A native structure of the cytochrome b(6)f complex with improved resolution was obtained from crystals of the complex grown in the presence of divalent cadmium. Two Cd(2+) binding sites with different occupancy were determined: (i) a higher affinity site, Cd1, which bridges His143 of cytochrome f and the acidic residue, Glu75, of cyt b(6); in addition, Cd1 is coordinated by 1-2 H(2)O or 1-2 Cl(-); (ii) a second site, Cd2, of lower affinity for which three identified ligands are Asp58 (subunit IV), Glu3 (PetG subunit) and Glu4 (PetM subunit). Binding sites of quinone analogue inhibitors were sought to map the pathway of transfer of the lipophilic quinone across the b(6)f complex and to define the function of the novel heme c(n). Two sites were found for the chromone ring of the tridecyl-stigmatellin (TDS) quinone analogue inhibitor, one near the p-side [2Fe-2S] cluster. A second TDS site was found on the n-side of the complex facing the quinone exchange cavity as an axial ligand of heme c(n). A similar binding site proximal to heme c(n) was found for the n-side inhibitor, NQNO. Binding of these inhibitors required their addition to the complex before lipid used to facilitate crystallization. The similar binding of NQNO and TDS as axial ligands to heme c(n) implies that this heme utilizes plastoquinone as a natural ligand, thus defining an electron transfer complex consisting of hemes b(n), c(n), and PQ, and the pathway of n-side reduction of the PQ pool. The NQNO binding site explains several effects associated with its inhibitory action: the negative shift in heme c(n) midpoint potential, the increased amplitude of light-induced heme b(n) reduction, and an altered EPR spectrum attributed to interaction between hemes c(n) and b(n). A decreased extent of heme c(n) reduction by reduced ferredoxin in the presence of NQNO allows observation of the heme c(n) Soret band in a chemical difference spectrum.

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The structure and function of the cytochrome b6 f complex is considered in the context of recent crystal structures of the complex as an eight subunit, 220 kDa symmetric dimeric complex obtained from the thermophilic cyanobacterium, Mastigocladus laminosus, and the green alga, Chlamydomonas reinhardtii. A major problem confronted in crystallization of the cyanobacterial complex, proteolysis of three of the subunits, is discussed along with initial efforts to identify the protease. The evolution of these cytochrome complexes is illustrated by conservation of the hydrophobic heme‐binding transmembrane domain of the cyt b polypeptide between b6 f and bc1 complexes, and the rubredoxin‐like membrane proximal domain of the Rieske [2Fe‐2S] protein. Pathways of coupled electron and proton transfer are discussed in the framework of a modified Q cycle, in which the heme cn, not found in the bc1 complex, but electronically tightly coupled to the heme bn of the b6 f complex, is included. Crystal structures of the cyanobacterial complex with the quinone analogue inhibitors, NQNO or tridecyl‐stigmatellin, show the latter to be ligands of heme cn, implicating heme cn as an n‐side plastoquinone reductase. Existing questions include (a) the details of the shuttle of: (i) the [2Fe‐2S] protein between the membrane‐bound PQH2 electron/H+ donor and the cytochrome f acceptor to complete the p‐side electron transfer circuit; (ii) PQ/PQH2 between n‐ and p‐sides of the complex across the intermonomer quinone exchange cavity, through the narrow portal connecting the cavity with the p‐side [2Fe‐2S] niche; (b) the role of the n‐side of the b6 f complex and heme cn in regulation of the relative rates of noncyclic and cyclic electron transfer. The likely presence of cyclic electron transport in the b6 f complex, and of heme cn in the firmicute bc complex suggests the concept that hemes bn‐cn define a branch point in bc complexes that can support electron transport pathways that differ in detail from the Q cycle supported by the bc1 complex.

Among machining chatter control methods, spindle speed variation (SSV) method is certified as a feasible and effective way: machining chatter can be suppressed by continuously and periodically fluctuating the spindle speed of the machine tool. Although with its substantial value, the SSV method has not yet gained enough acceptance in the industrial machining applications. One of the reasons is the inadequacy of theoretical and practical study on the mechanism of the SSV method for machining chatter suppression. In this paper the mechanism of the SSV method for chatter suppression is revealed by means of internal energy analysis. The theoretical analysis is based on a nonlinear delay differential equation (NLDDE) as the model of regenerative machining chatter. This approach can provide more practical and feasible analytical results than small perturbative approaches. The analytical results demonstrate the machining stability increment of SSV cutting in the mathematical and physical sense, and provide practical suggestions for selection of SSV cutting parameters. Numerical simulations and experimental SSV cutting testing outcomes have coincidence with the analytical results.

Pulsar Wind Nebula (PWN) DA 495 (G65.7+1.2) was detected in TeV gamma-rays by the High Altitude Water Cherenkov Observatory (HAWC) in 2017 (2HWC J1953+294). Follow-up observations by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) confirmed the association between 2HWC J1953+294 and DA 495 and found the TeV emission to be spatially coincident with the radio emission first reported in 1968. The detection of TeV gamma-rays from DA 495, along with past X-ray detection up to 10 keV, prompted high energy X-ray observations as part of the NuSTAR Galactic Legacy Survey. We present the results of these NuSTAR observations, combined with archival Chandra and XMM-Newton observations, and confirm the previous X-ray photon index of Γ 2−20 keV = 2.0 ± 0.1. We find no spectral cutoff up to 20 keV. With the spectral information for DA 495 extended to TeV gamma-rays, we were able to perform analytical modeling to test leptonic and hadronic emission scenarios. The leptonic models can explain the broadband emission, but also imply a diffuse X-ray nebula of similar extent to the radio and TeV nebulae, which cannot be confirmed by our observations. The hadronic models can simultaneously explain the spectrum and the spatial extent in all wavelengths; however, we need a very high magnetic field strength pervading the radio and TeV nebulae and a surprisingly high particle kinetic energy. These requirements deepen the mystery of the physical nature of DA 495. Future observations in radio to infrared bands and spatially resolved γ-rays can further constrain the physical conditions and radiation mechanisms in DA 495.