Kinetics of Guest-Induced Structural Transitions in Metal-Organic-Framework MIL-53(Al)-NH2 Probed by High-Pressure Nuclear Magnetic Resonance.

A nuclear magnetic resonance (NMR) study of a pore opening in amino-functionalized metal-organic framework (MOF) MIL-53(Al) in response to methane pressure variation is presented. Variations of both NMR signal intensities and transversal relaxation rates for methane are found to reveal hysteretic structural transitions in the MOF material, which are smeared out over broad pressure ranges. Experiments with pressure reversals upon an incomplete adsorption/desorption gave deeper insight into the microscopic transition mechanisms. These experiments have unequivocally proven that the non-stepwise pore opening/closing transitions observed in the experiments are governed by a distribution of the opening/closing pressures over different MOF crystallites, for example, due to a distribution of the crystal sizes or shapes. The slow kinetics of the structural transitions measured in the hysteresis regime revealed a complex free energy landscape for the phase transition process.

15N photo-CIDNP MAS NMR on both photosystems and magnetic field-dependent 13C photo-CIDNP MAS NMR in photosystem II of the diatom Phaeodactylum tricornutum.

Diatoms contribute about 20-25% to the global marine productivity and are successful autotrophic players in all aquatic ecosystems, which raises the question whether this performance is caused by differences in their photosynthetic apparatus. Photo-CIDNP MAS NMR presents a unique tool to obtain insights into the reaction centres of photosystems (PS), by selective enhancement of NMR signals from both, the electron donor and the primary electron acceptor molecules. Here, we present the first observation of the solid-state photo-CIDNP effect in the pennate diatoms. In comparison to plant PSs, similar spectral patterns have been observed for PS I at 9.4 T and PS II at 4.7 T in the PSs of Phaeodactylum tricornutum. Studies at different magnetic fields reveal a surprising sign change of the 13C photo-CIDNP MAS NMR signals indicating an alternative arrangement of cofactors which allows to quench the Chl a donor triplet state in contrast to the situation in plant PS II. This unusual quenching mechanism is related to a carotenoid molecule in close vicinity to the Chl a donor. In addition to the photo-CIDNP MAS NMR signals arising from the donor and the primary electron acceptor cofactors, a complete set of signals of the imidazole ring ligating to the magnesium of Chl a can be observed.

15N photo-CIDNP MAS NMR analysis of reaction centers of Chloracidobacterium thermophilum.

Photochemically induced dynamic nuclear polarization (photo-CIDNP) has been observed in the homodimeric, type-1 photochemical reaction centers (RCs) of the acidobacterium, Chloracidobacterium (Cab.) thermophilum, by 15N magic-angle spinning (MAS) solid-state NMR under continuous white-light illumination. Three light-induced emissive (negative) signals are detected. In the RCs of Cab. thermophilum, three types of (bacterio)chlorophylls have previously been identified: bacteriochlorophyll a (BChl a), chlorophyll a (Chl a), and Zn-bacteriochlorophyll a' (Zn-BChl a') (Tsukatani et al. in J Biol Chem 287:5720-5732, 2012). Based upon experimental and quantum chemical 15N NMR data, we assign the observed signals to a Chl a cofactor. We exclude Zn-BChl because of its measured spectroscopic properties. We conclude that Chl a is the primary electron acceptor, which implies that the primary donor is most likely Zn-BChl a'. Chl a and 81-OH Chl a have been shown to be the primary electron acceptors in green sulfur bacteria and heliobacteria, respectively, and thus a Chl a molecule serves this role in all known homodimeric type-1 RCs.

In Situ and in Operando Characterization of Mixing Dynamics in Liquid-Phase Reactions by 129 Xe NMR Spectroscopy.

129 Xe NMR spectroscopy is applied under in situ and in operando conditions to study the mixing process in a multicomponent liquid mixture with partially miscible components. The process of mixing of an oil-methanol mixture was triggered by an industrially relevant catalytic transesterification reaction to form fatty acid methyl esters and glycerol. Up to date, kinetic limitations in liquid-phase reactions originating from the poor miscibility of the reacting species have been addressed solely under ex situ conditions, typically by chromatography. In the approach presented here, xenon gas, solvated in the reacting species, acts as a sensor, providing information on the progress of mixing and on the composition during the course of the catalytic reaction. We believe that this study offers a new tool to the set of established techniques for addressing mixing and/or separation processes in liquids, including but not limited to the ones resulting from catalytic reactions.