Overall, the atomistic mechanism of the oxidation of Cys and Lys residues in the framework of NOS formation is not yet understood. This work is devoted to shed light into the reaction mechanism for the formation of the cross-link, exploring different oxidizing agents and intermediates. Particular focus is given to the question of how NOS formation proves to be competitive in relation to other oxidizing pathways of Cys, especially for the formation of the disulfide bond.