Singlet oxygen reactions in spread monolayers. A comparison of product yield and effects of reaction on surface parameters

Using high pressure liquid radiochromatography, the authors have directly determined quantum yields of photoreactions in spread monolayers and their dependence on such parameters as surface pressure, subphase composition and irradiation time. Singlet oxygen attack, investigated in this study, was initiated by photoexcitation of pheophytin incorporated into monolayers of cholesterol and of the singly unsaturated lipid, palmityloleylphosphatidyl choline. The products of singlet oxygen reaction, especially in cholesterol, are unique and allow ready identification of product formation with singlet oxygen reaction. Literature techniques were used for product separation; and accurate measurements of product at the nanomolar level were obtained. Quantum yields of photoproduct were found to be about 2-5 x 10-3. At pressures above the phase transition for the pheophytin-cholesterol system, the yield dropped. However, the yield was enhanced with increasing pressure in pheophytin-lipid systems. Both behaviors may be explained by reorganization of monolayer components upon compression. Effects with D2O were also obtained in both systems. While correlation between product yield and pressure behavior were observed in lipid systems, such relationship was found not straightforward in cholesterol monolayers