Surface‐enhanced Raman spectra of aromatic amino acids and proteins adsorbed by silver hydrosols

Abstract Surface‐enhanced Raman spectra of some amino acids (Phe, Tyr, Trp, His, Leu and Ala) and cytotoxin I from Naja naja oxiana adsorbed by silver hydrosols have been obtained. The spectra are enhanced from 100 to 200 times. Addition of aqueous solutions of the amino acids to the hydrosol resulted in the appearance of an absorption band in the visible spectral range (450–650 nm) due to formation of the hydrosol‐amino acid complex. The surface‐enhanced Raman spectrum and the absorption band disappear after short ultrasonic treatment of the complex. Comparison of electron microphotographs of the silver hydrosol and the hydrosolprotein complex clearly demonstrate coagulation of the silver micelles, apparently caused by protein adsorption and complex formation. The excitation profile of the Phe‐Ag hydrosol complex was obtained and this agreed well with the position of the longwave absorption band. From the analysis of the intensity ratios of the Stokes and anti‐Stokes components of the Raman spectra of Phe in the silver hydrosol and in aqueous solution the mechanism of the resonance Raman effect is strongly supported. The problem of possible conformational changes of the adsorbed molecules was considered. From CD spectra of cytotoxin I in aqueous solution and those in the silver hydrosol it was concluded that the backbone conformation of the protein did not change, but the environment of the aromatic residues altered. Experiments on the binding of an equimolar mixture of Phe and Ala with silver hydrosols showed that competitive binding took place, and thus the aromatic amino acid is mainly responsible for the surface‐enhanced Raman effect.