Dr James Baker
Our research interests centre on the areas of organic synthesis and chemical biology. Ongoing projects include; 1) The development of new reagents for the selective and reversible modification of proteins and peptides; 2) Harnessing the power of photochemistry for application to organic synthesis; 3) The design and synthesis of photoaffinity labelled antagonists of the GABAA ion channel.
1) New reagents for protein modification.
Reagents that can selectively modify individual amino acids are of widespread use in the study and manipulation of proteins. They enable; access to modified proteins as potential therapeutics; the installation of tags such as fluorophores for protein visualisation or biotin for immobilisation and purification; the conjugation of two or more biomolecules; and the mimicking of post-translational modification. Cysteine is often the most nucleophilic residue in a protein, and as such is generally regarded the easiest to modify in a selective manner. One of the most widely used reactive motifs for cysteine modification are maleimides. Notably however maleimides have limitations. They react irreversibly and thus cannot be cleaved in subsequent steps to regenerate the free cysteine. Such reversibility is extremely useful as it allows the modification to be temporary. They are also unable to bridge disulfide bonds.
We are developing ‘Next Generation Maleimides’ which offer powerful new opportunities in protein modification. We have reported on the use of bromomaleimide which reacts efficiently with cysteine to give thiomaleimide 1. We found that bromomaleimide is even more reactive to cysteine than maleimide itself, and it is highly selective for thiols over amines. We have also shown that bromomaleimides, dithiomaleimides and other related reagents can carry out highly selective and reversible modification of cysteine residues in peptides and proteins (collaboration with Prof Steve Caddick). Within this work we demonstrated that these reagents contain three points of attachment for bioconjugation by the construction of a glycoprotein mimic 2 and by the bridging and fluorescent labelling of the disulfide bond in somatostatin.
We are exploring this novel bioconjugation
technology broadly, and have established a spin-out company “Thiologics”
(www.thiologics.com) to translate this work towards therapeutic outcomes (see
2) Harnessing the power of photochemistry.
Photochemistry is an incredibly powerful tool for organic chemists. It enables reactions to be carried out that are impossible via standard thermal pathways. The widespread application of photochemistry is however hindered by the unpredictable yields of products obtained in reactions. One of the main causes for the low yields observed is that the products remain photoactive and thus over the course of the irradiation are consumed by secondary photoreactions. We are particularly interested in enone [2+2] photocycloadditions which enable the rapid construction of complex molecules incorporating highly strained cyclobutanes and cyclobutenes. Access to cyclobutanes is of high importance due to their presence in a number of biologically active natural products and their potential incorporation in chemotherapeutics. Furthermore the inherent strain in these four-membered rings can be exploited to provide a driving force for subsequent synthetic manipulations. We have been investigating new methodology to prevent secondary photoreactions in enone [2+2] photocycloadditions. Our approach has been to include chemical trapping agents that react selectively with the product removing the chromophore preventing further photon absorption leading to degradation. We have proven the potential of this approach by the use of a hydride reducing agent in the intramolecular photocycloaddition of 3.
3) The design and synthesis of photoaffinity labelled antagonists of the GABAA ion channel (collaboration with Professor Trevor Smart, UCL pharmacology).
The GABAA receptor is a chloride ion channel which has gamma-aminobutyric acid as its activating ligand. We are synthesising new antagonists that will, when irradiated with a UV light, covalently attach to the receptor completely blocking the active site. We want to then observe how the cell recovers its receptor activity - through digesting the receptor and replacing it with a new functional one. In this way we will gain new insights in to the ‘life cycle’ of receptors on the cells surface.
Thiologics is a UCL Business-owned company spun-out of the Department of Chemistry, UCL, in June 2011. The company aims to commercialise new bioconjugation technologies developed through collaboration between our group and Prof Steve Caddick’s. ThioLogics is particularly focused on delivering technology that will enable the construction of homogeneous antibody drug conjugate therapeutics (ADCs) (www.thiologics.com).