Head-group contributions to bilayer stability: monolayer and calorimetric studies on synthetic, stereochemically uniform glucolipids

Publisher Website
Google Scholar
Abstract
Monolayer and differential scanning calorimetry studies were performed on synthetic, stereochemically uniform glyceroglucolipids having saturated, ether-linked alkyl chains. The limiting area, A0 = 40 .ANG.2 .cntdot. molecule-1, resulting from the monolayer measurements of the glucolipids is comparable to the Ao value found for phosphatidylethanolamine lipids. The area corresponds to twice the value observed with saturated straight chain fatty acids, which indicates that at high surface pressure the space requirement of the glucose head group does not exceed that of the 2 alkyl chains. The apparant specific heat capacities of the glucolipid dispersions were found to be higher than those of corresponding phospholipids. They can be approximated from group parameters with the additional assumption that the experimental partial molar heat capacity of glucose is valid for the glucose head groups of the lipids. The transition enthalpies of the C16 and C18 glyceroglucolipids are clearly larger than the .DELTA.H values of corresponding phospholipids, while the C14 glycerolgucolipid has the same transition enthalpy as dimyristoylphosphatidylethanolamine or ditetradecylphosphatidylethanolamine. Glucolipids exhibit gel to lipid-crystalline phase transition temperatures which are only slightly lower than those of their phosphatidylethanolamine analogs, although they are uncharged molecules. Like phosphatidylethanolamine the glucolipids do not show a pretransition; however, with the C14 glucolipid a highly cooperative posttransition, .apprx. 5.degree. above the main transition, was found. Calorimetric experiments with a C14 glucolipid, in which the hydroxyl protons of the glucose moiety were exchanged by deuterium, suggest that the posttransition might reflect structural change of the head group. No unique correlations between transition parameters and the nature of the head group or the alkyl chain linkage can be evaluated on the basis of presently available data. This is probably due to the complexity introduced by the variety of opposing interactions which stabilize the structures of changed phospholipids. Agreement between transition temperatures estimated from monolayer studies at high surface pressures and those obtained from scanning calorimetry on bilayers was taken as supporting evidence for the hypothesis that monolayers behave similar to bilayers when subject to surface pressures around 50 mN .cntdot. m-1.

This publication has 34 references indexed in Scilit: