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In nature, an oxo‐bridged Mn4CaO5 cluster embedded in photosystem II (PSII), a membrane‐bound multi‐subunit pigment protein complex, catalyzes the water oxidation reaction that is driven by light‐i ...

We report the synthesis, characterization, and reactivity of [Lfe_3(PhPz)_3OMn(^sPhIO)][OTf]_x (3: x=2; 4: x=3), where 4 is one of very few examples of iodosobenzene–metal adducts characterized by X-ray crystallography. Access to these rare heterometallic clusters enabled differentiation of the metal centers involved in oxygen atom transfer (Mn) or redox modulation (Fe). Specifically, ^(57)Fe Mössbauer and X-ray absorption spectroscopy provided unique insights into how changes in oxidation state (Fe^(III)_2Fe^(II)Mn^(II) vs. Fe^(III)_3Mn^(II)) influence oxygen atom transfer in tetranuclear Fe_3Mn clusters. In particular, a one-electron redox change at a distal metal site leads to a change in oxygen atom transfer reactivity by ca. two orders of magnitude.

[Mn4O4{O2P(OtBu)2}6] (1), an Mn4O4 cubane complex combining the structural inspiration of the photosystem II oxygen-evolving complex with thermolytic precursor ligands, was synthesized and fully characterized. Core oxygen atoms within complex 1 are transferred upon reaction with an oxygen-atom acceptor (PEt3), to give the butterfly complex [Mn4O2{O2P(OtBu)2}6(OPEt3)2]. The cubane structure is restored by reaction of the latter complex with the O-atom donor PhIO. Complex 1 was investigated as a precursor to inorganic Mn metaphosphate/pyrophosphate materials, which were studied by X-ray absorption spectroscopy to determine the fate of the Mn4O4 unit. Under the conditions employed, thermolyses of 1 result in reduction of the manganese to Mn(II) species. Finally, the related butterfly complex [Mn4O2{O2P(pin)}6(bpy)2] (pin = pinacolate) is described.

ZnSn_(1-x)Ge_xN_2 direct bandgap semiconductor alloys, with a crystal structure and electronic structure similar to InGaN, are earth-abundant alternatives for efficient, high-quality optoelectronic devices and solar energy conversion. The bandgap is tunable almost monotonically from 2 eV (ZnSnN_2) to 3.1 eV (ZnGeN_2) by control of the Sn/Ge ratio.

The design and synthesis of catalysts, especially for the production of solar fuels, is a major challenge in developing sources of renewable energy. Catalyst development requires an understanding of the mechanism(s) involved and the nature of the active site. While platinum group metals have unrivalled activity for both hydrogen and oxygen evolution, they are scarce and expensive. Photocatalytic systems relying on earth-abundant materials are therefore desirable for large scale energy production. Herein, we examine the structure and electronic properties of an amorphous molybdenum sulfide species and its possible use for photocatalytic hydrogen evolution. The catalyst was grown on a seeded quantum-rod sensitizer, a model system for investigating the photophysics of solar fuel generation. This catalyst s activity is shown experimentally to be associated with under-coordinated molybdenum centers, and we document that a reduced form of MoS3 is an active species for hydrogen generation. Molybdenum sulfides are prevalent in both biological enzymes and industrial catalysts. Mo metalloenzymes are involved in carbon, nitrogen, and sulfur metabolism, while synthetic molybdenum sulfides serve as industrial hydrotreating catalysts and are proven electrocatalysts for the hydrogen evolution reaction (HER). MoS2, [13,14] incomplete cubane [Mo3S4] 4+ clusters, molecular molybdenum catalysts, and amorphous MoS2 made by a reduction of MoS3 [19] have been shown to be active HER catalysts. Highly active HER catalysts, including Pt, have a Gibbs free energy of H adsorption (DGH) close to zero. [14] Density functional theory calculations show that the equatorial sulfur atoms in Fe–Mo cofactors in nitrogenase enzymes as well as the bridging S atom on the edge sites of MoS2 bind H atoms with DGH 0. These calculations, coupled with scanning tunneling microscopy (STM) studies, have indicated that molybdenum sulfide based hydrodesulfurization and HER catalysts derive their activities from under-coordinated atoms. Recent investigations of MoS2 nanoparticles using STM combined with electrochemical measurements have revealed that HER activity scales with the number of edge sites, rather than nanoparticle area, adding substantial evidence that undercoordination is critical to activity. There is also substantial current interest in molecularly thick and structurally disordered metal oxide and sulfide layers supported on electrodes, surfaces, and nanoparticles as potential catalysts for the HER and oxygen evolution reaction (OER). Such ultrathin films can support a variety of unusual and possibly favorable bonding geometries and may retain flexibility in healing and recovering. Despite their potential, such systems remain very difficult to characterize, impeding reproducibility and the communication of results between groups. Mechanisms are difficult to pin down when structural and electronic characterization is lacking. In this work, we use X-ray absorption techniques to obtain structural information on a catalytically active disordered molybdenum chalcogenide species that was grown on a wellcontrolled seeded quantum rod photosensitizer system with very high surface area. The high surface area of the colloidal system enables us to employ a variety of X-ray characterization techniques. Yet the system is also well-defined: Amorphous layers of MoS3 are deposited on quantumcontrolled photosensitizers. We take advantage of recent work showing that cadmium chalcogenide nanocrystals can be engineered to systematically control the separation of photogenerated holes and electrons, thus allowing us to modulate the photochemical yield of hydrogen. Nanorods of CdS grown on CdSe seeds with varying diameters and pure CdS nanorods of differing length were synthesized by a seeded-growth method previously reported. These particles have been of interest as a model system for investigating photochemical HER because their [*] Dr. M. L. Tang, D. C. Grauer, Dr. L. Amirav, Prof. J. R. Long, Prof. A. P. Alivisatos Department of Chemistry, University of California, Berkeley Material Sciences, Lawrence Berkeley National Laboratory (LBNL) Berkeley, CA 94720 (USA) E-mail: alivis@berkeley.edu jyano@lbl.gov

