Probing the ethanol-induced chain interdigitations in gel-state bilayers of mixed-chain phosphatidylcholines

Abstract

Using high-resolution differential scanning calorimetry (DSC), we have studied the effects of ethanol concentrations, [EtOH], on the main phase transition temperatures (T[m]) of the following mixed-chain phosphatidylcholines (PCs): C(15):C(17)PC, C(17):C(15)PC, and C(12):C(20)PC. These lipids have a common molecular weight; however, their apparent acyl chain-length differences between the sn-1 and sn-2 acyl chains, delta C, are distinctively different. The delta C values for these three mixed-chain PCs are, respectively, 0.5, 3.5, and 6.5 C-C bond lengths. DSC results show that the T(m) profiles for C(15):C(17)PC and C(17):C(15)PC bilayers in the plot of T(m) versus [EtOH] are V-shaped biphasic curves, with the minimum T(m) occurring at 50 and 73 mg/ml of ethanol, respectively. In contrast, the C(12):C(20)PC bilayer exhibits a nearly linear decrease in T(m) with increasing [EtOH]. In addition, x-ray diffraction experiments were also performed to assess the structural changes of these three mixed-chain PCs in the gel-state bilayers, at 20 degrees C, in response to high concentrations of ethanol. X-ray diffraction data indicate that, in the absence of ethanol, these three lamellar lipids are all packed in the normal (L beta') gel phase in aqueous media. In the presence of 120 mg/ml of ethanol, however, the C(15):C(17)PC and C(17):C(15)PC lamellae are packed in the fully interdigitated (L beta[I]) gel phase. The V-shaped T(m) curves detected calorimetrically for these two lipids in response to [EtOH] can thus be explained by the ethanol-induced L beta' --> L beta[I] isothermal phase transition. Interestingly, the results of x-ray diffraction study reveal, for the first time, that an ethanol-induced L beta' --> L(MI) (mixed interdigitated phase) isothermal phase transition occurs in the gel-state bilayer of highly asymmetrical C(12):C(20)PC. Therefore, the chain asymmetry is recognized to play an important role in the ethanol-induced chain interdigitation at T < T(m).

Studies of the Structure and Organization of Cationic Lipid Bilayer Membranes: Calorimetric, Spectroscopic, and X-Ray Diffraction Studies of Linear Saturated P-O-Ethyl Phosphatidylcholines

“…For a centrosymmetric electron density profile such as occurs with a lipid bilayer, the phase information for each diffraction order is either ϩ1 or Ϫ1. The only combination of phases that gave a profile consistent with a phospholipid bilayer structure is (Ϫ Ϫ ϩ), in agreement with the observations of numerous x-ray diffraction studies of gel phase DPPC (e.g., see Wiener et al, 1989) and of chain-interdigitated lipid gel phases (for examples, see Furuike et al, 1999;Nambi et al, 1988;Hirsh et al, 1998;Huang and McIntosh, 1997). Fig.…”

Mechanisms of antimicrobial peptide action: Studies of indolicidin assembly at model membrane interfaces by in situ atomic force microscopy

“…It is possible that these aggregates are not indolicidin aggregates but rather pure lipid or lipid-peptide structures. It has been reported that modiWcations to the acyl chains of the phosphatidylcholine that aVect chain length asymmetry or the position of the unsaturation can prevent small amphiphiles such as alcohols from inducing lipid interdigitation, in which the acyl chains of the upper leaXet lipids cross the mid-plane of the bilayer to interpenetrate the acyl chains of the lower leaXet lipids (Huang and McIntosh, 1997;Li et al, 1996;McIntosh et al, 2001). It is likely that the asymmetric chain conWguration of the SM lipid prevents indolicidin from reducing the thickness of the Chol/SM enriched domains.…”

Partitioning of ABA into Bilayers of Di-Saturated Phosphatidylcholines as Measured by DSC

“…Solute permeability is determined by the magnitude of membrane/water partition coefficients and is effected by both solute structure and membrane composition (Sarmento et al, 1993, Finkelstein, 1976, Diamond and Katz, 1974, Xiang and Anderson, 1994. Not only does the membrane dictate total solute permeability, but the solute likewise influences the structural and physical properties of the membrane (Huang and McIntosh, 1997).…”