Effect of the addition of alkanols of different topology to dipalmitoyl-phosphatidylcholine vesicles in the presence of gramicidin

Abstract

In the present work, we analyze the effect of incorporation of the nonanol family (e.g., 1-Nonanol (1-N), 5-Nonanol (5-N), and 2,6-Dimethyl-4-Heptanol (2,6-DH)) into DPPC LUVs in the presence of different gramicidin concentrations. The principal aim of this work is to study the effect of alkanols solubilization on the physicochemical properties of lipid bilayers in the presence of peptide trans-membrane channels, that is, the effects of nonanol family in the interface of lipid–peptide region, considering that the study provides the analysis of a ternary system by direct excitation as well as by Fluorescence Resonance Energy Transfer. Fluorescence measurements were carried out at 20°C after direct excitation of the extrinsic probe or by Fluorescence Resonance Energy Transfer (FRET) from the tryptophan group of gramicidin. Alkanol incorporation decreases with increasing gramicidin content and branching of the additives. 1-N generates most important changes in the inner part of the bilayer, where it produces an increase in bulk acyl chain mobility. Similarly, 1-N significantly modifies the properties of the hydrophilic–hydrophobic interface region sensed by Laurdan, increasing the polarity of the probe microenvironment and/or increasing the relaxation time of interfacial water molecules. On the other hand, 1-N produces a decrease in PDA fluorescence lifetime, a result that can be explained by a significant amount of water entrance to the inner part of the bilayer. The same behavior was observed when pseudo-first-order quenching rate constants by oxygen were measured. 1-N produces an increase in mobility/solubility of the oxygen in the lipid membrane, an effect that is more noticeable in the deep region of the bilayer sensed by PDA, in the absence and in the presence of 2mol% of Gr. 1-N incorporation produces a greater reduction in GP value than 5-N and 2,6-DH when Laurdan was excited by FRET. These results show that 1-N has the greatest effect in the lipidic domains near the gramicidin channel. On the other hand, excimer–monomer ratios of PDA obtained by FRET show that 1-N reduces the lateral mobility of acyl chains near the lipid–gramicidin interface when gramicidin concentration in the lipid bilayer increases. This effect is more noticeable than that obtained by direct irradiation of the probe in the presence of 5-N and 2,6-DH. On the other hand, the addition of the three alkanols in the presence of Gr produces a noticeable increase in the water permeability, particularly for 1-N. In this context, we propose a scheme that represents the effect of 1-nonanol on the water outflow in DPPC LUVs in the absence and in the presence of Gr.