Innate and adaptive immunity are considered critical to protection against mucosal candidal infections. Among innate anti-Candida mechanisms, oral and vaginal epithelial cells have antifungal activity. The mechanism is fungistatic, acid-labile and includes a requirement for cell contact by intact, but not necessarily live, epithelial cells. The purpose of this study was to use the acid-labile property to further characterize the effector moiety. Surface material extracted from phosphate-buffered saline (PBS) -treated, but not acid-treated, epithelial cells significantly inhibited the growth of Candida blastoconidia in a dose-dependent manner which was abrogated by prior heat and protease treatment. Proteins extracted from PBS-treated cells bound blastoconidia and hyphae more intensely than those from acid-treated cells. Proteins from PBS-treated cells eluted from Candida revealed two unique bands of approximately 33 and 45 kDa compared with acid-treated cells. Mass spectrometry identified these proteins as Annexin-A1 and actin, respectively. Oral epithelial cells stained positive for Annexin-A1, but not actin. Western blots showed reduced Annexin-A1 in proteins from acid-treated epithelial cells compared with those from PBS-treated epithelial cells. Lastly, it was demonstrated that immunoprecipitation of Annexin-A1 from proteins extracted from PBS-treated oral epithelial cells resulted in abrogation of inhibitory activity. Taken together, these results indicate that Annexin-A1 is a strong candidate for the epithelial cell anti-Candida effector protein.

