Hydration-induced changes of structure and vibrational frequencies of methylphosphocholine studied as a model of biomembrane lipids

The chemical characteristics of the polar parts of phospholipids as the main components of biological membranes were investigated by using infrared (IR) spectroscopy and theoretical calculations with water as a probe molecule. The logical key molecule used in this study is methylphosphocholine (MePC) as it is not only a representative model for a polar lipid headgroup but itself has biological significance. Isolated MePC forms a compact (folded) structure which is essentially stabilized by two intramolecular C-H...O type hydrogen bonds. At lower hydration, considerable wavenumber shifts were revealed by IR spectroscopy: the frequencies of the (O-P-O)- stretches were strongly redshifted, whereas methyl and methylene C-H and O-P-O stretches shifted surprisingly to blue. The origin of both red- and blueshifts was rationalized, on the basis of molecular-dynamics and quantum-chemistry calculations. In more detail, the hydration-induced blueshifts of C-H stretches could be shown to arise from several origins: disruption of the intramolecular C-H...O hydrogen bonds, formation of intermolecular C-H...O(water) H-bonds. The stepwise disruption of the intramolecular hydrogen bonds appeared to be the main feature that causes partial unfolding of the compact structure. However, the transition from a folded to extended MePC structure was completed only at high hydration. One might hypothesize that the mechanism of hydration-driven conformational changes as described here for MePC could be transferred to other zwitterions with relevant internal C-H...O hydrogen bonds.