In M P ro , it was shown that a single NOS takes place with Cys22 and that further oxidation leads to the SONOS formation with Cys44. In order to study the reaction mechanism for the double NOS formation and the preference for Cys22 or Cys44 herein, thermodynamic and kinetic results are presented, see Figure . The overall reaction mechanism is divided into two steps. In step (a), the H atom abstraction from the N atom by the newly formed •OH radical is studied. The kinetic barrier is slightly lower for the single NOS with Cys44, by about 4 kcal/mol and no difference is observed in the thermodynamic stability of the formed intermediate. However, the reactant state for the single NOS with Cys22 is thermodynamically favoured by 8 kcal/mol, which could favour its formation over the single NOS with Cys44. Step (b) corresponds to the recombination of the radicals leading to the SONOS formation. No significant differences are observed for the kinetic barriers and so the preference for the single NOS formation with Cys22 is thereby attributed to the relative reactant stability.