The Application of 199Hg NMR and 199mHg Perturbed Angular Correlation (PAC) Spectroscopy to Define the Biological Chemistry of HgII: A Case Study with Designed Two- and Three-Stranded Coiled Coils

• Published in: Chemistry : a European journal 2007-11, Vol.13 (33), p.9178-9190
• Main Authors: Iranzo, Olga, Thulstrup, Peter W., Ryu, Seung-baek, Hemmingsen, Lars, Pecoraro, Vincent L.
• Format: Article
• Language: English
• Subjects: coordination modes; mercury; metalloproteins; NMR spectroscopy; perturbed angular correlation (PAC) spectroscopy
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200701208

The use of de novo designed peptides is a powerful strategy to elucidate HgII–protein interactions and to gain insight into the chemistry of HgII in biological systems. Cysteine derivatives of the designed α‐helical peptides of the TRI family [Ac‐G‐(LaKbAcLdEeEfKg)4‐G‐NH2] bind HgII at high pH values and at peptide/HgII ratios of 3:1 with an unusual trigonal thiolate coordination mode. The resulting HgII complexes are good water‐soluble models for HgII binding to the protein MerR. We have carried out a parallel study using 199Hg NMR and 199mHg perturbed angular correlation (PAC) spectroscopy to characterize the distinct species that are generated under different pH conditions and peptide TRI L9C/HgII ratios. These studies prove for the first time the formation of [Hg{(TRI L9C)2‐(TRI L9CH)}], a dithiolate–HgII complex in the hydrophobic interior of the three‐stranded coiled coil (TRI L9C)3. 199Hg NMR and 199mHg PAC data demonstrate that this dithiolate–HgII 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–HgII 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 L9CH)}] is the result of a compromise between the high affinity of HgII to form dithiolate complexes and the preference of the peptide to form a three‐stranded coiled coil. Mercury speciation: The sensitivity to different HgII environments and the different timescales of 199Hg NMR and 199mHg perturbed angular correlation (PAC) spectroscopy have been crucial to determine the solution structures of HgII bound to three‐ and two‐stranded de novo designed coiled coils. The combination of both spectroscopic techniques can provide valuable results in the study of mercury solution‐state coordination chemistry and speciation.