Excited-state proton transfer of equilenin and dihydroequilenin: interaction with bilayer vesicles

Abstract

The two-state excited-state proton-transfer process for d-equilenin [d-3-hydroxyestra-1,3,-5(10),6,8-pentaen-17-one] and dihydroequilenin is found to depend both on pH and on proton acceptor concentration. Both the protonated and deprotonated forms of the excited molecule are fluorescent. As is the case for 2-naphthol, the excited-state pKa (pKa*) is substantially lower than the ground-state pKa. Fluorescence decay studies have been performed as a function of emission wavelength in aqueous solutions at pH 6.9 in the presence of acetate anion (0.1 M). At this pH, both back-reaction from the excited-state and ground-state heterogeneity are minimal. A monoexponential decay is found in the blue region of the spectrum and a biexponential decay on the red edge. The lifetimes measured across both regions are constant, with a negative preexponential term, characteristic of an excited-state reaction, evident at longer wavelengths. Decay-associated spectra (DAS), the preexponential terms associated with the measured lifetimes, have been acquired for these aqueous solutions. Equilenin and dihydroequilenin are found to adsorb to dimyristoyllecithin (DML) vesicles. Rates for excited-state proton transfer are greatly reduced when dihydroequilenin adsorbs to vesicles. The accessibility of the bound probe to acetate as a proton acceptor depends on the cholesterol content of the vesicles.

Excited-State Reactions

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In the case of 2-naphthol, the pKa decreases from 9.5 in the ground state to 2.8 in the excited state. 18 In acid solution, the emission is from naphthol, with an emission maximum of 357 nm ( Figure 18.1).…”

Molecular-Size Fluorescence Emitters

“…The dyes displaying these bands of high and comparable intensities can be used for λ-ratiometric sensing. Since the acidity of dissociating groups is commonly much higher in the excited than in the ground state, the pH range of sensitivity in emission spectra can be dramatically shifted to lower pH (Laws and Brand 1979;Davenport et al 1986). This allows providing the wavelength-ratiometric recording in an extended pH range.…”

Comparative Analysis of Fluorescence Reporter Signals Based on Intensity, Anisotropy, Time-Resolution, and Wavelength-Ratiometry

“…These dyes exist as neutral molecules and their excited-state deprotonation with the rate faster than the emission results in new red-shifted bands in emission spectra [48]. Such properties can be explored in the same manner as the ground-state deprotonation with the shift of observed spectral effect to more acidic pH values.…”