A study of the structure and dynamics of complexes between polymyxin B and phosphatidylglycerol in monolayers by fluorescence

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Abstract
Interactions between the antibiotic polymyxin B and monolayers of dipalmitoylglycerophosphoglycerol have been reinvestigated through a study of the structure and dynamics of the complexes by means of a fabricated interface fluorimeter. A fluorescence technique was developed where the use of linearly polarized incident beams gives the simultaneous determination of the orientation and the lateral diffusion rate of a fluorescent probe inserted in the film. The present investigated was carried out with 12-(9-anthroyloxy)-stearic acid, a fluorescent compound which forms non-fluorescent photodimers upon illumination. Orientation of the probe was studied by computing the ratio of the 2 dimerization constants KD and the ratio of the fluorescence intensities obtained with crossed linearly polarized incident lights. The lateral diffusion rate of the probe was obtained by measuring fluorescence recovery after photobleaching (photodimerization) of the probe. Control experiments, carried out with dimyristoylglycerophosphocholine, a lipid which does not interact with polymyxin B, show that the antibiotic does not significantly modify the behavior of the probe. Both in terms of orientation and dynamics, with respect to dipalmitoylglycerophosphoglycerol, when the antibiotic is present in the subphase (1 .mu.M, saturating conditions), data indicate that the lipid remains in a liquid-expanded state. This is true even at a high surface pressure (.pi. .apprxeq. 37 mN .cntdot. m-1), above the apparent transition which can be observed at 30-35 mN .cntdot. m-1 on its compression isotherm. Computation of the contribution of polymyxin B to the film expansion leads to the conclusion that this transition would be a structural transition between 2 models of interaction: one, below the ''transition'', where the polypeptide ring penetrates between the film-forming lipid molecules and another one, above the transition, were the antibiotic is adsorbed at the lipid-water interface with only its hydrocarbon chain penetrating the film.