Steady-state and time-resolved studies of site-selective photocleavage of lysozyme by cobalt(III) complexes [Co(NH3)(5)Br](2+) and ([Co(NH3)(4)CO3](+) are reported. Photocleavage resulted in two fragments of molecular masses similar to 10.5 kDa and similar to 3.5 kDa. and the yield increased (8-33%) with irradiation time (0.16-0.8 h) as well as with the metal complex concentration (0.1-5 mM). The reaction proceeded to a significant extent even when nearly stoichiometric amounts of the reagents were used. Photocleavage wits effective at wavelengths ranging from 310 to 390 nm, and cleavage was inhibited by the addition of selected metal ions such its Gd(III)at moderate concentrations (2 mM). Gd(III) is known to bind at Asp52/Glu35 residues Oil lysozyme, and these residues are located at the enzyme active Site. Current and previous studies suggest that Co(III) metal complexes bind at this site oil lysozyme. Consistent with this hypothesis, [Co(NH3)(4)CO3](+) (8 mM) inhibited lysozyme activity by 67%. Laser flash photolysis Studies show that excitation of the metal complexes [Co(NH3)(5)Br](2+) and ([Co(NH3)(4)CO3](+) (308 nm, 20 ns pulse width) resulted in the corresponding ligand-derived radical intermediates. For example, photoexcitation of an aqueous Solution of [Co(NH3)(5)Br](2+) at 308 nm resulted in the formation of Br-2(-center dot). When the excitation was carried out in the presence of lysozyme, Br-2(-center dot) was quenched with a bimolecular rate constant of 1.4 x 10(9) M-1 s(-1). Quenching resulted in protein-derived radicals (Trp(+center dot) and Tyr(+)center dot), as identified by their characteristic known transient absorption bands. Steady-state Studies correlated with the time-resolved data, and taken together, these illustrated the reactivities of Co(III) metal complexes to direct protein photocleavage with high selectivity.