Toward New Designed Proteins Derived from Bovine Pancreatic Trypsin Inhibitor (BPTI):  Covalent Cross-Linking of Two ‘Core Modules' by Oxime-Forming Ligation

A 25-residue disulfide-cross-linked peptide, termed ‘oxidized core module' (OxCM), that includes essentially all of the secondary structural elements of bovine pancreatic trypsin inhibitor (BPTI) most refractory to hydrogen exchange, was shown previously to favor nativelike β-sheet structure [Carulla, N., Woodward, C., and Barany, G. (2000) Synthesis and Characterization of a β-Hairpin Peptide That Represents a ‘Core Module' of Bovine Pancreatic Trypsin Inhibitor (BPTI). <i>Biochemistry</i> <i>39</i>, 7927−7937]. The present work prepares to explore the hypothesis that the energies of nativelike conformations, relative to other possible conformations, could be decreased further by covalent linkage of two OxCMs. Optimized syntheses of six ∼50-residue OxCM dimers are reported herein, featuring appropriate monomer modifications followed by oxime-forming ligation chemistry to create covalent cross-links at various positions and with differing lengths. Several side reactions were recognized through this work, and modified procedures to lessen or mitigate their occurrence were developed. Particularly noteworthy, guanidine or urea denaturants that were included as peptide-solubilizing components for some reaction mixtures were proven to form adducts with glyoxylyl moieties, thus affecting rates and outcomes. All six synthetic OxCM dimers were characterized by 1D <sup>1</sup>H NMR; three of them showed considerable chemical shift dispersion suggestive of self-association and mutual stabilization between the monomer units.