Hunt for singlet oxygen under in vivo conditions.

It is widely accepted that applying exogenous photosensitizers in living organism the photodynamic effect is mediated by singlet oxygen formed by energy transfer from excited sensitizer to oxygen present in the cells (Type II mechanism). Though in homogeneous solutions singlet oxygen was detected during sensitization, efforts to observe it in vivo failed, so far. Kinetic considerations based on the assumption that triplet sensitizer molecules interact with (doublet) free radicals accumulated in the tissue as well as on literature data indicate that the steady state concentration of singlet oxygen can decrease by more than one order of magnitude at transition from homogeneous systems to in vivo conditions making it undetectable.