The Rayleigh—Schrödinger BK method applied to the lower electronic states of pyrrole

A second-order Rayleigh—Schrödinger B_K method (RS B_K) is described and applied to computing the lower electronic states of pyrrole. This method is more nearly size consistent than the previously used Brillouin—Wigner B_K (BW B_K) method. Pyrrole RS B_K excitation energies compare closely to BW B_K, second-order perturbation theory, and extrapolated CI energies. To provide insight into the origin of the bands in the experimental absorption spectrum, absorption coefficients and second moments are also reported.     

Simultaneous saccharification and fermentation of pretreated wheat straw to ethanol with selected yeast strains and β-glucosidase supplementation

Previous shake flask and stirred tank evaluations of temperature tolerant (37–43°C) yeasts in simultaneous saccharification and fermentation (SSF) on Sigmacell-50 cellulose substrates to ethanol have identified several good microorganisms for further SSF studies (27). Of these, the glucose fermenting yeastCandida acidothermophilum, C. brassicae, Saccharomyces cerevisiae, S. uvarum, and a mixed culture of the cellobiose fermenting yeastBrettanomyces clausenii withS. cerevisiae as a control were chosen for shake flask SSF screening experiments with pretreated wheat straw. This study indicates that theSaccharomyces strainscerevisiae anduvarum, give very good performance at high cellulase loadings or when supplemented with Novo-188 β-glucosidase. In fact, with the higher enzyme loadings these yeast will give complete conversion of cellulose to ethanol. Yet at the lower, more economical enzyme loadings, the mixed culture ofBrettanomyces clausenii andS. cerevisiae performs better than any single yeast.

     

Methyl propionate radical cation

Examination of the reactions of the long-lived (>0.5-s) radical cations of CD₃CH₂COOCH₃ and CH₃CH₂COOCD₃ indicates that the long-lived, nondecomposing methyl propionate radical cation CH₃CH₂C(O)OCH ₃ ^+· isomerizes to its enol form CH₃CH=C(OH)OCH ₃ ^+· (ΔH _{isomerization} ? ?32 kcal/mol) via two different pathways in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. A 1,4-shift of a β-hydrogen of the acid moiety to the carbonyl oxygen yields the distonic ion ^·CH₂CH₂C⁺ (OH)OCH₃ that then rearranges to CH₃CH=C(OH)OCH ₃ ^+· probably by consecutive 1,5- and 1,4-hydrogen shifts. This process is in competition with a 1,4-hydrogen transfer from the alcohol moiety to form another distonic ion, CH₃CH₂C⁺(OH)OCH ₂ ^· , that can undergo a 1,4-hydrogen shift to form CH₃CH=C(OH)OCH ₃ ^+· . Ab initio molecular orbital calculations carried out at the UMP2/6-31G** + ZPVE level of theory show that the two distonic ions lie more than 16 kcal/mol lower in energy than CH₃CH₂C(O)OCH ₃ ^+· . Hence, the first step of both rearrangement processes has a great driving force. The 1,4-hydrogen shift that involves the acid moiety is 3 kcal/mol more exothermic (ΔH _{isomerization}=?16 kcal/mol) and is associated with a 4-kcal/mol lower barrier (10 kcal/mol) than the shift that involves the alcohol moiety. Indeed, experimental findings suggest that the hydrogen shift from the acid moiety is likely to be the favored channel.     

EVIDENCE FOR A MAJOR STRONG BINDING SITE FOR TETRACHLOROSALICYLANILIDE ON HUMAN SERUM ALBUMIN

Abstract— Solutions of human serum albumin(HSA) monomer were irradiated with UV light(360 nm) in the presence of [₁₄C]-3,3.4'S-tetrachlorosalicylanilide([₁₄C]-T₄CS).The [₁₄C]-T₄ CS-labeiled HSA was cleaved by cyanogen bromide and separated into two fractions. These fractions were reduced carboxymethylated and separated into their seven characteristic peptides and monitored for radioactivity. Tetrachlorosalicylanilide was found to bind mainly to one region of the sequence of HSA and this covalent binding site was located in residues 124 (Cys) to 298 (Met) of the molecule. The binding of 3,5-dichlorosalicylamido-4-(2,2,6.6-tetramethylpiperidine-l-oxyl (DCS-TEMPO),a spin-label analogue of T₄CS, to HSA was studied by electron spin resonance spectroscopy. In the absence of UV light. DCS-TEMPO bound non-covalently (k = 6.1 times 10₆M₁) to one major binding site on HSA. These results are evidence for the existence of a major strong binding site for the photochemical binding of T₄CS to HSA.     

