Smita B. Gunnoo

The modification of proteins with non-protein entities is important for a wealth of applications, and methods for chemically modifying proteins attract considerable attention. Generally, modification is desired at a single site to maintain homogeneity and to minimise loss of function. Though protein modification can be achieved by targeting some natural amino acid side chains, this often leads to ill-defined and randomly modified proteins. Amongst the natural amino acids, cysteine combines advantageous properties contributing to its suitability for site-selective modification, including a unique nucleophilicity, and a low natural abundance--both allowing chemo- and regioselectivity. Native cysteine residues can be targeted, or Cys can be introduced at a desired site in a protein by means of reliable genetic engineering techniques. This review on chemical protein modification through cysteine should appeal to those interested in modifying proteins for a range of applications.

Dehydroalanine is a synthetic precursor to a wide array of protein modifications. We describe multiple methods for the chemical conversion of cysteine to dehydroalanine on peptides and proteins. The scope and limitations of these methods were investigated with attention paid to side reactions, scale, and aqueous- and bio-compatibility. The most general method investigated—a bis-alkylation–elimination of cysteine to dehydroalanine—was applied successfully to multiple proteins and enabled the site-selective synthesis of a glycosylated antibody.

A mild visible-light-mediated strategy for cysteine arylation is presented. The method relies on the use of eosin Y as a metal-free photocatalyst and aryldiazonium salts as arylating agents. The reaction can be significantly accelerated in a microflow reactor, whilst allowing the in situ formation of the required diazonium salts. The batch and flow protocol described herein can be applied to obtain a broad series of arylated cysteine derivatives and arylated cysteine-containing dipeptides. Moreover, the method was applied to the chemoselective arylation of a model peptide in biocompatible reaction conditions (room temperature, phosphate-buffered saline (PBS) buffer) within a short reaction time.

The pharmaceutical market has largely been dominated by small molecule drugs; however, larger biomolecules have recently become important contenders. Of these biomolecules, protein and peptide therapeutics are proving useful due to their often improved pharmacokinetic properties. In many circumstances, functionalisation of the protein or peptide therapeutics results in performance enhancement, and various methodologies are applied. In addition, introducing unnatural amino acids for structural reinforcement via chemical modification is also common. These strategies are discussed in this review.