Introduction: The <em>York Cycle</em> Then and Now

Biological and Bioinspired Inorganic N–N Bond-Forming Reactions

“…389 Although the Cu Z site architecture was only recently structurally resolved, it can be confidently concluded in hindsight that few Cu Z -containing N 2 OR homologs have been isolated over the previous years, although the identity of the Cu Z1 /Cu Z4 μ 2 -ligand was not known at the time. These species could never reach the super-reduced state, even after prolonged incubation with excess reductant, 392,419 and exhibited very low steady-state turnover rates (up to 106-fold lower compared to fully reduced Cu Z *) under physiological conditions. 357,376,380,381,389,399,[419][420][421][422] Presently, the super-reduced [4Cu Z 1+ ] state of the Cu Z * center is the only species that has exhibited substantial kinetic competency during steady-state turnover conditions, 419 and the physiological relevance of the μ 2 -S Z2 ligand of the Cu Z center still awaits investigation.…”

“…These species could never reach the super-reduced state, even after prolonged incubation with excess reductant, 392,419 and exhibited very low steady-state turnover rates (up to 106-fold lower compared to fully reduced Cu Z *) under physiological conditions. 357,376,380,381,389,399,[419][420][421][422] Presently, the super-reduced [4Cu Z 1+ ] state of the Cu Z * center is the only species that has exhibited substantial kinetic competency during steady-state turnover conditions, 419 and the physiological relevance of the μ 2 -S Z2 ligand of the Cu Z center still awaits investigation.…”

“…418 All of the mechanistic studies mentioned above have been based on the [4Cu-1S] Cu Z * center architecture that contains a μ 2 -water (or -hydroxo) ligand in the Cu Z1 /Cu Z4 edge, without accounting for the S Z2 ligand of the Cu Z center. 357,368,374,419 In the N 2 O-associated PsN 2 OR structure, N 2 O is positioned in a linear side-on manner within a structurally defined cleft between Cu Z and Cu A that is formed by Phe621, Met627, and the Cu A -ligating His626 (Figure 40). The O atom of N 2 O is oriented towards a solvent-filled cavity while the N end is within about 3.5 Å from the Cu Z site.…”