Formation of Long-Lived Dark States during Electronic Relaxation of Pyrimidine Nucleobases Studied Using Extreme Ultraviolet Time-Resolved Photoelectron Spectroscopy.

Ultrafast electronic relaxation of nucleobases from 1ππ* states to the ground state (S0) is considered essential for the photostability of DNA. However, transient absorption spectroscopy (TAS) has indicated that some nucleobases in aqueous solutions create long-lived 1nπ*/3ππ* dark states from the 1ππ* states with a high quantum yield of 0.4-0.5. We investigated electronic relaxation in pyrimidine nucleobases in both aqueous solutions and the gas phase using extreme ultraviolet (EUV) time-resolved photoelectron spectroscopy. Femtosecond EUV probe pulses cause ionization from all electronic states involved in the relaxation process, providing a clear overview of the electronic dynamics. The 1nπ* quantum yields for aqueous cytidine and uracil (Ura) derivatives were found to be considerably lower (<0.07) than previous estimates reported by TAS. On the other hand, aqueous thymine (Thy) and thymidine exhibited a longer 1ππ* lifetime and a higher quantum yield (0.12-0.22) for the 1nπ* state. A similar trend was found for isolated Thy and Ura in the gas phase: the 1ππ* lifetimes are 39 and 17 fs and the quantum yield for 1nπ* are 1.0 and 0.45 for Thy and Ura, respectively. The result indicates that single methylation to the C5 position hinders the out-of-plane deformation that drives the system to the conical intersection region between 1ππ* and S0, providing a large impact on the photophysics/photochemistry of a pyrimidine nucleobase. The significant reduction of 1nπ* yield in aqueous solution is ascribed to the destabilization of the 1nπ* state induced by hydrogen bonding.

Ultrafast Electronic Relaxation in 6-Methyluracil and 5-Fluorouracil in Isolated and Aqueous Conditions: Substituent and Solvent Effects.

We report ultrafast extreme ultraviolet photoelectron spectroscopy of 6-methyluracil (6mUra) and 5-fluorouracil (5FUra) in the gas phase and 6mUra and 5-fluorouridine in an aqueous environment. In the gas phase, internal conversion (IC) occurs from 1ππ* to 1nπ* states in tens of femtoseconds, followed by intersystem crossing to the 3ππ* state in several picoseconds. In an aqueous solution, 6mUra undergoes IC almost exclusively to the ground state (S0) in about 100 fs, which is essentially the same process as that for unsubstituted uracil, but much faster than that for thymine (5-methyluracil). The different dynamics for C5 and C6 methylation suggest that IC from 1ππ* to S0 is facilitated by out-of-plane (OOP) motion of the C5 substituent. The slow IC for C5-substituted molecules in an aqueous environment is ascribed to the solvent reorganization that is required for this OOP motion to occur. The slow rate for 5FUrd may arise in part from an increased barrier height due to C5 fluorination.

Inhibitory effect of astraxanthin combined with Flavangenol on oxidative stress biomarkers in streptozotocin-induced diabetic rats.

In this study, the effect of dietary antioxidants, such as astaxanthin and Flavangenol, and a combination of both, in counteracting oxidative stress in streptozotocin-induced diabetes was investigated. Streptozotocin-induced diabetic rats were divided into four groups: control, astaxanthin, Flavangenol, and combined astaxanthin and Flavangenol (mix group). Each group other than the control group was fed with an astaxanthin diet (0.1 g/kg), Flavangenol diet (2.0 g/kg), or an astaxanthin (0.1 g/kg)-Flavangenol (2.0 g/kg) mixture diet, respectively. After 12 weeks of feeding, the results showed that the lipid peroxide levels of plasma and lens and the plasma triglyceride (TG) level in the mix group were significantly decreased by 44%, 20%, and 20%, respectively, compared with the control group. In the mix group, lipid peroxidation was also significantly reduced by 70% in the liver and 20% in the kidney compared with the control group. Furthermore, the level of urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the mix group was significantly lower, 36%, than the control group. The alpha-tocopherol concentrations in the plasma, liver, and kidney in the astaxanthin and mix groups were significantly higher, 3-9 times, than in the control group. The degree of cataract formation in the Flavangenol and mix groups tended to be lower than the control group. These results indicate that the combination of astaxanthin with Flvangenol has an improved protective effect on oxidative stress associated with streptozotocin-induced diabetes than either agent used alone. Thus, this combination may be beneficial in preventing the progression of diabetic complications.