pH-dependent chirality of L-proline studied by Raman optical activity and density functional theory calculation

The characteristic pyrrolidine ring structure of proline (Pro) and its conformational flexibility are important to its biological function in peptides and proteins. Properties of Pro in aqueous solution vary with the acid-base conditions. In this work, Raman optical activity (ROA) spectroscopy is used to carefully study the chirality of L-Pro in aqueous solution with different pH values. Raman shifts and ROA intensities are found to be directly correlated with the acidic or basic environment. Pyrrolidine ring torsions and carbonyl group twists differ in cation, zwitterion, and anion forms (denoted as ProCA, ProZW, and ProAN, respectively), as confirmed by the agreement between the experimental results and density functional theory (DFT) calculation. From analysis of vibrational modes and group couple matrices (GCMs), the local carbonyl group stretching modes may reflect electronic structure changes, and the couplings between the three CH2 group with others mainly contribute to the ROA intensities in the low wavenumber region (1400-700 cm(-1)). The dramatic changes of the distributions and dispersion of the ROA intensities indicate that an acidic or basic environment not only leads to the simple abstraction or addition of a proton but also changes the electron delocalization throughout the molecule. Isolation of the two symmetric anisotropic invariants shows the predominant contributions of the electric dipole-magnetic dipole invariant to the ROA signals in L-Pro. These results will be useful for interpreting the ROA spectra of proteins and at least proline-rich peptides.