Characteristic Raman Markers of the Neutral and Protonated Histidine Side Chain: Data from l-Histidine and Tripeptide...

Due to the presence of imidazole in its structure, the histidine side chain is responsible for well-resolved and strong Raman lines appearing within the middle wavenumber spectral region. Imidazole undergoes, furthermore, a protonation-deprotonation close to neutral pH. Two neutral states, H0(NδH) and H0(NεH), and one protonated state, H1+, might participate in this process. To analyze this effect, pH-dependent off-resonance Raman spectra from zwitterionic l-histidine and tripeptide Gly-His-Gly having a cationic backbone were acquired. Drastic changes appearing upon the H0 ↔ H1+ process allowed estimation of the associated pKa values. Density functional theory calculations were performed on two representative samples formed by 18 histidine conformers, as well as 90 tripeptide conformers, all explicitly hydrated on their polar sites. Notable differences were found between the energy landscapes of H0 and H1+ species of histidine, depending basically on the aromatic side chain orientation. In the tripeptide, the backbone conformation was also shown to affect the energy landscapes. Aqueous populations of tripeptide conformers relative to the five considered backbone types have been assessed on the basis of their thermal (Boltzmann) weights. While helical and extended chain (β-like) rotamers form the major populations in H0 residue-containing tripeptides, polyproline-II and extended chains dominate when the H1+ side chain type is present. Classic and inverse γ-turn folds represent minor/negligible populations, whatever the histidine protonation state. Importantly, the H0(NεH) state was revealed as the most populated neutral histidine residue. Raman intensities were calculated upon a multiconformational analysis, leading to the selection of the characteristic Raman markers of neutral and protonated aromatic side chains.