Synthetic peptide models for protein secondary structures β-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 β-strand conformations in acyclic sequences. NMR studies in (CD₃)₂ SO provide evidence for the solvent inaccessible nature of the Val (2) NH group in peptides 1 and 2. J_HNCHxH values are indicative of extended structures. Sequential interresidue nuclear Overhauser effects support the population of β-strand structures in both peptides. The fluorescence quantum yield of tryptophan determined in methanol follows the order 2 > 1 ñ 3. Reduction of the disulfides with NaBH₄ results in large enhancements of emission intensity, with the changes following the order 1 > 3 > > 2. The order of quenching is a function of the disposition of the indole and disulfide sidechains in an extended beta;-sheet structure.