Mass spectrometric studies on 17β‐estradiol‐17‐fatty acid esters: Evidence for the formation of anion‐dipole intermediates

Abstract

AbstractThe behaviour towards low collision energy processes (eV range) of [M  H]− prepared under negative ion chemical ionization (NICI) ammonia conditions from 17β‐estradiol‐17‐fatty acid esters has been investigated. From such bifunctional compounds containing two acidic sites (i.e. phenol and ester groups), two isomeric forms (i.e. phenoxide and enolate forms) characterize the [M  H]− ion structures, whose distribution depends on the ion preparation mode. Here NICI (ammonia) provides both phenoxide and enolate forms as the [M  H]− species. This behaviour contrasts with the regioselectivity observed for proton abstraction from phenol under NICI (N2O) and fast atom bombardment conditions. Production of both phenoxide and enolate forms in NICI (ammonia) is demonstrated under NICI (ND3) conditions in which DO‐labelled [Md  H]− enolate ions are produced in a similar yield to unlabelled [Md  D]− phenoxide ions. Collisionally activated dissociation (CAD) spectra of both isomeric deprotonated molecules differ strongly by the presence of two different pairs of complementary daughter ions, suggesting that these ionic species are unconvertible. This is due to a steric hindrance effect on the long‐distance proton transfer. A mechanistic investigation on the formation of fragment ion pairs produced under CAD was performed with various deuterium‐labelled molecules. From these experiments, evidence is provided for molecular isomerizations into ion‐dipole complexes (prior to dissociation) which are structurally dependent on the initial charge location. Direct dissociation of these intermediates competes with the occurrence of exothermic proton transfer(s) yielding the formation of other isomeric intermediate forms. The orientation of these proton transfers is dictated by the relative acidities of both moieties of the complex.