Quantum-mechanical study of lead coordination in sulfur-rich proteins: mode and structure recognition in UV resonance Raman spectra

Resonance Raman spectra are computed applying the weighted gradient methodology with CIS and DFT gradients to determine the characteristic spectral patterns for Hg(II) and Pb(II) loaded sulfur-rich proteins while excited to a characteristic LMCT electronic transition band. A framework of structure-spectrum relationships is established to assess lead coordination modes via vibrational spectroscopy. Illustrative calculations on Hg(II) complexes agree with experimental data demonstrating reliability and accuracy of the applied methodology. In contrast to Hg(II) complexes, a unique 3-center-4-electron hypervalent C(β)H···S interaction present in lead-sulfur complexes was established and suggested to play a key role in the strong preference for lead versus other metal ions in lead specific proteins such as PbrR691. The characteristic Pb-S symmetric stretching bands, predicted without additional refinements such as scaling of a force field or frequencies, are found around 238 cm(-1) for 3-coordinated lead-sulfur domains and around 228 cm(-1) for 4-coordinated lead-sulfur domains. These results present an experimental challenge for clear detection of lead coordination via solely UVRR spectroscopy. In addition to predicted UVRR spectra, UVRR excitation profiles for relevant vibrational bands of lead-sulfur domains are presented.