Effect of Stacking Interactions on the Translation of Structurally Related Bis(thiosemicarbazonato)nickel(II) HER Catalysts to Modified Electrode Surfaces

A series of crystalline nickel(II) complexes (1-3) based on inexpensive bis(thiosemicarbazone) ligands diacetylbis(4-methyl-3-thiosemicarbazone) (H₂ATSM), diacetylbis(4,4-dimethyl-3-thiosemicarbazone) (H₂ATSDM), and diacetylbis[4-(2,2,2-trifluoroethyl)-3-thiosemicarbazone] (H₂ATSM-F₆) were synthesized and characterized by single-crystal X-ray diffraction and NMR, UV-visible, and Fourier transform infrared spectroscopies. Modified electrodes GC-1-GC-3 were prepared with films of 1-3 deposited on glassy carbon and evaluated as potential hydrogen evolution reaction (HER) catalysts. HER studies in 0.5 M aqueous H₂SO₄ (10 mA cm^-2) revealed dramatic shifts in the overpotential from 0.740 to 0.450 V after extended cycling for 1 and 2. The charge-transfer resistances for GC-1-GC-3 were determined to be 270, 160, and 630 Ω, respectively. Characterization of the modified surfaces for GC-1 and GC-2 by scanning electron microscopy and Raman spectroscopy revealed similar crystalline coatings before HER that changed to surface-modified crystallites after conditioning. The surface of GC-3 had an initial glasslike appearance before HER that delaminated after HER. The differences in the surface morphology and the effect of conditioning are correlated with crystal-packing effects. Complexes 1 and 2 pack as columns of interacting complexes in the crystallographic a direction with short interplanar spacings between 3.37 and 3.54 Å. Complex 3 packs as columns of isolated molecules in the crystallographic b direction with long-range interplanar spacings of 9.40 Å.

Establishing the oxidative tolerance of Thermomyces lanuginosus xylanase

AIMS

This work aims to determine the tolerance of xylanase towards enzyme-generated oxidative conditions, such as those produced by the peroxidase or laccase mediator systems (LMS).

METHODS AND RESULTS

The activity of Thermomyces lanuginosus xylanase was measured after incubation with lignin peroxidase, manganese peroxidase or laccase plus various mediators. The laccase system, using mediators such as 1-hydroxybenzotriazole and violuric acid, resulted in complete loss of xylanase activity, accompanied by an increase in the solution potential. However, an increase in solution potential alone was not sufficient to inactivate xylanase, nor was loss of xylanase activity always accompanied by a significant increase in solution potential, as observed with N-hydroxyphthalimide as the mediator. Neither lignin peroxidase nor manganese peroxidase impacted xylanase activity; only extended treatment with elevated hydrogen peroxide concentration promoted modest xylanase activity loss. The mechanism of inactivation as determined by the tryptophan-modifying reagent N-bromosuccinimide (NBS) indicated that oxidation of just one of the eight tryptophan residues of T. lanuginosus xylanase would be sufficient to result in complete loss of xylanase activity, since xylanase is completely inactivated at 1 : 1 molar ratio of NBS to xylanase.

CONCLUSIONS

While showing tolerance to peroxidase-based enzyme systems, T. lanuginosus xylanase is readily inactivated in the presence of the LMS. Based upon treatment with NBS as the oxidant, inactivation can be attributed to modification of a single tryptophan residue.

SIGNIFICANCE AND IMPACT OF THE STUDY

The simultaneous application of mixed hydrolytic and oxidative enzyme systems is of importance to biomass processing industries. Understanding the tolerance of xylanase to oxidative conditions will facilitate the design of reaction conditions or enzyme variants to maximize the impact of mixed enzyme systems.