| Abstract
| - Density functional theory methods have been employed to investigate possible first steps in the second half-reaction of the mechanism of nitric oxide synthases (NOSs). In particular, reactions and complexes formedvia transfer of either or both hydrogens of the substrates (NHA) −NHOH group to the Fe-bound O2 wereconsidered. For each of these pathways, the effect of adding an extra electron from tetrahydrobiotperin (H4B)was also examined. The preferred initial pathway involves the simultaneous transfer of both hydrogens of the−NHOH group to the Feheme−O2, without an additional electron, to give the Feheme−HOOH species whichlies only marginally higher in energy, 2.5 kcal mol-1 or less, than the initial bound active site. An alternativemechanism in which only the −NH− proton of the −NHOH group is transferred to the Feheme−O2 to givean Feheme−OOH derivative is found to require only slightly more energy, approximately 2 kcal mol-1. However,transfer of the proton back to the −NOH nitrogen occurs without a barrier at 298.15 K. Tetrahedral intermediatesin which the Feheme−O2 has attached at the guanidinium carbon (Cguan) of NHA, that is, forms an Feheme−O2−Cguan link, have also been investigated. All examples of such species considered, that is, with or withouthydrogen or electron transfers, lie significantly higher in energy by at least 29.0 kcal mol-1 than the initialbound active site. Thus, it is suggested that such complexes are not mechanistically feasible. The implicationsof the present findings for the second half-reaction are also discussed.
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