Non-resonant SERS study of the adsorption of cytochrome c on a silver electrode

Non-resonant surface-enhanced Raman spectroscopy (SERS) using 632.8 nm excitation was applied to the study of adsorption and redox changes of cytochrome c (cyt c) on a silver electrode. The bands from specifically adsorbed phosphate anions, heme group, and aromatic residues have been assigned in SER spectra. An electrode deactivation procedure was devised to enhance the signal of cyt c relative to that from adsorbed phosphate anions. The intensity of the B-1g modes has been found to be preferentially enhanced via the pre-resonance enhancement mechanism. The redox marker band nu(4)A(1g), not observed in solution cyt c Raman spectra with 632.8 nm excitation, was surface enhanced and seen in SERS at 1370 cm(-1) for cyt c(3+) and 1364cm(-1) for cyt c(2+). The nu(15)B(1g) mode at 742 cm(-1) (cyt c(2+)) was the most intense SERS peak and could serve as an indicator for the redox state of the adsorbed protein. The heme associated propionate modes have been found to be preferentially enhanced in SER spectra compared with solution, indicating the close proximity of the propionate group relative to the surface. Irreversible changes in SER spectra have been observed at potentials more negative than -0.7 V. The diminished intensity of the heme modes, and the appearance of bands associated with tyrosine and phenylalanine residues, as well as amide III mode, have been explained in terms of a new conformational state where the protein is partially unfolded.