Stability of the Disulfide Bond in Cystine Adsorbed on Silver and Gold Nanoparticles As Evidenced by SERS Data

In relation with the difficulties encountered in previous works concerning the preservation of the S S linkage in cystine (Cys-Cys dimer) on Ag nanoparticles (NPs), we present here a systematic investigation on both cysteine and cystine as a function of various parameters governing the preparation of metal substrates. Surface-enhanced Raman scattering (SEAS) was used as a probe for analyzing (i) the integrity of the disulfide bonds of the adsorbed dimers, (ii) the influence of the metal nature, the reduction protocol as well as the Cys-Cys concentration on the adsorption, (iii) the terminal groups through which the interaction with metal surfaces take place, and (iv) the side chain conformation of the adsorbed molecules. From the whole set of experimental data collected in this work, it appears that large size Au NPs, prepared at low citrate concentration, can be considered as the most appropriate substrates for ensuring the integrity of disulfide linkages. Although Ag NPs prepared with hydroxylamine lead to the cleavage of dimers at low concentration, they have shown their adequacy to keep intact S-S bonds beyond a concentration threshold of similar to 200 mu M. Based on the examination of the SERS data recorded as a function of dimer concentration, we can now assume that this effect is mainly due to the bidentate and monodentate binding of Cys-Cys dimers at low and high concentrations, respectively, facilitating or not their cleavage on Ag surfaces.