Оn the Ability of Bacteriochlorophyll a to Generate Singlet Oxygen upon Photoexcitation in Aqueous Environment

Аннотация

The quantum yields of singlet oxygen generation (Ф_∆) photosensitized by bacteriochlorophyll a have been measured in diethyl ether and aqueous (D₂O and H₂O) dispersions, in which hydrophobic bacteriochlorophyll molecules are solubilized by a detergent Triton X-100. According to the absorption spectra, bacteriochlorophyll molecules are monomeric in all samples studied. For measurements, both detection of photosensitized phosphorescence of singlet oxygen at 1270 nm and singlet oxygen trapping by 1,3-diphenylisobenzofuran were applied. In ether, both methods gave the highest quantum yields (Ф_∆=0.60±0.05) and slightly lower Ф_∆=0.3‒0.4 was obtained in micellar dispersions in deuterium oxide. The lowest, but still reasonable, value of Ф_∆ was found in aqueous (H₂O) dispersions (Ф_∆=0.12±0.02). In detergent-free water, where aggregated pigment molecules predominate, ¹O₂ generation was not detected. Generation of ¹O₂  was always accompanied by active quenching of ¹O₂  by bacteriochlorophyll with the rate constants k_q=(1.4‒2.2)·10⁹ M^-1s^-1. The data indicate that monomeric bacteriochlorophyll molecules actively photosensitize generation of ¹O₂ in organic solvents and do not lose this ability in aqueous environment, which most likely determines photodynamic action and PDT by bacteriochlorophyll and its derivatives.