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THE FLUORESCENCE PROPERTIES OF ORTHO AMINOBENZOATE ANESTHETICS IN DEFINED SOLVENTS AND PHOSPHOLIPID VESICLES

Abstract— The fluorescence properties of three ortho aminobenzoate local anesthetics have been determined in a variety of solvents. Results from these studies have been used to deduce how these drugs interact with phosphatidylcholine bilayers. The emission energy, fluorescence quantum yield and lifetime exhibited a biphasic dependence on solvent polarity. In aprotic solvents, alcohols and in ethanol‐water mixtures containing less than 40% water, quantum yields and lifetimes were high (approximately 0.55 and 8.5 ns respectively). In ethanol‐water mixtures containing >40% water, the strong fluorescence quenching was primarily due to an increase in the rate of non‐radiative deactivation of the excited state. Both the radiative (kr) and non‐radiative (knr) rate constants show a biphasic dependence on solvent polarity. These studies suggest the presence of two singlet excited states for these molecules, a polar singlet excited state, S1‐p and a charge transfer excited state, S1‐ct with the latter predominating in ethanol‐water mixture containing >40% water. In egg phosphatidylcholine bilayers, the fluorescence, lifetime and quantum yield are consistent with the view that these drugs are localized within the lipid head group region where the charge‐transfer excited state can be stabilized by intermolecular hydrogen bonding.