PHOTOCHEMISTRY OF ROUSSINS RED SALT, NA-2[FE2S2(NO)4], AND OF ROUSSINS BLACK SALT, NH4[FE4S3(NO)7] - IN-SITU NITRIC-OXIDE GENERATION TO SENSITIZE GAMMA-RADIATION INDUCED CELL-DEATH

The quantitative photoreactivities in solution of Roussin's red salt ( RRS, Na-2[Fe2S2(NO)(4)]) and of Roussin's black salt (RES, NH4[Fe4S3(N O)(7)]) are described; Photolysis of the red Roussinate anion Fe2S2(NO )(4)(2-) in aerobic aqueous solution leads to quantitative formation o f the black Roussinate anion Fe4S3(NO)(7)(-). The quantum yield for di sappearance of Fe2S2(NO)(4)(2-) (Phi(I) = 0.14) is independent of exci tation 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 blac k Roussinate anion is much less photoactive (Phi(II) = L1 x 10(-3)) bu t does undergo photodecomposition in aerobic solution to give, eventua lly, ferric precipitates plus NO. These studies were initiated with th e goal of developing photochemical strategies for delivering NO to bio logical targets on demand. In this context, the photolability of Fe2S2 (NO)(4)(2-) was examined as a possible candidate for exploiting the kn own nitric oxide sensitization of gamma-radiation induced cell killing in V79 cell cultures (Mitchell, J. B.; et al. Cancer Res. 1993, 53, 5 845-5848). Hypoxic cell cultures treated with RRS solution (1.0 mM) an d then subjected to gamma-radiation (15 Gy) demonstrated strikingly lo wer survival rates when simultaneously exposed to white light irradiat ion than did control systems treated identically but in the dark. The black salt was similarly probed, but its greater toxicity and lower qu antum yields for NO release make this a less likely candidate for such photochemically induced radiation sensitization.