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The Application of 199 Hg NMR and 199m Hg Perturbed Angular Correlation (PAC) Spectroscopy to Define the Biological Chemistry of Hg II : A Case Study with Designed Two‐ and Three‐Stranded Coiled Coils
Author(s) -
Iranzo Olga,
Thulstrup Peter W.,
Ryu Seungbaek,
Hemmingsen Lars,
Pecoraro Vincent L.
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200701208
Subject(s) - chemistry , deprotonation , protonation , nuclear magnetic resonance spectroscopy , crystallography , cysteine , coordination complex , peptide , spectroscopy , stereochemistry , metal , ion , organic chemistry , biochemistry , enzyme , physics , quantum mechanics
The use of de novo designed peptides is a powerful strategy to elucidate Hg II –protein interactions and to gain insight into the chemistry of Hg II in biological systems. Cysteine derivatives of the designed α‐helical peptides of the TRI family [Ac‐G‐(L a K b A c L d E e E f K g ) 4 ‐G‐NH 2 ] bind Hg II at high pH values and at peptide/Hg II ratios of 3:1 with an unusual trigonal thiolate coordination mode. The resulting Hg II complexes are good water‐soluble models for Hg II binding to the protein MerR. We have carried out a parallel study using 199 Hg NMR and 199m Hg perturbed angular correlation (PAC) spectroscopy to characterize the distinct species that are generated under different pH conditions and peptide TRI L9C/Hg II ratios. These studies prove for the first time the formation of [Hg{(TRI L9C) 2 ‐(TRI L9CH)}], a dithiolate–Hg II complex in the hydrophobic interior of the three‐stranded coiled coil (TRI L9C) 3 . 199 Hg NMR and 199m Hg PAC data demonstrate that this dithiolate–Hg II complex is different from the dithiolate [Hg(TRI L9C) 2 ], and that the presence of third α‐helix, containing a protonated cysteine, breaks the symmetry of the coordination environment present in the complex [Hg(TRI L9C) 2 ]. As the pH is raised, the deprotonation of this third cysteine generates the trigonal thiolate–Hg II complex Hg(TRI L9C) 3 − on a timescale that is slower than the NMR timescale (0.01–10 ms). The formation of the species [Hg{(TRI L9C) 2 (TRI L9CH)}] is the result of a compromise between the high affinity of Hg II to form dithiolate complexes and the preference of the peptide to form a three‐stranded coiled coil.