The electron impact induced cleavage of the C-17--C-20 bond and D-ring in trimethylsilyl derivatives of C21 steroids. Reciprocal exchange of trimethylsilyl groups

MASS SPECTROMETRIC FRAGMENTATION OF TRIMETHYLSILYL AND RELATED ALKYLSILYL DERIVATIVES

This review describes the mass spectral fragmentation of trimethylsilyl (TMS) and related alkylsilyl derivatives used for preparing samples for analysis, mainly by combined gas chromatography and mass spectrometry (GC/MS). The review is divided into three sections. The first section is concerned with the TMS derivatives themselves and describes fragmentation of derivatized alcohols, thiols, amines, ketones, carboxylic acids and bifunctional compounds such as hydroxy- and amino-acids, halo acids and hydroxy ethers. More complex compounds such as glycerides, sphingolipids, carbohydrates, organic phosphates, phosphonates, steroids, vitamin D, cannabinoids, and prostaglandins are discussed next. The second section describes intermolecular reactions of siliconium ions such as the TMS cation and the third section discusses other alkylsilyl derivatives. Among these latter compounds are di- and trialkyl-silyl derivatives, various substituted-alkyldimethylsilyl derivatives such as the tert-butyldimethylsilyl ethers, cyclic silyl derivatives, alkoxysilyl derivatives, and 3-pyridylmethyldimethylsilyl esters used for double bond location in fatty acid spectra. © 2019 Wiley Periodicals, Inc. Mass Spec Rev 0000:1-107, 2019.

Steroids excreted in urine by neonates with 21-hydroxylase deficiency: characterization, using GC-MS and GC-MS/MS, of the D-ring and side chain structure of pregnanes and pregnenes

Steroid metabolites in urine from neonates with 21-hydroxylase deficiency are predominantly polyhydroxylated 17-hydroxyprogesterone and androgen metabolites, and most have incompletely defined structure. This study forms part of a comprehensive project to characterize and identify these in order to enhance diagnosis and to further elucidate neonatal types of steroid metabolism. Steroids were analyzed, after extraction and enzymatic conjugate hydrolysis, as methyloxime-trimethylsilyl ether derivatives on gas-chromatographs coupled to quadrupole and ion-trap mass-spectrometers. GC-MS and GC-MS/MS spectra, obtained with constant excitation conditions, were used together to determine the structure of the D-ring and the side chain of 20-oxo and 20-hydroxy pregnane(ene)s without oxo groups on the A-, B-, and C-ring. All possible combinations of D-ring and side chain configuration were considered. Most fragmentations could be interpreted as partial or complete D-ring cleavages with loss of the side chain, aided by comparison with spectra of deuterated derivatives and of borohydride reduced metabolites. Possible rearrangement ions are also discussed. More than 140 endogenous metabolites were characterized. GC-MS/MS was especially beneficial for characterization of compounds with 16,17-dihydroxy-20-oxo structure, interpreted as markers of intra-uterine enzyme induction. It also assisted the differentiation of 16-hydroxy-20-oxo metabolites, present in urine of non-affected neonates, from the diagnostic 17-hydroxy-20-oxosteroids and enabled the detection of 15,17-dihydroxy-20-oxo compounds in low concentrations. The presence of 17,21-dihydroxylated pregnane(ene)s despite the deficit in CYP21A2 is discussed. We conclude that GC-MS combined with GC-MS/MS allows reliable identification of the structure of the D-ring and side chain of pregnane(ene)s without prior isolation, even when in low concentrations in urine.

Mass spectrometric analysis of androstan-17beta-ol-3-one and androstadiene-17beta-ol-3-one isomers

Mass spectrometric identification and characterization of steroids using electrospray ionization and tandem mass spectrometry has advantages in drug testing and doping control analysis attributable to limitations of gas chromatography followed by electron ionization mass spectrometry. Steroids with an androstadiene-17beta-ol-3-one nucleus and double bonds located either at C-1 and C-4, C-4 and C-9, or C-4 and C-6 were used to determine characteristic fragmentation pathways. Diagnostic dissociation routes are proposed using deuterium labeling, MS3 experiments, and analyses of structurally closely related compounds. Steroids such as boldenone (androst-1,4-diene-17beta-ol-3-one) produced characteristic product ions at m/z 121, 135, and 147. Compounds with double bonds at C-4 and C-9 generated abundant product ions at m/z 145 and 147. Conjugated double bonds at C-4 and C-6 gave rise to an intense and characteristic signal at m/z 133. Stereochemical differentiation between 5alpha- and 5beta-isomers of androstan-17beta-ol-3-ones was possible because of significant differences in relative abundance of product ions generated by elimination of acetone from alpha,beta-saturated 3-keto steroids.

A high-performance liquid chromatography-electrospray-tandem mass spectrometry analysis of cortisol and metabolites in placental perfusate

A reversed-phase high-performance liquid chromatography-electrospray-tandem mass spectrometry assay (HPLC-ESI-MS/MS) was developed to quantitate cortisol, cortisone, 20 alpha- and beta-dihydrocortisol, 20 alpha- and beta-dihydrocortisone, tetrahydrocortisol, and tetrahydrocortisone. The technique was used to analyze perfusate from the isolated human placental lobule for cortisol and its metabolites. Analytes were prepared from the perfusion medium using C18 solid-phase extraction cartridges. The internal standard for the analyses was 6 alpha-methylprednisolone. Chromatography was performed on a Novapak C18 column at ambient temperature using 53% methanol and 47% 10 mM ammonium formate buffer (pH 4.0) as mobile phase, at a flow rate of 80 microL/min. A PE-SCIEX API III triple quadrupole instrument was used for mass spectrometric detection. An ionspray (pneumatically assisted electrospray) interface was used in negative and positive ionization mode. The assay was linear over the range 100-2000 micrograms/L for each analyte. The instrumental limit of detection was 50 pg. Assay imprecision at 400 and 800 micrograms/L was < or = 10% (total coefficient of variation). Accuracy ranged between 83.2% for 20 beta-dihydrocortisone to 102.6% for cortisone. Recovery of 1000 micrograms/L analyte ranged from 91.3% for cortisone to 109.7% for tetrahydrocortisol.

17O nuclear magnetic resonance spectra of steroids

The 17O NMR spectra of cholesterol and 31 other steroid alcohols, esters, ketones, and acids enriched by synthesis with 17O from H2(17)O have been observed under ordinary operating conditions, and correlations between 17O chemical shift and structure have been adduced. Spectra-structure correlations for these steroids are in conformity with those previously adduced with simpler compounds by others.

Gas chromatography mass spectrometry of tert-butyldimethylsilyl ethers of phthiocerols and mycocerosic alcohols from Mycobacterium tuberculosis

Mycobacteria synthesize a variety of unusual long-chain fatty acid esters and the systematic analysis of these is of great potential in the classification and identification of these bacteria. This paper describes the extraction, reductive fission and derivatization to tert-butyldimethylsilyl ethers of beta-diesters, the phthiocerol dimycocerosates. The mass spectra of these ethers are characteristic of the parent alcohols and selected ion monitoring techniques have been applied to mixtures extracted from strains of Mycobacterium tuberculosis.