Photosensitized isomerization of resveratrol: Evaluation of energy and electron transfer pathways.

Resveratrol (3,5,4'-trihydroxystilbene, RSV) is a natural stilbene synthetized as trans-isomer in plants exposed to oxidative stress. In order to understand the mechanism involved during photosensitized degradation of trans-resveratrol, steady-state and time-resolved experiments were performed and compared with quantum-chemical calculations using density functional theory (DFT). Pterin (Ptr), a well-known photosensitizer, under UV-A radiation induces the oxidation of several biomolecules mainly through electron-transfer mechanisms. On the one hand, it was observed that trans-RSV participates in an energy-transfer pathway with Ptr triplet excited state (3Ptr*) forming 3trans-RSV*, which dissipates the energy by isomerization to cis-RSV. On the other hand, RSV neutral radical (trans-RSV(-H)•) was detected in laser flash photolysis experiments, evidencing an electron-transfer mechanism. The electron-transfer from 3Ptr* to trans-RSV is a barely feasible reaction, however, more favorable is the formation of trans-RSV(-H)• in a reaction between trans-RSV and Ptr radical cation (Ptr•+), which is produced during irradiation. The combination of experimental and theoretical approaches evidences the capability of trans-RSV to undergo energy-transfer (feasible by DFT calculations) and/or one-electron transfer pathways with 3Ptr*. These findings reveal the mechanisms involved in the interaction of trans-RSV and pterin excited states and provide information on the antioxidant action of resveratrol during photosensitized oxidation of biomolecules.

Avoiding One-Electron Oxidation of Biomolecules by 3,4-Dihydroxy-L-Phenylalanine (DOPA)†.

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 (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.

Photosensitized Dimerization of Tyrosine: The Oxygen Paradox.

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.

Pterin-photosensitization of thymine under anaerobic conditions in the presence of guanine.

Pterin (Ptr) is a model photosensitizer that acts mainly through type I mechanism and is able to photoinduce the one-electron oxidation of purine and pyrimidine nucleobases. However, under anaerobic conditions Ptr reacts with thymine (T) to form photoadducts (Ptr-T) but does not lead to the photodegradation of guanine (G), which is the nucleobase with the lowest ionization potential. Accordingly, G is thermodynamically able to reduce the radicals of the other nucleobases and has been described in this sense as the "hole sink" of the DNA double helix. Here we analyze by steady-state and time-resolved studies the effect of G in the anaerobic photosensitization of T by Ptr, using nucleotides and oligonucleotides of different sequences. We demonstrated that G is able to reduce T radicals but does not prevent the formation of Ptr-T adducts. Our results suggest that after the encounter between the excited Ptr and T, and completion of the electron transfer step, part of the radicals escape from the solvent cage, to further react with other species. However, a proportion of radicals do not escape and evolve to photoadducts before separation. We provide new evidence that contributes to understand the photosensitizing properties of Ptr in the absence of O2, the mechanism of formation of photoadducts in the DNA and the protective role of G towards the photodamage in other nucleobases.

Oxidation of tyrosine: Antioxidant mechanism of l-DOPA disclosed.

Tyrosine is an amino acid related to crucial physiological events and its oxidation, that produce beneficial or detrimental effects on biological systems, has been extensively studied. Degradation of tyrosine often begins with the loss of an electron in an electron transfer reaction in the presence of a suitable electron acceptor. The reaction is facilitated by excited states of the acceptor in photosensitized processes. Several products of tyrosine oxidation have been described, the main ones being 3,4-dihydroxy-l-phenylalanine (commonly known as DOPA) and tyrosine dimers. Here, we report tyrosine recovery from tyrosyl radical, after one-electron oxidation, in the presence of DOPA. We propose that under high oxidative stress the oxidation of tyrosine may be controlled, in part, by one of its oxidation products. Also, we present strong evidence of antioxidant action of DOPA by preventing tyrosine dimerization, one of the most serious oxidative protein modifications, and the origin of structural modifications leading to the loss of protein functionality.

Evidence of the effectiveness of Resveratrol in the prevention of guanine one-electron oxidation: possible benefits in cancer prevention.

Over the past few years, the interest in Resveratrol (3,4',5,-trihydroxystilbene, RSV) has increased due to the evidence found of its antioxidant action that protects biomolecules and cells from oxidative damage. The interest has been further exacerbated by the natural presence of RSV in some fruits and derivatives, especially in red wine. In this paper we present evidence of RSV capacity in protecting a deoxynucleotide, an essential constituent of DNA, from one-electron oxidation. This article evaluates the mechanism responsible for the antioxidant action of RSV, after one-electron oxidation of 2'-deoxyguanosine 5'-monophosphate (dGMP), by kinetic analysis during steady-state irradiation and laser flash photolysis experiments. Results showed that RSV protects dGMP by recovering the nucleotide from its radical, which is formed after the reaction of dGMP with the triplet excited state of the photosensitizer. In the absence of RSV, dGMP is irremediably oxidized, and if the damage occurs in dGMP located in DNA molecules, the consequences can be as serious as mutations and subsequent carcinogenic lesions.