The purpose of this contribution is two-fold: (1) to show the complementary application of the techniques of time resolved fluorescence quenching (TRFQ) and electron spin resonance (ESR) in the investigation of the physicochemical properties of micellar interfaces and (2) present results of such a study on mixed micelles of phospholipids and detergents. We have characterized mixed micelles of the phospholipid, dimyristoyl phosphatidyl choline (DMPC) and (i) the bile salt sodium deoxycholate (NaDC); and (ii) the zwitterionic dodecyl dimethyl ammonio propanesulfonate (DPS). These mixed micelles are model substrates for studying phospholipase activity. Results of characterization of enzyme activity are also presented.

Aggregation numbers of DPS/DMPC and NaDC/DMPC micelles and micellar quenching reaction rate constants are determined by TRFQ. The hydration and microviscosity of the micelle/water interface are measured by ESR of spin-probes inserted into the micelles. These properties vary with composition and total concentration. The ESR and TRFQ data are interpreted together within the framework of a polar shell model of the micelle to derive its features. In the case of NaDC/DMPC, small globular micelles are formed at low NaDC to DMPC ratios (<5), whereas DPS/DMPC forms stable globular micelles for a wide range of compositions. These properties are found to contribute to the observed rate enhancement of micellar lipid hydrolysis by lipolytic enzymes.