Membrane Lipid-Polypeptide Molecular Associations in Non Aqueous Solvent. Effect of Phosphatidylcholine Concentration and Temperature and their Influence on the Gramicidin a Dimer-Monomer Conformational Equilibrium

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Abstract
The use of an Ultrastyragel 500 Å column for the study of interactions between phosphatidylcholine and gramicidin A in tetra-hydrofuran is described. Analysis of vacant peak has allowed to establish the influence that eluent lipid composition, concentration of injected gramicidin and temperature have on the interaction. At 20[ddot] C, for the assayed phospholipid concentration range (0.04 to 0.14 %, w/v), the lipid/polypeptide molar binding ratio, BR, varies from 1.3 to 6.9. An increase in temperature from 20 to 40[ddot] C causes a decrease in BR of about 20 %. On the other hand, the interaction releases some of the water bound to the lipid polar head, suggesting that the binding involves at least this phospholipid moiety. It has been also found that the lipid alters the gramicidin dimer-monomer conformational equilibrium resulting in a higher extent of monomeriz-ation, more pronounced as temperature increases. Additional spectro-fluorometric measurements are consistent with the experimental evidence deduced from HPLC analysis.