Comparative studies on IR, Raman, and surface enhanced Raman scattering spectroscopy of dipeptides containing $\Delta$Ala and $\Delta$Phe

Three dipeptides containing dehydroresidues ($\Delta$Ala, $\Delta^{Z}$Phe, and $\Delta^{E}$Phe) were examined by IR, Raman, and surface-enhanced Raman techniques for the first time. The effect of the size and isomer type of the $\beta$-substituent in the dehydroresidue on the conformational structure of the peptide was evaluated by using the analysis of IR and Raman bands. Additionally, SERS spectroscopy provided insight into the adsorption mechanism of these species on the metal surface. SERS spectra were recorded at alkaline pH on the silver sol using visible light excitation. The dehydroresidues studied here strongly influenced the SERS profile of the peptides. The most pronounced SERS signal for all dipeptides was assigned to the symmetric stretching vibration of the carboxylate ions. This indicates that the dehydropeptides studied here primarily adsorb via the deprotonated carboxylic group. Additionally, the enhanced SERS bands in the range 1550-1650 cm$^{-1} $ show differences in contribution of the dehydroresidue to the adsorption mechanism of the studied peptides.