Gas-liquid chromatography-mass spectrometry of carbon-13 enriched amino acids as trimethylsilyl derivatives

Mass spectral fragmentation of trimethylsilylated small molecules

Mass spectrometry-based untargeted metabolomics detects many peaks that cannot be identified. While advances have been made for automatic structure annotations in LC-electrospray-MS/MS, no open source solutions are available for hard electron ionization used in GC-MS. In metabolomics, most compounds bear moieties with acidic protons, for example, amino, hydroxyl, or carboxyl groups. Such functional groups increase the boiling points of metabolites too much for use in GC-MS. Hence, in GC-MS-focused metabolomics, derivatization of these groups is essential and has been employed since the 1960s. Specifically, trimethylsilylation is known as mild and universal method for GC-MS analysis. Here, we comprehensively compile accurate mass fragmentation rules and pathways of trimethylsilylated small molecules from 80 research articles over the past 5 decades, including diagnostic fragment ions, neutral losses, and typical ion ratios, for alcohols, carboxylic acids, amines, amino acids, sugars, steroids, thiols, and phosphates. These fragmentation rules were subsequently validated by specificity and sensitivity assessments using the NIST 14 nominal mass library and a new in-house GC-QTOF MS library containing 589 accurate mass spectra. From 556 tested fragmentation patterns, 228 rules yielded true positive hits within 4 mDa mass accuracy. These rules can be applied to assign substructures for mass spectra computation and unknown identification. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 37:245-257, 2018.

Conversion of prohistidine decarboxylase to histidine decarboxylase: peptide chain cleavage by nonhydrolytic serinolysis

Unlabeled prohistidine decarboxylase and prohistidine decarboxylase containing L-[carboxyl-(18)O]serine or L-[hydroxyl-(18)O]serine were isolated in homogeneous form from mutant 3 of Lactobacillus 30a grown with the appropriately labeled serine. There was no randomization or redistribution of label during growth, isolation of the protein, or enzymatic hydrolysis and reisolation of the labeled amino acids. These proteins were used to show that during proenzyme activation, in which individual pi subunits of the proenzyme are converted to alpha and beta subunits of the active enzyme [Formula: see text] (in which pi, alpha, and beta subunits have the partial structures shown and Prv designates a pyruvoyl group), no (18)O from H(2) (18)O is incorporated into the newly formed carboxyl terminus (Ser-81) of the beta chain, although no labilization of (18)O from proenzyme labeled with L-[carboxyl-(18)O]serine occurred when the proenzyme was activated in H(2) (16)O by the same procedures. The additional oxygen atom present in the carboxyl group of Ser-81 of the beta subunit is transferred from the hydroxyl group of Ser-82 of the proenzyme during the activation reaction. The same result was obtained with wild-type enzyme formed intracellularly. Peptide bond cleavage during activation of the proenzyme thus proceeds by a hitherto unobserved direct or indirect "serinolysis" coupled to alpha,beta-elimination at Ser-82 to yield the pyruvoyl group of the alpha subunit, rather than by hydrolysis. Possible mechanisms for the reaction are discussed briefly.

Synthesis and back exchange of 18O labeled amino acids for use as internal standards with mass spectrometry

Amino acids with two oxygen-18 atoms in the carboxyl moiety can be synthesized easily from the amino acids and H2 18O through exchange. The exchange conditions are simply solution of the amino acid (1-4 mg) in acidic H2 18O (0.1-0.2ml) followed by heating at 60-70 degrees C for several days. Eighteen oxygen-18 amino acids with incorporations typically greater than 90% 18O2 have been achieved and characterized by mass spectrometry. The exchange conditions do not lead to racemization. Loss of the oxygen-18 atoms through back exchange was found to be pH and temperature dependent, but yet slow enough to allow these isotopimers to be used in most studies as internal standards for quantitative mass spectrometry. The 18O carboxylic atoms were not exchanged upon incubation in plasma for three days at 37 degrees C, PH 7.4. The exchange reaction is general enough to allow the simultaneous synthesis of several [18O] amino acids.