Role of Ionic Strength on n-Butanol Partitioning into Anionic Dipalmitoyl Phosphatidylcholine/Phosphatidylglycerol Vesicles

Alkanols Regulate the Fluidity of Phospholipid Bilayer in Accordance to Their Concentration and Polarity

In spite of the widespread use of alkanols as penetration enhancers, their effect on vesicular formulations remains largely unexplored. These can affect the stability and integrity of the phospholipid bilayers. In this study, we have investigated the interaction of linear (ethanol, butanol, hexanol, octanol) and branched alkanols (t-amylol and t-butanol) with three phospholipids (soya lecithin, SL; soy L-α-phosphatidylcholine, SPC; and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC). Thermodynamic and structural aspects of these interactions were studied as a function of the alkanol concentration and chain length. Our interpretations are based on isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) experiments. We observed one-site interactions wherein hydroxyl and acyl groups interacted with the polar and nonpolar regions of the phospholipid, respectively. The stability and structural integrity of bilayers appeared to be dependent upon (a) the hydrocarbon chain length and concentration of alcohols, and (b) the degree of unsaturation in the phospholipid molecule. We found that these interactions triggered a reduction in the enthalpy which was compensated by increased entropy, keeping free energy negative. Drop in enthalpy indicates reversible disordering of the bilayer which enables the diffusion of alcohol without triggering destabilization. Ethanol engaged predominantly with the interface, and it resulted in higher enthalpic changes. Interactions became increasingly unfavorable with longer alcohols - a cutoff point was recorded with hexanol. The overall sequence of membrane disordering capability was recorded as follows: ethanol < butanol < octanol < hexanol. Octanol's larger size restricted its penetration in the bilayer, and hence it caused less enthalpic changes relative to hexanol. This could also be verified from the trends in the area ratio of these vesicles obtained from the DLS data. Branched alkanols displayed a lower binding affinity with the phospholipids relative to their linear counterparts. These data are useful while contemplating the inclusion of short-chain alcohols as penetration enhancers in phospholipid vesicles.

Structural disruption of phospholipid bilayers over a range of length scales by n-butanol

We report on the exposure of planar multicomponent lipid bilayers supported on mica to n-butanol. The bilayer contains 49 mol % 1,2-dioleoyl-sn-phosphatidylcholine (DOPC), 10 mol % cholesterol, 40 mol % sphingomyelin, and 1 mol % sulforhodamine-tagged 1,2-dioleoyl-sn-phosphatidylethanolamine (SR-DOPE). Phase separation of the cholesterol domains is seen within the bilayer structure, and exposure of this supported bilayer to controlled amounts of n-butanol in the aqueous overlayer produces morphological changes over a range of length scales. We report steady state fluorescence imaging, fluorescence lifetime imaging, and fluorescence anisotropy decay imaging for these bilayers. These data are consistent with literature reports on the interactions of lipid bilayers with n-butanol and provide molecular-scale insight relative to bilayer organization that has not been available to date. The exposure of these bilayers to n-butanol leads to more extensive disruption of the bilayer than is seen for their exposure to ethanol.