Ligands play critical roles during the catalytic reactions in inorganic systems and in metalloproteins through bond formation/breaking, protonation/deprotonation, and electron/spin delocalization. There are well-defined element-specific spectroscopic handles, such as X-ray spectroscopy and EPR, to follow the chemistry of metal catalytic sites. However, directly probing particular ligand atoms like C, N, and O, especially in a large protein matrix, is challenging due to their abundance in the protein. FTIR/Raman and ligand-sensitive EPR techniques such as ENDOR and ESEEM have been applied to study metal-ligand interactions. X-ray absorption spectroscopy (XAS) can also probe the ligand environment; its element-specificity allows us to focus only on the catalytic metal site, and EXAFS and XANES provide metal-ligand distances, coordination numbers, and symmetry of ligand environments. However, the information is limited, because one cannot distinguish among ligand elements with similar atomic number (i.e. C, N. and O). As an alternative and a more direct method to probe the specific metal-ligand chemistry in the protein matrix, we investigated the application of X-ray emission spectroscopy (XES). Using this technique we have identified the oxo-bridging ligands of the Mn4Ca complex of photosystem II (PS II), a multisubunit membrane protein, that catalyzes the water oxidizing reaction in photosynthesis.[1] The catalytic mechanism has been studied intensively by Mn XAS.[2] The fundamental challenge, however, is to learn how the water molecules are ligated to the Mn4Ca cluster and how O-O bond formation occurs before the evolution of O2.[3-5] This implies that it is necessary to follow the chemistry of the oxygen ligands to understand the mechanism. XES, which is a complementary method to XAS, has the potential to directly probe ligation modes.[6] Among the several emission lines, Kβ1,3 and Kβ′ lines originate from the metal 3p to 1s transition, and they have been used as an indicator of the charge and spin states on Mn in the oxygen-evolving complex (OEC) (Figure 1).[7, 8] The higher energy region corresponds to valence-to-core transitions just below the Fermi level and can be divided into the Kβ″ and the Kβ2,5 emission (Fig.1 left scheme). Kβ2,5 emission is predominantly from ligand 2p (metal 4p) to metal 1s, and the Kβ″ emission is assigned to a ligand 2s to metal 1s; both are referred to as cross-over transitions.[9-11] Therefore, only direct ligands to the metal of interest are probed with Kβ,2,5/Kβ″ emission; i.e. other C, N, and O atoms in the protein media do not contribute to the spectra. In this report, we focus on the Kβ″ spectral region to characterize metal-ligand interactions, in particular contributions from ligated oxygens. The energy of the Kβ″ transition depends on the difference between the metal 1s and ligand 2s binding energies, which reflects the environment of the ligand owing to orbital hybridization. Therefore the Kβ″ energy is affected by the charge density on the metal, the ligand protonation state, and changes in the coordination environment. The Kβ″ intensity is influenced by the spatial overlap between the wavefunction that describes the Mn 1s orbital and the molecular orbitals on the ligands. The Kβ″ intensity is affected by the metal-to-ligand distance, and the number of ligands per metal ion. Shorter distances (e.g. from higher bond order or deprotonation) result in increased Kβ″ intensity with an approximate exponential dependence on distance.[9] A spread of the molecular wavefunction over next-nearest neighbor atoms will decrease the Kβ″ spectral intensity. Therefore single-atom ligands such as oxo-bridges, or terminal oxo ligands bonded to Mn have predominant contributions to the spectra (see below). This combination of factors makes the Kβ″ spectrum a powerful tool for detection and characterization of oxo-bridges in the Mn4Ca cluster of PS II. Figure 1 (A) Energy diagram of Mn Kβ transitions in MnO. The Kβ″ and Kβ2,5 transitions are from valence molecular orbitals; Kβ″ is the O 2s to Mn 1s ‘cross-over’ transition. (B) Logarithmic plot of ... However, because of the weak intensity of the Kβ″ spectrum, obtaining such spectra from biological samples as dilute as PS II (800μM Mn) have been difficult. For O ligation in a typical model compound the signal is ~103 times weaker than that of Kα, and there is an additional large background from both the Kβ1,3 and the Kβ2,5 spectral features (Fig. 1). Furthermore the work is challenging because of the high sensitivity of the Mn4Ca cluster to radiation damage.[12] This study of PS II became possible by using a new high resolution spectrometer equipped with 8-14 analyzer crystals collecting over a large solid angle (see experimental section). Fig. 2 shows the Kβ″ spectrum of a sample of PS II in the S1 state compared with a series of Mn oxide spectra. Each spectrum is normalized by the Kβ1,3 peak intensity which is proportional to the number of Mn atoms in the system. The peak position of the PS II S1 state falls between those of the MnIII and MnIV oxides. The energy of the Kβ″ feature may be influenced by charge screening effects that depend on the charge density on the Mn ion, i.e. the Mn oxidation state. However, the formal oxidation state, is only an approximation to the actual charge density.[13] Figure 2 Mn Kβ″ emission spectra from Mn oxides and the PS II S1 state. These spectra referred to as cross-over peaks are assigned to emission from ligand 2s2p levels to the Mn 1s core level. Fig. 3 shows a comparison of the PS II S1 state spectrum with those of a series of Mn coordination compounds: MnV-oxo (a), di-μ-oxo bridged Mn2III,IV and Mn2IV (b, c), cubane-type Mn2III Mn2IV and MnIIIMn3IV (d, e), and a μ-alkoxide bridged Mn2II (f). These compounds have oxo-bridged Mn (except for MnV-oxo) with O or N/O terminal ligands. In Fig. 3, there is no detectable Kβ″ peaks from the μ-alkoxide/carboxylate bridged Mn2II complex (f). This is likely due to peak broadening originating from the delocalization of the oxygen 2s electron into a molecular orbital that is spread over the whole methyl/carboxylate group, i.e. atoms that are next-nearest to Mn, and to a lesser degree due to the longer Mn-ligand bridging interactions (~2.0-2.1 A).[14, 15] Similarly, contributions from O and N terminal ligands from carboxylates or histidine/amines are weak, because the molecular orbitals that contain O and N 2s are strongly delocalized. Moreover, terminal ligands are generally at longer distances to Mn than the bridging ligands, and their contributions are smaller.[9] A similar effect has been observed in Fe cyanides.[6] Theoretical studies regarding X-ray emission spectra from transition metal complexes are emerging,[14, 15] yet at present the interpretation of the Kβ″ spectra has remained largely empirical. There is a strong peak in the MnV-oxo compound, indicating that the Kβ″ peak intensity is predominantly sensitive to single atom ligands and short metal ligand atom distances, namely, bridging O ligands and double/triple bonds which all have localized 2s orbitals. The sensitivity of the spectra to even one-electron changes is illustrated in Fig. 3. The spectra are different for the two binuclear and the two cubane molecules in which one of two or four Mn is oxidized from (III) to (IV). The Kβ″ peak of the PS II S1 state is relatively intense compared to other Mn2III,IV di-μ-oxo bridged compounds, which is suggestive that there are several μ-oxo bridged Mn-O bonds in the S1 state.[2] Figure 3 The Kβ″ emission spectra from a series of multinuclear Mn complexes with oxo-bridging groups, a MnV-oxo complex and PS II in the S1 state. The cross-over peak from the O ligand is prominent when short bridging Mn-O distances are present. ... The involvement of bridging oxo groups[4] or the high valent MnIV=O• or MnV≡O species[3, 5] have all been implicated in the mechanism for the formation of the critical O-O bond in the water-oxidation reaction of PS II. As we have demonstrated here, it is now feasible to obtain such spectra for the Mn4Ca cluster in PS II, and the outlook for Kβ″ spectroscopy as a tool for studying the nature of the O ligand binding modes and therefore the mechanism of the water-splitting reaction seems promising. Finally, new powerful X-ray lasers such as the LINAC Coherent Light Source, Stanford, can be used for future real-time XES studies of the S-state cycle. In contrast to XAS, XES does not require scanning of the incident X-ray beam, and with a dispersive XES spectrometer, a full spectrum can be collected at once, and the time-evolution of the spectrum can be monitored.

