SALT-MEDIATED EFFECTS IN NONIONIC LIPID BILAYERS CONSTITUTED OF DIGALACTOSYLDIACYLGLYCEROL STUDIED BY FTIR SPECTROSCOPY AND MOLECULAR MODELLIZATION

Digalactosyldiacylglycerol (DGDG;Galp alpha 1-6Galp beta 1-3DAG) is one of the two nonionic species of the thylakoid membrane of higher plant chloroplasts where it constitutes about 29% of the total lipid content; This high concentration led to the conjecture that DGDG intervenes in the molecular organization and function of the thylakoid membrane. In this respect, we show that the DGDG membranes undergo aggregation but not fusion, at salt concentrations in the incubation media above a threshold characteristic of every ionic species, i.e., about 1.0 and 4.5-4.7 mM for Ca2+ and Mg2+, respectively. We also give evidence that the ions' effect is 2-fold: on one hand, the electric field created by the divalent cations affects the sn(2) ester C=O dipole in the DGDG head group but is not responsible for the onset of membrane aggregation; on the other hand, the initial step in DGDG aggregation is an ion-induced decrease in interfacial polarity. The results point toward a;general mechanism of ionic control of the interfacial polarity and membrane aggregation in lipid bilayers; that is, the ions derived from atoms with [Ar]4s(x) configurations, where x is 1 or 2, are more effective polarity modifiers and membrane aggregation inducers than those with [Ne]3s(x) or [He]2s(x) configurations. We determined also the minimal energy conformation of the DGDG molecule that corresponds to the structure in zero compression conditions and found that the beta-anomer of the digalactosyl moiety adopts an almost perpendicular geometry in relation to the cu-anomer orientation which is about parallel to the membrane plane. These geometrical characteristics confer unexpected structure-forming properties On DGDG which provide a novel basis to explain (i) the galactolipid participation in close approach interactions, or adhesion, and (ii) the inability of DGDG membranes to undergo fusion.