Developments in the Understanding of the Dinitrosyl Iron Unit: Its Stabilization, Reactivity, and Nitric Oxide Release

Dinitrosyl iron complexes (DNICs), as well as S-nitrosothiols (RSNOs), form endogenously to provide a stable means of storage and transport for the highly reactive signaling molecule, nitric oxide. Through the development of biomimetic complexes, the fundamental chemistry of such DNICs is established within a range of ligand sets as anionic and neutral donors that stabilize the dinitrosyl iron unit, DNIU, in oxidized {Fe(NO)₂}⁹ and reduced {Fe(NO)₂}¹⁰ (Enemark-Feltham notation) redox levels. This amorphous electronic characteristic is readily accommodated by amino acid residues, cysteinyl S- and histidine N-donors, and their surrogates, as ligands to iron. My research targets a greater understanding of DNIC reactivity with components of cellular environments, the possible connections between the electronically similar d⁹ , Cu(II) and d¹⁰, Cu(I), redox couple, the DNIC/Copper/RSNO connections, structural studies of aggregates of DNICs with metallodithiolates, as well as design strategies for NO-release therapeutic development of DNICs. A stable, reduced, {Fe(NO)₂}¹⁰ DNIC containing a bipyridyl ligand, [(neo)Fe(NO)₂] (neo = 2,9-dimethyl-1,10-phenanthroline) was used to explore redox switches and N₂ ligand exchange between copper and the Fe(NO)₂. Both Cu^I and Cu^II sources were found to promote neo ligand transfer from the DNIU with concomitant NO release. With Cu^II, redox processes were also involved, as evidenced by the formation of a mixture of oxidized {Fe(NO)₂}⁹ DNICs, in addition to both Cu^I- and Cu^II-neo complexes. Copper is known to catalyze the release of NO from RSNOs, and our discoveries found here are the first biomimetic investigation of copper’s reactivity with DNICs. Reaction of an RSNO with reduced, {Fe(NO)₂}¹⁰ DNICs, [(L)₂Fe(NO)₂] (L = CO or neo), results in NO release and the formation of a unique (-S)(SR)₂[Fe(NO)₂]₄ cluster, containing an interstitial sulfide. Such a cluster can be visualized as a plausible intermediate in the known conversion of DNICs to FeS clusters. The NO released showed complete scrambling with ¹⁵N-labeled RSNO. Although DNICs are promising NO-delivery candidates, the lack of biocompatible examples has hindered their development into useful therapeutics. We reported the first DNICs with pendant thiosugars, isolated in both monomeric and dimeric form. Cytotoxicity towards endothelial cells was low and steady NO release over several hours was observed in aqueous media