SERS and DFT Study of Noble-Metal-Anchored Cys-Trp/Trp-Cys Dipeptides: Influence of Main-Chain Direction and Terminal Modifications

Abstract

Tiny changes in the covalent structure of a noble-metal-anchored peptide may affect its surface-enhanced Raman scattering (SERS) signature. While SERS spectra of Cys-Trp peptides are dominated by the bands arising from the aromatic Trp side chain, a thorough study of the impact of modifications in SERS-silent regions concerning such dipeptides was yet to be performed. Thus, here we conduct an extensive SERS study of a series of Cys- and Trp-containing dipeptides, varying both the main-chain direction (Cys-Trp/Trp-Cys) and the presence of typical covalent modifications at N- and C-termini (acetylation/amidation). We use three different SERS-active substrates: oxidation–reduction cycling-roughened silver (Ag ORC), gold (Au ORC), and chemically synthesized colloidal silver nanoparticles (Ag NPs). Interpretation of the experimental data was aided with density functional theory (DFT) calculations. Potential energy distribution (PED) analysis was used for the assignment of dipeptide vibrational bands. IR and normal Raman spectra were also examined to get a complete vibrational analysis. This work may be considered the first report on the effect of terminal group modification and reversing the peptide sequence on the adsorptive behavior of dipeptide on silver and gold surface studied by SERS spectroscopy, supported by full vibrational assignment by means of the DFT method.

Spectroscopic Investigations of 316L Stainless Steel under Simulated Inflammatory Conditions for Implant Applications: The Effect of Tryptophan as Corrosion Inhibitor/Hydrophobicity Marker

“…The reference RS and FT-IR spectra of the dried Trp solution (10 −2 M Trp/PBS, pH 3.0) are shown in Figures 4a and 5a, respectively. The proposed band assignments were based on many theoretical and experimental data from the literature on the molecular structure of Trp and Trp derivatives obtained under various conditions [42,[52][53][54][55][56][57][58][59][60][61][62][63][64][65][66]. As mentioned in our previous paper [28], the preparation of samples for experiments may affect the information obtained from spectral patterns (i.e., the state of the sample condition, pH of the surrounding environment).…”

“…Based on the clear changes in the band that appear below 880 cm −1 during the adsorption process of Trp on the corroded SS surface (Figure 4b,c) with respect to the ordinary RS spectrum, it can be concluded that there are hydrogen bonding interactions between the indole ring and the metallic surface [52,53,[62][63][64]. This spectral feature, which can be assigned to the NH group deformation, exhibits a higher relative intensity, red-shift (∆υ = 3-1 cm −1 ), and substantial broadening (∆ FWHM = 10-16 cm −1 ) compared to those observed in the normal RS spectrum.…”

“…The Raman modes of the Trp functional group are very sensitive to, i.e., the conformation and hydrophobicity of the environment surrounding the indole ring [52,63]. The Trp doublet in the 1360-1338 cm −1 spectral range has been reported to arise from the Fermi resonance between the fundamental in-plane N-C stretching band and combinations of the indole ring out-of-plane vibrations [52,62,66].…”

“…The intensity ratio of the above-mentioned doublet in Raman, as well as in the SERS spectra, is believed to be a sensitive probe of the local environment of the indole ring [52,62]. The intensity ratio of the high-wavenumber band to the low-wavenumber band of the Fermi doublet (I~1 360 /I~1 339 ) is a marker of the hydrophobicity of the environment [52,62].…”

“…The intensity ratio of the above-mentioned doublet in Raman, as well as in the SERS spectra, is believed to be a sensitive probe of the local environment of the indole ring [52,62]. The intensity ratio of the high-wavenumber band to the low-wavenumber band of the Fermi doublet (I~1 360 /I~1 339 ) is a marker of the hydrophobicity of the environment [52,62]. In the SERS spectrum obtained for Trp adsorbed on the corroded metallic surface after 2 h (see Figure 4b), it is observed that the band at 1357 cm −1 is the most enhanced band (the comparable relative intensity among the spectrum) and becomes dominant compared to the component at 1338 cm −1 (I 1357 /I 1338 = 1.8) which can indicate the presence of a hydrophobic surrounding during the adsorption process of Trp onto the metallic surface under acidic conditions obtained after PDP tests (Section 3.1).…”