Fluorescence anisotropy studies of dibucaine.HCl in micelles and bacteriorhodopsin

Emission and excitation spectra for the local anesthetic drug, dibucaine.HCl in neutral and charged surfactant solutions and in bacteriorhodopsin (bR) have been investigated for lambda(ex) = 266 nm at room temperature. The total fluorescence and fluorescence anisotropy decays of the anesthetic in the same environments were also measured using a picosecond laser/streak camera system (lambda (ex = 266 nm)). The total fluorescence decay gave two components for dibucaine micellar and dibucaine bR solutions, one component in the range of 200-500 ps and the other in the range of 1200-3400 ps. Only the nanosecond timescale component was found for the dibucaine monomer surfactant solutions (1200-3000 ps), indicating that the anesthetic resides in the bulk solution. The fluorescence anisotropy decays of dibucaine in Triton X-100 and in lithium dodecyl sulfate (LDS) micelles are approximately 200 ps, which is attributed to dibucaine solubilized in the micellar environment. Dibucaine.HCl in anionic monomer solution exhibits an unusually large fluorescence anisotropy, r(t)max = 0.22 and a depolarization decay of less than 100 ps. This presumably results from a head-to-tail exciplex aggregation between the positively charged dibucaine and negatively charged dodecyl sulfate surfactant molecules. The anisotropy decay of dibucaine in bR is 300 ps. This solution was the only one which exhibited a residual fluorescence anisotropy, r(infinity) - 0.08. This implies that dibucaine is restricted in its rotational motion and suggests protein binding rather than lipid solubility.