UVRR Spectroscopy and Vibrational Analysis of Mercury Thiolate Compounds Resembling d(10) Metal Binding Sites in Proteins.

Abstract

Raman, ultraviolet resonance Raman (UVRR) and far-IR spectra are reported for the mercury-cysteamine complex, Hg(SCH(2)CH(2)NH(2))(2). Band assignments are made for Hg(SCH(2)CH(2)NH(2))(2), and also for [Hg(SBu(t))(3)](-) and [Hg(SMe)(3)](-) on the basis of ab initio calculations with the effective core potential approximation and also on the basis of comparison with vibrational data of corresponding thiols. The calculations show that geometry-optimized [Hg(SBu(t))(3)](-) and [Hg(SMe)(3)](-) have virtually the same Hg-S bond lengths, but very different nu(s) HgS frequencies, 196 and 268 cm(-)(1), in good agreement with the experimental data. The exceptionally low HgS frequency for [Hg(SBu(t))(3)](-) compared to [Hg(SMe)(3)](-) and to the Hg-MerR protein results from kinematic interactions of the Hg-S stretching and S-C-C bending coordinates when all three substituents at C(alpha) are carbon atoms. For Hg(SCH(2)CH(2)NH(2))(2), the HgS stretching coordinate is distributed over three modes, at 339, 273, and 217 cm(-)(1), all of which exhibit UVRR enhancement. The other contributors to these modes are angle bending and torsional coordinates of the chelate rings. Involvement of the CCN bending coordinates is supported by observed and calculated frequency shifts in D(2)O. The excitation profiles track the main UV absorption band, associated with S-->Hg charge transfer. Enhancement is attributable to the weakening of the Hg-S bonds in the excited state, and probably to changes in the SCC bond angle. Also enhanced, albeit weakly, is the nu(CS) mode at 658 cm(-)(1), reflecting C-S bond shortening in the excited state. The mingling of metal-sulfur and internal ligand coordinates is reminiscent of the mingling seen in RR spectra of type 1 Cu proteins. In both cases the phenomenon may be associated with elevated torsional contributions associated with the rigidity of the ligands.