Oximes and Hydrazones in Bioconjugation: Mechanism and Catalysis

Abstract

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.

Leveraging the Knorr Pyrazole Synthesis for the Facile Generation of Thioester Surrogates for use in Native Chemical Ligation

“…The reaction at pH 8.5 yielded only starting material with minute amounts of hydrazone intermediate being observed, consistent with mechanistic studies on the pH dependence of hydrazone formation. [24,25] …”

Dynamic Covalent Self‐Assembly Based on Oxime Condensation

“…Thepartially negatively-charged carbon atom is therefore less electrophilic,p rotecting the C=Nb ond from nucleophilic attack by water molecules or other nucleophiles. Oxime, [21] namely -C= N À O-, is reported [22] to be more robust than hydrazone both thermodynamically and kinetically.T here are af ew reasons that can explain the enhanced stability of oxime compared to its hydrazone counterparts.F irst, the NH 2 unit in either the alkoxyamino (-OÀNH 2 )o rh ydrazide (-NHÀNH 2 )p recursor could undergo protonation in water, which could be considered as ac ompetitive process of nucleophilic addition. Thed ynamic nature of hydra-zone,however, jeopardizes the inherent stabilities of the selfassembled products.F or example,w eo bserved that the catenanes [18,19] containing hydrazone undergo decomposition via hydrazone exchange during counteranion exchange or solvent removal, even in the condition of low temperature and/or in the absence of acid catalyst.…”

“…Avariety of macrocycles, [15,16] cages, [17] catenanes [18,19] as well as knots [20] were obtained in water based on hydrazone condensation in pure water. Oxime, [21] namely -C= N À O-, is reported [22] to be more robust than hydrazone both thermodynamically and kinetically.T here are af ew reasons that can explain the enhanced stability of oxime compared to its hydrazone counterparts.F irst, the NH 2 unit in either the alkoxyamino (-OÀNH 2 )o rh ydrazide (-NHÀNH 2 )p recursor could undergo protonation in water, which could be considered as ac ompetitive process of nucleophilic addition. Oxime, [21] namely -C= N À O-, is reported [22] to be more robust than hydrazone both thermodynamically and kinetically.T here are af ew reasons that can explain the enhanced stability of oxime compared to its hydrazone counterparts.F irst, the NH 2 unit in either the alkoxyamino (-OÀNH 2 )o rh ydrazide (-NHÀNH 2 )p recursor could undergo protonation in water, which could be considered as ac ompetitive process of nucleophilic addition.…”

Switching Multivalent DNA Complexation using Metal‐Controlled Cationic Supramolecular Self‐Assemblies

“…[3] The first challenge in this application is to condense therapeutic oligonucleotides (siRNA,m iRNA, etc) into small nanoparticles that are effectively taken up by cells. [17] However,t od ate, the biological applications of grids have been hardly explored. [4] The secondc hallenge in the delivery process is the subsequent step-thea ctive release of the nucleic acid cargo-and this is wherem ost artificial deliverys ystems fail.…”