Effects of cholesterol on conformational disorder in dipalmitoylphosphatidylcholine bilayers. A quantitative IR study of the depth dependence

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Abstract
A method originally proposed by Synder and Poore [(1973) Macromolecules 6, 708-715] as a specific probe of trans-gauche isomerization in hydrocarbon chains and recently applied [Mendelsohn et al. (1989) Biochemistry 28, 8934-8939] to the quantitative determination of phospholipid acyl chain conformational order is utilized to monitor the effects of cholesterol at various depths in dipalmitoylphosphatidylcholine (DPPC) bilayers. The method is based on the observation that the CD2 rocking modes from the acyl chains of specifically deuterated phospholipids occur at frequencies in the Fourier transform infrared spectrum which depend upon the local geometry (trans or gauche) of the C-C-C skeleton surrounding a central CD2 group. Three specifically deuterated derivatives of DPPC, namely, 4,4,4'',4''-d4 DPPC (4-d4 DPPC), 6,6,6'',6''-d4 DPPC (6-d4 DPPC), and 12,12,12'',12''-d4 DPPC (12-d4 DPPC), have been synthesized, and the effects of cholesterol addition at 2:1 DPPC/cholesterol( mol:mol) on acyl chain order at various temperatures have bee determined. At 48.degree. C, cholesterol inhibits gauche rotamer formation by factors of .apprx.9 and .apprx.6 at positions 6 and 4, respectively, of the acyl chains, thus demonstrating a strong ordering effect in regions of the bilayers where the sterol rings are presumed to insert parallel to the DPPC acyl chains. In contrast, the ability of the sterol to order the acyl chains is much reduced at the 12-position. The sterol demonstrates only a slight disordering of phospholipid gel phases. Finally, the contributions of different classes of gauche conformers to the spectra have been determined. Kinks and single gauche bends dominate the disordering process as the temperature is increased, although acyl chains with multiple gauche forms are noted at low temperatures. These data are discussed in terms of current models for the effect of cholesterol on phospholipid order and phase properties.