Formation of NO and SO3•– radical by Photoactivation of trans-[Ru(NH3)4 (Isn)(N(O)SO3)]+ as an Evidence of N(O)SO3• photodissociation

The nitrososulfonate ion (N(O)SO 3 − ) has been proposed to be formed in stomach by the reaction of nitrite and sulfite ions. Nevertheless, very little is known about the chemistry of N(O)SO 3 − , which is unstable and generates reactive species. Aiming to provide a better knowledge on the chemistry and biological relevance of the N(O)SO 3 − , we synthesized a nitrosylsulphito complex trans-[Ru(NH 3 ) 4 (isn)(N(O)SO 3 )] + (isn=isonicotinamide) as a stable and delivery agent of N(O)SO 3 − via photoactivation. This complex was characterized by spectroscopic means, single-crystal X-ray diffraction, and supported by DFT calculations. Electronic and NMR spectroscopy revealed that trans-[Ru(NH 3 ) 4 (isn)(N(O)SO 3 )] + is stable in phosphate buffer with a k = 3.0×10 -5 s -1 (t 1/2 ~385 min, at 25 °C) for isn labilization. At acidic conditions (1.0 M HTFA) trans-[Ru(NH 3 ) 4 (isn)(N(O)SO 3 )] + is found to be converted back to the synthetic precursor trans-[Ru(NH 3 ) 4 (isn)(NO)] 3+ . This process is proposed to be related to the protonation of the SO 3 fragment in the coordinated N(O)SO 3 ligand (pK a ~5) favoring the S–N bond cleavage. An intense absorption band at 363 nm (e=6.0×10 3 M −1 cm −1 ) observed for trans-[Ru(NH 3 ) 4 (isn)(N(O)SO 3 )] + was assigned by DFT as a metal-to-ligand charge transfer (Ru→N(O)SO 3 ) transition. Upon photolysis at 355 nm in phosphate buffer, trans-[Ru(NH 3 ) 4 (isn)(N(O)SO 3 )] + releases NO and SO 3 ●− , by a probable mechanism in which N(O)SO 3 − ligand is photoreleased and further dissociates into NO • and SO 3 ●− species. SO 3 ●– is one of the few sulfur radical species found in cells, which despite its relative low reduction potential (Eo~0.63 V) may promote oxidative damage by acting as a prooxidant and precursor of sulfate radical (SO 4 ●– ), a strong oxidant (Eo ~ 2.5 V vs. NHE). Thus, the newly synthesized nitrosylsulphito complex seems to be a valuable source of N(O)SO 3 − , nitric oxide and sulfite radical (SO 3 ●– ).