Indirect detection of mercury-199 nuclear magnetic resonance spectra of methylmercury complexes

Measurement of 199Hg NMR spectra of methylmercury species by the heteronuclear multiple quantum coherence (HMQC) indirect proton detection method and its application to the study of the solution chemistry of CH3HgII-thiol ligand complexes are described. A sensitivity enhancement factor of 16 is obtained for measurement of the 199Hg NMR spectrum of a 4 mM solution of the CH3HgII-glutathione complex by the indirect detection method, as compared to a theoretical enhancement of 74. The less-than-theoretical enhancement is attributed to loss of signal by relaxation during the HMQC pulse sequence. Longitudinal relaxation of the 199Hg in CH3HgII-thiolate complexes is fast at the field strength used in this research (11.7 T) due to efficient relaxation by the chemical shift anisotropy mechanism. This in turn causes the transverse relaxation rate for the 199Hg spin-coupled methyl protons to be fast due to efficient relaxation by another mechanism, scalar relaxation of the second kind. Depending on the rate of exchange of CH3HgII among its complexed forms, the 199Hg line width can also include a large contribution from exchange broadening. In such cases, it is shown that extremely broad 199Hg resonances, which would be difficult to detect by direction observation, can be observed by the indirect detection method. For example, 199Hg resonances with exchange-broadened line widths up to 8000 Hz are observed by the indirect detection method for CH3HgII in mixtures of CH3HgOD and the CH3HgII-mercaptoethanol or CH3HgII-glutathione complex. The chemical shift of 199Hg in CH3HgII-thiolate ligand complexes is found to be extremely sensitive to the nature of the thiolate ligand.(ABSTRACT TRUNCATED AT 250 WORDS)