Infrared dichroism investigations on the acyl chain ordering in lamellar structures II. The effect of diene groups in membranes of dioctadecadienoylphosphatidylcholine

The infrared (IR) linear dichroism of selected absorption bands of the acyl chains of the diene lipid dioctadecadienoylphosphatidylcholine (DODPC) has been studied as a function of relative humidity (RH) at T = 25 degrees C. The IR order parameters of the C-H wagging, and of the C=C and C-C stretching modes of the diene groups yield the mean orientation of these moieties within the membranes. The tendency of the plate-like diene groups to align mutually parallel induces the formation of tightly packed subgel phases. The arrangement of the octadecadienoyl chains is characterised by a considerable tilt with respect to the bilayer normal ((theta) approximate to 40 degrees) and by a skew rotation between the diene and polymethylene parts. The packing requirements of the chains in the subgels force the area of the membranes to increase and the PC headgroups to dehydrate partially. The presence of the diene groups near the polar/apolar interface promotes lyotropic solid/solid phase transitions. Consequently, the packing of the acyl chains can be modulated by hydration. Two modifications of the subgels with different tilt directions of the chain axes are found in each of the two subgel phases. (C) 1999 Elsevier Science B.V. All rights reserved.