Effects of short-chain alcohols on the phase behavior and interdigitation of phosphatidylcholine bilayer membranes

Abstract

Solute effects on the polymorphism and phase transitions in the suspensions of dipalmitoylphosphatidylcholine (DPPC) were studied by means of carboxyfluorescein (CF) and phosphatidylethanolamine rhodamine (PERho) fluorescence, differential scanning calorimetry, and X-ray diffraction. Specifically, the shifts of the lipid chain-melting phase transition, pretransition and subtransition temperature as a function of the bulk alcohol concentration were determined calorimetrically. The chain-melting phase transition temperature, Tm, was found to depend on the chain-length of the added alcohol: for short-chain alcohols (up to n-propanol), Tm first decreases and then increases with increasing alcohol concentration. For longer-chain alcohols, however, Tm decreases over the whole investigated alcohol concentration range. The pretransition and the subtransition temperature of DPPC both decrease monotonously (but non-linearly) with increasing alcohol concentration, but the former transition disappears at some characteristic, chain-length dependent alcohol concentration, cL beta i. This point in the solute-dependent phase diagram of DPPC is diagnostic of the complete hydrocarbon interdigitation. It was determined for a series of short-chain alcohols ranging from methanol through to 1-hexanol. A quantitative formula for the calculation of such limiting alcohol concentration is introduced. This formula relates the cL beta i values to the free energy of transfer of alcohols from the aqueous sub-phase into the DPPC sub-phase. By using the concept of an apparent chain-length this formalism can also be used for the alcohols with polar OH-groups at the second or third position on the hydrocarbon chain. Alcohol-induced hydrocarbon interdigitation in the phospholipid bilayers is thus shown to result chiefly from the solute-induced perturbation (lateral expansion) in the lipid headgroup region. Longer-chain alcohols, which balance this effect by disordering the phospholipid chains, therefore do not induce chain interdigitation.

Phase behavior of selected artificial lipids

“…GIXD experiments have shown that the effective area per head group decreases with an increase in ethanol concentration leading to a reduction of the chain tilt angle. The influence of ethanol on self-assembled aggregates of phospholipid molecules has been reported in [33][34][35][36][37][38][39]. In these investigations, lipid bilayers dispersed as unilamellar or multilamellar vesicles in an aqueous solution have been used.…”

Differential Scanning Calorimetry Studies of Phospholipid Membranes: The Interdigitated Gel Phase

“…In particular, the carbonyl groups of the glycerol backbone of phospholipids are thought to be the favored hydrogen bonding sites for ethanol [40]. Ethanol displaces water when it adsorbs to the head group, which increases the head group volume and decreases the order of the hydrocarbon chains [13,41,42]. The increase in head group volume leads to increased chain tilting and creates energetically unfavorable voids in the hydrocarbon region of non-interdigitated membranes, encouraging the creation of the L I phase at high concentrations [13,39,[42][43][44].…”

“…Ethanol displaces water when it adsorbs to the head group, which increases the head group volume and decreases the order of the hydrocarbon chains [13,41,42]. The increase in head group volume leads to increased chain tilting and creates energetically unfavorable voids in the hydrocarbon region of non-interdigitated membranes, encouraging the creation of the L I phase at high concentrations [13,39,[42][43][44]. Once the L I phase is formed, ethanol can bind to the exposed hydrocarbon chains, replacing the unfavorable interaction of the acyl chains with water [45].…”

Mesoscopic Simulations of Biological Membranes

“…To study the effect of these molecules on the properties studies on various model membranes have been carried out by various groups [66,[71][72][73][74][75][76]. These studies show that at low temperatures adding alcohol leads the formation of an interdigitated or L βI phase in stead of the commonly formed L c or L β (gel phases).…”

“…The critical bulk concentration at which interdigitation is complete decreases with increasing length of the alcohol [71,75,77]. Various theories have been used to relate the bulk concentration of alcohol to the concentration in the lipid.…”