QUANTITATIVE-DETERMINATION OF HYDROCARBON CHAIN CONFORMATIONAL ORDER IN BILAYERS OF SATURATED PHOSPHATIDYLCHOLINES OF VARIOUS CHAIN LENGTHS BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

The infrared spectra of aqueous dispersions of a homologous series of symmetric-chain, disaturated phosphatidylcholines, with fatty acyl chain lengths ranging from 12 to 19 carbons, have been measured at comparable reduced temperatures in their liquid-crystalline phases. The infrared spectra of these compounds contain bands that are dependent on the conformation of the fatty acyl chains. In particular, in the 1400-1300-cm−1 spectral region, there are bands due to CH2 wagging which are specific for the different types of gauche conformers. Thus, gauche-trans-gauche' sequences (or kinks) give a band at 1367 cm−1, end-gauche conformers a band at 1341 cm−1, and double-gauche conformers a band at 1355 cm−1. The intensities of these bands were determined and normalized to the intensity of the conformation-insensitive band due to symmetric methyl bending at 1378 cm−1. The intensities of the different “gauche” bands yield a “per chain” intensity, which is directly related to the concentration of the different types of conformational defects. We find that, within experimental error, the concentration of end-gauche and double-gauche conformers is relatively low and practically invariant with chain length when a series of homologous phosphatidylcholines are compared at the same reduced temperature. In contrast, the concentration of gauche-trans-gauche' sequences (kink defects) is much higher and increases as the chain length increases. For dipalmitoylphosphatidylcholine we find that there are about 1.2 kink, 0.5−0.6 end-gauche, and 0.4 double-gauche conformers per hydrocarbon chain. Thus, for this phospholipid, the total number of gauche bonds per chain is about 3.7, in excellent agreement with that measured recently by Mendelsohn et al. [Mendelsohn, R., Davies, M. A., Brauner, J. W., Schuster, H. F., & Dluhy, R. A. (1989) Biochemistry 28, 8934-8939] and with earlier estimations derived from spectroscopic and thermodynamic measurements. The number of gauche bonds per hydrocarbon chain in liquid-crystalline bilayers is less than that in liquid n-alkanes or detergent micelles of comparable hydrocarbon chain length, due primarily to the lower concentration of double-gauche conformers present in the former. © 1990, American Chemical Society.