FOURIER-TRANSFORM INFRARED SPECTROSCOPIC STUDY OF ION-BINDING AND INTRAMOLECULAR INTERACTIONS IN THE POLAR HEAD OF DIGALACTOSYLDIACYLGLYCEROL

Lipid bilayers composed of digalactosyldiacylglycerol (DGDG), that is, Galpalpha1-6Galpbeta1-3DAG, a non-ionic lipid of the thylakoid membra ne of chloroplasts, aggregate in aqueous media containing mono- and di valent cations in amounts above a threshold concentration (C(t)) of ab out 1.0, 4.7 and 10.0 mM for Ca2+, Mg2+ and Na+, respectively. In this work, we found that above C(t) the DGDG membranes do not undergo fusi on and that the aggregation can be reversed, or disrupted. This means that the perturbation induced by the salts results from adsorption, or complexation of the ions in the polar head of DGDG. To investigate th is question, we used Fourier transform infrared (FTIR) spectroscopy to identify the molecular sites in DGDG which are modified by interactio n, or adduct formation with CaCl2, MgCl2 and NaCl. We also determined whether the ions affect the intramolecular hydrogen bonding between th e sn2 ester C = O and the carbon-6 of the alpha-anomer of galactose (G al). The major conclusions are: (i) the salts do not affect, at least directly, the ester carbonyl region of DGDG, (ii) the most probable si tes of binding, or adsorption, for the ions are the ring oxygen, and ( iii) the ring hydroxyls are the sites of either ion complexation or in tra- and intermolecular H-bonding in interacting DGDG membranes. Withi n this framework, the complexation of the ions with Gal might induce t otal or partial dehydration of the galactolipid headgroup and thus pro vides the means to overcome the repulsive hydration forces that hinder aggregation of the DGDG membranes.