Nonreducible cystine isosteres represent important peptide design elements in that they can maintain a near-native tertiary conformation of the peptide while simultaneously extending the in vitro and in vivo half-life of the biomolecule. Examples of these cystine mimics include dicarba, diselenide, thioether, triazole, and lactam bridges. Each has unique physicochemical properties that impact upon the resulting peptide conformation. Each also requires specific conditions for its formation via chemical peptide synthesis protocols. While the preparation of peptides containing two lactam bonds within a peptide is technically possible and reported by others, to date there has been no report of the chemical synthesis of a heterodimeric peptide linked by two lactam bonds. To examine the feasibility of such an assembly, judicious use of a complementary combination of amine and acid protecting groups together with nonfragment-based, total stepwise solid phase peptide synthesis led to the successful preparation of an analogue of the model peptide, insulin-like peptide 3 (INSL3), in which both of the interchain disulfide bonds were replaced with a lactam bond. An analogue containing a single disulfide-substituted interchain lactam bond was also prepared. Both INSL3 analogues retained significant cognate RXFP2 receptor binding affinity. 1. Introduction Cysteine-rich peptides such as conotoxins and insulin-like peptides are an increasingly important class of biomolecules. They usually possess intricately folded, sometimes knotted, structures and some have been developed as treatments for a variety of conditions, such as pain [1, 2], cancer , diabetes mellitus , and heart failure [5, 6]. As such, much work is being undertaken to optimise their pharmacological properties so that new lead compounds are developed for preclinical evaluation. Disulfide bonds play a critical role in maintaining the peptide conformation and biological activity of these molecules. However, they are susceptible to reduction in vivo, as part of the normal degradative process which, in turn, can disrupt the three-dimensional structure and lead to loss of activity. In order to stabilise peptide structures, numerous disulfide bond mimics have been developed. Guo et al. substituted a diselenide for a disulfide bond in a sunflower trypsin inhibitor which retained high potency . Armishaw et al. also applied this further to an α-conotoxin, which maintained full biological activity and had enhanced stability under biologically reducing conditions . This same model peptide was also
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