TOPOCHEMISTRY FOR PREPARING LIGANDS THAT DIMERIZE RECEPTORS

Background: The cyclic, disulfide-containing peptide, cyclo-Ac-[Cys-Hi s-Pro-Gln-Gly-Pro-Pro-Cys]-NH2, binds to streptavidin with high affini ty. In streptavidin-peptide cocrystals of space group I222, cyclic pep tide monomers are bound on adjacent streptavidin tetramers related by a crystallographic two-fold symmetry axis. We set out to determine whe ther the disulfide bonds of the peptide, presented close to one anothe r in the crystal, could undergo disulfide interchange to form a dimer. Results: When juxtaposed, the disulfides of neighboring peptides unde rgo disulfide interchange, breaking and forming covalent disulfide bon ds, to produce a peptide dimer adopting the symmetry of the crystal. T his is the first example of a chemical transformation mediated by a pr otein crystal lattice. The structure of the streptavidin-bound monomer , and that of the dimer that was eventually produced from it in the cr ystal, were both determined from the same single crystal studied at di fferent times. The two-fold symmetric peptide dimer was independently synthesized and shown to form crystals of dimerized streptavidin.Concl usions: We have shown that formation of a covalently linked peptide di mer can be mediated by a protein crystal lattice. The dimer thus produ ced dimerizes its target, streptavidin, suggesting that solid-state (o r topochemical) reactions of this kind may be broadly useful for the p reparation of ligands that can dimerize other protein targets.