Аннотация:One of important stages of the substrate bond breaking in the active site (AS) of α-chymotrypsin (ACT) is considered. Three tasks are solved by methods of quantum mechanics and stochastic molecular dynamics: the loosening of peptide bond of a substrate attacked by O- ion of Ser195 of catalytic group; the opportunity of increase of a peptide bond (PB) breaking probability; the increase of this probability related to nonlinear interacting modes (or Fermi resonance (FR)) of oscillations of group N-H in PB. It is shown also that the splitting of vibrational levels Amide A and Amide B in a spectrum of an amide group pays off due to FR.