Determination of the partition coefficients of the nonionic detergent C12E 7 between lipid-detergent mixed membranes and water by means of Laurdan fluorescence spectroscopy.

The membrane-water partition coefficient of the detergent C12E7 between water and C12E7/POPC mixed membranes has been determined by means of steady-state fluorescence spectroscopy. The emission spectra of the fluorescent probe Laurdan were used as an indicator of membrane composition at different membrane concentrations in the sample. The partition coefficient expressed as the ratio of the mole fractions of the detergent in the membrane and water phases is about 6(*)10(5) at low molar ratios of C12E7/POPC (R c ) and decreases rapidly with increasingR c . The limiting detergent content of the lamellar phase (R c (*) >≈0.8) is indicated by a minimum ofP(R c ).