Background: Candida albicans is the causative agent of oral and vaginal candidiasis. Innate host defenses against C. albicans are important against each infection. Among these are oral and vaginal epithelial cells that have anti-Candida activity. The mechanism of action includes a requirement for cell contact with no role for soluble factors, and a putative role for carbohydrates based on the sensitivity of the activity to periodic acid. Methods: Periodic acid treatment of epithelial cells as well as the property of partial resistance of antifungal activity to fixation was used to further dissect the mechanism of action. Results: The results herein effectively now challenge a role for carbohydrates alone. Firstly, the putative carbohydrate(s) released into supernatants of periodic acid-treated epithelial cells could not compete with fresh epithelial cells for activity, and equivalent abrogation of activity was observed by periodic acid-treated cells irrespective of the amount of carbohydrate released. Instead, the similar abrogation of activity following treatment with other acids or when cocultured under acidic conditions suggests that the activity is acid-labile. Finally, while activity requires intact epithelial cells, it does not require live cells; activity was minimally affected by fixing epithelial cells prior to coculture where the majority of cells remained impermeable to Trypan blue but were defined as non–viable by positive nuclear staining with propidium iodide. Conclusion: These results suggest that antifungal activity is dependent on contact by intact, but not necessarily live, epithelial cells through an acid-labile mechanism.

Polymorphic transformation behavior of sub-α1, sub-α2, α, and γ in 1,3-distearoyl-sn-2-linoleoyl-glycerol (SLS) has been studied with X-ray diffraction, differential scanning caloremetry, and Fourier-transform infrared spectroscopy. Synchrotron radiation X-ray beam was employed to observe rapid transformation processes from the sub-α and α forms to the γ form. The chain length structures were double in sub-α1, sub-α2, and α, whereas γ was of triple chain-length structure. The subcell packing was pseudohexagonal for the two sub-α forms, hexagonal for the α form, and parallel type for the γ form. In comparison with 1,3-distearoyl-sn-2-oleoyl-glycerol (SOS), the occurrence behavior of sub-α, α, and γ of SLS was the same as that of SOS. However, the absence of β′ and β was unique for SLS. The chain-chain interactions between the linoleoyl moieties may stabilize the γ form, prohibiting the transformation into β′ and β forms. The presence of two cis double bonds may cause this stabilization, revealing the disordered chain conformation of the unsaturated chains.

