Richards, DA; (2016) The Development of Novel Thiomaleimide Photochemical Transformations and Their Application to the Manipulation of Proteins. Doctoral thesis , UCL (University College London).

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Abstract

The unique spatiotemporal control provided by photochemical transformations allows for exquisite power over biochemical processes. Despite this, relatively little research involving novel biocompatible photochemical transformations has been described. This is due in part to the difficulty in discovering photochemical reactions which fulfil the strict criteria required for biocompatibility, and also a lack of available methods for the reliable site-selective modification of proteins. Thus, a method for achieving site selective attachment of novel photoactive chemical moieties to biomolecules would represent a significant contribution to the field. Recently published work has highlighted the photochemical reactivity of thiomaleimides, which can be easily and site-selectively installed on protein cysteine residues. This thesis describes the discovery and investigation of novel thiomaleimide based photochemical reactions, with special focus on utilising these reactions as phototriggers for peptide and protein manipulation. The initial work expands on a previously reported thiomaleimide [2+2] photocycloaddition to develop a method for photochemically rebridging peptides and proteins at the site of a disulfide bond. This method was employed for the photochemical activation of the therapeutic peptide Octreotide, and as a tool for generating highly thiol stable bis-modified antibody fragment conjugates. During the course of this study two novel thiomaleimide-mediated photochemical decarboxylation reactions were discovered and their utility as photolabile linkers for bioconjugation was explored. This led to the development of a cysteine selective photolabile linker based around the thiomaleimide scaffold, which was subsequently employed to release an analogue of the cytotoxic drug Doxorubicin from an anti-CEA scFv antibody fragment. This work highlights the previously unreported ability of thiomaleimides to act as electron acceptors in photoinduced electron transfer reactions, greatly expanding the reactivity profile of this chemical motif. The excellent photochemical reactivity of thiomaleimides, coupled with the relative ease of their installation on proteins, suggests that these reagents could play an important part in the future of photochemical protein and peptide manipulation.

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