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It has been proposed that 3,4‐dihydroxy‐L‐phenylalanine (DOPA) has antioxidant properties, and thus, the objective of this work was to evaluate the effect of adding DOPA during the photosensitized oxidation of tyrosine (Tyr), tryptophan (Trp), histidine (His), 2′‐deoxyguanosine 5′‐monophosphate (dGMP) and 2′‐deoxyadenosine 5′‐monophosphate (dAMP). It was observed that, upon pterin‐photosensitized degradation of a given biomolecule in acidic aqueous solutions, the rate of the biomolecule consumption decreases due to the presence of DOPA. Although DOPA deactivates the excited states of pterin (Ptr), biomolecules do as well, being the bimolecular quenching constants in the diffusional control limit, indicating that DOPA antioxidant mechanism is not a simple deactivation of Ptr excited states. Laser flash photolysis experiments provide evidence of the formation of DOPA radical (DOPA(–H)•, λMAX 310 nm), which is formed in a timescale longer than Ptr triplet excited state (3Ptr*) lifetime, ruling out its formation in a reaction between DOPA and 3Ptr*. The experimental results presented in this work indicate that the observed decrease on the rate of each biomolecule consumption due to the presence of DOPA is through a second one‐electron transfer reaction from DOPA to the biomolecule radicals. [ABSTRACT FROM AUTHOR]

It has been proposed that 3,4‐dihydroxy‐L‐phenylalanine (DOPA) has antioxidant properties, and thus, the objective of this work was to evaluate the effect of adding DOPA during the photosensitized oxidation of tyrosine (Tyr), tryptophan (Trp), histidine (His), 2′‐deoxyguanosine 5′‐monophosphate (dGMP) and 2′‐deoxyadenosine 5′‐monophosphate (dAMP). It was observed that, upon pterin‐photosensitized degradation of a given biomolecule in acidic aqueous solutions, the rate of the biomolecule consumption decreases due to the presence of DOPA. Although DOPA deactivates the excited states of pterin (Ptr), biomolecules do as well, being the bimolecular quenching constants in the diffusional control limit, indicating that DOPA antioxidant mechanism is not a simple deactivation of Ptr excited states. Laser flash photolysis experiments provide evidence of the formation of DOPA radical (DOPA(–H)•, λMAX 310 nm), which is formed in a timescale longer than Ptr triplet excited state (3Ptr*) lifetime, ruling out its formation in a reaction between DOPA and 3Ptr*. The experimental results presented in this work indicate that the observed decrease on the rate of each biomolecule consumption due to the presence of DOPA is through a second one‐electron transfer reaction from DOPA to the biomolecule radicals. [ABSTRACT FROM AUTHOR]

In electron‐transfer initiated photosensitization processes, molecular oxygen (O2) 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 O2 does not participate as a reactant in any step of the pathway yielding Tyr dimers (Tyr2). In the pterin (Ptr) photosensitized oxidation of Tyr, O2 does not directly participate in the formation of Tyr2 and quenches the triplet excited state of Ptr, the reactive species that initiates the process. However, O2 is necessary for the dimerization, phenomenon that we have named as the oxygen paradox. Here, we review the literature on the photosensitized formation of Tyr2 and present results of steady‐state and time resolved experiments, in search of a mechanistic model to explain the contradictory role of O2 in this photochemical reaction system. [ABSTRACT FROM AUTHOR]

In electron‐transfer initiated photosensitization processes, molecular oxygen (O2) 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 O2 does not participate as a reactant in any step of the pathway yielding Tyr dimers (Tyr2). In the pterin (Ptr) photosensitized oxidation of Tyr, O2 does not directly participate in the formation of Tyr2 and quenches the triplet excited state of Ptr, the reactive species that initiates the process. However, O2 is necessary for the dimerization, phenomenon that we have named as the oxygen paradox. Here, we review the literature on the photosensitized formation of Tyr2 and present results of steady‐state and time resolved experiments, in search of a mechanistic model to explain the contradictory role of O2 in this photochemical reaction system. [ABSTRACT FROM AUTHOR]

It has been proposed that 3,4-dihydroxy-L-phenylalanine (DOPA) has antioxidant properties, and thus, the objective of this work was to evaluate the effect of adding DOPA during the photosensitized oxidation of tyrosine (Tyr), tryptophan (Trp), histidine (His), 2'-deoxyguanosine 5'-monophosphate (dGMP) and 2'-deoxyadenosine 5'-monophosphate (dAMP). It was observed that, upon pterin-photosensitized degradation of a given biomolecule in acidic aqueous solutions, the rate of the biomolecule consumption decreases due to the presence of DOPA. Although DOPA deactivates the excited states of pterin (Ptr), biomolecules do as well, being the bimolecular quenching constants in the diffusional control limit, indicating that DOPA antioxidant mechanism is not a simple deactivation of Ptr excited states. Laser flash photolysis experiments provide evidence of the formation of DOPA radical (DOPA(-H) • , λ MAX 310 nm), which is formed in a timescale longer than Ptr triplet excited state ( 3 Ptr*) lifetime, ruling out its formation in a reaction between DOPA and 3 Ptr*. The experimental results presented in this work indicate that the observed decrease on the rate of each biomolecule consumption due to the presence of DOPA is through a second one-electron transfer reaction from DOPA to the biomolecule radicals., (© 2022 American Society for Photobiology.)

In electron-transfer initiated photosensitization processes, molecular oxygen (O 2 ) 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 2 does not participate as a reactant in any step of the pathway yielding Tyr dimers (Tyr 2 ). In the pterin (Ptr) photosensitized oxidation of Tyr, O 2 does not directly participate in the formation of Tyr 2 and quenches the triplet excited state of Ptr, the reactive species that initiates the process. However, O 2 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 2 and present results of steady-state and time resolved experiments, in search of a mechanistic model to explain the contradictory role of O 2 in this photochemical reaction system., (© 2021 American Society for Photobiology.)