Effect of unsaturated lipid chains on dimensions, molecular order and hydration of membranes

We conducted infrared linear dichroism and X-ray measurements to characterize membranes of sn-1 chain perdeuterated, polyunsaturated 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (18:0-22:6 omega3 PC, SDPC-d35) and of monounsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (16:0-18:1 PC, POPC-d31) as a function of hydration and temperature. Our novel approach is to use IR order parameters of C-H and C-D vibrations of both sn-1 and sn-2 chains for a separate estimation of the mean length and effective cross sectional area of the saturated and unsaturated chains in the bilayers, In the gel phase, the thickness of the hydrophobic core of SDPC bilayers exceeded the thickness of POPC bilayers, because the polyunsaturated chains adopted an extended conformation. However, in the fluid phase, the hydrophobic thickness of SDPC membranes, with the longer 18:0 and 22:6 acyl chains, was almost equal to the thickness of the 16:0-18:1 chains in POPC membranes. The isothermal compressibility modulus of the docosahexaenoyl chains was smaller than the modulus of the stearoyl chains of SDPC, whereas the palmitoyl and oleoyl chains of POPC are similarly compressible. Analysis in terms of a simple polymer brush model showed that unsaturation increases chain flexibility and chain mean area in lipid membranes. Temperature dependent measurements indicated that polyunsaturation decreased the sensitivity of the bilayer to temperature changes. The study of lipid hydration-sensitive spectral parameters suggested that this tendency may be at least partially explained by a weakening of water binding to polar moieties of polyunsaturated lipids with increasing temperature. Although the influence of hydrocarbon chain polyunsaturation on hydration of phosphate groups and the entire lipid was minor, we observed a modification of lipid carbonyl group hydration. Hydration properties and phase behavior of SDPC suggest an influence from polyunsaturation on the lateral pressure profile across the bilayer.