Raman spectroscopic study of polycrystalline mono- and polyunsaturated 1-eicosanoyl-d(39)-2-eicosenoyl-sn-glycero-3-phosphocholines: bilayer lipid clustering and acyl chain order and disorder characteristics.

Abstract

Polycrystalline lipid samples of a series of mono- and polyunsaturated, double bond positional isomers of 1-eicosanoyl-d(39)-2-eicosenoyl-sn-glycero-3-phosphocholines [C(20-d(39)):C(20:1 Delta(j))PC, with j = 5, 8, 11, or 13; C(20-d(39)):C(20:2 Delta(11,14))PC; and C(20-d(39)):C(20:3 Delta(11, 14,17))PC] were investigated using vibrational Raman spectroscopy to assess the acyl chain packing order-disorder characteristics and putative bilayer cluster formation of the isotopically differentiated acyl chains. Perdeuteration of specifically the saturated sn-1 acyl chains for these bilayer systems enables each chain's intra- and intermolecular conformational and organizational properties to be evaluated separately. Various saturated chain methylene CD(2) and carbon-carbon (C&bond;C) stretching mode peak height intensity ratios and line width parameters for the polycrystalline samples demonstrate a high degree of sn-1 chain order that is unaffected by either the double bond placement or number of unsaturated bonds within the sn-2 chain. In contrast, the unsaturated sn-2 chain spectral signatures reflect increasing acyl chain conformational disorder as either the cis double bond is generally repositioned toward the chain terminus or the number of double bonds increases from one to three. The lipid bilayer chain packing differences observed between the sn-1 and sn-2 chains of this series of monounsaturated and polyunsaturated 20 carbon chain lipids suggest the existence of laterally distributed microdomains predicated on the formation of highly ordered, saturated sn-1 chain clusters.