Structural properties of bombesin-like peptides revealed by surface-enhanced Raman scattering on roughened silver electrodes.

This work presents a Fourier-transform absorption infrared, Fourier-transform Raman, and surface-enhanced Raman scattering (SERS) study of the following peptides belonging to the bombesin-like family: phyllolitorin, [Leu(8)]phyllolitorin, NMB, NMC, and PG-L. The SERS study was undertaken to understand the adsorption mechanism of bombesin-like peptides on an electrochemically roughened silver electrode surface and to show changes in the adsorption mechanism with alterations in amino acids and small tertiary structures. The SERS spectra presented here shows bands mainly associated with the Trp(8) residue vibrations. The presence of mainly pyrrole coring vibrations for phyllolitorin and [Leu(8)]phyllolitorin and mainly benzene coring modes for NMB and NMC indicated that these groups interact with the roughened silver electrode surface. Furthermore, N(1)-C(8) and C(3)-C(9) bonds of the PG-L indole ring seemed to have nearly a vertical orientation on the electrode surface. In addition, distinct vibrations of the C-S fragment were observed in the SERS spectra of [Leu(8)]phyllolitorin and PG-L. The strong enhancement of the nu(C=O) vibration in the [Leu(8)]phyllolitorin SERS spectrum yielded evidence that the intact C=O bond(s) bind strongly to the silver electrode surface, whereas NMC, phyllolitorin, and NMB were located near the silver surface. This finding was supported by the presence of the nu(C-C(=O)) mode. The amide I band observed at 1642 and 1634 cm(-1) for NMB and NMC, respectively, and the Raman amide III band seen in the 1282-1249 cm(-1) range for all peptides except PG-L, indicate that the strongly hydrogen-bonded alpha-helical conformation and random-coil structure are favored for binding to the surface.