Chemical Strategies for Bicyclic Peptide Formation

The chemical constraint of proteins has been demonstrated to improve the binding affinity of peptide ligands as well as improving properties such as cell penetrability and resistance to proteases. In this work two new chemical methodologies are presented for such constraint, widening the diversity of peptide structures available starting from a single linear peptide sequence. The combination of these chemical modifications with a library of peptide sequences access new classes of constrained peptides which bind protein surfaces. Such molecules could potentially be used in similar applications to antibodies: therapeutics, diagnostics, and molecular biology research tools. A two-step strategy to produce stereochemically diverse bicyclic peptides has been developed where the first step involves the conversion of multiple cysteine residues to dehydroalanines. The application of methyl 2,5-dibromovalerate proved superior for this transformation when compared to other chemical modification methodologies. The second step of the strategy involved cyclisation via nucleophilic attack at the dehydroalanine residues by a small molecule core bearing multiple thiol functional groups – generating an alkylated L- or D- cysteine in the peptide sequence. The effect of scrambling the stereochemistry at each of these positions is exemplified via the use of a human plasma kallikrein inhibitor PK15 as a model system. An alternative strategy to create diverse bicyclic peptides involved the synthesis of a new core molecule based upon Barbas reagent. The core molecule bearing three 2 mesyl 1,3,4 oxadiazole moieties cyclised peptides containing three cysteines efficiently - constraining peptide loops in distinctly different structures from other established core molecules. Attempts were made to create a phagemid-encoded phage display library of peptides to subsequently modify with these chemical methodologies. A phage display library of Adhirons was screened for binders to the antifungal drug posaconazole. Adhirons are a non-antibody binding protein scaffold bearing variable peptide loops. The screening relied upon chemical derivatisation of the posaconazole structure to enable both screening and hit characterisation.