Wentworth et al. [Wentworth, P., Jones, L. H., Wentworth, A. D., Zhu, X. Y., Larsen, N. A., Wilson, I. A., Xu, X., Goddard, W. A., Janda, K. D., Eschenmoser, A. & Lerner, R. A. (2001) Science 293, 1806–1811] recently reported the surprising result that antibodies and T cell receptors efficiently catalyze the conversion of molecular singlet oxygen (¹O₂) plus water to hydrogen peroxide (HOOH). Recently, quantum mechanical calculations were used to delineate a plausible mechanism, involving reaction of ¹O₂ with two waters to form HOOOH (plus H₂O), followed by formation of HOOOH dimer, which rearranges to form HOO—HOOO + H₂O, which rearranges to form two HOOH plus ¹O₂ or ³O₂. For a system with ¹⁸O H₂O, this mechanism leads to a 2.2:1 ratio of ¹⁶O:¹⁸O in the product HOOH, in good agreement with the ratio 2.2:1 observed in isotope experiments by Wentworth et al. In this paper we use docking and molecular dynamics techniques (HierDock) to search various protein structures for sites that stabilize these products and intermediates predicted from quantum mechanical calculations. We find that the reaction intermediates for production of HOOH from ¹O₂ are stabilized at the interface of light and heavy chains of antibodies and T cell receptors. This inter Greek key domain interface structure is unique to antibodies and T cell receptors, but is not present in β₂-microglobulin, which does not show any stabilization in our docking studies. This result is consistent with the experimentally observed lack of HOOH production in this system. Our results provide a plausible mechanism for the reactions and provide an explanation of the specific structural character of antibodies responsible for this unexpected chemistry.