Interplays of electron and nuclear motions along CO dissociation trajectory in myoglobin revealed by ultrafast X-rays and quantum dynamics calculations

Ultrafast structural dynamics with different spatial and temporal scales were investigated during photodissociation of carbon monoxide (CO) from iron(II)–heme in bovine myoglobin during the first 3 ps following laser excitation. We used simultaneous X-ray transient absorption (XTA) spectroscopy and X-ray transient solution scattering (XSS) at an X-ray free electron laser source with a time resolution of 80 fs. Kinetic traces at different characteristic X-ray energies were collected to give a global picture of the multistep pathway in the photodissociation of CO from heme. In order to extract the reaction coordinates along different directions of the CO departure, XTA data were collected with parallel and perpendicular relative polarizations of the laser pump and X-ray probe pulse to isolate the contributions of electronic spin state transition, bond breaking, and heme macrocycle nuclear relaxation. The time evolution of the iron K-edge X-ray absorption near edge structure (XANES) features along the two major photochemical reaction coordinates, i.e., the iron(II)–CO bond elongation and the heme macrocycle doming relaxation were modeled by time-dependent density functional theory calculations. Combined results from the experiments and computations reveal insight into interplays between the nuclear and electronic structural dynamics along the CO photodissociation trajectory. Time-resolved small-angle X-ray scattering data during the same process are also simultaneously collected, which show that the local CO dissociation causes a protein quake propagating on different spatial and temporal scales. These studies are important for understanding gas transport and protein deligation processes and shed light on the interplay of active site conformational changes and large-scale protein reorganization.

The Photochemistry of Fe2(S2C3H6)(CO)6(µ-CO) and Its Oxidized Form, Two Simple [FeFe]-Hydrogenase CO-Inhibited Models. A DFT and TDDFT Investigation

FeIFeI Fe2(S2C3H6)(CO)6(µ-CO) (1a–CO) and its FeIFeII cationic species (2a+–CO) are the simplest model of the CO-inhibited [FeFe] hydrogenase active site, which is known to undergo CO photolysis within a temperature-dependent process whose products and mechanism are still a matter of debate. Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations, the ground state and low-lying excited-state potential energy surfaces (PESs) of 1a–CO and 2a+–CO have been explored aimed at elucidating the dynamics of the CO photolysis yielding Fe2(S2C3H6)(CO)6 (1a) and [Fe2(S2C3H6)(CO)6]+ (2a+), two simple models of the catalytic site of the enzyme. Two main results came out from these investigations. First, a–CO and 2a+–CO are both bound with respect to any CO dissociation with the lowest free energy barriers around 10 kcal mol−1, suggesting that at least 2a+–CO may be synthesized. Second, focusing on the cationic form, we found at least two clear excited-state channels along the PESs of 2a+–CO that are unbound with respect to equatorial CO dissociation.

The Photochemistry of Fe2(S2C3H6)(CO)6(µ-CO) and its Oxidized Form, Two Simple [FeFe]-Hydrogenase CO-Inhibited Models. A DFT and TDDFT Investigation

FeIFeI Fe2(S2C3H6)(CO)6(µ-CO) (1a-CO) and its FeIFeII cationic species (2a+-CO) are the simplest model of the CO-inhibited [FeFe] hydrogenase active site, which is known to undergo CO photolysis within a temperature- dependent process whose products and mechanism are still a matter of debate. Using Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) computations, the ground state and low-lying excited state potential energy surfaces (PESs) of 1a-CO and 2a+-CO have been explored aimed at elucidating the dynamics of the CO photolysis yielding Fe2(S2C3H6)(CO)6 (1a) and Fe2(S2C3H6)(CO)6+ (2a+), two simple models of the catalytic site of the enzyme. Two main results came out from these investigations. First, a-CO and 2a+-CO are both bound with respect to any CO dissociation with lowest free energy barriers around 10 kcal mol-1, suggesting that at least 2a+-CO might be synthetized. Second, focusing on the cationic form, we found at least two clear excited state channels along the PESs of 2a+-CO that are unbound with respect to equatorial CO dissociation.

Theoretical Study of CO Adsorption and Activation on Orthorhombic Fe7C3(001) Surfaces for Fischer–Tropsch Synthesis Using Density Functional Theory Calculations

Fischer–Tropsch synthesis (FTS), which converts CO and H2 into useful hydrocarbon products, has attracted considerable attention as an efficient method to replace crude oil resources. Fe-based catalysts are mainly used in industrial FTS, and Fe7C3 is a common carbide phase in the FTS reaction. However, the intrinsic catalytic properties of Fe7C3 are theoretically unknown. Therefore, as a first attempt to understand the FTS reaction on Fe7C3, direct CO* dissociation on orthorhombic Fe7C3(001) (o-Fe7C3(001)) surfaces was studied using density functional theory (DFT) calculations. The surface energies of 14 terminations of o-Fe7C3(001) were first compared, and the results showed that (001)0.20 was the most thermodynamically stable termination. Furthermore, to understand the effect of the surface C atom coverage on CO* activation, C–O bond dissociation was performed on the o-Fe7C3(001)0.85, (001)0.13, (001)0.20, (001)0.09, and (001)0.99 surfaces, where the surface C atom coverages were 0.00, 0.17, 0.33, 0.33, and 0.60, respectively. The results showed that the CO* activation linearly decreased as the surface C atom coverage increased. Therefore, it can be concluded that the thermodynamic and kinetic selectivity toward direct CO* dissociation increased when the o-Fe7C3(001) surface had more C* vacancies.