Monosaccharide Isomer Interconversions Become Significant at High Temperatures

Theoretical Studies on the Reaction Mechanism of Schiff Base Formation from Hexoses

The Maillard reaction is one of the nonenzymatic post-translational modifications of proteins. Products of this reaction are considered to be related to aging diseases and the sensation of taste. In the initial stage of the Maillard reaction, Schiff base formation first occurs by the nucleophilic attack of amine nitrogen in proteins, and then, the reaction proceeds through the formation of 1,2-eminal and Amadori compounds. In this study, we computationally investigated the reaction pathway of Schiff base formation from hexoses. The optimized geometries of energy minima and transition states were calculated by using the density functional theory with the CAM-B3LYP/6-311+G(2d,2p) level of theory. The Schiff base formation progressed through three steps: two steps of carbinolamine formation and one step of dehydration. The dehydration is considered to be the rate-determining step in all hexoses because the activation barrier of the dehydration was higher than that of the carbinolamine formation. Furthermore, the steric configuration of the OH group at positions 2 and 3 affected the activation barrier.

Intramolecular Catalytic Hydrogen Atom Transfer (CHAT)

Intramolecular catalysis (IntraCat) is the acceleration of a process at one site of a molecule catalyzed by a functional group in the same molecule; an external agent such as a solvent typically facilitates it. Here, we report a general first-principles-based IntraCat mechanism, which strictly occurs within a single molecule with no coreagent being involved─we call it intramolecular catalytic transfer of hydrogen atoms (CHAT). A reactive part of a molecule (chat catalyst moiety or chat agent, represented by -OOH, -COOH, -SH, -CH2OH, -HPO4, or another bifunctional H-donor/acceptor group) catalyzes an interconversion process, such as keto-enol or amino-imino tautomerization, and cyclization in the same molecule, while being regenerated in the process. It can thus be regarded as an intramolecular version of the intermolecular H atom transfer processes mediated by an external molecular catalyst, e.g., dihydrogen, water, or a carboxylic acid. Earlier, we proposed a general mechanistic systematization of intermolecular processes, illustrated in the simplest case of the H2-mediated reactions classified as dihydrogen catalysis [Asatryan, R.; et al. Catal. Rev.: Sci. Eng., 2014, 56, 403-475]. Following this systematization, the CHAT catalysis belongs to the category of relay transfer of H atoms, albeit in an intramolecular manner. A broader class of intramolecular processes includes all types of H-transfer reactions stimulated by an H-migration, which we call self-catalyzed H atom transfer (SC-HAT). The CHAT mechanism comprises a subset of SC-HAT in which the catalytic moiety is regenerated (i.e., acts as a true catalyst and not a reagent). We provide several characteristic examples of CHAT mechanism based on detailed analysis of the corresponding potential energy surfaces. All such cases showed a dramatically reduced activation barrier relative to the corresponding uncatalyzed H-transfer reactions. For example, we show that CHAT can facilitate long-range H-migration in larger molecules and can occur multiple times in one molecule with multiple interconverting groups. It also facilitates amino-imino tautomerization of unsaturated GABA-analogues and peptides, as well as intramolecular cyclization processes to form heterocycles, e.g., oxygenated rings. CHAT pathways may also explain the pH-dependent increase of mutarotation rate of glucose-6-phosphate demonstrated in pioneering experiments that introduced the classical IntraCat concept. In addition, we identify a ground electronic state CHAT pathway as an alternative to the UV-promoted long-range molecular crane keto-enol conversion with a remarkably low activation energy. To initially assess the possible impact of the new keto-enol conversion pathway on combustion of n-alkanes, we present a detailed kinetic analysis of isomerization and decomposition of pentane-2,4-ketohydroperoxide (2,4-KHP). The results are compared with key alternative reactions, including direct dissociation and Korcek channels (for which a new alkyl group migration channel is also identified), revealing the competitiveness of the CHAT pathway across a range of conditions. Taken together, this work provides insight into a general class of reaction pathways that has not previously being systematically considered and that may occur in a broad range of contexts from combustion to atmospheric chemistry to biochemistry.

Mutarotation of aldoses: Getting a deeper knowledge of a classic equilibrium enabled by computational analyses

The mutarotation equilibrium, by which reducing carbohydrates exist in solution as the α and β anomers of cyclic (furanoid and pyranoid) structures, along with open-chain (aldehyde and hydrate) forms, and whose ratios are depending on factors such as temperature, pH and solvent, portraits a phenomenon involved in numerous processes of chemical and biological importance. Herein, we have developed a DFT-based rationale that provides a broader landscape for anomerizations and ring-open chain interconversions, together with the pivotal role exerted not only by the aldehyde intermediate (essentially the only acyclic structure taken into account so far), but also the hydrate form (often more abundant at the equilibrium). These calculations reveal a more complex and richer scenario than was thought, and identify different mutarotation mechanisms that hinge on every monosaccharide. It is noteworthy that pyranose-furanose interconversion may actually occur without the intermediacy of open-chain forms. For the aldoses evaluated, namely d-glucose, d-ribose, and d-xylose, all structures involved in mutarotation undergo interconversion pathways, whose energy barriers calculated at the M06-2X/6-311++G(d,p) level, are in good agreement with previous experimental measurements.