Abstract
The effect of up to 20 mol% incorporation of α-tocopherol on acyl chain order and dynamics in liquid crystalline phosphatidylcholine (PC) membranes was studied as a function of acyl chain unsaturated by electron spin resonance (ESR) of 5-, 7-, 12- and 16-doxyl spin labelled stearic acids intercalated into the membrane. Order parameters S in the upper portion of the chain (positions 5 and 7) and correlation times τ C in the lower portion (positions 12 and 16) determined from the ESR spectra indicate that in general α-tocopherol restricts acyl chain motion within the membrane. The magnitude of the increases in order appears to be dependent upon phospholipid molecular area, being the greatest (up to 15%) in saturated dimyristoylphosphatidylcholine (14:0-14:0 PC) which possesses a relatively small area per molecule as opposed to much smaller increases (<3%) in unsaturated PC membranes of larger molecular area. This behaviour is interpreted as incompatible with the hypothesis of Lucy and coworkers (A.T. Diplock and J.A. Lucy (1973) FEBS Lett. 29, 205–210), who proposed that membranes are structurally stabilized by interactions between the phytyl side chain of α-tocopherol and the polyunsaturated chains of phospholipids.
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