DIFFERENTIAL EFFECTS ON PHOSPHOLIPID PHASE-TRANSITIONS PRODUCED BY STRUCTURALLY RELATED LONG-CHAIN ALCOHOLS

The thermotropic behavior of aqueous dispersions of dipalmitoylphosphatidylcholine and, in a few cases, dimyristoylphosphatidylcholine and distearoylphosphatidylcholine, was measured spectroscopically by using the spin probe, 2, 2, 6, 6-tetramethylpiperidine-1-oxyl (Tempo1). From the resulting sigmoidal phase transition profiles, the main gel to liquid-crystalline transition temperature (Tm) was obtained, an estimate was made of the mean transition half-width (W1/2) and, where it was observed, the small pretransition (T1) was also determined. The effects on these parameters of incorporating the long-chain alcohols, C14:0, cis- and trans-C14:1, C16;0, and cis- and trans-C16:1, were studied as a function of the concentration of alcohol. In DPL, the saturated alcohols produced a concentration-dependent elevation, the trans unsaturated alcohols, a smaller elevation, while the cis unsaturated alcohols produced a substantial depression of Tm. All six alcohols broadened the main transition. The latter effect was large in the case of the saturated alcohols but significantly smaller in the case of three out of four of the unsaturated alcohols. The unsaturated hexadecenols were also incorporated into DML and DSL. As with DPL, the trans isomer raised, while the cis isomer lowered, the main transition temperature. In each case, there was an increase in the mean transition half-width (W 1/2). Spin-labeled phospholipid (PC(7, 6)) was used to determine the order parameter of DPL vesicles in the presence and absence of 33 mol % cis- and transhexadecenol. Above Tm, both alcohols ordered the lipid membrane slightly, whereas, below Tm, the cis isomer disordered, while the trans isomer expelled the spin label from the lipid bilayer. In contrast to their effect on Tm, all three of the C16 alcohols shifted the pretransition (T1) to higher temperatures such that ∣ΔT1∣ was usually greater than ∣ΔTm∣. The manner and extent to which the phase transition parameters were modified were found to depend not only on the length and shape of the added alcohol but also on the chain length of the lipid into which it was incorporated. The results are discussed in terms of a thermodynamic model describing the differential partitioning of the alcohols into the gel and liquid-crystalline phases of the respective lipids. © 1979, American Chemical Society. All rights reserved.