Verification study for an empirical rule in diverse helical conformational behaviors of asymmetric 1,2-diacyl-sn-glycerols in the solution state

Cell-membrane glycerophospholipids and glycolipids have a common asymmetric skeleton of 1,2-diacyl-sn-glycerols. The 1,2-diacyl moiety in solutions permits a rapid equilibrium among three helical conformers, namely gt(+), gg(−), and tg, to display diverse conformational properties. The conformational property changes variably not only by head groups at the sn-3 position, but also by the solvent conditions applied. Recently, we came across an empirical rule in the conformational diversity in the solution state when we assumed the term of ‘helical disparity’ for the equilibrium between gt(+) and gg(−) conformers with reversed helical signs for each other. The sign and magnitude of the helical disparity (%) governs the (+)- or (−)-chirality around the lipid tail and corresponds to the magnitude of the exciton couplet CD (circular dichroism) bands. The empirical rule expresses that the disparity (%) changes linearly by the function of gt(+) population (%). Herein, the rule was verified by ¹H NMR spectroscopy using different types of 1,2-diacyl-sn-glycerols as model compounds. The present paper describes that the rule is formulated with a general equation (Eq-1): ‘helical disparity (%)’ = [gt(+)−gg(−)] (%) = A[gt(+)−B], in which A and B are constants taking values around 1.3 and 38, respectively. This rule is maintained regardless of the 1,2-diacyl and sn-3 substituent groups as far as examined here, while affording several exceptions. With Eq-1 (A = 1.3, B = 38), a conformational diagram can be obtained. This allows us to overview the diverse helical conformational properties of the asymmetric 1,2-diacyl-sn-glycerols in the solutions state.