Differentiation of stereoisomeric steroids by reactions with phosphenium ions

Abstract

A chemical ionization method is reported for distinction of diastereomeric hydroxysteroids by using Fourier-transform ion cyclotron mass spectrometry (FT-ICR). Certain phosphenium ions are demonstrated to react with stereoisomeric steroids to yield qualitatively different product ions. For example, 1,3,5(10)-estratriene-3,16␤,17␤-triol (cis-estriol) reacts with the dimethoxy phosphenium ion to form a diagnostic product ion (not formed for the trans-estriol) through addition followed by the loss of two molecules of methanol. In an analogous manner, the 1,3-dioxolan-2-phosphenium ion produces a diagnostic product ion through the loss of ethylene glycol from the adduct of cis-estriol only. The 1,3,5(10)-estratriene-3,16␣,17␤-triol (trans-estriol), on the other hand, reacts with each phosphenium ion only via hydroxide abstraction-initiated pathways that indicate the presence of at least two hydroxyl groups in the molecule. These specific reactions take place for all hydroxysteroids examined, independent of their stereochemistry. Another isomer pair, cholestan-3␣,5␣-diol (cis-cholestandiol) and cholestan-3␤,5␣-diol (trans-cholestandiol), is differentiated based on selective elimination of water only from the adduct of the cis-isomer. However, the method does not allow distinction between the stereoisomeric 5␤-pregnane-3␣,17␣,20␣-triol and 5␤-pregnane-3␣,17␣,20␤-triol. The different reactivities of the three pairs of steroid isomers and of each diastereomeric compound pair are rationalized by reaction enthalpies and steric effects based on straightforward and predictable reaction mechanisms. T he ability to distinguish stereoisomers of biologically active compounds is important since their reactivity is often greatly influenced by their stereochemistry. The need for fast, accurate, and specific methods for the determination of the stereochemical structures of compounds is especially important in the area of drug development because the activity of a drug depends greatly upon its three-dimensional structure [1]. HPLC is commonly used for this task, but long analysis times and the lack of standards for unknown compounds are some of the drawbacks of this technique [2]. Stereoisomer differentiation by mass spectrometry offers several advantages, including speed, sensitivity, and the ability to obtain structural information for unknown compounds in complex mixtures. As a result, many research efforts have focused on the development of mass spectrometric approaches for the analysis of stereoisomers [3][4][5][6][7][8] . The ability to differentiate these types of compounds is important because amino and hydroxyl groups are some of the most common functional groups present in biological molecules and natural products.Electron ionization mass spectrometry is widely used to obtain structural information on molecules but it is insensitive to their stereochemical structure. For example, the electron ionization mass spectra of cis -and trans-1,2-cyclopentanediols [3a] and cis-and trans-1,2-diaminocyclohexanes [5] are identical. ...

Polar Acetalization and Transacetalization in the Gas Phase: The Eberlin Reaction

“…Another gas-phase reaction closely related to the Eberlin reaction is that of the phosphenium ion, P(OCH 3 ) 2 + , with cyclic vicinal diols to form cyclic phosphenium ions. This P-centered ion reacts via polar acetalization in a stereoselective reaction, which has been applied to the gas-phase differentiation of cis and trans isomeric steroids (Scheme a) . As shown in Figure , in a FT-ICR reaction cell a diagnostic cyclic product ion of m / z 317 was observed in the reaction of the dimethoxyphosphenium ion of m / z 93 with cis -estriol rather than trans -estriol through the loss of two molecules of methanol .…”

Probing isomeric differences of phosphorylated carbohydrates through the use of ion/molecule reactions and FT-ICR MS

“…The analytical application of ion/molecule reactions has long been realized and is currently of much interest as is evidenced by two recent reviews [18,19]. Other recent examples include those by Kenttämaa and coworkers who utilized ion/molecule reactions to differentiate isomeric species such as diols [20,21] and steroids [22]. In other studies, ion/molecule reactions have been utilized to develop a potential gas phase sequencing method for peptides [23], to determine enantiomeric excess [24 -26], and to explore non-covalent interactions [27].…”

Differentiation of Protonated Aromatic Regioisomers Related to Lignin by Reactions with Trimethylborate in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

“…Hence, these reactions follow pseudo-first order kinetics, which allows for the derivation of the second-order reaction rate constants (k exp ) from semilogarithmic plots of the relative abundances of the reactant ions versus reaction time and the concentration of the neutral reagent [16][17][18]. This concentration was determined from the measured pressure change when the neutral reagent was introduced into the instrument.…”

“…For example, reactions of protonated trimethylborate (TMB) and its complexes, (2TMB+H -CH 3 OH) + and (2TMB+H) + , have been shown to allow differentiation of some neutral isomeric cyclic glycols and mono-and disaccharides [14]. Phosphenium ions have been employed to distinguish neutral diastereomeric hydroxysteroids, cyclic vicinal diols and cis-and trans-1,2-diaminocyclohexanes [16][17][18].…”