Structure and binding of specifically mutated neurotensin fragments on a silver substrate: vibrational studies

Here, we report a systematic study showing an analogy between the activities of peptide structural component interactions with both a metal substrate and a G-protein-coupled seven-transmembrane receptor. In the present work, N-terminal fragments of human neurotensin (NT), NT(1-6), NT(1-8), and NT(1-11), and C-terminal fragments of human neurotensin, NT(8-13) and NT(9-13), as well as six specifically mutated analogues with the following modifications, Acetyl-NT(8-13), [Dab(9)]NT(8-13), [Lys(8),Lys(9)]NT(8-13), [Lys(8)-(®)-Lys(9)]NT(8-13), [Lys(9),Trp(11),Glu(12)]NT(8-13), and Boc[Lys(9),Leu(13)OMe]NT(9-13), were immobilized onto an electrochemically roughened silver electrode surface in an aqueous solution. The orientation of the adsorbed molecules and the adsorption mechanism were determined from surface-enhanced Raman scattering (SERS) spectra. A comparison was made between the structures of the mutated fragments to determine how changes in the mutation of the structure influenced the adsorption properties. The contribution of the structural components to the peptides' ability to interact with the NTR1 receptor was correlated with the SERS patterns. The SERS spectra revealed that the substitution of native amino acids in the investigated peptides slightly influenced their adsorption state on an electrochemically roughened silver surface. Thus, all of the investigated peptides, excluding [Lys(9),Trp(11),Glu(12)]NT(8-13), tended to adsorb to the surface mainly via the oxygen atom of the deprotonated phenol group, and the phenyl ring became rearranged in a slightly different edge-on manner (NT(1-8), NT(1-11), NT(8-13), Acetyl-NT(8-13), [Dab(9)]NT(8-13), [Lys(8),Lys(9)]NT(8-13), [Lys(8)-(®)-Lys(9)]NT(8-13), NT(9-13), and Boc[Lys(9), Leu(13)OMe]NT(9-13)) or in an almost horizontal manner (N(1-6)) of the tyrosine residue. Meanwhile, [Lys(9),Trp(11),Glu(12)]NT(8-13) bound to this substrate through the tilted phenyl coring of the tryptophan residue. Small changes in the enhancement of the CCNH(2), COO(-), and -CONH- group modes upon adsorption, which were consistent with the adsorption of these peptides, also occurred (with slightly different strengths) through the nitrogen and oxygen lone pair of electrons in these groups. However, for NT(1-8), a greater preferential interaction between the guanidine group of Arg(8) and the roughened silver substrate was observed in comparison to that between the guanidine moiety of the other investigated peptides and the substrate. Vibrational spectroscopy was also used to produce an extensive table of Raman and absorption infrared spectra to allow for a rapid and accurate structural determination of these biomolecules and to allow the reader to easily follow the proposed SERS assignments.