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Identification of the distinctive electron paramagnetic resonance signal at g = 2.03 in the yeast cells and liver of mice treated with carcinogens opened the discovery and investigation of the natural [Fe­(NO)2] motif in the form of dinitrosyliron complexes (DNICs). In this Viewpoint, a chronological collection of the benchmark for the study of DNIC demonstrates that the preceding study of its biological synthesis, storage, transport, transformation, and function related to NO physiology inspires the biomimetic study of structural and functional models supported by thiolate ligands to provide mechanistic insight at a molecular level. During the synthetic, spectroscopic, and theoretical investigations on the structure-to-reactivity relationship within DNICs, control of the Fe–NO bonding interaction and of the delivery of NO+/•NO/HNO/NO– by the supporting ligands and nuclearity evolves into the “redesign of the natural [Fe­(NO)2] motif” as a strategy to develop DNICs for NO-related biomedical application and therapeutic approach. The revolutionary transformation of covalent a [Fe­(NO)2] motif into a translational model for hydrogenase, triggered by the discovery of redox interconversion among [{Fe­(NO)2}9-L•] ↔ {Fe­(NO)2}9 ↔ {Fe­(NO)2}10 ↔ [{Fe­(NO)2}10-L•]−, echoes the preceding research journey on [Fe]/[NiFe]-hydrogenase and completes the development of an electrodeposited-film electrode for electrocatalytic water splitting. Through the 50-year journey, bioinorganic chemistry of DNIC containing the covalent [Fe­(NO)2] motif and noninnocent/labile NO ligands highlights itself as a unique metallocofactor to join the longitudinal study between biology/chemistry/biomedical application and the lateral study toward multielectron (photo/electro)­catalysis for industrial application. This Viewpoint discloses the potential [Fe­(NO)2] motif awaiting continued contribution in order to emerge as a novel application in the next 50 years, whereas the parallel development of bioinorganic chemistry, guided by inspirational Nature, moves the science forward to the next stage in order to benefit the immediate needs for human activity.

The water-soluble Roussin's red ester [(NO)(2)Fe(mu-SCH(2)CH(2)P(O)(CH(2)OH)(2))(2)Fe(NO)(2)] (1), a potential photochemical prodrug of an NO precursor, was synthesized from the reaction of HSCH(2)CH(2)P(O)(CH(2)OH)(2) (F) and [Fe(CO)(2)(NO)(2)]. The IR v(NO) stretching frequencies of complex 1 appear at 1759 (s), 1784 (s) and 1816 (w) cm(-1) in buffer (pH = 7.4). NO was released with a stoichiometry ratio Delta[NO]/Delta[1] = 3.6 +/- 0.2 when complex 1 was exposed to UV in deaerated aqueous phosphate buffer solution. Here light acts as an On/Off switch for NO release. Incubation of pBR322 supercoiled DNA with complex 1, followed by irradiation, produced DNA strand breakage. In contrast to the addition of carboxy-PTIO (NO radical scavenger), DNA strand breakage was not inhibited when the scavengers of hydroxyl radical and singlet oxygen were added. Complex 1 irradiated under a N(2) atmosphere exhibited the same cleavage efficiency as complex 1 irradiated under air. The results show that DNA strand cleavage efficiency depends on the concentration of complex 1, the pH value of the buffer, and the duration of the photolysis of complex 1. The conversion rate from supercoiled (SC form) to nicked circular (NC form) of complex 1 was 2.96 x 10(-2) s(-1). The results of a T4 ligase enzymatic assay reveals the nonhydrolytic DNA breakage mechanism. The NO-release ability of complexes 1, 2, and 3 follows the order 1 > 2 > 3. Upon UV-irradiation, complex 1 exhibits cytotoxicity against B16-F10 mouse melanoma cells.

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The title complex, [Fe2(C2H5OS)2(NO)4], lies on a crystallographic inversion center. The Fe—Fe distance is characteristic of a metal–metal bond. In the crystal structure, inter­molecular O—H⋯O hydrogen bonds link complex mol­ecules into a two-dimensional network.