Effect of water on the molecular structure of a phosphatidylcholine hydrate. Raman spectroscopic analysis of the phosphate, carbonyl and carbon-hydrogen stretching mode regions of 1,2-dipalmitoylphosphatidylcholine dihydrate.

The Raman spectra of 1,2-dipalmitoylphosphatidylcholine (DPPC) dihydrate crystals were examined in (a) the 1250 and 1080 cm-1 PO-2 antisymmetric and symmetric stretching mode regions, (b) the 1730 cm-1 carbonyl stretching mode region and (c) the 2800-3100 cm-1 C-H stretching mode interval. No evidence was observed for hydrogen bonding effects involving the nonester phosphate oxygen atoms. Spectra of the DPPC dihydrate crystal in the 1250 cm-1 region are compared to spectra of both anhydrous and successively hydrated systems and to spectra of crystalline glycerylphosphorylcholine. The splitting pattern in the 1240 to 1280 cm-1 interval for the DPPC dihydrate was interpreted in terms of the different bonding arrangements assumed by the PO-2 groups in the two molecule asymmetric unit of the eight molecule unit cell. An unusual triplet is observed in the carbonyl stretching mode region around 1730 cm-1. These features reflect differing bond environments, relative to the polar and hydrophobic regions of the bilayer, of the 1- and 2-chain carbonyl groups. Spectra for crystalline DPPC dihydrate in the 3000 cm-1 C-H stretching mode region suggest a hybrid chain packing lattice which is analogous to the packing arrangement determined for the isostructurally similar DMPC dihydrate species.