Fluorescence depolarization in a model system: carbonic anhydrase-dansyl sulphonamide

The model system, carbonic anhydrase to which dansyl sulphonamide is specifically adsorbed, was chosen to test the fundamentals of the fluorescence depolarisation method. Fluorescence decay was shown to be satisfactorily represented by a single exponential decay term and the derived lifetime did not seem to vary significantly with mean excitation or emission wavelength. Depolarisation measurements were quite reproducible and those made with varying temperature were consistent with those performed with varying solution viscosity, thus excluding localised dye rotation. The calculated relaxation time, which was independent of wavelength of excitation or emission, was in conformity with a rigid spherical molecule of molecular weight 29 000 with 30% hydration.