Monomeric phospholipids, micellization detergents

V. Critical Micelle Concentration Determinations and Micellization of Monomeric Phospholipids by Detergents... 

Similarly, incorporation of monomeric phospholipids by detergents into mixed micelles results in a chemical-shift change in P NMR, and this can be used to follow the micellization process. This is illustrated in Fig. 8, where the micellization of dihexanoyl-PC by titration with Triton X-lOO was followed by changes in the P-NMR chemical shift. In contrast, with the same amount of Triton X-100, dibutyryl-PC is hardly micellized at all. A control titration of the completely water-soluble analog glycerophosphoryl-choline is also included. For the dihexanoyl-PC, a partition coefficient between Triton X-100 micelles and free solution was calculated as a function of the Triton X-100 concentration, using the phase-separation approximation for mixed-micelle formation and considering the mixed micelles to be a pseudo-phase. With the data in Fig. 8, the fraction of dihexanoyl-PC... 

We have now extended these studies to synthetic phospholipids that contain short chain fatty acyl groups and which are water soluble, such as dibutyryl and dihexanoyl phosphatidylcholine (PC). These phospholipids are monomeric below their critical micelle concentration (cmc), yet activate the enzyme. In order to carry out kinetic studies, the long chain phospholipid substrate must generally be solubilized by a detergent such as Triton X-100 which serves as an inert matrix. Further understanding of the mechanism of the activation by short-chain phospholipids requires first a quantitation of the solubilization of these compounds by detergent ... 

We found the 31p-NMR chemical shift of monomeric dihexanoyl PC increases upon the addition of the nonionic detergent Triton X-100. This phenomenon was used to quantitate the solubilization of this phospholipid by the detergent micelles as a function of detergent concentration using a simple phase separation model ( 5). Similar studies were carried out on dibutyryl PC. At a phospholipid concentration of 7 mM and 56 mM detergent, 85% of the dihexanoyl PC, but only 3% of the dibutyryl PC was incorporated into the micelles. 

This chapter limits itself to P-NMR studies on phospholipids and lysophospholipids in monomeric and micellar states and focuses on the identification of species and aggregation states, as well as on the dynamic processes of migration and reaction kinetics. For this purpose, micelles and mixed micelles are defined as dilute isotropic solutions of phospholipids, either with or without detergents that form spontaneously and are at thermodynamic equilibrium diis definition specifically excludes sonicated or small unilamellar vesicles as well as membranes. These are covered by Smith and Ekiel (Chapter 15). This limitation in scope necessarily requires heavy reliance on work from the laboratory of the authors of this chapter. 

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