Relationships between conformational and vibrational features of tryptophan characteristic Raman markers

Tryptophan (Trp) residue provides characteristic vibrational markers to the middle wavenumber spectral region of the Raman spectra recorded from peptides and proteins. In this report, we were particularly interested in eight Trp Raman markers, referred to as Wi (i = 1,…,8). All responsible for pronounced Raman lines, these markers originate from indole moiety, a bicyclic conjugated segment involved in the Trp structure. Numerous investigations have previously attempted to relate the variations observed in the spectral features of these markers to the environmental changes of Trp residues. To emphasize the most important points we can mention (i) the variations in the Raman profile of W4 (∼1360 cm-1) and W5 (∼1340 cm-1), frequently observed as a doublet with variable intensity ratio. These two markers were thought to result from a Fermi-resonance effect between certain planar and nonplanar modes; (ii) the changes observed in the wavenumbers and relative intensities of W4, W7 (∼880 cm-1) and W8 (∼760 cm-1) were supposed to be related to the accessibility of Trp to surrounding water molecules; and (iii) the wavenumber fluctuations of W3 (∼1550 cm-1), taken as a Trp side chain orientational marker. However, some ambiguities still exist regarding the interpretation of these markers, needing further clarification. Herein, upon a joint experimental and theoretical analysis based on a multiconformational approach, attention was paid to the relationships between structural and vibrational features of three indole-containing compounds with increasing structural complexity, i.e., skatole (3-methylindole), tryptophan, and tripeptide Gly-Trp-Gly. This study clearly shows that the existing assignments given to certain Trp Raman markers should be reconsidered, especially those based on the Fermi-resonance origin of W4-W5 (∼1360-1340 cm-1) doublet, as well as the purely environmental dependence of W7 and W8 markers.

The relationship between the tyrosine residue 850-830 cm-1 Raman doublet intensity ratio and the aromatic side chain χ1 torsion angle

Tyrosine (Tyr) residue in a peptide chain is characterized by the presence of seven Raman markers, referred to as Yi (i = 1, …, 7), distributed over the middle wavenumber spectral region. Particularly, the changes observed in the relative intensity of Y5 and Y6 markers, appearing as a side by side doublet at ca. 850-830 cm-1, has received a great attention. Primarily assigned to a Fermi-resonance effect between phenol ring planar and nonplanar modes, former density functional theory calculations led us to affiliate the Y5-Y6 doublet to two distinct fundamental modes. Furthermore, despite the previous assumptions, it was evidenced that the reversal of the doublet intensity ratio cannot be solely explained by hydrogen bonding on the phenol hydroxyl group involved in Tyr. Herein, upon analyzing the observed and theoretical data collected from the cationic species of the tripeptide Gly-Tyr-Gly, the crucial effect of the aromatic side chain orientation, especially that of the χ1 torsion angle defined around the CαCβ bond, on the Tyr doublet intensity ratio has been evidenced.

Multiconformational analysis of tripeptides upon consideration of implicit and explicit hydration effects

During the last two decades, numerous observed data obtained by various physical techniques, also supported by molecular modeling approaches, have highlighted the structuring features of tripeptides, as well as their aggregation properties. Herein, we focus on the structural dynamics of four trimers, i.e., Gly-Gly-Gly, Gly-Ala-Gly, Ala-Ala-Ala and Ala-Phe-Ala, in an aqueous environment. Density functional theory calculations (DFT) were carried out to assess the stability of four types of secondary structures, i.e., β-strand, polyproline-II (pP-II), α-helix and γ-turn, of which the formation had been described in these tripeptides. Both implicit and explicit hydration effects were analyzed on the conformational and energetic features of trimers. It has been shown that the use of M062X functional (versus B3LYP) improve the stability of intramolecular H-bonds, especially in inverse γ-turn structures, as well as the energetic and conformational equilibrium in all tripeptides. Explicit hydration reflected by the presence of five water molecules around the backbone polar sites (NH₃⁺, N-H, CO and NH₂) considerably changes the conformational landscapes of the trimers. Characteristic intramolecular and intermolecular interactions evidenced by the calculations, were emphasized.