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 degrees C at -0.600 and -1.000 V potentials. It was found that intensities of peptide bands decrease at temperatures above 30 degrees 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 C-O and N-H groups above 38 degrees C for -0.600 V, and increase in H-bonding interaction strength between 12 and 72 degrees 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.