ACYL-CHAIN CONFORMATIONAL ORDERING IN LIQUID-CRYSTALLINE BILAYERS - COMPARATIVE FT-IR AND H-2-NMR STUDIES OF PHOSPHOLIPIDS DIFFERING IN HEADGROUP STRUCTURE AND CHAIN-LENGTH

FT-IR spectroscopy has been used to evaluate the acyl chain conformati onal ordering of DMPC, DMPE, DMPA (pH 6 and 12), DMPG (pH 1 and 7), an d DPPC, DPPE, DPPA (pH 6). The frequencies of the symmetric and antisy mmetric methylene stretching vibrations were determined as a function of temperature. In the liquid-crystalline phase the frequencies show a qualitative dependence on the amount of chain disorder. Quantitative data for trans-gauche isomerization were obtained from the integral in tensities of the conformation sensitive methylene wagging absorptions at ca. 1368 cm(-1) (gtg' and gtg sequences), 1356 cm(-1) (double gauch e) and 1342 cm(-1) (end gauche). The integral band intensities were co nverted to the number of gauche conformers per acyl chain using the ca libration factors published by Senak et al. (1991). At 69 degrees C th e highest number of gauche conformers excluding contributions from sin gle gauche conformers and jogs (g'tttg) are found for PCs (DMPC:2.6; D PPC:2.4), followed by DMPG(-) (2.0), phosphatidylethanolamines (DMPE:1 .4; DPPE:2.0), protonated DMPG (1.5), and phosphatidic acids (DPPA(-): 1.7; DMPA(-):1.4, DMPA(2-):1.7). From H-2-NMR measurements of perdeute rated samples of DMPC, DMPA, DPPC, and DPPA the quadrupolar splittings Delta nu(Q perpendicular to i) and the order parameter S-CDi of the C D2-segments close to the chain ends could be determined whereas splitt ings in the plateau region of the chains could not be resolved. The qu adrupolar splittings are affected by trans-gauche isomerization, long axis rotation, and restricted wobbling motions of the acyl chains. In the simplest assumption, the order parameter S-CD can be expressed as a product of a segmental order parameter S-gamma and a chain order par ameter S-alpha. For comparison of the different lipids we used average order parameters ($) over bar S-CD, obtained by averaging over all S- CDi-values, and ($) over bar S-gamma determined from the total number of gauche conformers per chain by FT-IR-spectroscopy, to calculate an empirical average chain order parameter ($) over bar S-alpha. The comb ination of H-2-NMR and FT-IR results allows the estimation of the rela tive extent of chain wobbling for the different lipid molecules. ($) o ver bar S-alpha, is lowest for PCs (($) over bar S-alpha approximate t o 0.475) while PEs (($) over bar S-alpha approximate to 0.51) and PAs (($) over bar S-alpha approximate to 0.52) show less chain wobbling.