In photosynthesis, (bacterio)chlorophylls play crucial roles in light-harvesting and transmembrane electron transport. (B)Chls, however, are known to be potentially dangerous due to the high efficiency in generating the toxic singlet oxygen (1O2). Under sunlight illumination, unprotected monomeric (B)Chl degrades within few minutes. In this dissertation, I present and discuss several newly discovered strategies nature exploits in photosynthesis to avoid the danger of 1O2. I study the optical properties of triplet excited states of (B)Chl responsible for 1O 2 generation using the highly sensitive nanosecond optical pump-probe spectrometer we developed as a primary experimental tool. In photosynthetic pigment-protein complexes, a carotenoid is typically positioned within a distance of ∼4 Å of individual (B)Chls, allowing rapid triplet energy transfer from (B)Chl to Car, which prevents the formation of toxic 1O2. In the cytochrome b 6f complex that contains a single chlorophyll a molecule, this chlorophyll is distant (14 Å) from the single β-carotene according to atomic structures. Despite this separation, I observed a rapid (\u3c 8 ns) long-range triplet energy transfer from the chlorophyll a to β-carotene, in seeming violation of the existing theory for the distance dependence of such transfer. We infer that a third molecule, possibly oxygen trapped in an intra-protein channel connecting the chlorophyll a and β-carotene, can serve as a mediator of the energy transfer. In chlorosomes, the light-harvesting antenna from the green photosynthetic bacteria, whose core structure is self-assembled aggregates of BChl, I detected the long-lived triplet-like states that are not quenched by oxygen. Based on model excitonic simulations and experiments, we infer that these states are triplet excitons formed in closely packed BChls. This is the first study of triplet excitons in native biological systems and of their function. The high intrinsic photostability of BChl oligomer makes them especially suitable for artificial photosynthesis. Finally, the development of a food pathogen biosensor is described utilizing the color-changing molecular beacon probes to detect the target DNA sequence, which eliminates the risk of a false negative signal and is suitable for multiplex detection for hundreds of nucleotide sequences if spatially arrayed
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