Raman spectroscopy of sulphur‐containing amino acids and their derivatives adsorbed on silver

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Abstract
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 105. 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.