Investigation of Anisotropic Motion of Radical Ions in Reversed Micelles

Burkhard Kirste, Dominique Niethammer and Harry Kurreck

Institut für Organische Chemie, Freie Universität Berlin, D-1000 Berlin 33

Reversed micelles have recently found interest in enzymatic studies because they are capable of hosting biomolecules and other polar water-soluble species within aqueous compartments in organic solvents. In contrast to normal micelles ("oil in water"), reversed micelles are "water-in-oil" microemulsions consisting of an internal aqueous microphase (water pool) surrounded by a surfactant layer and the external organic phase. Reversed micelles are also an attractive medium for EPR/ENDOR studies of water-soluble radical ions [1]. They are superior to purely aqueous phases which render EPR investigations quite difficult because the strong microwave absorption of water puts tight restrictions on the sample volume. This problem is even more severe with respect to ENDOR studies and is aggravated by the limited viscosity range available in aqueous solutions. EPR/ENDOR investigations of spin probes in reversed micelles should also provide a better understanding of the physicochemical properties of these systems, i.e., the preferred location and the mobility of the guest molecules. Whereas nitroxide spin labels have been used for that purpose in previous studies [2], the suitability of semiquinone radical ions will be explored in the present contribution.

We have investigated a series of substituted benzosemiquinone radical anions in reversed micelles. In comparison with EPR studies in purely aqueous phases or in alcoholic solutions we found small but significant changes of g values and hyperfine splittings. Particularly interesting are pronounced anisotropic linebroadening effects observed in inverse-micellar solutions. Computer simulations of the EPR spectra demonstrate that different molecular positions make distinctly different contributions to this effect. This peculiar behaviour is also revealed in the ENDOR spectra which exhibit a selective broadening of some of the signal pairs. Obviously the motion of the probe molecules is anisotropic and significantly slowed down as compared to the aqueous solution. It is concluded that the semiquinone anion radicals are immobilized at the surfactant-water interface. Models for the preferred location and anisotropic motion are discussed.

[1] N. Bretz, I. Mastalsky, M. Elsner and H. Kurreck, Angew. Chem. 99, 357 (1987).
[2] G. Häring, P. L. Luisi and H. Hauser, J. Phys. Chem. 92, 3574 (1988).

Burkhard Kirste, 1995/10/03