Bioconjugation reaction availability

The technology of bioconjugation has affected every conceivable discipline in the life sciences. The application of a myriad of available chemical reactions and reagent systems for creating novel complexes with unique activities has made possible the assay of minute quantities of substances, the in vivo targeting of molecules, and the modulation of specific biological processes. Modified or conjugated molecules also have been used for purification, detection, or location of specific substances, and in the treatment of disease. 

These considerations form the basis for the numerous techniques that are now available for the chemical modification of proteins. The sections that follow will examine these techniques and the reactive principles by which they function. A section describing reactions that display orthogonal reactivity to native protein functional groups has also been included because of the growing importance of these reactions as tools to label proteins in complex mixtures. Because it is not practical to summarize all protein bioconjugation methods here, this information instead is intended to serve as an introduction to the concepts that drive the development of these reactions. Several additional reviews and books on protein modification have been listed in the Further Reading section. 

Given this structural diversity, the continued development of new reactions is also crucial. Even in cases where a modification strategy is already in place for a particular functional group, alternative reactions can allow expansions in substrate scope, alterations in modification selectivity, synthetic convenience, and perhaps even greater biocompatibility. Just as a well-trained synthetic chemist must know a dozen methods for the oxidation of an alcohol to an aldehyde, protein bioconjugation will be approached with much more success if many techniques are available to address the situation at hand. 

Non-covalent insertion of several modified metal cofactors and synthetic metal complexes into protein cavities such as serum albumin (SA) and Mb has been reported [5, 24, 28, 30, 69], If synthetic metal complexes, whose structures are very different from native cofactors, can be introduced into protein cages, the bioconjugation of metal complexes will be applicable to many proteins and metal complexes. Mn(corrole) and Cn(phthalocyanine) are inserted into SA by non-covalent interactions and the composites catalyze asymmetric sulfoxidation and Diels-Alder reactions with up to 74 and 98% ee, respectively (Fig. 2c) [28, 30], Since the heme is coordinated to Tyrl61 in the albumin cavity, determined by X-ray crystal structure [20], it is expected that both Mn(corrole) and Cu(phtalocyanine) are also bound to albumin with the same coordination. The incorporation of synthetic metal complexes in protein cavities using these methods is a powerful approach for asymmetric catalytic reactions. However, there are still some difficulties in further design of the composites for improving reactivities and understanding reaction mechanisms because detailed structural analyses are not available for most of the composites. 

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