The organometallic anticancer complex [(eta(6)-bip)Ru(en)Cl](+) (1; bip = biphenyl, en = ethylene-diamine) selectively binds to guanine (N7) bases of DNA (Novakova, O.; Chen, H.; Vrana, O.; Rodger, A.; Sadler, P. J.; Brabec, V. Biochemistry 2003, 42, 11544-11554). In this work, competition between the tripeptide glutathione (gamma-(L)-Glu-(L)-Cys-Gly; GSH) and guanine (as guanosine 3',5'-cyclic monophosphate, cGMP) for complex 1 was investigated using HPLC, LC-MS and H-1, N-15 NMR spectroscopy. In unbuffered solution (pH ca. 3), the reaction of 1 with GSH gave rise to three intermediates: an S-bound thiolato adduct [(eta(6)-bip)Ru(en)(GS-S)] (4) and two carboxylate-bound glutathione products [(eta(6)-bip)Ru(en)(GSH-O)](+) (5, 6) during the early stages (<6 h), followed by en displacement and formation of a tri-GS-bridged dinuclear Ru-11 complex [((eta(6)-bip)RU)(2)(GS-mu-S)(3)](2-) (7). Under physiologically relevant conditions (micromolar Ru concentrations, pH 7, 22 mM NaCl, 310 K), the thiolato complex 4 was unexpectedly readily oxidized by dioxygen to the sulfenato complex [(eta(6)-bip)Ru(en)(GS(O)-S)] (8) instead of forming the dinuclear complex 7. Under these conditions, competitive reaction of complex 1 with GSH and cGMP gave rise to the cGMP adduct [(eta(6)-bip)Ru(en)(cGMP-N7)](+) (10) as the major product, accounting for ca. 62% of total Ru after 72 h, even in the presence of a 250-fold molar excess of GSH. The oxidation of coordinated glutathione in the thiolato complex 4 to the sulfenate in 8 appears to provide a facile route for displacement of S-bound glutathione by G N7. Redox reactions of cysteinyl adducts of these Ru-11 arene anticancer complexes could therefore play a significant role in their biological activity.