RAMAN-SPECTROSCOPY OF SULFUR-CONTAINING AMINO-ACIDS AND THEIR DERIVATIVES ADSORBED ON SILVER

Surface-enhanced Raman scattering (SERS) of sulphur-containing amino acids and peptides (L-cysteine, L-cystine, L-methionine, L-methionylglycine and glycyl-L-methionine) in silver sol was investigated. The Raman signal was enhanced by a factor of about 10(5). In the pH range 5-11, L-cysteine was found to absorb on the silver surface through the sulphur atom and also the carboxylate and amino groups. The changes in the SER spectrum of L-cysteine observed on lowering the pH of the sol solution below 5 suggested that the amino group was desorbed from the surface on protonation. The SER spectrum of L-cystine was similar to that of L-cysteine, indicating cleavage of the S-S bond on adsorption on the silver surface. L-Methionine, L-methionylglycine and glycyl-L-methionine also seemed to adsorb via the sulphur atom and also the carboxylate and amino groups in the pH range 5-9. The SER spectra of these compounds showed no concentration dependence in the ranges investigated.