Photocatalytic Generation of Oxygen Radicals by the Water-Soluble Bacteriochlorophyll Derivative WST11, Noncovalently Bound to Serum Albumin

Abstract

Light-induced radical generation is the hallmark of fundamental processes and many applications including photosynthesis and photodynamic therapy (PDT). In this manuscript, we present two novel observations made upon monitoring light-induced generation of reactive oxygen species (ROS) in aqueous solutions by WST11, a water-soluble derivative of the photosynthetic pigment Bacteriochlorophyll a (Bchl). Using a host of complementary experimental techniques including time-resolved spectroscopy at the subpicosecond to the millisecond range, ESR spectroscopy, electrochemistry, spectroelectrochemistry, oximetry, and protein mass spectroscopy, we first show that in aqueous solutions WST11 generates only superoxide (O 2 -•) and hydroxyl (OH • ) radicals with no detectable traces of singlet oxygen. Second, we show that WST11 makes a noncovalent complex with human serum albumin (HSA) and that this complex functions as a photocatalytic oxidoreductase at biologically relevant concentrations enabling approximately 15 cycles of electron transfer from the associated HSA protein to molecular oxygen in the solution. These findings rule out the paradigm that porphyrin and chlorophyll based PDT is mainly mediated by formation of singlet oxygen, particularly in vascular targeted photodynamic therapy (VTP) with sensitizers that undergo photoactivation during circulation in the plasma, like [Pd]-Bacteriopheophorbide (WST09, Tookad). At the same time, our findings open the way for new design paradigms of novel sensitizers, since O 2 -• and OH • radicals are well-recognized precursors of important pathophysiological processes that can be activated for achieving tumor eradication. Moreover, the finding that promiscuous protein scaffolds become sinks for holes and electrons when holding light-activated pigments provides a new insight to the evolution and action mechanism of natural light activated oxidoreductases (such as photosynthetic reaction centers) and new guidelines for the preparation of synthetic-light converting machineries.

ATG7 deficiency suppresses apoptosis and cell death induced by lysosomal photodamage

“…24 We recently confirmed that WST11 does not generate singlet oxygen in cultured 1c1c7 cells following photoactivation. 25 This represents a significant departure from the situation with other sensitizers, such as NPe6, where PDT initially generates a high quantum yield of singlet oxygen.…”

(Metallo)porphyrins as Potent Phototoxic Anti‐Cancer Agents after Irradiation with Red Light

“…Because paramagnetic porphyrin 3 did not show any luminescence nor generated any singlet oxygen species, we were surprised to observe that porphyrin 3 exhibited a phototoxic index between 9 and 31 (ratio of dark to light toxicity). Spintrapping electron paramagnetic resonance (EPR) experiments revealed that 3 produced hydroxyl radicals [28] upon light irradiation (see the Supporting Information), explaining the observed phototoxicity. We also investigated whether this effect could be observed inside living cells.…”

Photodynamic Therapy: Current Status and Future Directions

“…In principle, the probability of such species reacting with a potential target depends on the distance between the PS and the target, the abundance of the target and the specific reaction rate constants. Singlet oxygen is considered the main damaging factor in PDT, but PSs operating mainly by the type I mechanism can also be highly effective [50]. PSs rarely participate only in type I or type II processes; therefore, photodynamic activation usually generates a mixture of 1 O 2 and radicals [51].…”