Protonation of a histidine copper ligand in fern plastocyanin

Plastocyanin is a small blue copper protein that shuttles electrons as part of the photosynthetic redox chain. Its redox behavior is changed at low pH as a result of protonation of the solvent-exposed copper-coordinating histidine. Protonation and subsequent redox inactivation could have a role in the down regulation of photosynthesis. As opposed to plastocyanin from other sources, in fern plastocyanin His90 protonation at low pH has been reported not to occur. Two possible reasons for that have been proposed: pi-pi stacking between Phe12 and His90 and lack of a hydrogen bond with the backbone oxygen of Gly36. We have produced this fern plastocyanin recombinantly and examined the properties of wild-type protein and mutants Phe12Leu, Gly36Pro, and the double mutant with NMR spectroscopy, X-ray crystallography, and cyclic voltammetry. The results demonstrate that, contrary to earlier reports, protonation of His90 in the wild-type protein does occur in solution with a pKa of 4.4 (+/-0.1). Neither the single mutants nor the double mutant exhibit a change in protonation behavior, indicating that the suggested interactions have no influence. The crystal structure at low pH of the Gly36Pro variant does not show His90 protonation, similar to what was found for the wild-type protein. The structure suggests that movement of the imidazole ring is hindered by crystal contacts. This study illustrates a significant difference between results obtained in solution by NMR and by crystallography.     

α-L-Arabinofuranosylated pyrrolidines as arabinanase inhibitors

As part of continued efforts to understand the mechanisms of 1,5-α-l-arabinanases better, some arabinan-like iminosugar oligosaccharides were synthesized. An iminosugar analogue of arabinobiose was found to be a good inhibitor of the arabinanase Arb93A from Fusarium graminearum. Structures were determined for complexes of this inhibitor with wild-type Arb93A and a catalytically inactive mutant.     

Photosensitization by iodinated DNA minor groove binding ligands: Evaluation of DNA double-strand break induction and repair

Iodinated DNA minor groove binding bibenzimidazoles represent a unique class of UVA photosensitizer and their extreme photopotency has been previously characterized. Earlier studies have included a comparison of three isomers, referred to as ortho-, meta- and para-iodoHoechst, which differ only in the location of the iodine substituent in the phenyl ring of the bibenzimidazole. DNA breakage and clonogenic survival studies in human erythroleukemic K562 cells have highlighted the higher photo-efficiency of the ortho-isomer (subsequently designated UV(A)Sens) compared to the meta- and para-isomers. In this study, the aim was to compare the induction and repair of DNA double-strand breaks induced by the three isomers in K562 cells. Further, we examined the effects of the prototypical broad-spectrum histone deacetylase inhibitor, Trichostatin A, on ortho-iodoHoechst/UVA-induced double-strand breaks in K562 cells. Using γH2AX as a molecular marker of the DNA lesions, our findings indicate a disparity in the induction and particularly, in the repair kinetics of double-strand breaks for the three isomers. The accumulation of γH2AX foci induced by the meta- and para-isomers returned to background levels within 24 and 48 h, respectively; the number of γH2AX foci induced by ortho-iodoHoechst remained elevated even after incubation for 96 h post-irradiation. These findings provide further evidence that the extreme photopotency of ortho-iodoHoechst is due to not only to the high quantum yield of dehalogenation, but also to the severity of the DNA lesions which are not readily repaired. Finally, our findings which indicate that Trichostatin A has a remarkable potentiating effect on ortho-iodoHoechst/UVA-induced DNA lesions are encouraging, particularly in the context of cutaneous T-cell lymphoma, for which a histone deacetylase inhibitor is already approved for therapy. This finding prompts further evaluation of the potential of combination therapies.     

Stability effects on CO2 adsorption for the DOBDC series of metal-organic frameworks

Metal-organic frameworks with unsaturated metal centers in their crystal structures, such as Ni/DOBDC and Mg/DOBDC, are promising adsorbents for carbon dioxide capture from flue gas due to their high CO(2) capacities at subatmospheric pressures. However, stability is a critical issue for their application. In this paper, the stabilities of Ni/DOBDC and Mg/DOBDC are investigated. Effects of steam conditioning, simulated flue gas conditioning, and long-term storage on CO(2) adsorption capacities are considered. Results show that Ni/DOBDC can maintain its CO(2) capacity after steam conditioning and long-term storage, whereas Mg/DOBDC does not. Nitrogen isotherms for Mg/DOBDC show a drop in surface area after steaming, corresponding to the decrease in CO(2) adsorption, which may be caused by a reduction of unsaturated metal centers in its structure. Conditioning with dry simulated flue gas at room temperature only slightly affects CO(2) adsorption in Ni/DOBDC. However, introducing water vapor into the simulated flue gas further reduces the CO(2) capacity of Ni/DOBDC.