The renaissance of H/D exchange

The unusual hydrogen-deuterium exchange of α-carbon protons in N-substituted glycine-containing peptides

Hydrogens connected to α-carbon (α-C) of amino acid residues are usually resistant to hydrogen-deuterium exchange (HDX) unless reaction conditions promote racemization. Although N-methylglycine (sarcosine) residue has been found in biologically active peptide such as cyclosporine, to the best of our knowledge, the HDX of α-C protons of this residue was not explored yet. Here, we presented a new and efficient methodology of α-C deuteration in sarcosine residues under basic aqueous conditions. The deuterons, introduced at α-C atom, do not undergo back-exchange in acidic aqueous solution. The electrospray ionization-MS and MS/MS experiments on proposed model peptides confirmed the HDX at α-C and revealed the unexpected hydrogen scrambling in sarcosine-containing peptides. Although the observed HDX of α-C protons is only successful in N-acylglycine when the amide possesses a certain degree of alkylation, it offers a new approach to the analysis of sarcosine-containing peptides such as cyclosporine.

Using deuterium in drug discovery: leaving the label in the drug

Deuterium, the stable isotope of hydrogen, is known to medicinal chemists for its utility in mechanistic, spectroscopic, and tracer studies. In fact, well-known applications utilizing deuterium exist within every subdiscipline in pharmaceutical discovery and development. Recent emphasis on incorporation of deuterium into the active pharmaceutical ingredient has come about as a result of inquiries into the potential for substantial benefits of the deuterium kinetic isotope effect on the safety and disposition of the drug substance. This Perspective traces the author's experience in reviving and expanding this potential utility, first suggested many decades prior by the discoverer of this, the simplest of all isotopes.

Palladium-catalyzed ortho-selective C-H deuteration of arenes: evidence for superior reactivity of weakly coordinated palladacycles

We disclose a protocol for the palladium-catalyzed ortho-selective C-H deuteration of arenes. Phenylacetic acids and benzoic acids are suitable substrates for this reaction. This reaction offers a catalytic route to ortho-deuterated phenylacetic acids and benzoic acids and demonstrates the sharp difference in reactivity of palladacycle intermediates held together by weak and strong coordination.

Regioselective and stereospecific deuteration of bioactive aza compounds by the use of ruthenium nanoparticles

An efficient H/D exchange method allowing the deuteration of pyridines, quinolines, indoles, and alkyl amines with D2 in the presence of Ru@PVP nanoparticles is described. By a general and simple procedure involving mild reaction conditions and simple filtration to recover the labeled product, the isotopic labeling of 22 compounds proceeded in good yield with high chemo- and regioselectivity. The viability of this procedure was demonstrated by the labeling of eight biologically active compounds. Remarkably, enantiomeric purity was conserved in the labeled compounds, even though labeling took place in the vicinity of the stereogenic center. The level of isotopic enrichment observed is suitable for metabolomic studies in most cases. This approach is also perfectly adapted to tritium labeling because it uses a gas as an isotopic source. Besides these applications to molecules of biological interest, this study reveals a rich and underestimated chemistry on the surface of ruthenium nanoparticles.

Development of deuterium labeling method based on the heterogeneous platinum group metal-catalyzed C-H activation

Deuterium (D) labeled compounds are utilized in various scientific fields such as mechanistic elucidation of reactions, preparation of new functional materials, tracers for microanalysis, deuterium labeled heavy drugs and so on. Although the H-D exchange reaction is a straightforward method to produce deuterated organic compounds, many precedent methods require expensive deuterium gas and/or harsh reaction conditions. A part of our leading research agendas is intended to the development of novel and functional heterogeneous platinum-group catalysts and the reclamation of unknown functionalities of existing heterogeneous platinum-group catalysts. During the course of the study, benzylic positions of substrates were site-selectively deuterated under mild and palladium-on-carbon (Pd/C)-catalyzed hydrogenation conditions in heavy water (D2O). Heat conditions promoted the H-D exchange reactivity and facilitated the H-D exchange reaction at not only the benzylic sites but also inactive C-H bonds and heterocyclic nuclei. It is noteworthy that platinum-on-carbon (Pt/C) indicated a quite high affinity toward aromatic nuclei, and the H-D exchange reaction was strongly enhanced by the use of Pt/C as a catalyst under milder conditions. The mixed use of Pd/C and Pt/C was found to be more efficient in the H-D exchange reaction compared to the independent use of Pd/C or Pt/C. Furthermore, simple alkanes could also be efficiently deuterated under rhodium-on-carbon (Rh/C)-catalyzed conditions. The use of ruthenium-on-carbon (Ru/C) enabled the regiospecific and efficient deuterium incorporation at α-positions of alcohols and results were applied as a regio- and stereoselective multi-deuteration method of sugar derivatives.

