Jun Yi

CO2-reforming of methane on transition metal surfaces

Abstract The mechanisms of CO 2 -reforming of methane on Cu(111), Ni(111), Pd(111), Pt(111), Rh(111), Ru(001), Ir(111) and Fe(110) have been investigated by the the unity bond index-quadratic exponential potential (UBI-QEP) method. This method was named as the bond order conservation Morse potential (BOC-MP) approach before, but it has been generalized and renamed now. The heats of chemisorption ( Q ) for all involved adspecies, activation barriers (Δ E ) and enthalpy changes (Δ H ) for forward and reverse reactions were evaluated. The calculations indicated that both the dissociation of CH 4 and the dissociation of CO 2 are rate-determining steps and that they are promoted by each other. A small amount of OH radical may account for the lower activity for the CO 2 -reforming of methane. The activity sequence of catalysts is Fe>Ni>Rh>Ru>Ir>Pd>Pt>Cu. The most appropriate catalyst for CO 2 -reforming is Ru. The most suitable non-noble catalyst is Ni.

The nitrite anion binds to human hemoglobin via the uncommon O-nitrito mode.

The nitrite anion is known to oxidize and degrade hemoglobin (Hb). Recent literature reports suggest a nitrite reductase activity for Hb, converting nitrite into nitric oxide. Surprisingly, no structural information about Hb-nitrite interactions has been reported. We have determined the crystal structure of the ferric Hb-nitrite complex at 1.80 A resolution. The nitrite ligand adopts the uncommon O-nitrito binding mode. In addition, the nitrito conformations in the alpha and beta subunits are different, reflecting subtle effects of the distal His in orienting the nitrite ligand in the O-nitrito binding mode.

The distal pocket histidine residue in horse heart myoglobin directs the o-binding mode of nitrite to the heme iron.

It is now well-established that mammalian heme proteins are reactive with various nitrogen oxide species and that these reactions may play significant roles in mammalian physiology. For example, the ferrous heme protein myoglobin (Mb) has been shown to reduce nitrite (NO(2)(-)) to nitric oxide (NO) under hypoxic conditions. We demonstrate here that the distal pocket histidine residue (His64) of horse heart metMb(III) (i.e., ferric Mb(III)) has marked effects on the mode of nitrite ion coordination to the iron center. X-ray crystal structures were determined for the mutant proteins metMb(III) H64V (2.0 A resolution) and its nitrite ion adduct metMb(III) H64V-nitrite (1.95 A resolution), and metMb(III) H64V/V67R (1.9 A resolution) and its nitrite ion adduct metMb(III) H64V/V67R-nitrite (2.0 A resolution). These are compared to the known structures of wild-type (wt) hh metMb(III) and its nitrite ion adduct hh metMb(III)-nitrite, which binds NO(2)(-) via an O-atom in a trans-FeONO configuration. Unlike wt metMb(III), no axial H(2)O is evident in either of the metMb(III) mutant structures. In the ferric H64V-nitrite structure, replacement of the distal His residue with Val alters the binding mode of nitrite from the nitrito (O-binding) form in the wild-type protein to a weakly bound nitro (N-binding) form. Reintroducing a H-bonding residue in the H64V/V67R double mutant restores the O-binding mode of nitrite. We have also examined the effects of these mutations on reactivities of the metMb(III)s with cysteine as a reducing agent and of the (ferrous) Mb(II)s with nitrite ion under anaerobic conditions. The Mb(II)s were generated by reduction of the Mb(III) precursors in a second-order reaction with cysteine, the rate constants for this step following the order H64V/V67R > H64V >> wt. The rate constants for the oxidation of the Mb(II)s by nitrite (giving NO as the other product) follow the order wt > H64V/V67R >> H64V and suggest a significant role of the distal pocket H-bonding residue in nitrite reduction.

Linkage isomerization in heme-NOx compounds: understanding NO, nitrite, and hyponitrite interactions with iron porphyrins.