Redox chemistry and H-atom abstraction reactivity of a terminal zirconium(iv) oxo compound mediated by an appended cobalt(i) center

The reactivity of the terminal zirconium(iv) oxo complex, O Zr(MesNPⁱPr₂)₃CoCN^tBu (2), is explored, revealing unique redox activity imparted by the pendent redox active cobalt(i) center. Oxo complex 2 can be chemically reduced using Na/Hg or Ph₃C^• to afford the Zr^IV/Co⁰ complexes [(μ-Na)OZr(MesNPⁱPr₂)₃CoCN^tBu]₂ (3) and Ph₃COZr(MesNPⁱPr₂)₃CoCN^tBu (4), respectively. Based on the cyclic voltammogram of 2, Ph₃˙ should not be sufficiently reducing to achieve the chemical reduction of 2, but sufficient driving force for the reaction is provided by the nucleophilicity of the terminal oxo fragment and its affinity to bind Ph₃C⁺. Accordingly, 2 reacts readily with [Ph₃C][BPh₄] and Ph₃CCl to afford [Ph₃COZr(MesNPⁱPr₂)₃CoCN^tBu][BPh₄] ([5][BPh4]) and Ph₃COZr(MesNPⁱPr₂)₃CoCl (6), respectively. The chemical oxidation of 2 is also investigated, revealing that oxidation of 2 is accompanied by immediate hydrogen atom abstraction to afford the hydroxide complex [HOZr(MesNPⁱPr₂)₃CoCN^tBu]⁺ ([9]+). Thus it is posited that the transient [OZr(MesNPⁱPr₂)₃CoCN^tBu]⁺ [2]⁺ cation generated upon oxidation combines the basicity of a nucleophilic early metal oxo fragment with the oxidizing power of the appended cobalt center to facilitate H-atom abstraction.

Bimetallic cooperativity is demonstrated with a Co/Zr complex featuring both nucleophilic Zr(iv) oxo and redox active Co sites.     

Theoretical studies of manganese and iron superoxide dismutases: superoxide binding and superoxide oxidation

Density-functional calculations indicate that the second sphere of coordination around the metal centers of manganese and iron superoxide dismutases (MnSODs and FeSODs) plays an important role in the binding of O2(-). In these systems, O2(-) prefers to bind to Mn or Fe in end-on configurations. For human and E. coli MnSODs, the bound O2(-) forms hydrogen bonds with the tyrosine and glutamine amino acid residues in the second sphere of coordination. In the cases of E. coli and T. elongates FeSODs, hydrogen bonding occurs between the bound O2(-) and the tyrosine amino acid only because the glutamine is too far away for an effective bonding interaction. The manner in which the O2(-) binds to the metal center in MnSODs and FeSODs can affect the rate of subsequent protonation and determine the mechanism for the formation of H2O2. Both Mn- and Fe-containing superoxide dismutases contain a metal-bound solvent molecule that has been suggested to be involved in the uptake of a H+ upon reduction of the metal center [Bull, C.; Fee, J. A. J. Am. Chem. Soc. 1985, 107, 3295; Miller, A.-F.; Padmakumar, K.; Sorkin, D. L.; Karapetian, A.; Vance, C. K. J. Inorg. Biochem. 2003, 93, 71]. Using density-functional theory, we confirm this suggestion and show the involvement of the second sphere of coordination in the process. We show that the oxidation of superoxide by Mn- or Fe-containing superoxide dismutases is facilitated by a cooperative effect between superoxide binding, protonation of the OH- bound to the metal, and electron transfer from the superoxide molecule to the oxidized metal. In particular, proton transfer through tyrosine-34 on the absence of a bound superoxide is uphill while, once superoxide is bound, the energetic barrier is lowered. It is this barrier that likely keeps the resting state (Mn(III)SOD) of the enzyme with a bound hydroxide, instead of a water. This work provides a model for the mechanism of reaction of superoxide with the oxidized form of the metal within Mn- and FeSODs.     

Preparation of a series of benzothieno[3,2‐b]pyridine‐3‐carbonitriles and benzofuro[3,2‐b]pyridine‐3‐carbonitriles

New and shorter routes to the benzothieno[3,2‐b]pyridine‐3‐carbonitrile and benzofuro[3,2‐b]pyridine‐3‐carbonitrile ring systems are reported. These heterocycles may function as new templates for kinase inhibitors.