Effect of potential on temperature-dependent SERS spectra of neuromedin B on Cu electrode

Adsorption of decapeptide neuromedin B (NMB) on copper electrode has been investigated by in situ surface-enhanced Raman scattering (SERS) spectroelectrochemistry in the temperature interval from 12 to 72 °C at −0.600 and −1.000 V potentials. It was found that intensities of peptide bands decrease at temperatures above 30 °C with higher decrease slope at −1.000 V. Frequency of F12 mode (1004 cm−1) of non-surface-interactive phenylalanine residue was found to be insensitive to temperature variation at both studied electrode potentials, while frequency–temperature curves for surface-interactive groups (Amide-III, methylene) were found to be controlled by the potential. In particular, opposite frequency–temperature trends were detected for Amide-III (Am-III) mode indicating decrease in H-bonding interaction strength of amide CO and N–H groups above 38 °C for −0.600 V, and increase in H-bonding interaction strength between 12 and 72 °C for −1.000 V. Anomalous Am-III temperature-dependence of the frequency at −1.000 V was explained by temperature-induced transformation of a disordered secondary structure to a helix-like conformation. The potential-difference spectrum revealed interaction of methylene groups with Cu surface at sufficiently negative potential values because of the appearance of a soft C–H stretching band near 2825 cm−1 and a broad band near 2904 cm−1 assigned to vibration of a distal C–H bond of the surface-confined methylene group. Consequently, a rapid decrease in frequency of CH2-stretching band with temperature was observed at −1.000 V, while no essential frequency changes were detected for this mode at −0.600 V. The results show that electrode potential controls the temperature-dependence of the frequency for vibrations associated with surface-interactive molecular groups.