Nitric oxide (NO) and its derivatives such as nitrite and hyponitrite are biologically important species of relevance to human health. Much of their physiological relevance stems from their interactions with the iron centers in heme proteins. The chemical reactivities displayed by the heme-NOx species (NOx = NO, nitrite, hyponitrite) are a function of the binding modes of the NOx ligands. Hence, an understanding of the types of binding modes extant in heme-NOx compounds is important if we are to unravel the inherent chemical properties of these NOx metabolites. In this Forum Article, the experimentally characterized linkage isomers of heme-NOx models and proteins are presented and reviewed. Nitrosyl linkage isomers of synthetic iron and ruthenium porphyrins have been generated by photolysis at low temperatures and characterized by spectroscopy and density functional theory calculations. Nitrite linkage isomers in synthetic metalloporphyrin derivatives have been generated from photolysis experiments and in low-temperature matrices. In the case of nitrite adducts of heme proteins, both N and O binding have been determined crystallographically, and the role of the distal H-bonding residue in myoglobin in directing the O-binding mode of nitrite has been explored using mutagenesis. To date, only one synthetic metalloporphyrin complex containing a hyponitrite ligand (displaying an O-binding mode) has been characterized by crystallography. This is contrasted with other hyponitrite binding modes experimentally determined for coordination compounds and computationally for NO reductase enzymes. Although linkage isomerism in heme-NOx derivatives is still in its infancy, opportunities now exist for a detailed exploration of the existence and stabilities of the metastable states in both heme models and heme proteins.

Synchrotron X-ray-Induced Photoreduction of Ferric Myoglobin Nitrite Crystals Gives the Ferrous Derivative with Retention of the O-Bonded Nitrite Ligand.

Exposure of a single crystal of the nitrite adduct of ferric myoglobin (Mb) at 100 K to high-intensity synchrotron X-ray radiation resulted in changes in the UV-vis spectrum that can be attributed to reduction of the ferric compound to the ferrous derivative. We employed correlated single-crystal spectroscopy with crystallography to further characterize this photoproduct. The 1.55 A resolution crystal structure of the photoproduct reveals retention of the O-binding mode for binding of nitrite to the iron center. The data are consistent with cryogenic generation and trapping, at 100 K, of a ferrous d(6) Mb(II)(ONO)* complex by photoreduction of the ferric precursor crystals using high-intensity X-ray radiation.

Nitrite reduction by CoII and MnII substituted myoglobins: Towards understanding necessary components of Mb nitrite reductase activity

Nitrite reduction to nitric oxide by heme proteins is drawing increasing attention as a protective mechanism to hypoxic injury in mammalian physiology. Here we probe the nitrite reductase (NiR) activities of manganese(II)- and cobalt(II)-substituted myoglobins, and compare with data obtained previously for the iron(II) analog wt Mb(II). Both Mn(II)Mb and Co(II)Mb displayed NiR activity, and it was shown that the kinetics are first order each in [protein], [nitrite], and [H(+)], as previously determined for the Fe(II) analog wt Mb(II). The second order rate constants (k(2)) at pH 7.4 and T=25 °C, were 0.0066 and 0.015 M(-1)s(-1) for Co(II)Mb and Mn(II)Mb, respectively, both orders of magnitude slower than the k(2) (6M(-1)s(-1)) for wt Mb(II). The final reaction products for Mn(II)Mb consisted of a mixture of the nitrosyl Mn(II)Mb(NO) and Mn(III)Mb, similar to the products from the analogous NiR reaction by wt Mb. In contrast, the products of NiR by Co(II)Mb were found to be the nitrito complex Co(III)Mb(ONO(-)) plus roughly an equivalent of free NO. The differences can be attributed in part to the stronger coordination of inorganic nitrite to Co(III)Mb as reflected in the respective M(III)Mb(ONO(-)) formation constants K(nitrite): 2100 M(-1) (Co(III)) and <~0.4M(-1) (Mn(III)). We also report the formation constants (3.7 and 30 M(-1), respectively) for the nitrite complexes of the mutant metmyoglobins H64V Mb(III)(NO(2)(-)) and H64V/V67R Mb(III)(ONO(-)) and a K(nitrite) revised value (120 M(-1)) for the nitrite complex of wt metMb. The respective K(nitrite) values for the three ferric proteins emphasize the importance of a H-bonding residue, such as His64 in the Mb(III) distal pocket or the Arg67 in H64V/V67R Mb(III), in stabilizing nitrite coordination. Notably, the NiR activities of the corresponding ferrous Mbs follow a similar sequence suggesting that nitrite binding to these centers are analogously affected by the H-bonding residues.

