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.