Synthetic use of the primary kinetic isotope effect in hydrogen atom transfer 2: generation of captodatively stabilised radicals

Using C-3 di-deuterated morpholin-2-ones bearing N-2-iodobenzyl and N-3-bromobut-3-enyl radical generating groups, only products derived from the more stabilised C-3, rather than the less stabilised C-5 translocated radicals, were formed after intramolecular 1,5-hydrogen atom transfer, suggesting that any kinetic isotope effect present was not sufficient to offset captodative stabilisation.

Preparation of labeled human drug metabolites and drug-drug interaction-probes with fungal peroxygenases

Enzymatic conversion of a drug can be an efficient alternative for the preparation of a complex metabolite compared with a multi-step chemical synthesis approach. Limitations exist for chemical methods for direct oxygen incorporation into organic molecules often suffering from low yields and unspecific oxidation and also for alternative whole-cell biotransformation processes, which require specific fermentation know-how. Stable oxygen-transferring biocatalysts such as unspecific peroxygenases (UPOs) could be an alternative for the synthesis of human drug metabolites and related stable isotope-labeled analogues. This work shows that UPOs can be used in combination with hydrogen/deuterium exchange for an efficient one-step process for the preparation of 4'-OH-diclofenac-d6. The scope of the reaction was investigated by screening of different peroxygenase subtypes for the transformation of selected deuterium-labeled substrates such as phenacetin-d3 or lidocaine-d3. Experiments with diclofenac-d7 revealed that the deuterium-labeling does not affect the kinetic parameters. By using the latter substrate and H2 (18) O2 as cosubstrate, it was possible to prepare a doubly isotope-labeled metabolite (4'-(18) OH-diclofenac-d6). UPOs offer certain practical advantages compared with P450 enzyme systems in terms of stability and ease of handling. Given these advantages, future work will expand the existing 'monooxygenation toolbox' of different fungal peroxygenases that mimic P450 in vitro reactions.

Selected scientific topics of the 11th International Isotope Symposium on the Synthesis and Applications of Isotopes and Isotopically Labeled Compounds

This micro-review describes hot topics and new trends in isotope science discussed at the 11th International Isotope Symposium on the Synthesis and Applications of Isotopes and Isotopically Labeled Compounds from a personal perspective.

Arenium acid catalyzed deuteration of aromatic hydrocarbons

The arenium acid [mesitylene-H](+) has been shown to be an extraordinarily active H/D exchange catalyst for the perdeuteration of polycyclic aromatic hydrocarbons. The reactions take place under ambient conditions in C6D6 as an inexpensive deuterium source. High isolated yields and excellent degrees of deuterium incorporation were achieved using the substrates p-terphenyl, fluoranthene, pyrene, triphenylene, and corannulene.

Ruthenium-catalyzed selective α,β-deuteration of bioactive amines

A novel and convenient protocol for the catalytic hydrogen-deuterium exchange of biologically active tertiary amines utilizing the borrowing hydrogen methodology has been developed. In the presence of the readily available Shvo catalyst, excellent chemoselectivity toward α- and β-protons with respect to the nitrogen atom as well as high degree of deuterium incorporation and functional group tolerance is achieved. This allowed for the deuteration of complex pharmaceutically interesting substrates, including examples for actual marketed drug compounds. Notably, this method constitutes a powerful tool for the generation of valuable internal standard materials for LC-MS/MS analyses highly demanded for various life-science applications.

