THE FLUORESCENCE PROPERTIES OF ORTHO AMINOBENZOATE ANESTHETICS IN DEFINED SOLVENTS AND PHOSPHOLIPID VESICLES |
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Authors: | Woubalem Birmachu Juta K Reed † |
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Institution: | Department of Chemistry, Erindale Campus, University of Toronto, Mississauga, Ontario, L 5L 1C6, Canada |
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Abstract: | 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. |
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