Convenient preparation of deuterium-labeled analogs of peptides containing N-substituted glycines for a stable isotope dilution LC-MS quantitative analysis

5-Amino-3-methyl-Isoxazole-4-carboxylic Acid as a Novel Unnatural Amino Acid in the Solid Phase Synthesis of α/β-Mixed Peptides

The hybrid peptides consisting of α and β-amino acids show great promise as peptidomimetics that can be used as therapeutic agents. Therefore, the development of new unnatural amino acids and the methods of their incorporation into the peptide chain is an important task. Here, we described our investigation of the possibility of 5-amino-3-methyl-isoxazole-4-carboxylic acid (AMIA) application in the solid phase peptide synthesis. This new unnatural β-amino acid, presenting various biological activities, was successfully coupled to a resin-bound peptide using different reaction conditions, including classical and ultrasonic agitated solid-phase synthesis. All the synthesized compounds were characterized by tandem mass spectrometry. The obtained results present the possibility of the application of this β-amino acid in the synthesis of a new class of bioactive peptides.

Trends in the Hydrogen-Deuterium Exchange at the Carbon Centers. Preparation of Internal Standards for Quantitative Analysis by LC-MS

The application of internal standards in quantitative and qualitative bioanalysis is a commonly used procedure. They are usually isotopically labeled analogs of the analyte, used in quantitative LC-MS analysis. Usually, ²H, ¹³C, ¹⁵N and ¹⁸O isotopes are used. The synthesis of deuterated isotopologues is relatively inexpensive, however, due to the isotopic effect of deuterium and the lack of isotopologue co-elution, usually they are not considered as good internal standards for LC-MS quantification. On the other hand, the preparation of ¹³C, ¹⁵N and ¹⁸O containing standards of drugs and their metabolites requires a complicated multistep de novo synthesis, starting from the isotopically labeled substrates, which are usually expensive. Therefore, there is a strong need for the development of low-cost methods for isotope-labeled standard preparations for quantitative analysis by LC-MS. The presented review concentrates on the preparation of deuterium-labeled standards by hydrogen−deuterium exchange reactions at the carbon centers. Recent advances in the development of the methods of isotopologues preparation and their application in quantitative analysis by LC-MS are evaluated.

Application of Safirinium N-Hydroxysuccinimide Esters to Derivatization of Peptides for High-Resolution Mass Spectrometry, Tandem Mass Spectrometry, and Fluorescent Labeling of Bacterial Cells

Mass spectrometry methods are commonly used in the identification of peptides and biomarkers. Due to a relatively low abundance of proteins in biological samples, there is a need for the development of novel derivatization methods that would improve MS detection limits. Hence, novel fluorescent N–hydroxysuccinimide esters of dihydro-[1,2,4]triazolo[4,3-a]pyridin-2-ium carboxylates (Safirinium P dyes) have been synthesized. The obtained compounds, which incorporate quaternary ammonium salt moieties, easily react with aliphatic amine groups of peptides, both in solution and on the solid support; thus, they can be applied for derivatization as ionization enhancers. Safirinium tagging experiments with ubiquitin hydrolysate revealed that the sequence coverage level was high (ca. 80%), and intensities of signals were enhanced up to 8-fold, which proves the applicability of the proposed tags in the bottom–up approach. The obtained results confirmed that the novel compounds enable the detection of trace amounts of peptides, and fixed positive charge within the tags results in high ionization efficiency. Moreover, Safirinium NHS esters have been utilized as imaging agents for fluorescent labeling and the microscopic visualization of living cells such as E. coli Top10 bacterial strain.

The unexpected racemization and hydrogen–deuterium exchange of the hydrogen at the α-carbon of proline analogs containing the 5-azoniaspiro[4.4]nonyl-group

Herein, we present an unexpected racemization and the hydrogen–deuterium exchange (HDX) at the α-C atom of the proline derivative under basic aqueous conditions (1% water solution of triethylamine).