Synthetic peptide models for protein secondary structures. Beta-sheet formation in acyclic cystine peptides

The conformations of the symmetrical cystine peptides Boc-Cys-(Val)n-Trp-OMe Boc-Cys-(Val)n-Trp-OMe (n = 1, 1; 2, 2; 3, 3) have been examined in solution, in order to evaluate the use of disulfide crosslinks in stabilizing extended beta-strand conformations in acyclic sequences. NMR studies in (CD3)2SO provide evidence for the solvent inaccessible nature of the Val(2) NH group in peptides 1 and 2. JHNCH alpha H values are indicative of extended structures. Sequential interresidue nuclear Overhauser effects support the population of beta-strand structures in both peptides. The fluorescence quantum yield of tryptophan determined in methanol follows the order 2 greater than 1 approximately 3. Reduction of the disulfides with NaBH4 results in large enhancements of emission intensity, with the changes following the order 1 greater than 3 much greater than 2. The order of quenching is a function of the disposition of the indole and disulfide sidechains in an extended beta-sheet structure.