Photochemistry of Bacteriochlorophylls in Human Blood Plasma: 2. Reaction Mechanism Investigated by Product Analysis and Deuterium Isotope Effect

Transmetalated (Pd) bacteriochlorophyll derivatives are currently being clinically tested as sensitizers for photodynamic therapy. Protocols using short delay times between injection and irradiation generate interest in the photochemistry of these pigments in the blood. Using near‐infrared irradiation where these pigments absorb strongly, we have studied the mechanism of photo‐oxidation in two lipoprotein fractions, low‐ and high‐density lipoproteins, derived from human blood plasma that preferentially accumulate these pigments (Dandler et al. [2009] Photochem. Photobiol., 85, in press). Using quenchers of reactive oxygen species, and chemical reporters, in particular peroxides generated from cholesterol as an inherent component of the lipoproteins, a Type II mechanism generating singlet oxygen has been demonstrated for Pd‐ and Zn‐bacteriopheophorbides. In homogeneous systems, accelerated bleaching in D2O, compared with H2O, supports this mechanism. An unusual deuterium isotope effect was observed, by contrast, in heterogeneous amphiphilic‐water systems. In the early phase, and under high oxygen concentrations, again a positive D‐isotope effect is observed which later, in a second phase, is reversed to a negative D‐isotope effect. The latter cannot be explained by heterogeneous pigment populations in the amphiphilic system; we, therefore, conclude a mechanistic switch, and discuss a possible mechanism.