Enthalpy is a proper criterion for comparability of monolayer and bilayer studies: isobaric temperature scanning measurements on glycolipid monolayers.

The thermotropic behaviour of glycolipid monolayers has been studied by isobaric temperature scanning measurements to elucidate conditions under which monolayers exhibit thermodynamic and structural properties comparable to those observed in bilayers. A selection of synthetic, stereochemically pure, glyceroglycolipids with identical, ether-linked alkyl chains of 12, 14, or 16 CH2-groups has been investigated. The head groups of the glycolipids consisted of glucose, galactose, maltose, lactose or maltotriose moieties with beta-configuration of the glycosidic bond. These glycolipids were chosen to permit a quantitative characterization of three effects, (i) the role of the length of the aliphatic chains, (ii) the influence of the size of the head group, and (iii) the influence of the stereochemistry of the sugar moieties on the structure and stability of the monolayers. To probe the effects of stereochemical alterations in the glycerol moiety 2,3-O-ditetradecyl-1-O-beta-D-glucosyl-sn-glycerol (14-2,3-Glc) was compared with 1,2-O-ditetradecyl-3-O-beta-D-glucosyl-sn-glycerol (14-1,2-Glc). It has been shown that in general several features of bilayers can be obtained from monolayer studies with reasonable accuracy, provided the proper parameters are chosen. The monolayer is stabilized by elongation of the aliphatic chains of the lipids and destabilized when the monosaccharide read groups is replaced by a di-, or trisaccharide, in a similar manner as in the bilayer. The stabilizing effect that has been observed in bilayer studies, when galactose instead of glucose is introduced as head group, has also been established in the monolayer studies. This stabilizing effect is even retained in the lipids having disaccharide head groups. On the basis of these monolayer studies in connection with WAXS and SAXS measurements on multilamellar systems, we suggest that identity of transition enthalpies of the chain melting L beta-L alpha transition is an appropriate criterion for estimating molecular areas and area changes of bilayers from monolayer measurements and vice versa. However, estimates of transition temperatures are poor using the enthalpy criterion. If identity of transition temperature is introduced as criterion, glycolipid monolayers must be compressed to about 43 +/- 3 mN m-1. Under these conditions the agreement between the calculated enthalpies and structural properties of monolayers and multilayers is poor. As a general conclusion it can be emphasized that for monolayer and bilayer systems of glycolipids there exists no such parameter as a universal pressure or a universal temperature that automatically renders monolayer data identical to bilayer data. Depending on which property (transition temperatures, transition enthalpies, lateral areas and transitional area changes) one wants to extrapolate from monolayer to bilayer different lateral pressures have to be applied.