UVRR spectroscopy and vibrational analysis of mercury thiolate compounds resembling d(10) metal binding sites in proteins

Raman, ultraviolet resonance Raman (UVRR) and far-IR spectra are reported f or the mercury-cysteamine complex, Hg(SCH2CH2NH2)(2). Band assignments are made for Hg(SCH2CH2NH2)(2), and also for [Hg(SBut)(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(SB ut)(3)](-) and [Hg(SMe)(3)](-) have virtually the same Hg-S bond lengths, b ut very different nu(s) HgS frequencies, 196 and 268 cm(-1), in good agreem ent with the experimental data. The exceptionally low HgS frequency for [HS (SBut)(3)](-) compared to [Hg(SMe)(3)](-) and to the Hg-MerR protein result s from kinematic interactions of the Hg-S stretching and S-C-C bending coor dinates when all three substituents at C-alpha are carbon atoms. For Hg(SCH 2CH2NH2)(2), the HgS stretching coordinate is distributed over three modes, at 339, 273, and 217 cm(-1), all of which exhibit UVRR enhancement. The ot her contributors to these modes are angle bending and torsional coordinates of the chelate rings. Involvement of the CCN bending coordinates is suppor ted by observed and calculated frequency shifts in D2O. The excitation prof iles track the main UV absorption band, associated with S-Hg charge transfe r. Enhancement is attributable to the weakening of the Hg-S bonds in the ex cited state, and probably to changes in the SCC bond angle. Also enhanced, albeit weakly, is the yes mode at 658 cm(-1), reflecting C-S bond shortenin g in the excited state. The mingling of metal-sulfur and internal ligand co ordinates is reminiscent of the mingling seen in RR spectra of type 1 Cu pr oteins. In both cases the phenomenon may be associated with elevated torsio nal contributions associated with the rigidity of the ligands.