Bioconjugation reactions

Figure 1.22 The solvent accessibility of lysine residues in the Fc region of an antibody is illustrated by highlighting the lysine groups in solid gray. Some lysine e-amine groups are extremely accessible to conjugation, while others are only partially exposed, making them difficult to modify in bioconjugation reactions.

The nitrogen group in fulleropyrrolidines can be used for conjugation with crosslinking agents or hydrophilic biotinylation compounds for subsequent use in bioconjugation reactions. 

Although the chemical modification of tyrosine residues has enjoyed a long history, this residue remains an underused target for bioconjugation reactions. It is typically modified through electrophilic aromatic substitutions (EAS), which makes its reactivity distinct from other amino acid side chains. This reaction... 

Fig. 10.3-1 A survey of molecules and materials that are commonly attached to proteins through bioconjugation reactions.

The above studies emphasize the ability of diazonium-coupling reactions to modify proteins with extremely high efficiency, but one of the limitations of this method is the lack of selectivity that can be obtained when there are multiple tyrosines on the surface of a single protein. This has not been problematic for the viral capsids shown above, as only one tyrosine is accessible on each monomer, but many applications demand higher levels of selectivity than allowed by these coupling reactions. To address this need, and to increase the substrate scope for bioconjugation reactions in general, a versatile Mannich-type reaction has been developed for tyrosine modification, Fig. 10.3-5 [25]. In this reaction, aldehydes and anilines are mixed to form... 

The labeling of a single biomolecule in a complex protein mixture presents a particularly difficult challenge, as no bioconjugation reaction targeting a... 

Fig. 10.3-16 Introduction of unnatural functional groups through posttranslational modification, (a) Ketones and azides can be introduced onto cell surfaces by feeding cells with unnatural sialic acid precursors, such as mannosamine derivatives 40b and c. These are incorporated into cell-surface glycans, which can be further elaborated using additional bioconjugation reactions, (b) Specific amino acid sequences can be modified using biotin ligase. Interestingly, ketobiotin" is also recognized as a substrate for the enzyme, allowing a...

In terms of the reactions themselves, there are several areas that are likely to see improvement. First, there are still many native functional groups for which reliable modification strategies are yet to be developed (including disulfides, asparagines (potentially allowing an efficient synthesis of N-linked glycoproteins), and methionines). Secondly, many bioconjugation reactions currently in use do not reach full conversion in a reasonable period of time. 

Aziridination of alkenes in the absence of conventional metal catalysts is particularly attractive from a green chemistry perspective. In this regard, the aziridination of norbornenes using electron-deficient sulfonyl azides was reported as a new bioconjugation reaction enabling the attachment of tags... 

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