Both fluorescence emission and singlet oxygen production were almost completely shut down, while an excess of complimentary sequence was able to recover 85 % of the singlet oxygen after hybridization.
Pyropheophorbide and Black Hole Quencher were conjugated to two separate complimentary oligonucleotide strands to serve as photosensitizer and quencher, respectively. ĭNA-based activatable photosensitizers have been constructed as a means to control on-and-off switching of singlet oxygen in response to nucleic acid target binding. The principles in designing photodynamic molecular beacons for controlling singlet oxygen generation are essentially the same as for fluorescence beacons and must enable beacon activation upon target recognition. The localization of photosensitizers is also important, and partitioning into organelles like the mitochondria and endoplasmic reticulum can improve efficacy. As for the photosensitizer spectral properties, those absorbing at near-infrared wavelengths (650–900 nm) are usually selected since deeper tissue penetration and lower background fluorescence are possible in this range due to intrinsic optical properties of tissues. Surrounded by diffused oxygen molecules, triplet-state photosensitizers can interact with these to yield singlet oxygen. Compared to fluorophores of conventional molecular beacons, the excited singlet state can alternatively enter a lower-energy triplet state via intersystem crossing. Cytotoxic singlet oxygen is only produced upon irradiation if the photosensitizer is liberated from the quencher. For instance, activatable photosensitizers can control how singlet oxygen is quenched and activated via protease control.
Historically, development of photodynamic molecular beacons has not been based on nucleic acids. With the same sensitive detection ability as molecular beacons, photodynamic molecular beacons feature light-activated singlet oxygen generating properties of photosensitizers, which are altered, reduced, or intensified by molecular recognition or environmental changes.
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