Adsorption of neurotensin‐family peptides on SERS‐active Ag substrates

Abstract

Kinetensin (KN) and its amino acids 1–8 fragment ([des‐Leu9]KN), neuromedin N (NMN), and xenopsin (XP) and its two analogs (XP‐1 and XP‐2) belong to the neurotensin family of peptides and are known to stimulate the growth of human tumors. In this work, we report Fourier transform‐Raman and surface‐enhanced Raman scattering (SERS) studies of these peptides and discuss their structures, orientation, and mode of adsorption onto a highly specific, electrochemically roughened SERS‐active Ag electrode that is characterized by the formation of a 50–150 nm Ag island on its surface. We show that the investigated peptides bind preferentially to this surface by substantial electronic overlap between the metal surface and the π‐orbitals of the benzene rings of the Phe, Tyr, and Trp residues, which forces them to take parallel or almost parallel orientations with respect to the surface. In addition, the –CH2–, –CNH2, and –COO‐ molecular fragments are involved in interactions with (binding to or in close proximity with) the Ag surface. The SERS data show that the adsorption modes in each of these cases are very similar. In addition, we show that the specific differences in the amino acid sequences do not significantly affect the orientation of the investigated peptides on the Ag substrate. This result implies that the N‐termini of the neurotensin‐family peptides do not influence the mode for adsorption onto the Ag substrates. Copyright © 2012 John Wiley & Sons, Ltd.

Recent advances in linear and nonlinear Raman spectroscopy. Part VII

“…The geometry of these molecules etched on the silver surface was deduced from the observed changes in the Raman bands for the bound versus free species . In a second paper, Podstawka‐Proniewicz and co‐workers studied adsorption of neurotensin‐family peptides on SERS‐active Ag substrates and found that the investigated peptides bound preferentially to the surface by substantial electronic overlap between the metal surface and the pi‐orbitals of the benzene rings of the Phe, Tyr, and Trp residues, which forced them to take close to parallel orientations with respect to the surface . Sassi and co‐workers carried out studies of reversible and irreversible denaturation processes in globular proteins using a variety of spectroscopic probes.…”

Adsorption mode of neurotensin family peptides onto a colloidal silver surface: SERS studies

“…This proposed assignment agrees with the comprehensive systematic SERS study on the human NT structure in an aqueous solution of nanometer‐sized colloidal silver particles under various environmental conditions, including excitation wavelengths (488.0, 514.5, and 785.0 nm), peptide concentrations (10 ‐4 – 10 ‐6 M), pH levels of the solutions (from pH = 2 to pH = 11), H 2 O/D 2 O solvent exchange conditions, single‐ and multi‐site synthetic structural mutations, and length of the NT fragments . Discussed here normal mode assignments also corresponds with the band assignments for human NT, its mutated fragments and analogues, and related peptides in the solid state and immobilized onto the electrochemically roughened silver substrate …”

“…Based on the intensity of the above Tyr and –COO ‐ bands, we propose that together with Phe, the Tyr and –COO ‐ moieties of KN were in close contact with the silver substrate but those of [des‐Leu 9 ]KN were slightly farther from the surface. This finding supports the earlier conclusion about the importance of the Leu 13 COO ‐ residue in the interaction with the silver substrate . The influence of the Leu 13 COO ‐ residue on receptor binding have been also demonstrated in the NT biological activity studies…”

“…Among these bands, the 757 cm ‐1 spectral future (W18) was predominantly enhanced, whereas second characteristic Trp mode, namely W16, showed 75% of the relative intensity of W18 (the reversed intensities in comparison to those in the XP Raman spectrum). These findings, together with the only slight band width widening and no wavenumber shift of the aforementioned Trp bands in comparison with those of the corresponding Raman spectrum, may suggest an approximately vertical position of the XP Trp ring on the colloidal silver surface that is oriented via the co‐pyrrole ring toward the silver surface. The 1362 and 1440 cm ‐1 SERS signals related to the co‐pyrrole –C 8 –N 1 H–C 2 – unit vibrations also imply an orientation of the indole ring that allows the free electron pair on the nitrogen atom to interact with the silver nanoparticles.…”

Structure Characterization of [N-Phenylamino(2-boronphenyl)-R-methyl]phosphonic Acid by Vibrational Spectroscopy and Density Functional Theory Calculations

“…Because the absorption infrared method gives complementary information to the Raman method, supports the Raman analysis, and helps to solve ambiguities during this analysis, the absorption infrared spectra are also examined. Both these methods are commonly employed in both experimental investigations [17][18][19][20] and theoretical calculations [21][22][23] to analyze and compare structures for a large number of conformers of the investigated compounds. Interpreting the Raman and absorption infrared spectra involves explaining spectral regions and wavenumbers based on likely vibrational modes.…”