FTIR-spectroscopic characterization of phosphocholine-headgroup model compounds

The polar headgroups are an important molecular subregion of phospholipids since they mediate a substantial part of the interactions emerging with other constituents of biological membranes, be it inherent macromolecules (proteins) or water as forming the natural environment. FTIR spectroscopy is well proven for characterizing aspects of weak interactions, above all hydrogen bonding. We have used this method to study solid deposits of a number of selected lipid models, such as choline, acetylcholine and methylphosphocholine (MePC), and compared them with the common phospholipid dimyristoylglycerophosphocholine (DMPC), at different degrees of hydration, which were varied via relative humidity (RH). At low RHs, only MePC and DMPC take up considerable amounts of water, thus elucidating the essential role of phosphate groups in the first stages of lipid hydration. The progress of PC-headgroup hydration can be sensitively monitored by the wavenumber decrease of the band owing to antisymmetric PO2- stretching vibration. Concomitant variations of the spectral parameters of nu CH bands of MePC reveal that conformational changes may simultaneously occur in the headgroup. Surprisingly to us, the model compounds display qualitative differences in the appearance of their nu CH bands, which are most probably a result of substituent effects. (C) 2001 Elsevier Science B.V. All rights reserved.