chapter  4
30 Pages

## Singlet-Oxygen Reactions: Solvent and Compartmentalization Effects

I. INTRODUCTION Singlet molecular oxygen (!Ag) reactions are important in biological systems, where it can play deleterious (damaging valuable biomolecules) and/or beneficial roles [1,2]. A main characteristic of biological systems is their microheterogene­ ity. Most singlet oxygen reactions are not diffusion-controlled processes. If it is accepted that singlet oxygen is equilibrated between the lipidic regions and the surrounding aqueous media, the extent of damage to a given target will depend both on the steady-state singlet-oxygen concentration in the vicinity of the target and the bimolecular rate constant value in the microenvironment where the target is localized. The damage can be then related to thermodynamic (distribution of the oxygen) and kinetic (specific rate constant) parameters. Furthermore, in situations where diffusion must take place prior to reaction (when singlet oxygen is produced far from the target) or to avoid reaction (if singlet oxygen is produced near the target), other factors, such as diffusion through the different microenvi­ ronments and/or exchange rates between microphases, become relevant. The same factors are also relevant in diffusion-controlled processes. This complex situation can be partially mimicked in well-defined systems, such as micellar solutions or liposome suspensions [3-5].