Surface-enhanced Raman scattering studies on bombesin, its selected fragments and related peptides adsorbed at the silver colloidal surface

Abstract SERS studies presented in this work on BN 8–14 , [ d -Phe 6 ,β-Ala 11 ,Phe 13 ,Nle 14 ]BN 6–14 , [ d -Tyr 6 , β-Ala 11 ,Phe 13 ,Nle 14 ]BN 6–14 , BN and its modified analogues, as well as NMB , NMC , and PG-L show that these molecules at pH 8.3 bind to a colloidal silver surface mainly through Trp 8 and Met 14 residues. Trp 8 adsorbs at the surface almost perpendicularly. Met 14 appears on the surface mainly as a P C -G conformer. His 12 , as is evident from the spectra, practically does not take part in the adsorption process. Substitution of l -leucine at the 13 position of amino acid sequence with l -phenylalanine does not change substantially the pattern of the adsorption mechanism; however, substitution of phenylalanine at the 12 position (instead of l -histidine) causes changes in the SERS spectra that show that Phe 12 takes parallel orientation to the surface upon adsorption of [ d -Phe 12 ]BN , while in the case of [Tyr 4 , d -Phe 12 ]BN this residue is perpendicular to the surface and influences the orientation of the bound Trp 8 . On the other hand, substitution of Asn with Tyr in the 6 position in nonapeptide fragment causes changes in the adsorption mechanism. In this case, the discussed fragment binds to the silver colloidal surface by Tyr 6 , Trp 8 , and Met 14 . The SERS spectrum of NMC is very similar to that of BN ; although it differs by the binding orientation of the amide bond towards the surface. Appearance of Phe 13 in NMB and PG-L causes that this residue competes successfully with Trp 8 forcing it to take tilted orientation. As seen from the enhancement of the characteristic Phe vibrations this moiety in NMB and PG-L adsorbs on the silver surface in a tilted fashion. This arrangements cause that the 8–14 peptide chain in all these studied compounds takes almost a parallel orientation to the surface while the 1–5 fragment of the peptide chain is removed from the silver surface vicinity.