All cryptochromes are currently classified as flavoproteins. In animals their best-described role is as components of the circadian clock. This circadian function is variable, and can be either light-dependent or -independent; the molecular origin of this difference is unknown. Type I animal cryptochromes are photoreceptors that entrain an organism’s clock to its environment, whereas Type II (including mammals) regulate circadian timing in a light-independent manner. Here, we reveal that, in contrast to Type I, Type II animal cryptochromes lack the structural features to securely bind the photoactive flavin cofactor. We provide a molecular basis for the distinct circadian roles of different animal cryptochromes, which also has significant implications for the putative role of Type II cryptochromes in animal photomagnetoreception.

Hydride transfer plays a crucial role in a wide range of biological systems. However, its mode of action (concerted or stepwise) is still under debate. Light‐dependent NADPH: protochlorophyllide oxidoreductase (POR) catalyzes the stereospecific trans addition of a hydride anion and a proton across the C17−C18 double bond of protochlorophyllide. Time‐resolved absorption and emission spectroscopy were used to investigate the hydride transfer mechanism in POR. Apart from excited states of protochlorophyllide, three discrete intermediates were resolved, consistent with a stepwise mechanism that involves an initial electron transfer from NADPH. A subsequent proton‐coupled electron transfer followed by a proton transfer yield distinct different intermediates for wild type and the C226S variant, that is, initial hydride attaches to either C17 or C18, but ends in the same chlorophyllide stereoisomer. This work provides the first evidence of a stepwise hydride transfer in a biological system.

Hydride transfer plays acrucial role in awide range of biological systems.H owever,i ts mode of action (concerted or stepwise) is still under debate.L ight-dependent NADPH: protochlorophyllide oxidoreductase (POR) catalyzest he stereospecific trans addition of ah ydride anion and ap roton across the C 17 À C 18 double bond of protochlorophyllide.T imeresolved absorption and emission spectroscopyw ere used to investigate the hydride transfer mechanism in POR. Apart from excited states of protochlorophyllide,t hree discrete intermediates were resolved, consistent with as tepwise mechanism that involves an initial electron transfer from NADPH. As ubsequent proton-coupled electron transfer followed by aproton transfer yield distinct different intermediates for wild type and the C226S variant, that is,i nitial hydride attaches to either C 17 or C 18 ,b ut ends in the same chlorophyllide stereoisomer.This work provides the first evidence of astepwise hydride transfer in abiological system.

Photoactivation of Drosophila melanogaster cryptochrome results in tryptophan decomposition, conformational changes, and final FAD release.

Background:In plants, a key regulatory step in chlorophyll biosynthesis is catalyzed by two light-dependent isozymes. Results: The two isozymes operate via the same reaction mechanism but differ in their catalytic efficiencies. Conclusion: Different substrate affinities and conformational flexibilities modulate the catalytic reaction. Significance: Detailed understanding of light-driven reactions in nature has a big impact on the development of artificial energy conversion systems.

Dedicated to Professor František Liška on the occasion of his 80th birthday. Flavinium salts are frequently used in organocatalysis but their application in photoredox catalysis has not been systematically investigated to date. We synthesized a series of 5-ethyl-1,3dimethylalloxazinium salts with different substituents in the positions 7 and 8 and investigated their application in lightdependent oxidative cycloelimination of cyclobutanes. Detailed mechanistic investigations with a coumarin dimer as a model substrate reveal that the reaction preferentially occurs via the triplet-born radical pair after electron transfer from the substrate to the triplet state of an alloxazinium salt. The very photostable 7,8-dimethoxy derivative is a superior catalyst with a sufficiently high oxidation power (E* = 2.26 V) allowing the conversion of various cyclobutanes (with E ox up to 2.05 V) in high yields. Even compounds such as all-trans dimethyl 3,4-bis (4-methoxyphenyl)cyclobutane-1,2-dicarboxylate can be converted, whose opening requires a high activation energy due to a missing pre-activation caused by bulky adjacent substituents in cis-position.

The syntheses of two novel, organic, and chiral photocatalysts are presented. By combining donor−acceptor cyanoarene-based photocatalysts with a chiral phosphoric acid, bifunctional catalysts have been designed. In preliminary proof-ofconcept reactions, their use in both enantioselective energy transfer and photoredox catalysis is demonstrated.

The Cover Feature shows the photocatalytic oxidative cycloelimination of a coumarin dimer by a flavinium salt inspired by the light‐induced photolyase DNA repair reaction in nature. Light absorption and subsequent intersystem crossing (isc) enable diffusion‐controlled electron transfer (eT) from the coumarin dimer to the excited triplet flavinium salt. The oxidized coumarin dimer dissociates and back electron transfer (beT) completes the photocatalytic cycle. More information can be found in the Full Paper by R. Cibulka, R. Kutta, and co‐workers.