Polymorphic transformations of sn-1,3-distearoyl-2-ricinoleyl-glycerol (SRS) have been studied with differential scanning calorimetry, X-ray powder diffraction (XRD), synchrotron radiation X-ray diffraction, and Fourier transform infrared spectroscopy (FTIR) techniques by using a 99.8% pure sample. Four polymorphs, α, γ, β′2, and β′1, were isolated. The thermal behavior of the four forms showed that the fusion of α at 25.8°C was followed by the crystallization of γ which melts at 40.6°C, and β′2 and β′1 revealed melting peaks at 44.3 and 48.0°C, respectively. No β form was observed, even when the two β′ forms were annealed around their melting points over one week. The XRD long spacing indicates that α packs into a double chain-length structure; however, γ and the two β′ phases pack into a triple chain-length structure. The polarized and nonpolarized FTIR spectra in methylene scissoring and methylene rocking regions indicated a parallel subcell packing in γ, and a mixture of orthorhombic perpendicular and parallel or hexagonal subcells in the β′2 and β′1 phases. Consequently, SRS exhibits quite a unique polymorphic behavior, compared to tristearoyl glycerol and sn-1,3-distearoyl-2-oleoyl-glycerol.

Thermodynamic and polymorphic behavior of POP (sn-1,3-dipalmitoyl-2-oleoylglycerol) and OPO (sn-1,3-dioleoyl-2-palmitoylglycerol) binary mixtures was examined using differential scanning calorimetry and conventional and synchrotron radiation X-ray diffraction. A molecular compound, βC, was formed at the 1:1 (w/w) concentration ratio of POP and OPO, giving rise to two monotectic phases of POP/compound and compound/OPO in juxtaposition. βC has a long-spacing value of 4.2 nm with a double chainlength structure and the melting point of 31.9°C. A structural model of the POP-OPO compound is proposed, involving the separation of palmitoyl and oleoyl chain leaflets in the double chainlength structure. In the polymorphic occurrence of the POP-OPO mixtures, the POP fraction transformed from α to β′ with no passage through γ, then transformed to β. The presence of OPO in POP promoted the β′-β transformation of POP during the melt-mediated crystallization.

Molecular properties of polymorphic forms of gondoic acid [cis-C20:1Δ11ω9 (GOA)] have been studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), optical microscopy, and Raman scattering, in comparison to those of six principal unsaturated fatty acids: oleic acid [cis-C18:1Δ9ω9 (OA)], erucic acid [cis-C22:1Δ13ω9 (ERA)], petroselinic acid [cis-C18:1Δ6ω12 (PSA)], asclepic acid [cis-C18:1Δ11ω7 (APA)], palmitoleic acid [cis-C16:1Δ9ω7 (POA)], and elaidic acid [trans-C18:1Δ9ω9 (ELA)]. In addition, phase behavior of binary mixtures of GOA and APA and OA was examined by XRD and DSC. The polymorphic structures of GOA are quite similar to those of APA, ERA, POA, and partly to OA. In particular, DSC and Raman scattering studies have shown that gondoic acid exhibits conformational disordering on heating at the ω-chain, a chain segment between the double bond and CH3 group, as a transition from all-trans (γ form) to gauche-rich (α form) conformations. A miscible mixing phase was observed in the mixture of GOA and APA, yet eutectic phases were observed in the GOA and OA mixtures. This is a remarkable contrast because the binary mixture systems of varying combinations of cis-unsaturated fatty acids examined so far exhibited either eutectic nature or molecular compound formation. It is expected that specific molecular interactions between GOA and APA that originate from the equivalence of the length of the Δ-chain, the chain segment between the cis-double bond and COOH group, and also from the presence of the γ-α order-disorder transformation would be operating to form the miscible mixing phase.

The phase behavior of thesn-1,3-dipalmitoyl-2-oleoylglycerol (PPP-POP) binary mixture system was studied by powder X-ray diffraction with synchrotron radiation and by differential scanning calorimetry. The results showed that the immiscible phases were observed in metastable and in the most stable forms. In particular, synchrotron X-ray diffraction enabled us to reveal the monotectic nature of α as a kinetic phase behavior. The equilibrium phase diagram of the PPP-POP mixture is divided into two regions. In POP concentration ratios below 40%, solid-state transformation from α to β was observed, indicating that the α-β transition of PPP was promoted in the presence of POP. By contrast, the polymorphic transition proceeds from α to β through the occurrence of the intermediate β′ form at POP concentration ratios above 50%.