Computational Studies of Photobiological Keto-Enol Reactions and Chromophores

This thesis presents computational chemistry studies of keto-enol reactions and chromophores of photobiological significance. The first part of the thesis is concerned with two protein-bound chromophores that, depending on the chemical conditions, can exist in a number of different ketonic and enolic forms. The first chromophore is astaxanthin, which occurs in the protein complex responsible for the deep-blue color of lobster carapace. By investigating how different forms of astaxanthin absorb UV-vis radiation of different wavelengths, a model is presented that explains the origin of the dramatic color change from deep-blue to red upon cooking of live lobsters. The second chromophore is the oxyluciferin light emitter of fireflies, which is formed in the catalytic center of the enzyme firefly luciferase. To date, there is no consensus regarding which of the possible ketonic and enolic forms is the key contributor to the light emission. In the thesis, the intrinsic tendency of oxyluciferin to prefer one particular form over other possible forms is established through calculation of keto-enol and acid-base excited-state equilibrium constants in aqueous solution. The second part of the thesis is concerned with two families of biological photoreceptors: the blue-light-absorbing LOV-domain proteins and the red-lightabsorbing phytochromes. Based on the ambient light environment, these proteins regulate physiological and developmental processes by switching between inactive and active conformations. In both families, the conversion of the inactive into the active conformation is triggered by a chemical reaction of the respective chromophore. The LOV-domain proteins bind a flavin chromophore and regulate processes such as chloroplast relocation and phototropism in plants. An important step in the activation of these photoreceptors is a singlet-triplet transition between two electronically excited states of the flavin chromophore. In the thesis, this transition is used as a prototype example for illustrating, for the first time, the ability of first-principles methods to calculate rate constants of inter-excited state phosphorescence events. Phytochromes, in turn, bind bilin chromophores and are active in the regulation of processes like seed germination and flowering time in plants. Following two systematic studies identifying the best way to model the UV-vis absorption