Membrane Order

2H-Nuclear membrane resonance was used to determine the orientational order parameter of a series of PCs with perdeuterated stearic acid in the sn-1 chain and 18:1, 18:2, 18:3, 20:4, 20:5, and 22:6 in the sn-2 position (Holte et al., 1995). The profiles showed that unsaturation causes inhomogeneous disordering along the sn-1 chain. Going from one to six double bonds in the sn-2 chain results in a 0.01 reduction in SCD (order parameter) in the plateau region and a maximum difference of 0.03 at C14 of the sn-1 chain. From these studies, it was determined that the molecular area of the sn-1 chain increases three to four times more around carbon-14 than it does for carbons in the top part of the chain. This suggests a wedge shape for 18:0,22:6 PC (a slightly wider acyl chain base than polar head) that loosens packing at the interface but is not so pronounced as to cause formation of a nonlamellar inverted Hu phase, Consequently, a "free volume" exists at the lipid— water interface. This space would be more available for small solutes such as water (Holte et al., 1995). These studies could detect only a slight change in membrane thickness.

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