A Caged,Destabilized, Free Radical Intermediate in the Q‐Cycle

The Rieske/cytochrome b complexes, also known as cytochrome bc complexes, catalyze a unique oxidant‐induced reduction reaction at their quinol oxidase (Q_o) sites, in which substrate hydroquinone reduces two distinct electron transfer chains, one through a series of high‐potential electron carriers, the second through low‐potential cytochrome b. This reaction is a critical step in energy storage by the Q‐cycle. The semiquinone intermediate in this reaction can reduce O₂ to produce deleterious superoxide. It is yet unknown how the enzyme controls this reaction, though numerous models have been proposed. In previous work, we trapped a Q‐cycle semiquinone anion intermediate, termed SQ_o, in bacterial cytochrome bc₁ by rapid freeze‐quenching. In this work, we apply pulsed‐EPR techniques to determine the location and properties of SQ_o in the mitochondrial complex. In contrast to semiquinone intermediates in other enzymes, SQ_o is not thermodynamically stabilized, and can even be destabilized with respect to solution. It is trapped in Q_o at a site that is distinct from previously described inhibitor‐binding sites, yet sufficiently close to cytochrome b_L to allow rapid electron transfer. The binding site and EPR analyses show that SQ_o is not stabilized by hydrogen bonds to proteins. The formation of SQ_o involves “stripping” of both substrate ‐OH protons during the initial oxidation step, as well as conformational changes of the semiquinone and Q_o proteins. The resulting charged radical is kinetically trapped, rather than thermodynamically stabilized (as in most enzymatic semiquinone species), conserving redox energy to drive electron transfer to cytochrome b_L while minimizing certain Q‐cycle bypass reactions, including oxidation of prereduced cytochrome b and reduction of O₂.     

Photocatalytic Activity of Pure and Zinc Doped Tin Oxide Nanoparticles Synthesized by One Step Direct Injection Flame Synthesis

A very simple and rapid Direct Injection Flame Synthesis (DIFS) method is effectively used to synthesize pure tin oxide (SnO₂) and zinc doped tin oxide (Zn:SnO₂) nanoparticles from the metallic tin (Sn) and zinc (Zn) powders for the photocatalytic degradation of methylene blue (MB) dye. The DIFS nanoparticles were characterized using XRD, Raman, UV–Vis, FESEM, PL and EDX studies. The X-ray diffraction analysis indicated that the synthesized SnO₂ and Zn:SnO₂ nanoparticles have pure tetragonal phases and their average crystallite size decreases when Zn was doped with SnO₂. Raman study confirmed the various mode of vibrations and the crystal structure of the synthesized nanoparticles. Purity, atomic percentage and chemical composition were analysed using Energy dispersive X-ray analysis and found to be free from impurities. The band gap energy increases from 3.5 to 3.6 eV upon doping which was revealed from the UV–Visible spectroscopic analysis. Photoluminescence analysis confirms the red shifted emission for Zn:SnO₂ due to the oxygen deficiency. The CIE chromaticity (x,y) for SnO₂ and Zn:SnO₂ was calculated from the emission spectra and the co-ordinates represents blue and violet region respectively. Field Emission Scanning Electron Microscopy analysis showed that the pure SnO₂ nanoparticles have irregular, agglomerated, nanoflowered and nanoclustered formation whereas Zn:SnO₂ nanoparticles has more crystalline, cubical and nanoflake structure. The photocatalytic activity was enhanced due to the presence of Zn in SnO₂ under UV light irradiation. The efficiency of MB degradation by SnO₂ was found to be 82% and enhanced to 88% upon doping. Thus the Zn doped SnO₂ nanoparticles synthesized by DIFS was found to be an effective photocatalyst than the pure SnO₂.