Chemical Conjugation

The thiol groups in the cysteine side chains can undergo oxidation to form covalent disulfide bridges, which contribute to the secondary and tertiary structures of proteins. During protein conjugation reactions, these disulfide bonds must often be reduced to their free sulfhydryl forms to facilitate the desired chemical modifications. Commonly employed strategies for cysteine-based protein modification include Michael addition reactions with maleimide or vinyl sulfone moieties, disulfide exchange reactions, and alkylation reactions.

Polypeptide chains typically contain two types of primary amines, including the α-amino group at the N-terminus and the ε-amino group in the lysine side chain. These positively charged amines are typically located on the outer surface of proteins, facilitating various chemical conjugation reactions without significantly altering the protein structure. A wide range of reactive groups, such as N-hydroxysuccinimide esters (the most commonly used), isothiocyanates, isocyanates, sulfonyl chlorides, epoxides, imide esters, and fluorophenyl esters, are commonly employed to target these primary amines. The conjugation reactions mediated by these reactive groups primarily involve two pathways: acylation or alkylation.

Carboxylic acids are found at the C-terminus and within the side chains of aspartic acid and glutamic acid in polypeptide chains. Similar to primary amines, carboxyl groups are usually located on the surface of protein structures. Carboxylic acids are reactive towards carbodiimide (EDC) and dicyclohexylcarbodiimide (DCC).

Aldehydes play an important role in the chemical modification of target macromolecules, such as proteins and polysaccharides. While aldehydes are not naturally present in these biomolecules, they can be generated through selective oxidation processes, such as the use of sodium periodate to oxidize vicinal diols within polysaccharide chains. The resulting aldehydes then serve as reactive sites for the attachment of hydrazide or alkoxyamine moieties. Furthermore, the aldehyde group can undergo Schiff base formation with primary amines, further reduced by sodium borohydride or sodium cyanoborohydride to form a stable imine bond.

The small molecule compounds containing photoreactive groups can covalently bind to biological macromolecules induced by UV light. Photoreactive crosslinkers are widely used for non-specific conjugation. The most commonly employed photoreactive moieties are aromatic azides and bisacridines, which undergo photochemical transformations to generate highly reactive intermediates. These intermediates turn initiate addition reactions to double bonds, insertion into C-H and N-H sites, or subsequent ring expansion to react with nucleophiles (e.g., primary amines).

The reactions of chemoselective conjugation involve the specific pairing of unique reactive groups, offering a remarkable degree of selectivity and minimal background interference in complex biological systems. Two prominent examples are the "Click" reaction of azide-alkynes and the Staudinger reaction of azide-phosphines, which are often used for in vivo metabolic labeling.