Effect of a linear (1-octanol) and a branched (2,6-dimethyl-4-heptanol) alkanol upon the properties of dipalmitoylphosphatidylcholine large unilamellar vesicles

Abstract

We have investigated the effect of the incorporation of a linear (1-octanol) and a branched (2,6-dimethyl-4-heptanol) alkanol into large unilamellar vesicles of dipalmitoylphosphatidylcholine in the gel state upon several properties of the bilayer. The effect was evaluated by measurements of the rate of water outflow following a hypertonic osmotic shock, generalized polarization of 6-dodecanoyl-2-dimethyl)aminonaphthalene (Laurdan), absorption spectra of merocyanine 540 (MC), fluorescence anisotropies of 2-anthroyloxystearic acid (2-AS), 12-anthroyloxystearic acid (12-AS) and diphenylhexatriene (DPH), the lifetimes of pyrene (Py) and pyrenedodecanoic acid (PyD), and the pseudo-first order rate constant of the fluorescence quenching of Py and PyD by oxygen, k ox, as a function of the intravesicular alkanol concentration. The data obtained indicate that the degree of incorporation, and the effect of a given intravesicular concentration of the additive, are extremely dependent on the molecular shape of the alkanol. In particular, the branched alkanol is less soluble and causes greater disorganization of the central parts of the bilayer than the linear one. The incorporation of both additives at low alkanol/lipid ratios produces a notable increase in the rate of water outflow following a hypertonic shock, the effect being more noticeable at low concentrations of the branched alcohol. Both alkanols produce, at least at low concentrations, an increase in the value of 2-AS fluorescence anisotropy, suggesting a more compact organization of the interface. In this region, MC is expelled from the vesicle. At higher 1-octanol concentrations, the fluorescence anisotropy of 2-AS decreases and MC incorporation increases, indicating a biphasic behaviour for the linear alcohol. Significant differences between both alkanols are also observed with respect to their effect upon the values of k ox for PyD, a probe that can be considered to be localized in the inner parts of the bilayer.