Polyvalent Catalysts Operating on Polyvalent Substrates: A Model for Surface-Controlled Reactivity

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Dansyl–NA3 conjugates for glycoprotein detection through fluorescent tagging and native gel electrophoresis

An aldehyde-reactive fluorophore has been prepared that can afford the fluorescent detection of serum glycoproteins by native gel electrophoresis.

Mapping aldehydic load in vivo by positron emission tomography with [18F]NA3BF3

A new radiotracer, [¹⁸F]NA₃BF₃, capable of rapid, stable, and catalyst-free complexation of aldehydes in vivo is reported. [¹⁸F]NA₃BF₃ was shown to bind aldehydes in live subjects using locally administered aldehyde-presenting microparticles, and was then applied to mapping aldehydic load in a mouse model of sepsis. [¹⁸F]NA₃BF₃ may enable the direct investigation of the chemical biology of aldehydes in living subjects, and may open avenues for the adoption of endogenous aldehydic load as an imaging biomarker of inflammatory pathology.

Methyl 5-MeO-N-aminoanthranilate, a minimalist fluorogenic probe for sensing cellular aldehydic load

A minimalist fluorogenic probe is presented capable of the mapping of aldehydic load through live cell microscopy.

Aromatic Aminocatalysis

Aromatic aminocatalysis refers to transformations that employ aromatic amines, such as anilines or aminopyridines, as catalysts. Owing to the conjugation of the amine moiety with the aromatic ring, aromatic amines demonstrate distinctive features in aminocatalysis compared with their aliphatic counterparts. For example, aromatic aminocatalysis typically proceeds with slower turnover, but is more active and conformationally rigid as a result of the stabilized aromatic imine or iminium species. In fact, the advent of aromatic aminocatalysis can be traced back to before the renaissance of organocatalysis in the early 2000s. So far, aromatic aminocatalysis has been widely applied in bioconjugation reactions through transamination; in asymmetric organocatalysis through imine/enamine tautomerization; and in cooperative catalysis with transition metals through C-H/C-C activation and functionalization. This Focus Review summarizes the advent of and major advances in the use of aromatic aminocatalysis in bioconjugation reactions and organic synthesis.

Jose G, Shanbhag C, Kalia J. Rapid and reversible hydrazone bioconjugation in cells without the use of extraneous catalysts

Rapid, catalyst-free and reversible bioconjugation in mammalian cells.

Model-driven engineering of supramolecular buffering by multivalency

The ability of chemists to regulate the concentration of molecules is extremely important. However, as reactions are slowly superseded by more complex reaction networks, new ways of regulating molecular concentrations are needed. Recently, we described a system in which the concentration of a monovalent molecule with catalytic activity was buffered over a wide concentration range by its binding to a divalent molecule. Guided by model predictions, we are able to experimentally optimize the system by increasing the valency of the buffer, with even-numbered valencies displaying superior buffering capabilities. These results allow us to understand and gain more control over the activities of molecules in complex molecular systems, thereby obtaining insights into natural systems as well as creating adaptive artificial systems.

A supramolecular system in which the concentration of a molecule is buffered over several orders of magnitude is presented. Molecular buffering is achieved as a result of competition in a ring–chain equilibrium of multivalent ureidopyrimidinone monomers and a monovalent naphthyridine molecule which acts as an end-capper. While we previously only considered divalent ureidopyrimidinone monomers we now present a model-driven engineering approach to improve molecular buffering using multivalent ring–chain systems. Our theoretical models reveal an odd–even effect where even-valent molecules show superior buffering capabilities. Furthermore, we predict that supramolecular buffering can be significantly improved using a tetravalent instead of a divalent molecule, since the tetravalent molecule can form two intramolecular rings with different “stabilities” due to statistical effects. Our model predictions are validated against experimental ¹H NMR data, demonstrating that model-driven engineering has considerable potential in supramolecular chemistry.

Oximes and Hydrazones in Bioconjugation: Mechanism and Catalysis

The formation of oximes and hydrazones is employed in numerous scientific fields as a simple and versatile conjugation strategy. This imine-forming reaction is applied in fields as diverse as polymer chemistry, biomaterials and hydrogels, dynamic combinatorial chemistry, organic synthesis, and chemical biology. Here we outline chemical developments in this field, with special focus on the past ∼10 years of developments. Recent strategies for installing reactive carbonyl groups and α-nucleophiles into biomolecules are described. The basic chemical properties of reactants and products in this reaction are then reviewed, with an eye to understanding the reaction's mechanism and how reactant structure controls rates and equilibria in the process. Recent work that has uncovered structural features and new mechanisms for speeding the reaction, sometimes by orders of magnitude, is discussed. We describe recent studies that have identified especially fast reacting aldehyde/ketone substrates and structural effects that lead to rapid-reacting α-nucleophiles as well. Among the most effective new strategies has been the development of substituents near the reactive aldehyde group that either transfer protons at the transition state or trap the initially formed tetrahedral intermediates. In addition, the recent development of efficient nucleophilic catalysts for the reaction is outlined, improving greatly upon aniline, the classical catalyst for imine formation. A number of uses of such second- and third-generation catalysts in bioconjugation and in cellular applications are highlighted. While formation of hydrazone and oxime has been traditionally regarded as being limited by slow rates, developments in the past 5 years have resulted in completely overturning this limitation; indeed, the reaction is now one of the fastest and most versatile reactions available for conjugations of biomolecules and biomaterials.