Selective Protein Functionalisation at Methionine
The development of novel bioconjugation strategies for the functionalisation of polypeptides and proteins has been of great benefit to the fields of chemical biology and medicine. Selective protein functionalisation has enabled the development of technologies such as targeted drug delivery, enzyme activity profiling and imaging of cells in vivo. Many strategies for bioconjugation target the nucleophilic side chains of the amino acids cysteine and lysine. However, many strategies remain sequence specific and some require pre-installation of recognition units to achieve high selectivity.
Due to the low natural abundance and ancillary protein function of methionine, selective functionalisation of this amino acid presents an opportunity to develop a strategy complementary to current technologies. Strategies targeting methionine remain under-explored, with few truly biocompatible methods reported to date. This thesis describes the development of a novel bioconjugation method employing hypervalent iodine reagents for the selective functionalisation of methionine residues. The broad scope and excellent biocompatibility of the reaction is demonstrated, with a number of different polypeptide and protein substrates tolerated. Additionally, through the synthesis of different hypervalent iodine reagents, varying the ester group of the iodonium salt (R), a number of different payloads can be transferred to polypeptides and proteins.
The highly reactive diazo group which is introduced using this methodology can be exploited in subsequent bioorthogonal transformations. Firstly, phosphine reagents can be employed for the cleavage of the methionine label, in a stimuli-responsive reversal of the conjugation. Secondly, a visible light-mediated reduction was developed, forming trialkylsulfonium products which exhibited improved stability. Finally, a C-benzylation has been developed using photoredox catalysis to enable functionalisation of this diazo moiety.
The methionine bioconjugation strategy has also been used in tandem with a literature procedure for the functionalisation of tryptophan residues. Through the combination of two bioconjugation techniques, a simultaneous dual functionalisation of polypeptides has been developed, forming dually- or triply-functionalised scaffolds in a single step from native polypeptides.