Upon photolysis at 355 nm, dioxygen is released from a (-peroxo)(,-hydroxo)bis[bis(bipyridyl)cobalt- (III)] complex in aqueous solutions and at physiological pH with a quantum yield of 0.04. The [Co(bpy)2(H20)z2+ (bpy = bipyridyl) photoproduct was generated on a nanosecond or faster time scale as determined by time-resolved optical absorption spectroscopy. A linear correspondence between the spectral changes and the oxygen production indicates that 02 is released on the same time scale. Oxyhemoglobin was formed from deoxyhemoglobin upon photodissociation of the (,iperoxo)(,u-hydroxo)bis[bis(bipyridyl)cobalt(III)I complex, verifying that dioxygen is a primary photoproduct. This complex and other related compounds provide a method to study fast biological reactions involving 02, such as the reduction of dioxygen to water by cytochrome oxidase.Many biological reactions involving dioxygen are so fast that studies by conventional stopped-flow techniques are impractical. Initiation of these and other fast reactions by photolysis offers a different approach. For example, the reaction of cytochrome c oxidase with dioxygen can be studied by a flow-flash method in which the reaction is initiated upon photodissociation of carbon monoxide (CO) bound to cytochrome a3 (1, 2). However, this method is potentially compromised by the fate of the photodissociated CO and, therefore, may not represent the physiological reaction of the enzyme under turnover conditions (3, 4). It is also not applicable to nonheme oxygen-activating systems. An alternative involves 02 production in situ by photodissociating synthetic caged dioxygen carriers. An ideal system would involve a direct source of dioxygen that does not react prematurely with the protein of interest but would release oxygen upon photodissociation on any relevant time scale. There should be no electrochemical or chemical reactions of the dioxygen carrier and its photoproduct with the protein, and interference from the photoproduct in the spectral region of the protein should be negligible. High solubility in aqueous solutions at physiological pH and high oxygen quantum yield are also desirable.Synthetic caged dioxygen carriers such as dicobalt ,u-peroxoor ,u-superoxo-bridged derivatives have the potential to fulfill the above requirements. Many of these complexes are soluble in aqueous solutions and are amenable for study by various spectroscopic probes (5-7). The most extensively studied have been cobalt(II) complexes of polyamines (8-13), amino acids (8, 14-16), and dipeptide ligands (8, 15, (11,15,18). The hydroxo bridge has been proposed to "lock in" the peroxo bridge, thereby shifting the equilibrium toward the oxygenated species (18). An advantage of the ,u-peroxo and ,-superoxo cobalt complexes is that such complexes have their major absorbances in the UV region. Therefore, electronic transitions of the protein in the visible region can be investigated without serious interference from transitions due to the cobalt complexes.Several studies of the photochemical...