Shape complementarity, binding-site dynamics, and transition state stabilization: a theoretical study of Diels-Alder catalysis by antibody 1E9

Antibody 1E9 is a protein catalyst for the Diels-Alder reaction between tetrachlorothiophene dioxide and N-ethylmaleimide. Quantum mechanical calculations have been employed to study the 1E9-catalyzed Diels-Alder reaction in the gas phase. The transition states and intermediates were all determined at the B3LYP/6-31G*//HF/6-31G* level. The cycloaddition step is predicted to be rate-determining, and the endo reaction pathway is strongly favored. Binding of the reactants and the transition states to antibody 1E9 was investigated by docking and molecular dynamics simulations. The linear interaction energy (LIE) method was adopted to estimate the free energy barrier of the 1E9-catalyzed Diels-Alder reaction. The catalytic efficiency of antibody 1E9 is achieved by enthalpic stabilization of the transition state, near-perfect shape complementarity of the hydrophobic binding site for the transition state, and a strategically placed hydrogen-bonding interaction.     

Identification of metal cations, metal complexes, and anions by electrospray mass spectrometry in the negative ion mode

Pneumatically assisted electrospray mass spectrometry (ES-MS) is used in the negative ion mode for aqueous metal (M) solutions in an excess of hydrochloric or nitric acid, where the major anion X = Cl- or NO3-. A collision energy of approximately 20 eV removes anion-solvent clusters for most elements and leaves negative complex ions of the general form (Mn+Xn+1)-. Complexation with anions prevents charge reduction reactions at least to n = 3, even in cases where the third ionization energy of M greatly exceeds the first ionization energy of the solvent. These negative ions thus preserve the charge state of the metal cation from the solution and allow identification of both cations and anions in a single set of electrospray conditions. Cations such as Fe3+ or Cu2+ that have a lower oxidation state in solution produce a distribution of negative ions, each with a single negative charge overall; e.g., an Fe3+ solution produces both Fe(III)X4- and Fe(III)X3-. This distribution of FeIII and FeII species is attributed to electrochemical reduction of Fe3+ at the negatively charged ES needle. "Native" anions such as perrhenate or molybdate produce singly charged analogues such as ReO4- or HMoO4-. Metal-EDTA complexes are seen as M(III)Y- or M(II)HY-. The sensitivity for these "native" anions is suppressed by competition with the excess chloride or nitrate used to produce the metal-containing complex ions.     

Oxidation of 5-hydroxymethylfurfural over supported Pt, Pd and Au catalysts

Supported Pt, Pd, and Au catalysts were evaluated in the aqueous-phase oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) at 295 K and high pH in a semibatch reactor. The intermediate reaction product 5-hydroxymethyl-2-furancarboxylic acid (HFCA) was formed in high yield over Au/C and Au/TiO₂ at 690 kPa O₂, 0.15 M HMF and 0.3 M NaOH, but did not continue to react substantially to FDCA at the specified O₂ pressure and base concentration. In contrast, the final reaction product FDCA was formed over Pt/C and Pd/C under identical conditions. The initial turnover frequency of HMF conversion was an order of magnitude greater on Au catalysts compared to either Pt or Pd. Increasing the O₂ pressure and NaOH concentration facilitated the conversion of HFCA to FDCA over the supported Au. The significant influence of base concentration on the product distribution indicates an important role of OH⁻ in the activation, oxidation and degradation of HMF.     

High-resolution mass spectrometry with a multiple pass quadrupole mass analyzer

The peak shape narrows and the resolution improves if the ions are simply reflected back and forth through a conventional quadrupole mass analyzer. CO(+) and N(2)(+) at m/z = 28 are separated to 50% valley with half of the original signal remaining. These two ions can be resolved to baseline (m/Δm) = 5000 with 1% of the original signal remaining.     

Combinatorial screening of heterogeneous catalysis in selective oxidation of naphthalene by laser-induced fluorescence imaging

Heterogeneous catalysis is one of the most important processes in the petroleum and the chemical industries. To be able to screen catalysts at high throughput will dramatically improve performance and reduce costs. Here we used laser-induced fluorescence imaging as a high-throughput screening technique in the combinatorial discovery of active catalysts for naphthalene oxidation. Binary catalysts of V-Mo-O, V-Sn-O, V-Ti-O, and V-W-O in various 15-member libraries were screened. Laser ablation ICPMS was employed to confirm the composition of the individual catalysts in the combinatorial library. The addition of MoO3, WO3, SnO2, and TiO2 to V2O5 did not improve the catalytic activity in the conversion of naphthalene to naphthoquinone, but the overall activity was found to increase for certain binary samples. The screening of ternary catalysts of V-Sn-Mo-O revealed that the combination of V (45%)-Sn (45%)-Mo (10%) gave 70% higher catalytic activity than pure V2O5 in converting naphthalene to naphthoquinone. Reaction temperature and sample preparation effects on the activity and selectivity of catalysts are also studied in a combinatorial manner.