pH and Substrate Effect on Adsorption of Peptides Containing <i>Z</i> and <i>E</i> Dehydrophenylalanine. Surface-Enhanced Raman Spectroscopy Studies on Ag Nanocolloids and Electrodes

The silver substrates and pH dependent surface-enhanced Raman scattering (SERS) spectra of unsaturated derivatives of di- and tripeptides (dehydropeptides) are investigated. Experimental spectra were interpreted with the help of DFT calculations and normal-mode analysis. We choose as objects of our studies modified but natural peptides containing <i>Z</i> or <i>E</i> dehydrophenylalanine (^(Z)/(E)ΔPhe) residue to study an effect of the type of the isomer on the interaction between the peptide and silver surfaces in the form of nanocolloidal particles and an electrochemically roughened electrode. We also observed that the SERS profile is sensitive to both the type of the studied SERS active substrate and pH, especially for the adsorption on the silver colloid. In general, all dehydropeptides interact with both SERS substrates upon deprotonation of the C-end of the molecule. The participation of the other fragments of the adsorbates such as the N-terminal amino group and the dehydroresidue is also manifested in the SERS spectra. Their orientation with respect to the silver surfaces is discussed in detail