Photochemistry of Roussin's Red Salt, Na₂[Fe₂S₂(NO)₄], and of Roussin's Black Salt, NH₄[Fe₄S₃(NO)₇]. In Situ Nitric Oxide Generation To Sensitize γ-Radiation Induced Cell Death¹

The quantitative photoreactivities in solution of Roussin's red salt (RRS, Na 2 [Fe 2 S 2 (NO) 4 ]) and of Roussin's black salt (RBS, NH 4 [Fe 4 S 3 (NO) 7 ]) are described. Photolysis of the red Roussinate anion Fe 2 S 2 (NO) 4 2 - in aerobic aqueous solution leads to quantitative formation of the black Roussinate anion Fe 4 S 3 (NO) 7 - . The quantum yield for disappearance of Fe 2 S 2 (NO) 4 2 - (Φ I = 0.14) is independent of excitation wavelength over a broad range (313−546 nm). Real time detection of nitric oxide by electrochemical sensors in the photolysis solution demonstrated the release of NO with a quantum yield of 0.07. The black Roussinate anion is much less photoactive (Φ II = 1.1 × 10 - 3 ) but does undergo photodecomposition in aerobic solution to give, eventually, ferric precipitates plus NO. These studies were initiated with the goal of developing photochemical strategies for delivering NO to biological targets on demand. In this context, the photolability of Fe 2 S 2 (NO) 4 2 - was examined as a possible candidate for exploiting the known nitric oxide sensitization of γ-radiation induced cell killing in V79 cell cultures (Mitchell, J. B.; et al. Cancer Res. 1993, 53, 5845−5848). Hypoxic cell cultures treated with RRS solution (1.0 mM) and then subjected to γ-radiation (15 Gy) demonstrated strikingly lower survival rates when simultaneously exposed to white light irradiation than did control systems treated identically but in the dark. The black salt was similarly probed, but its greater toxicity and lower quantum yields for NO release make this a less likely candidate for such photochemically induced radiation sensitization.