Perturbation of egg phosphatidylcholine and dipalmitoylphosphatidylcholine multilamellar vesicles by n-alkanols. A fluorescent probe study.

The perturbing effects of n-alkanols (pentanol, decanol and tetradecanol) in egg phosphatidylcholine and dipalmitoylphosphatidylcholine multilamellar vesicles were studied with five fluorescent probes, 1-(4'-trimethylaminophenyl)-6-phenylhexa-1,3,5-triene (TMA-DPH), 1,6-diphenyl-1,3,5-hexatriene, and 2-, 7-, and 12-(9-anthroxyloxy)stearic acid (2-, 7-, and 12-AS). These probes localize at various depths in the membrane, enabling study of the membrane-order gradient. Phase-modulation fluorescence spectroscopy was used to measure steady-state anisotropies, excited-state lifetimes and differential polarized lifetimes from which the limiting hindered anisotropies (r infinity) and the logarithm of the rotational rate (log R) were calculated. The probes that localize at about the same depth in the membrane (TMA-DPH and 2-AS, diphenylhexatriene and 12-AS) generally, but not always, showed similar changes in r infinity and log R with added alkanols. However, the absolute values of r infinity and log R were usually different. The inconsistencies are attributed to differences in the probes' sizes, structures, photophysical properties and perturbing abilities. The perturbation of membranes by alkanols is chain-length-dependent. Pentanol disorders the membrane at all depths but is more effective in the membrane center than nearer to the polar headgroups of the phospholipids, tetradecanol can be accommodated into the membrane without effect or with increased order and the effects of decanol are intermediate between pentanol and tetradecanol. Our results with alkanols indicate that: a single perturber can have different effects on membrane order at different depths in the bilayer; the perturbation is observed at and distant from the perturbers' location in the membrane, and the bilayer center is more susceptible to perturbation by alkanols than the region of the bilayer near the phospholipid headgroups.