The folding dynamics and infrared spectra of a photocleaved tetrapeptide predicted by theoretical simulations

We present the theoretical investigation of the folding dynamics of a photocleaved tetrapeptide with a disulfide bridge by using combined semiempirical quantum-mechanical and molecular-mechanical molecular dynamics simulations and high-leveled CASPT2//CASSCF/Amber calculations. We find that in acetonitrile solvent, aside from the recombination of the sulfur biradicals, the peptide can refold to a double sulfur-heterocyclic ring structure or a fully opened structure. The radical bicyclization reaction and the intramolecular hydrogen transfer are responsible for the low recombination rate of the cysteinyl radicals, respectively. On the other hand, in methanol solvent, the formation of the solvent cages around the sulfur radicals reduces the possibility of the close contact of the radicals. The calculated infrared spectra of the amide I mode corresponding to the conformation changes of the peptide can well explain the experimental observation.