Table 1. Relative amount (%) of Hg(Cys)n species in
199
solutions A – E from EXAFS, Raman and Hg NMR.
Solution H2Cys/Hg(II) Hg(Cys)2 Hg(Cys)3 Hg(Cys)42.2
94
6A
3.3
4258
B
4.3
126226
C
5.3
52273
D
10.1~15~85EReference[1] F. Jalilehvand et al., Inorg. Chem. 45 (2006) 66.
* faridehj@ucalgary.ca.