Proton relay in cyclic 7‐hydroxyquinoline–(alcohol)2 complexes (see picture) has been explored in nonpolar solvents. The asymmetric triple proton transfer has an unusually large, temperature‐independent, and viscosity‐dependent kinetic isotope effect. Heavy‐atom motions (wavy arrow) of solvent and bridging molecules allow proton tunneling by assisting the complex to reach the optimal precursor configuration.

We investigated the ultrafast structural transitions of the heme induced by nitric oxide (NO) binding for several heme proteins by subpicosecond time-resolved resonance Raman and femtosecond transient absorption spectroscopy. We probed the heme iron motion by the evolution of the iron-histidine Raman band intensity after NO photolysis. Unexpectedly, we found that the heme response and iron motion do not follow the kinetics of NO rebinding. Whereas NO dissociation induces quasi-instantaneous iron motion and heme doming (<0.6 ps), the reverse process results in a much slower picosecond movement of the iron toward the planar heme configuration after NO binding. The time constant for this primary domed-to-planar heme transition varies among proteins (∼30 ps for myoglobin and its H64V mutant, ∼15 ps for hemoglobin, ∼7 ps for dehaloperoxidase, and ∼6 ps for cytochrome c) and depends upon constraints exerted by the protein structure on the heme cofactor. This observed phenomenon constitutes the primary structural transition in heme proteins induced by NO binding.time-resolved Raman spectroscopy | allostery | structural dynamics T he role of diatomics endogenously generated in cells as messengers in various signaling pathways has been demonstrated for nitric oxide (NO) (1, 2) and carbon monoxide (CO) (3, 4) and various gas-sensors heme proteins have been discovered in the last two decades (5). In these signaling pathways, the binding of diatomics to their respective heme sensor domain triggers a response of the associated enzymatic domain by modulating the catalytic or the gene regulating activity (5). Recently, myoglobin (Mb) was proposed to be involved in the regulation of NO level (6), and neuroglobin was proposed as a sensor regulating the NO∕O 2 balance (7). Similarly to Hb (8-10), the signaling mechanisms are based at the molecular level on an allosteric structural change of the receptor/sensor protein coupled to a change of activity or affinity and are induced by diatomic ligation to the heme iron cofactor. The signal of diatomic binding/release can be transmitted to the overall protein structure either by in-plane/out-of-plane motion of the iron through the bound proximal histidine or by the cleavage of the iron-histidine (Fe-His) bond. In particular, for Hb and its monomeric counterpart Mb, the out-of-plane iron motion occurs in less than 1 ps after gaseous ligand release (11,12) and represents the first event of the allosteric R → T transition (9). This primary heme iron motion is followed by overall tertiary changes, notably motion of α-helices (13). Upon gaseous ligand binding, the reverse T → R allosteric transition is coupled to the iron motion toward the heme plane, and this primary motion is also considered quasi-instantaneous since the stereochemical description of Hb allostery by Perutz (9, 10) although up to now it has never been observed.The ultrafast events in the interaction of heme proteins with various gaseous ligands were studied extensively at molecular level in the last two decades by femtosec...

In vivo, F-actin flows are observed at different cell life stages and participate in various developmental processes during asymmetric divisions in vertebrate oocytes, cell migration, or wound healing. Here, we show that confinement has a dramatic effect on F-actin spatiotemporal organization. We reconstitute in vitro the spontaneous generation of F-actin flow using Xenopus meiotic extracts artificially confined within a geometry mimicking the cell boundary. Perturbations of actin polymerization kinetics or F-actin nucleation sites strongly modify the network flow dynamics. A combination of quantitative image analysis and biochemical perturbations shows that both spatial localization of F-actin nucleators and actin turnover play a decisive role in generating flow. Interestingly, our in vitro assay recapitulates several symmetry-breaking processes observed in oocytes and early embryonic cells.

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