MITOGENS-DPPC-VESICLES INTERACTION - A SPIN-LABEL ESR STUDY

The electron spin resonance spectroscopy with the spin labelling techn ique has been used to investigate the interaction of sonicated di-palm itoyl-phosphatidylcholine (DPPC) small unilamellar vesicles with conca navalin A (Con A), phytohemagglutinin (PHA) and staphylococcal enterot oxin B (SEB) in the temperature range 0-50-degrees-C. Conventional ESR measurements show that the addition of mitogens to the vesicles suspe nsions up to the number of mitogens/number of vesicles ratio of 60 inc reases the orientational degree of order of the lipid molecules in the gel phase and the gel-->liquid-crystalline phase transition temperatu re. No effects on the liquid-crystalline phase have been observed, als o no matter for the mitogen used. The effectiveness results in the ord er: Con A > SEB > PHA. Saturation transfer-ESR measurements evidence m ore restricted rotation of the DPPC molecules in the gel phase in the presence of Con A and SEB than in the presence of PHA if compared to p ure DPPC-vesicles. The effective rotational correlation time, tau(c), of the spin label 5-SASL results about one order of magnitude higher i n the presence of 60 mitogen molecules/vesicle. The ESR findings sugge st a mitogen-induced increase in the packing density of lipid bilayers which is attributed to a morphological change from sonicated vesicles , with a small radius of curvature, to a more planar mesophase. Con A is more effective than SEB and PHA in inducing this process.