A Raman spectroscopy and theoretical study of zinc–cysteine complexation

The Raman spectrum of the 1:2 zinc–cysteine complex in aqueous solution is compared to those of cysteine zwitterion and cysteine anion in order to identify specific complexation effects. Band assignment is based on frequency calculation and normal modes analysis using the density functional theory (DFT) method. Two bands specific to the complex were detected in the 200–400 cm−1 spectral range. It is shown that the corresponding vibrations are not pure metal–ligand modes but that they result from the coupling between the Zn–S and Zn–N stretching modes with some cysteine internal modes. Raman spectra analysis also provides direct evidence for the deprotonation of the SH and NH3+ groups of cysteine upon zinc binding. It is found in addition that complexation significantly affects the cysteine internal mode mixing in the 500–1500 cm−1 spectral range. The results are considered in connection with the spectral characterization of zinc–protein complexes of biological interest.