Temporal profile of the singlet oxygen emission endogenously produced by photosystem II reaction centre in an aqueous buffer

The temporal profile of the phosphorescence of singlet oxygen endogenously photosensitized by photosystem II (PSII) reaction centre (RC) in an aqueous buffer has been recorded using laser excitation and a near infrared photomultiplier tube. A weak emission signal was discernible, and could be fitted to the functional form a[exp( - t/t₂ ) - exp( - t/t₁ )] a[exp ( - t/tau_{2} ) - exp ( - t/tau_{1} )] , with $ a > 0 $ a > 0 and $ tau_{2} > tau_{1} $ tau_{2} > tau_{1} . The value of t₂ tau_{2} decreased from 11.6 ± 0.5 μs under aerobic conditions to 4.1 ± 0.2 μs in oxygen-saturated samples, due to enhanced bimolecular quenching of the donor triplet by oxygen, whereas that of t₁ tau_{1} , identifiable with the lifetime of singlet oxygen, was close to 3 μs in both cases. Extrapolations based on the low amplitude of the emission signal of singlet oxygen formed by PSII RC in the aqueous buffer and the expected values of t₁ tau_{1} and t₂ tau_{2} in chloroplasts indicate that attempts to analyse the temporal profile of singlet oxygen in chloroplasts are unlikely to be rewarded with success without a significant advance in the sensitivity of the detection equipment.