Genetically encodable proteins that photosensitize the production of singlet oxygen, O 2 (a~1△g ), will play an increasingly important role in elucidating mechanisms of cellular processes modulated by reactive oxygen species, ROS, and changes in redox balance. In the development of such tools, it is essential to characterize the oxygen-dependent photophysics of the protein-encased chromophore. Of the O 2 (a~1△g )-photosensitizing systems recently developed, a protein-bound derivative of Malachite Green has several desirable features: (1) it absorbs light at wavelengths longer than those typically absorbed by endogenous molecules, and (2) the chromophore becomes a viable sensitizer only when bound to the activating protein. However, we now demonstrate that the photophysics of this Malachite Green system is not simple. Our data indicate that, with an increase in the concentration of ground-state oxygen, O 2 (X~3Σg~- ), the yield of O 2 (a~1△g ) does not increase in a pro- portional manner. Moreover, the lifetime of O 2 (a~1△g ) decreases as the O 2 (X~3Σg~- ) concentration is increased. One mechanism that could account for our observations involves the concomitant photo-initiated formation of O 2 (a~1△g ) and the superoxide radical anion. We propose that the superoxide ion acts as a dynamic diffusion-dependent quencher to influence the O 2 (a~1△g ) lifetime and as a static quencher within the protein enclosure to influence the measured O 2 (a~1△g ) yield. Thus, in the least, caution should be exercised when using this Malachite Green system to probe mechanisms of ROS-mediated processes. Our results contribute to a better understanding of the general photophysics of protein-bound O 2 (a~1△g ) sensitizers which, in turn, facilitates the further development of these useful mechanistic tools.
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