The ability to sense light is one of the most important and common features throughout the kingdoms of life. Organisms use light receptors to obtain information about their surroundings and seek out ideal living conditions. These receptors are of great interest to the research community, not least due to their applicability in optogenetics. Light activation often involves a chain of slight changes, be it, e.g. isomerisation or protonation states, that culminate in structural shifts of the tertiary protein structure. This last step, namely the shift in the structure that fascilitates function, is the focus of this work. Electron Paramagnetic Resonance (EPR) is a valuable tool to achieve this goal. By exploiting naturally existing radical states or by side-directed spin labeling, the distances between distinct protein sites can be determined. Among others, two protein systems were studied within this work: LOV proteins are widely known blue light receptors, that possess a flavin chromophore for light absorption. The newly discovered PAL receptor regulates RNA expression, a function not yet known in this class of proteins. Here, the distance of the flavin chromophores in the illuminated protein was determined by means of double resonance EPR. Thus, valuable insights into structural changes upon activation were gained. Channelrhodopsins are a group of light-driven ion channels found in green algae that have inspired growing interest. The structural aspects of the opening channel of Channelrhodopsin-2 are not fully understood: Although previous EPR studies have resolved the movement of the inner helices, the shift of the outermost has not yet been established. Measuring long distances by means of EPR can be challenging, as the dipolar time traces recorded need to be of sufficient lengths. To address this problem, a thorough methodological examination is provided in which several approaches to elongate dipolar time traces are compared. Another part of this thesis concerns itself with light-induced catalysis. Light is a valuable reactant, as it can be produced in a sustainable manner and is applicable in mild conditions. Metal-center based catalysis, especially those used for cross-coupling reactions, play a decisive roll in pharmaceutical applications. Although dual photo-/metal catalysis, i.e. a photocatalyst combined with a metal-center system, is a common strategy, direct photocatalysis, where a single catalyst fulfils both roles, has inspired increasing interest. Photocatalysis often involves radical species, that mediate charge and energy transport. In this thesis, the novel nickel catalyst based on a bipyridil ligand Ni(czbpy)Cl 2 was studied and such radical species successfully captured by EPR. The results are consistent with a radical species that is formed by chloride and the surrounding solvent. This suggests the homolysis of the aryl-nickel bond and therefore provides evidence for the hypothesized catalytic reaction mechanism.
