Model membranes, reverse micelles

The protein-containing colloidal solutions of water-in-organic solvents are optically transparent. Hence, absorption spectroscopy, circular dichroism spectroscopy and fluorescence spectroscopy are found to be convenient for studying biocatalysis [53]. The reversed micelles are interesting models for studying bioconversion, since the majority of the enzymes in vivo act inside or on the surface of biological membranes. 

Martinek, K., Levashov, A. V, Pantin, V. I., and Berezin, I. V. (1978). Model of biological membranes or surface-layer (active center) of protein globules (enzymes) - reactivity of water solubilized by reversed micelles of aerosol OT in octane during neutral hydrolysis of picrylchloride. Doklady Akademii Nauk SSSR, 238, 626-9. 

The lipidic cubic phase has recently been demonstrated as a new system in which to crystallize membrane proteins [143, 144], and several examples [143, 145, 146] have been reported. The molecular mechanism for such crystallization is not yet clear, but the interfacial water and transport are believed to play an important role in nucleation and crystal growth [146, 147], Using a related model system of reverse micelles, drastic differences in water behavior were observed both experimentally [112, 127, 128, 133-135] and theoretically [117, 148, 149]. In contrast to the ultrafast motions of bulk water that occurs in less than several picoseconds, significantly slower water dynamics were observed in hundreds of picoseconds, which indicates a well-ordered water structure in these confinements. 

There are a variety of other types of nonbilayer lipid structures such as reversed micelles sandwiched between monolayers of the lipid bilayers in vivo, while the main structural pattern of biological membranes is the flat bilayer of lipid molecules. These nonbilayer structures can explain many processes occurring in the living cell, such as fusion, and exo- and endo-cytosis. Because the water in the reversed micelle resembles that adjacent to biological membranes or biological reversed micelle-like microcompartments, reversed micelles may be an appropriate model for investigating biological catalysis at the molecular level [3-5]. 

While photosensitizers for PDT have been mostly studied in model membrane systems to understand how a membrane interface affects localization, photophysics, and reaction rates with oxygen, they also turn out to be useful as probes for the microenvironment of the model membrane systems. The properties of micelles, reverse micelles, and liposomes are of special interest in understanding biological membrane systems and in the... 

The above studies have demonstrated that various photosensitizers, in conjunction with many available experimental techniques, can be used to probe different regions of the colloidal model membranes systems. Careful choice of sensitizers is important in determining different regions of the micelles, reverse micelles, or liposomes, and their different dynamic and structural features. 

Charge and proton relay through hydrogen bonds have been proposed to contribute to the catalytic efficiency of enzymes, and in this sense reversed micelles provide an appropriate model to delineate the importance of such factors at the enzyme active site. Micellar surfaces also provide a convenient means for the reduction in dimensionality, an important factor in enhancing reaction rates. They also serve as good models to demonstrate the feasibility of ultrafast proton transfer when the reactants are localized in a suitable environment such as membrane surfaces and other complex biomacromolecules. 

It is commonly assumed that transfer processes can be modeled by bulk phase thermodynamics and that surface or interfacial effects are negligible. These assumptions may, in the case of partitioning into amphiphilic structures formed by micelles or bilayer membranes, not always be appropriate. These interfacial solvents have a large surface to volume ratio, similar to interfacial solvents used in reversed-phase liquid chromatography. The partitioning into such phases is the basis of the chromatographic separation. 

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