Conformation of the choline chain of phosphatidylcholines - A FTIR study of the symmetric C-N (CH

Infrared spectra of phosphatidylcholines in various forms of aggregation (multilamellar films, liposomes, unilamellar vesicles, micelles) exhibit a composite band in the 931-904cm-1 region which is ascribed to the C-N(CH3)3 stretching of the choline terminal. Mathematical resolution enhancements yields three components that can be associated with isomers concerning the rotation about the PO-CH2 bond (α4 torsion). The assignment of the components is based on single bands appearing in the spectra of the dry phosphatidylcholine-EuCl3 complex (931 cm-1, antiplanar), two crystalline phosphatidylcholine hydrates (916 cm-1, anticlinal) and Fringeli's crystalline C phase (904 cm-1, synclinal). These assignments are supported by the published normal coordinate analysis of acetylcholine halides. Integrated absorptions of the resolved components permit the estimation of rotamer population. The effect of temperature variation on the latter is very small indicating that rotamers differ very little in energy. Hydration effects are more drastic. This demonstrates the general usefulness of the C-N(CH3)3 stretching bands for conformational studies of the choline chain of phosphatidylcholines