Influence of trivalent metal ions on the surface structure of a copper-based catalyst for methanol synthesis

Abstract The method of doping trivalent metal ions into a copper-based catalyst for methanol synthesis is effective in modifying the surface structure of the catalyst. The promotion effect and its relation to catalytic activity for hydrogenation of CO to methanol after doping with trivalent metal ions such as Al 3+ , Sc 3+ , and Cr 3+ into Cu–ZnO have been investigated by XRD, ESR, XPS, TPR, and the evaluation of catalytic activity. The results show that doping trivalent metal ions into ZnO assists in the formation of monovalent cationic defects on the surface of ZnO. These monovalent cationic defects both enrich and stabilize monovalent copper on the surface of copper-based catalysts for methanol synthesis during reduction and reaction. They increase catalytic activity for methanol synthesis and extend the life of catalysts.

Plasmonic photoluminescence for recovering native chemical information from surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) spectroscopy has attracted tremendous interests as a highly sensitive label-free tool. The local field produced by the excitation of localized surface plasmon resonances (LSPRs) dominates the overall enhancement of SERS. Such an electromagnetic enhancement is unfortunately accompanied by a strong modification in the relative intensity of the original Raman spectra, which highly distorts spectral features providing chemical information. Here we propose a robust method to retrieve the fingerprint of intrinsic chemical information from the SERS spectra. The method is established based on the finding that the SERS background originates from the LSPR-modulated photoluminescence, which contains the local field information shared also by SERS. We validate this concept of retrieval of intrinsic fingerprint information in well controlled single metallic nanoantennas of varying aspect ratios. We further demonstrate its unambiguity and generality in more complicated systems of tip-enhanced Raman spectroscopy (TERS) and SERS of silver nanoaggregates.

Crystallographic Trapping of Heme Loss Intermediates during the Nitrite-Induced Degradation of Human Hemoglobin.

Heme is an important cofactor in a large number of essential proteins and is often involved in small molecule binding and activation. Loss of heme from proteins thus negatively affects the function of these proteins but is also an important component of iron recycling. The characterization of intermediates that form during the loss of heme from proteins has been problematic, in a large part, because of the instability of such intermediates. We have characterized, by X-ray crystallography, three compounds that form during the nitrite-induced degradation of human α(2)β(2) hemoglobin (Hb). The first is an unprecedented complex that exhibits a large β heme displacement of 4.8 Å toward the protein exterior; the heme displacement is stabilized by the binding of the distal His residue to the heme Fe, which in turn allows for the unusual binding of an exogenous ligand on the proximal face of the heme. We have also structurally characterized complexes that display regiospecific nitration of the heme at the 2-vinyl position; we show that heme nitration is not a prerequisite for heme loss. Our results provide structural insight into a possible pathway for nitrite-induced loss of heme from human Hb.

Energetics of chemisorption and conversion of methane on transition metal surfaces

Abstract The chemisorption and conversion of methane on Pt(111), Rh(111), Ru(0001), Ir(111), Cu(111) and Ni(111) were investigated using the unity bond index-quadratic exponential (UBI-QEP) method. Following conclusions were found from the analyses. (1) The main dissociate species of methane on the metal surfaces is CH 3 . The dissociation of CH x species on Ru(0001) is the easiest but the dissociation on Cu(111) is very difficult. (2) Coupling of CH 3 may produce ethane, and then dehydrogenation of ethane may produce ethylene. The coupling of CH 3 is the easiest on Pt(111) and Cu(111), but it is very difficult on Ru(0001). It indicated that copper was favorable to the C 2 selectivity. Non-oxidative reactions of methane coupling may produce ethane but it is difficult to get ethylene. (3) There are two competitive reaction pathways for part oxidation of methane to CO, directly heat-cracking pathway on Ni(111) and burning–reforming pathway on other metal surfaces. The selectivity of CO can be increased at elevated temperatures. (4) Carbon-deposit is the cause of the dissociation of CH. It can easily take place on Ni(111) and Ru(0001), but it is weak on other metal surfaces because that CH can fast produce CO without passing the surface carbon.