Determination of the glycosaminoglycan and collagen contents in tissue samples by high-resolution 1H NMR spectroscopy after DCl-induced hydrolysis

The determination of the collagen and glycosaminoglycan (GAG) contents of native and particularly bioengineered tissues is of considerable interest because the collagen-to-GAG ratio determines the water content of the tissue, which is crucial regarding its mechanical properties. (1)H NMR spectroscopy subsequent to the hydrolysis of the sample by aqueous 6 M DCl at 353 K is used to determine the GAG and collagen contents simultaneously. Under these strongly acidic conditions the biopolymers of the extracellular matrix, collagen, and GAG are fragmented into their individual monomers, that is, free amino acids from collagen and monosaccharides from the polymer repeat units of GAGs. The amino acid amount can be easily determined in the presence of an internal standard by (1)H NMR spectroscopy because amino acids proved to be stable under acidic conditions. The carbohydrates are subject to charring in the presence of concentrated DCl, but glucosamine and galactosamine were found to be sufficiently stable for quantification under the chosen conditions.

The hydrogen-deuterium exchange at α-carbon atom in N,N,N-trialkylglycine residue: ESI-MS studies

Derivatization of peptides as quaternary ammonium salts (QAS) is a known method for sensitive detection by electrospray ionization tandem mass spectrometry. Hydrogens at α-carbon atom in N,N,N-trialkylglycine residue can be easily exchanged by deuterons. The exchange reaction is base-catalyzed and is dramatically slow at lower pH. Introduced deuterons are stable in acidic aqueous solution and are not back-exchanged during LC-MS analysis. Increased ionization efficiency, provided by the fixed positive charge on QAS group, as well as the deuterium labeling, enables the analysis of trace amounts of peptides.

Selective deuteration of (hetero)aromatic compounds via deutero-decarboxylation of carboxylic acids

A practical, mild and highly selective protocol for the monodeuteration of a variety of arenes and heteroarenes is presented. Catalytic amounts of Ag(I) salts in DMSO/D(2)O are shown to facilitate the deutero-decarboxylation of ortho-substituted benzoic and heteroaromatic α-carboxylic acids in high yields with excellent levels of deuterium incorporation.

Exploration of possible binding sites of nanoparticles on protein by cross-linking chemistry coupled with mass spectrometry

For the first time, the possible binding site of nanoparticles on protein was revealed by cross-linking chemistry coupled with mass spectrometry. The peptides located very close to the poly(acrylic acid) (PAA)-coated Fe(3)O(4) nanoparticles (NPs) during interaction with human serum albumin (HSA) were cross-linked to the surface of NPs. Following protease digestion, the attached peptides were cleaved off the particle surface and identified by matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry (MALDI-TOF-MS). The peptides were found to be part of the so-called drug binding site 2 of HSA; and the competitive binding to HSA between the corresponding drug, ibuprofen, and the NPs was observed. Our results demonstrated that cross-linking chemistry coupled with MS was a quick and simple method for locating the possible binding sites of NPs on protein. Information on NP-protein binding interface will benefit the study of how the interactions are governed by the physicochemical properties of NPs, for guiding the design of functional bionano constructs. It can also help to predict the biological consequence of protein adsorption on NPs, for obtaining more knowledge on nanotoxicity.

Pincer Ru and Os complexes as efficient catalysts for racemization and deuteration of alcohols

The pincer complexes [MX(CNN)(PP)] (M = Ru, Os; X = Cl, OTf; HCNN = 1-(6-arylpyridin-2-yl)methanamine; PP = diphosphine) have proven to efficiently catalyze both racemization and deuteration of alcohols in the presence of a base. Chiral alcohols have been racemized at 30-50 °C using 1 mol% of Ru or Os pincer complexes and 5 mol% of KOtBu in 2-propanol. Primary and secondary alcohols are efficiently deuterated at the α position, with respect to the OH group, using 2-propanol-d(8) as solvent with Ru or Os pincer complexes and KOtBu at 30-50 °C. For secondary alcohols incorporation of deuterium at the β position has also been observed. In 2-propanol-d(8) the pincer complexes catalyze the simultaneous deuteration and racemization of (S)-1-phenylethanol, the two processes being strictly correlated. For both reactions much the same activity has been observed with the Ru and Os complexes. The pincer complexes display a superior activity with respect to the related compounds [MCl(2)(NN)(PP)] (NN = bidentate amine or pyridine ligand). The synthesis of the new complexes [MCl(CNN)(PP)] (M = Ru, 2, 4 and Os, 6, 7; PP = dppb, dppf) and [Ru(OTf)(CNN)(dppb)] (3) is also reported.