Structure and Hydrogen-Bonding Ability of Estrogens Studied in the Gas Phase

Abstract

The structures of estrogens (estrone­(E1), β-estradiol­(E2), and estriol­(E3)) and their 1:1 hydrogen-bonded (hydrated) clusters with water formed in supersonic jets have been investigated by various laser spectroscopic methods and quantum chemical calculations. In the S₁–S₀ electronic spectra, all three species exhibit the band origin in the 35 050–35 200 cm^–1 region. By use of ultraviolet–ultraviolet hole-burning (UV–UV HB) spectroscopy, two conformers, four conformers, and eight conformers, arising from different orientation of OH group(s) in the A-ring and D-ring, are identified for estrone, β-estradiol, and estriol, respectively. The infrared–ultraviolet double-resonance (IR–UV DR) spectra in the OH stretching vibration are observed to discriminate different conformers of the D-ring OH for β-estradiol and estriol, and it is suggested that in estriol only the intramolecular hydrogen bonded conformer exists in the jet. For the 1:1 hydrated cluster of estrogens, the S₁–S₀ electronic transition energies are quite different depending on whether the water molecule is bound to A-ring OH or D-ring OH. It is found that the water molecule prefers to form an H-bond to the A-ring OH for estrone and β-estradiol due to the higher acidity of phenolic OH than that of the alcoholic OH. On the other hand, in estriol the water molecule prefers to be bound to the D-ring OH due to the formation of a stable ring-structure H-bonding network with two OH groups. Thus, the substitution of one hydroxyl group to the D-ring drastically changes the hydrogen-bonding preference of estrogens