Photophysical Study of Local Anesthetics in Reverse Micelles and Water-Ethanol Mixtures

Fluorescence emission from three tertiary amine local anesthetics, dibucaine (D), tetracaine (T), and procaine (P) was investigated in the membrane-mimetic environment of AOT/isooctane reverse micelles and water-ethanol mixtures. At room temperature all three show broad UV absorption bands and large Stokes' shifts in emission from aqueous medium. In reverse micelles the emission bands are blue-shifted, with the magnitude of this shift decreasing continuously as the waterto-surfactant molar ratio (w₀) of the micelles is increased. The emission intensity of D is much stronger in water and in the interior of micelles than in ethanol, whereas for T and P the emission in water is much weaker than in ethanol and in micelles. Whereas the behavior in micelles is the result of the increased hydrophobicity with respect to bulk water in their interior, that in waterethanol mixtures may be explained on the basis of state reversal of the excited electronic levels of D with increasing hydrogen bonding capability of the solvent mixture. The steady-state anisotropy for both T and P is unusually high in bulk water (0.27) and increases with increasing w₀ in micelles. However, the rotational correlation time of the anesthetics, as calculated using Perrin's equation, decreases with increasing w₀, as expected from physical considerations relating the size of the water pool to the ease of rotational relaxation of probe molecules inside micelles. Together, these results point to a marked difference in emission properties between D on one hand and T and P on the other, arising from differences in electronic characters of their primary emitting chromophores—quinoline for D, para amino benzoate for T and P.