Photosensitized Dimerization of Tyrosine: The Oxygen Paradox.

In electron-transfer initiated photosensitization processes, molecular oxygen (O ) is not involved in the first bimolecular event, but almost always participates in subsequent steps giving rise to oxygenated products. An exception to this general behavior is the photosensitized dimerization of tyrosine (Tyr), where O does not participate as a reactant in any step of the pathway yielding Tyr dimers (Tyr ). In the pterin (Ptr) photosensitized oxidation of Tyr, O does not directly participate in the formation of Tyr and quenches the triplet excited state of Ptr, the reactive species that initiates the process. However, O is necessary for the dimerization, phenomenon that we have named as the oxygen paradox. Here, we review the literature on the photosensitized formation of Tyr and present results of steady-state and time resolved experiments, in search of a mechanistic model to explain the contradictory role of O in this photochemical reaction system.