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Through the use of ambient pressure X-ray photoelectron spectroscopy (APXPS) and a single-sided solid oxide electrochemical cell (SOC), we have studied the mechanism of electrocatalytic splitting of water (H 2 O + 2e − → H 2 + O 2− ) and electro-oxidation of hydrogen (H 2 + O 2− → H 2 O + 2e − ) at ∼700°C in 0.5 Torr of H 2 /H 2 O on ceria (CeO 2−x ) electrodes. The experiments reveal a transient buildup of surface intermediates (OH − and Ce 3+ ) and show the separation of charge at the gas−solid interface exclusively in the electrochemically active region of the SOC. During water electrolysis on ceria, the increase in surface potentials of the adsorbed OH − and incorporated O 2− differ by 0.25 eV in the active regions. For hydrogen electro-oxidation on ceria, the surface concentrations of OH − and O 2− shift significantly from their equilibrium values. These data suggest that the same charge transfer step (H 2 O + Ce 3+ ⇔ Ce 4+ + OH − + H • ) is rate limiting in both the forward (water electrolysis) and reverse (H 2 electrooxidation) reactions. This separation of potentials reflects an induced surface dipole layer on the ceria surface and represents the effective electrochemical double layer at a gas−solid interface. The in situ XPS data and DFT calculations show that the chemical origin of the OH − /O 2− potential separation resides in the reduced polarization of the Ce−OH bond due to the increase of Ce 3+ on the electrode surface. These results provide a graphical illustration of the electrochemically driven surface charge transfer processes under relevant and nonultrahigh vacuum conditions. ■ INTRODUCTIONUnderstanding the mechanisms of charge separation and charge transfer at electrochemical interfaces is essential for the rational development of electrochemical devices, such as batteries, fuel cells, electrolyzers, and supercapacitors. 1,2 However, the materials and operating conditions employed in real world applications of these technologies are usually quite different from those used in surface science studies on model systems (i.e., the "pressure and materials gap"). 3−5 This disconnect is particularly problematic with high temperature electrochemical energy conversion devices with multicomponent materials (e.g., solid oxide fuel cells, electrolyzers, and electrocatalytic fuel processors) 6 for which in situ surface experiments at cell operating temperatures (typically >500°C) are challenging. 7 Because of the experimental constraints of most surface science experiments, the knowledge and understanding of the surface processes at relevant conditions are limited and rely on extrapolations from ultrahigh vacuum (UHV) conditions and modeling studies. 8 As a result, the electrochemical surface processes are not well understood. For example, the nonFaradaic electrochemical modification of catalytic activity (NEMCA or EPOC) 9 can significantly enhance the rates of catalytic transformation of over 100 reactions, 3,10 yet the origins of this enhancement are not fully understood. 3 Even the mechanism ...

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bThe entomopathogenic bacteria Xenorhabdus nematophila and Photorhabdus temperata subsp. temperata suppress insect immune responses by inhibiting the catalytic activity of phospholipase A 2 (PLA 2 ), which results in preventing biosynthesis of immune-mediating eicosanoids. This study identified PLA 2 inhibitors derived from culture broths of these two bacteria. Both X. nematophila and P. temperata subsp. temperata culture broths possessed significant PLA 2 -inhibitory activities. Fractionation of these bacterial metabolites in the culture broths using organic solvent and subsequent chromatography purified seven potent PLA 2 inhibitors, three of which (benzylideneacetone [BZA], proline-tyrosine [PY], and acetylated phenylalanine-glycine-valine [FGV]) were reported in a previous study. Four other compounds (indole, oxindole, cis-cyclo-PY, and p-hydroxyphenyl propionic acid) were identified and shown to significantly inhibit PLA 2 . X. nematophila culture broth contained these seven compounds, while P. temperata subsp. temperata culture broth contained three compounds (BZA, acetylated FGV, and cis-cyclo-PY). BZA was detected in the largest amount among these PLA 2 compounds in both bacterial culture broths. All seven bacterial metabolites also showed significant inhibitory activities against immune responses, such as phenoloxidase activity and hemocytic nodulation; BZA was the most potent. Finally, this study characterized these seven compounds for their insecticidal activities against the diamondback moth, Plutella xylostella. Even though these compounds showed relatively low toxicities to larvae, they significantly enhanced the pathogenicity of Bacillus thuringiensis. This study reports bacterial-origin PLA 2 inhibitors, which would be applicable for developing novel insecticides. Insect immune responses consist of innate cellular and humoral components (2). Upon pathogen infection, pattern recognition receptors recognize nonself and the recognition signal is transferred by immune mediators to effector tissues, such as hemocytes and fat body (7). Eicosanoids form a chemical group of oxygenated polyunsaturated C 20 fatty acids, which are derived from arachidonic acid, a catalytic product of phospholipase A 2 (PLA 2 ) on phospholipid (PL) substrate (3). Two types of eicosanoids, prostaglandins and leukotrienes, are known to mediate the nonself recognition signals against various insect pathogens, including bacteria, fungi, viruses, and parasitoid eggs in insects (38). Inhibition of PLA 2 prevents eicosanoid biosynthesis and suppresses immune responses in response to microbial pathogens (23, 37).Xenorhabdus nematophila and Photorhabdus temperata subsp. temperata are insect pathogens that are symbiotic to the entomopathogenic nematodes Steinernema carpocapsae and Heterorhabditis megidis, respectively (1, 13, 25). Infective juvenile-stage nematodes enter target insect hemocoel through natural openings (such as mouth, anus, and spiracles) and then release their symbiotic bacteria from the nematode intestine. The releas...

X-ray ptychography and fluorescence imaging reveal 3D elemental composition and ultrastructure in frozen-hydrated green algae.

We analyzed mitochondrial (mt) DNA restriction fragment length polymorphisms (RFLPs) associated with cytochrome oxidase, subunit I (coxI)-related gene sequences in 268 trees derived from 19 natural populations of three species of pines from California (USA): Monterey pine (Pinus radiata D. Don), bishop pine (P. Muricata D. Don), and knobcone pine (P. attenuata Lemm.). Total genomic DNA was digested with four restriction endonucleases and probed with a 750-bp fragment of the mitochondrialcoxI gene amplified fromP. attenuata via the polymerase chain reaction (PCR). ThecoxI gene is repeated at least 4 times in some populations, and all variants that we observed resulted from complex rearrangements rather than from point mutations. There was limited intrapopulation variation, but strong differentiation among populations. When applied to haplotype frequencies, Nei's gene diversity within populations (Hs) averaged 7% (±3), and Gst varied from 75% forP. Radiata to 96% forP. muricata. The high degree of population differentiation for mtDNA suggests that it can be a powerful marker of population differences, but its rapid rate of structural evolution appears to result from recombination among a limited number of repetitive elements-giving frequent homoplasious fragment phenotypes. The phylogenetic trees disagreed with results from chloroplast DNA, nuclear gene, and morphological studies.

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