Surface-enhanced Raman difference between bombesin and its modified analogues on the colloidal and electrochemically roughen silver surfaces

In this article, surface-enhanced Raman scattering (SERS) spectra of bombesin (BN) and its six modified analogues (D-Phe(12)]BN, Tyr(4)]BN, Tyr(4),D-Phe(12)]BN, D-Phe(12),Leu(14)]BN, Leu(13)-(R)-Leu(14)]BN, and Lys(3)]BN) on a colloidal silver surface are reported and compared with SERS spectra of these species immobilized onto an ellectrochemically roughen silver electrode. Changes in enhancement and wavenumber of proper bands upon adsorption on different silver surfaces are consistent with BN and its analogues adsorption primarily through Trp(8). Slightly different adsorption states of these molecules are observed depending upon natural amino acids substitution. For example, the indole ring in all the peptides interacts with silver nanoparticles in a edge-on orientation. It is additionally coordinated to the silver through the N(1)--H bond for all the peptides, except Phe(12)]BN. This is in contrary to the results obtained for the silver roughen electrode that show direct but not strong N(1)--H/Ag interaction for all peptides except D-Phe(12),Leu(14)]BN and Leu(13)-(R)-Leu(14)]BN. For BN only C==O is not involved in the chemical coordination with the colloidal surface. Lys(3)]BN and BN also adsorb with the C--N bond of NH(2) group normal and horizontal, respectively, to the colloidal surface, whereas C--NH(2) in other peptides is tilted to this surface. Also, the Trp(8) --CH(2)-- moiety of only Tyr(4)]BN, Lys(3)]BN, and Tyr(4),D-Phe(12)]BN coordinates to Ag, whereas the Phe(12) ring of Phe(12)]BN, Tyr(4),D-Phe(12)]BN, and D-Phe(12),Leu(14)]BN assists in the peptides binding only on the colloidal silver.