Interactions of N-hydroxyamphetamine with an iron porphyrin: A unique intramolecular H-bond probed by DFT calculations.

Hydroxylamine (NHOH) and its N-substituted derivatives (RNHOH) are important biological intermediates in the global N cycle. Heme plays a central role in the binding and activation of these hydroxylamines. We report the crystal structures of N-hydroxyamphetamine (AmphNHOH) in complex with Fe and Co heme models. We demonstrate a previously unrecognized internal H-bond interaction between a hydroxylamine RNHO-H group and a porphyrin N-atom. We utilize density functional theoretical (DFT) calculations to show that the conformations with the internal H-bond represent global minima along the potential energy surfaces for both the Fe and Co heme models. A natural bond orbital (NBO) analysis reveals a donor π (por) to acceptor σ* (O-H) interaction of 3.04 kcal/mol for Fe, accounting for 11% of the total heme-AmphNHOH interaction energy. Our DFT calculations with the parent Fe-NHOH suggests that the presence of internal H-bonds between hydroxylamine (R/H)NHOH moieties and heme N-atoms may be more common than previously recognized.