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Characterization of the Photodynamics of Photoswitches: A Study of a Photomagnetic Molecular Photoswitch, Phytochromes, Cyanobacteriochromes, and a Cellular Retinoic Acid Binding Protein II Rhodopsin Mimic



Photoreceptors play essential roles in initiating a wide array of biological responses based on external light conditions. Photoreceptors have been shown to be involved in phototropism, shade avoidance, seed germination, vision, and chromic acclimation. These photosensitive proteins initiate these various responses by reversibly photoconverting between a thermally stable dark- adapted state and a meta-stable light-adapted state when a covalently bound chromophore absorbs a photon of light. The chromophore causes the large-scale reorganization of the protein by undergoing a structural or electronic change. Since photoreceptors have evolved to initiate various biological responses as well as improve the efficiency of photosynthesis via chromatic acclimation (converting higher energy light to lower energy light that can be used for photosynthesis), they are also of interest to serve as a basis for designing novel fluorescent probes for medicinal imaging applications or optogenetic tools to selectively initiate and control biological functions in living tissue and study neural circuits at a more fundamental level.

To better understand biological signal transduction and establish the practicality of these photoreceptors for the application outlined above, a molecular level understanding of these photoreceptors is of paramount importance. Key to developing a molecular level understanding of these photoinduced events is the characterization of the photoreceptor forward and reverse reactions with time and temperature dependent spectroscopic techniques. These experiments track changes in the absorption spectra due to the photoreaction, which allows for a mapping of the underlying mechanism and estimations of transient properties (e.g., timescale, energy barriers, quantum efficiencies). Photoreceptors are especially well suited for being studying by time- resolved techniques since the reaction is initiated at a known time point (i.e., photoexcitation). Theanalysis of the resulting time-dependent data sets reveals complex mechanisms, including ground state inhomogeneity, unproductive pathways, and estimation of the quantum efficiencies. Cryokinetic UV-Vis spectroscopy complements the room-temperature primary and secondary transient absorption spectroscopic results as well as isolates quickly evolving intermediates difficult to resolve at room temperature.

In the first part of this dissertation a brief overview to photoswitches, including select molecular photoswitches and biological photoreceptors, is provided and followed by a description of the experimental protocols used in the subsequent chapters. The second part of this dissertation is a collection of studies on the photoinduced dynamics of: i) a synthetic cobalt-dioxolene based photomagnetic molecular photoswitch and ii) several biological photoreceptors including comparative studies on the forward and reverse dynamics of several red/green cyanobacteriochromes (CBCRs) isolated from Nostoc punctiforme, a comparison of the secondary forward and reverse dynamics of the photosensory core of cyanobacterial phytochrome 1 (Cph1∆) isolated from Synechocystis sp. PCC 6803 and the canonical red/green CBCR NpR6012g4 isolated from Nostoc punctiforme, primary forward dynamics of a wild-type and mutated far-red/x CBCR isolated from Anabaena cylindrica PCC 7122, and the photocycle of a novel semisynthetic rhodopsin mimic.

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