Vibrational characterization of L-leucine phosphonate analogues: FT-IR, FT-Raman, and SERS spectroscopy studies and DFT calculations.

This study reports the Raman (FT-Raman) and absorption infrared (FT-IR) spectra, based on calculated wavenumbers and normal modes of vibrations, of the following compounds: L-Leu-D-NH-CH(Me)-PO(3)H(2) (LI), L-Leu-NH-C(Me)(2)-PO(3)H(2) (LII), L-Leu-D-NH-CH(Et)-PO(3)H(2) (LIII), L-Leu-L-NH-CH(Et)-PO(3)H(2) (LIV), L-Leu-L-NH-CH(EtOH)-PO(3)H(2) (LV), L-Leu-NH-C(Me)(Et)-PO(3)H(2) (LVI), L-Leu-L-NH-CH(PrA)-PO(3)H(2) (LVII), L-Leu-L-NH-CH(c-Pr)-PO(3)H(2) (LVIII), L-Leu-L-NH-CH(t-Bu)-PO(3)H(2) (LIX), L-Leu-L-NH-CH(BuA)-PO(3)H(2) (LX), L-Leu-L-NH-CH(c-Bu)-PO(3)H(2) (LXI), and L-Leu-L-NH-C(Adm)-PO(3)H(2) (LXII). The equilibrium geometries and vibrational wavenumbers were calculated using density functional theory (DFT) at the B3LYP, 6-311++G** level using Gaussian 03, Raint, GaussSum 0.8, and Gar2ped software. We briefly compare and analyze the experimental and calculated vibrational wavenumbers in the range 4000-400 cm(-1). In addition, the Raman wavenumbers are compared to those from the surface-enhanced Raman scattering (SERS) spectra for the phosphono analogues of l-leucine (l-Leu) adsorbed on a colloidal silver surface in an aqueous solution. The geometries of these molecules etched on the silver surface were deduced from observed changes in both the intensity and broadness of Raman bands in the spectra of the bound versus free species. For example, LVI appears to adsorb onto the colloidal silver particles mainly through the amine group and amide bond, which assists in the adsorption process, whereas LII shows strongly enhanced SERS bands due to the rocking, twisting, and stretching vibrations of the N(amid)C(sg)(Me)(2)P fragment, suggesting that this peptide's interaction with the silver surface occurs mainly via this fragment. On the other hand, the most dominant SERS bands of LIII and LIV due to the P═O bond stretches reflect P═O···Ag complex formation.

Potential induced changes in neuromedin B adsorption on Ag, Au, and Cu electrodes monitored by surface-enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS), electrochemistry, and generalized two-dimensional correlation analysis (G2DCA) methods were used to define neuromedin B (NMB) ordered superstructures on Ag, Au, and Cu electrode surfaces at different applied electrode potentials in an aqueous solution at physiological pH. The orientation of NMB and the adsorption mechanism were determined based on the analysis of enhancement, broadness, and shift in wavenumber of particular bands, which allow drawing some conclusions about NMB geometry and changes in this geometry upon change of the electrode type and applied electrode potential. The presented data demonstrated that NMB deposited onto the Ag, Au, and Cu electrode surfaces showed bands due to vibrations of the moieties that were in contact/close proximity to the electrode surfaces and thus were located on the same side of the polypeptide backbone. These included the Phe(9) and Trp(4) rings, the sulfur atom of Met(10), and the -CCN- and -C═O units of Asn(2). However, some subtle variations in the arrangement of these fragments upon changes in the applied electrode potential were distinguished. The Amide-III vibrations exhibited an electrochemical Stark effect (potential dependent frequencies) with Stark tuning slope sensitive to the electrode material. Potential-difference spectrum revealed that the imidazole ring of His(8) was bonded to the Cu electrode surface at relatively positive potentials.

Structure of monolayers formed from neurotensin and its single-site mutants: vibrational spectroscopic studies.

The human, pig, and frog neurotensins and four single-site mutants of human neurotensin (NT), having the following modifications, [Gln(4)]NT, [Trp(11)]NT, [D-Trp(11)]NT, and [D-Tyr(11)]NT, were immobilized onto an electrochemically roughened silver electrode surface in an aqueous solution. The orientation of adsorbed molecules was determined from surface-enhanced Raman scattering (SERS) measurements. A comparison was made between these structures to determine how the change upon the mutation of the neurotensin structure influences its adsorption properties. The SERS patterns were correlated with the contribution of the structural components of the aforementioned peptides to the ability to interact with the NTR1 G-protein receptor. Briefly, the SERS spectra revealed that the substitution of native amino acids in investigated peptides influenced slightly their adsorption state on an electrochemically roughened silver surface. Thus, human, pig, and frog neurotensins and [Gln(4)]NT and [D-Tyr(11)]NT tended to adsorb to the surface via the tyrosine ring, the oxygen atom of the deprotonated phenol group of Tyr(11), and the -CH(2)- unit(s), most probably of Tyr(11), Arg(9), and/or Leu(13). The observed changes in the enhancement of the deprotonated Tyr residue SERS signals indicated a further parallel orientation of a phenol-O bond with regard to the silver surface normal for pig NT, [Gln(4)]NT, and [D-Tyr(11)]NT, whereas the orientation was slightly tilted for human and frog NT. In the case of [Trp(11)]NT and [D-Trp(11)]NT, the formation of a peptide/Ag complex was confirmed by strong SERS bands involving the phenyl co-ring of Trp(11)/d-Trp(11) and -CH(2)- vibrations and the tilted and flat orientations of the two compounds with respect to the surface substrate. The spectral features were accompanied by a SERS signal caused by vibrations of the carboxyl group of C-terminal Leu(13) and the guanidine group of Arg(9). Reported changes in SERS spectra of L and D isomers were fully supported by generalized two-dimensional correlation analysis. Additionally, a combination of mutation-labeling and vibrational spectroscopy (Fourier-transform Raman and absorption infrared) was used to investigate the possible peptide conformations and environments of the tyrosine residues.

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.

Neuromedin C: potential-dependent surface-enhanced Raman spectra in the far-red spectral region on silver, gold, and copper surfaces.

Neuromedin C (NMC) is a decapeptide (Gly-Asn-His-Trp-Ala-Val-Gly-His-Leu-Met-NH(2)) that acts as a growth factor in a wide range of tumors including carcinomas of the pancreas, stomach, breast, prostate, and colon. We report surface-enhanced Raman spectra (SERS) of NMC on electrochemically roughened Ag, Au, and Cu electrode surfaces over an electrode potential range varying from +0.200 to -1.200 V (depending on the electrode material). We compared the SERS spectra to the Raman spectrum of the corresponding solid species. The SERS spectra were dominated by L-tryptophan (Trp) vibrations. This indicates that Trp interacted with the metallic surfaces of the electrodes, either by binding directly to the surface or by staying in close proximity to the surface. Characteristic SERS bands showed that, in the case of the Ag electrode, the Trp residue was almost perpendicular to the surface. In contrast, the Trp residue was slightly tilted with respect to the Au electrode surface, and Trp remained some distance from the surface of the Cu electrode. These differences were due to differences in surface rheology and in the type of metal (Ag vs Au vs Cu) responsible for the observed enhancement mechanism. On the other hand, variations in the electrode potentials only had a slight influence on the SERS patterns and the observed changes were mainly due to the reorientation of the Trp ring with respect to the electrode surface. These findings were fully supported by generalized two-dimensional correlation analysis (G2DCA).