No abstract

No abstract

No abstract

Quantum yield of singlet oxygen formation for some sensitizers in aqueous solution has been determined by analyzing the concentration dependence of the quantum yield of 2,5-dimethylfuran oxidation in reference to that of methylene blue as a sensitizer.

No abstract

Flash photolysis of aqueous lysozyme has shown that the initial photochemical products are photo-oxidized tryptophan residues ( A , , , = 500 nm), hydrated electrons (Amax = 720 nm), and the cystine residue electron adduct ( A , , , = 420 nm). Comparisons with mixtures of the chromophoric amino acids show that 1 to 2 tryptophan residues provide electrons at a quantum yield of 0.018 (25 per cent). Part of the ejected electrons are captured by cystine residues via a short-range, intramolecular process with essentially unit efficiency. The remainder become hydrated and back react with oxidized tryptophan residues before sec. The cystine residue electron adduct decays with 2 msec halftime (25°C) and 1.5 kcal/mole activation energy. The surviving oxidized tryptophan residues decay with a comparable time constant in a hydroxyl ion catalyzed process. In acid solutions the oxidized tryptophan residue and long-lived H atom adduct are observed (A,,, = 380 nm). The quantum yield of lysozyme inactivation induced by xenon Rash irradiation above 250 nm is 0.023 (20 per cent), which is not sensitive to oxygen or pH. Comparison to the primary photochemical reactions indicates that electron ejection from the essential tryptophan residues inactivates the enzyme, irrespective of the electron trap and subsequent reactions. On the basis of the structure and supporting information it is proposed that the tryptophan residues of the active site are involved. Direct disruption of cystine residues does not contribute more than 10 per cent to the inactivation quantum yield in this wavelength region. Lysozyme inactivation may differ from other enzymes because the chromophores include essential residues located in the active center.

Photoreduction of methyl viologen (MV2+) by eosin-Y (EY2-) in the presence of triethanolamine (TEOA) has been investigated in water-methanol mixture by means of steady-state photolysis and laser-flash photolysis in the visible/near-infrared regions. The complete conversion to the persistent methyl viologen radical cation (MV.+) was observed in the presence of lower concentrations of EY2- and excess TEOA. By laser-flash photolysis measurements, electron transfer was confirmed to occur from the triplet state of EY2- [3(EY2-)*] to MV2+ in the rate constants of ca 2.0 x 10(10) M-1 s-1. The rates and efficiencies of production of MV.+ were found to be dependent on solvent compositions and concentrations of MV2+, ionic salt and TEOA. The back electron transfer reaction from MV.+ to EY.- was retarded in the presence of TEOA, which supports that EY2- is reproduced by accepting an electron from TEOA. In the presence of excess TEOA, the indirect formation of MV.+ from EY.3-, which was produced by accepting an electron from TEOA, was confirmed. The contributions of both the oxidative and reductive routes of 3(EY2-)* for the MV.+ formation have been confirmed.

The formation efficiencies of singlet oxygen for the oxygen quenching in excited singlet of aromatic hydrocarbons; A and for the quenching in the triplet T1; B were estimated. The hydrocarbons were classified into three types: (i) A = 1 and B = 1 for dicyanoanthracene i.e., total 200% efficiency, (ii) A ≈ 0.3 and B = 1 for anthracene, (iii) A ≈ 0 and B = 1 for pyrene, as examples. It is suggested that the energy levels of T2 and a dissociative exciplex; (T11Δg) correlated to A and B values from the energetic considerations.