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Biological systems usually contain cysteine, glutathione or other sulfur-containing biomolecules. These S-nucleophiles were found to affect drastically the $[Fe_{4}(\mu_{3}-3)_{3}(NO)_{7}]^{-}$ photolysis pathway generating products completely different from that of the neat cluster, which produces $Fe^{II}$ and NO and $S^{2-}$. The effect is interpreted in terms of formation of a pseudo-cubane adduct, $[Fe_{4}(\mu_{3}-3)_{3}(\mu _{3}-SR(NO)_{7}]^{2-}$, whose existence in equilibrium with the parent complex has no detectable influence on the spectral properties, whereas shifts the redox potential and induces photoconversion leading to the $Fe^{III}$ species and $N_{2}O$. Characteristic bond lengths, bond angles and atomic Mulliken charges were calculated using semi-empirical quantum chemical methods for the RBS anion and a series of pseudo-cubane complexes with S-donor or N-donor ligands. The results justify the hypothesis of the adduct formation and show that only in case of S-ligands the higher contribution of the $Fe^{III}-NO^{-}$ components in adduct than in RBS is observed, which on excitation can undergo heterolytic cleavage yielding $Fe^{III}$ and $NO^{-}$, converted rapidly into $N_{2}O$. These results are crucial in understanding the physiological activity of RBS. Fe(III) formation can be detected only when the S-ligand enables formation of a stable $Fe^{III}$ compound; the effect was recorded in the presence of sulfide, thioglycolate, 2-mercaptopropionate, mercaptosuccinate, penicillamine, 2,3-dimercaptosuccinate, 2,3-dimercaptopropanol, and thiocyanate. For all these S-ligands the $Fe^{III}$ photoproducts were identified and characterised. In the case of other thiolates, their excess results in fast reduction of $Fe^{III}$ to $Fe^{II}$, whereas $N_{2}O$ can be still detected. Quantum yields of $Fe^{III}$ formation in the presence of the S-ligands are considerably higher than that of the $Fe^{II}$ photoproduction from neat $[Fe_{4}(\mu_{3}-3)_{3}(NO)_{7}]^{-}$.
affiliation:
Pion Rektora : Jagiellońskie Centrum Rozwoju Leków