Mechanism of n-Butane Hydrogenolysis Promoted by Ta-Hydrides Supported on Silica

The mechanism of hydrogenolysis of alkanes, promoted by Ta-hydrides supported on silica via 2 Si-O-bonds, has been studied with a density functional theory (DFT) approach. Our study suggests that the initial monohydride ( Si-O-)(2)(TaH)-H-(III) is rapidly trapped by molecular hydrogen to form the more stable tris-hydride ( Si-O-)(2)(TaH3)-H-(v). Loading of n-butane to the Ta-center occurs through C-H. activation concerted with elimination of molecular hydrogen (sigma-bond metathesis). Once the Ta-alkyl species is formed, the C C activation. step corresponds to a beta-alkyl transfer transfer to the metal with elimination of an olefin. According to these calculations, an alpha-alkyl transfer to the metal to form a Ta-carbene species is of higher energy. The olefins formed during the C-C activation step can be rapidly hydrogenated by both mono- and tris-Ta-hydride species, making the overall process of alkane cracking thermodynamically favored

Mechanism of <i>n</i>‑Butane Hydrogenolysis Promoted by Ta-Hydrides Supported on Silica

The mechanism of hydrogenolysis of alkanes, promoted by Ta-hydrides supported on silica via 2 Si–O– bonds, has been studied with a density functional theory (DFT) approach. Our study suggests that the initial monohydride (Si–O−)₂Ta^(III)H is rapidly trapped by molecular hydrogen to form the more stable tris-hydride (Si–O−)₂Ta^(V)H₃. Loading of <i>n</i>-butane to the Ta-center occurs through C–H activation concerted with elimination of molecular hydrogen (σ-bond metathesis). Once the Ta-alkyl species is formed, the C–C activation step corresponds to a β-alkyl transfer to the metal with elimination of an olefin. According to these calculations, an α-alkyl transfer to the metal to form a Ta-carbene species is of higher energy. The olefins formed during the C–C activation step can be rapidly hydrogenated by both mono- and tris-Ta-hydride species, making the overall process of alkane cracking thermodynamically favored

DFT Study on the Impact of the Methylaluminoxane Cocatalyst in Ethylene Oligomerization Using a Titanium-Based Catalyst

A computational study within the framework of density functional theory is presented on the oligomerization of ethylene to yield 1-hexene using [(η⁵-C₅H₄CMe₂C₆H₅)]­TiCl₃/MAO] catalyst. This study explicitly takes into account a methylaluminoxane (MAO) cocatalyst model, where the MAO cluster has become an anionic species after having abstracted one chloride anion, yielding a cationic activated catalyst. Hence, the reaction profile was calculated using the zwitterionic system, and the potential energy surface has been compared to the cationic catalytic system. Modest differences were found between the two free energy profiles. However, we show for the first time that the use of a realistic zwitterionic model is required to obtain a Brønsted–Evans–Polanyi relationship between the energy barriers and reaction energies

C–H and C–C Activation of <i>n</i>‑Butane with Zirconium Hydrides Supported on SBA15 Containing N‑Donor Ligands: [(SiNH−)(SiX−)ZrH₂], [(SiNH−)(SiX−)₂ZrH], and[(SiN)(SiX−)ZrH] (X = −NH–, −O−). A DFT Study

Density functional theory (DFT) was used to elucidate the mechanism of <i>n</i>-butane hydrogenolysis (into propane, ethane, and methane) on well-defined zirconium hydrides supported on SBA15 coordinated to the surface via N-donor surface pincer ligands: [(SiNH−)­(SiO−)­ZrH₂] (<b>A</b>), [(SiNH−)₂ZrH₂] (<b>B</b>), [(SiNH−)­(SiO−)₂ZrH] (<b>C</b>), [(SiNH−)₂(SiO−)­ZrH] (<b>D</b>), [(SiN)­(Si–O−)­ZrH] (<b>E</b>), and [(SiN)­(SiNH−)­ZrH] (<b>F</b>). The roles of these hydrides have been investigated in C–H/C–C bond activation and cleavage. The dihydride <b>A</b> linked via a chelating [N,O] surface ligand was found to be more active than <b>B</b>, linked to the chelating [N,N] surface ligand. Moreover, the dihydride zirconium complexes are also more active than their corresponding monohydrides <b>C</b>–<b>F</b>. The C–C cleavage step occurs preferentially via β-alkyl transfer, which is the rate-limiting step in the alkane hydrogenolysis. The energetics of the comparative pathways over the potential energy surface diagram (PES) reveals the hydrogenolysis of <i>n</i>-butane into propane and ethane

Structure-Function Relationship of a Plant NCS1 Member – Homology Modeling and Mutagenesis Identified Residues Critical for Substrate Specificity of PLUTO, a Nucleobase Transporter from Arabidopsis

<div><p>Plastidic uracil salvage is essential for plant growth and development. So far, PLUTO, the plastidic nucleobase transporter from <i>Arabidopsis thaliana</i> is the only known uracil importer at the inner plastidic membrane which represents the permeability barrier of this organelle. We present the first homology model of PLUTO, the sole plant NCS1 member from Arabidopsis based on the crystal structure of the benzyl hydantoin transporter MHP1 from <i>Microbacterium liquefaciens</i> and validated by molecular dynamics simulations. Polar side chains of residues Glu-227 and backbones of Val-145, Gly-147 and Thr-425 are proposed to form the binding site for the three PLUTO substrates uracil, adenine and guanine. Mutational analysis and competition studies identified Glu-227 as an important residue for uracil and to a lesser extent for guanine transport. A differential response in substrate transport was apparent with PLUTO double mutants E227Q G147Q and E227Q T425A, both of which most strongly affected adenine transport, and in V145A G147Q, which markedly affected guanine transport. These differences could be explained by docking studies, showing that uracil and guanine exhibit a similar binding mode whereas adenine binds deep into the catalytic pocket of PLUTO. Furthermore, competition studies confirmed these results. The present study defines the molecular determinants for PLUTO substrate binding and demonstrates key differences in structure-function relations between PLUTO and other NCS1 family members.</p></div

Competition studies with PLUTO mutants putatively acting as specificity filter.

<p>Direct uptake studies were performed after heterologous expression of PLUTO (black) and PLUTO mutants W223A (white), F341A (spotted) and I226A (striped) in <i>E. coli</i> cells lacking the endogenous uracil transporter uraA. The uptake was measured for uracil (A), guanine (B) and adenine (C) with concentrations of 20 μM and several competitors were added with ten-fold excess (200 μM). The data represent the mean of net uptake rates of at least three independent experiments ± SE. The asterisks indicate significant differences between PLUTO mutants and the control based on Student's t-test (*  = p<0,05; **  = p<0,01; ***  = p<0,005).</p

Alignment of PLUTO with other NCS1-type protein sequences from different organisms and PLUTO homology model.

<p>(A) NCS1 proteins from <i>Arabidopsis thaliana</i> PLUTO (AED90625), <i>Microbacterium liquefaciens</i> MHP1 (2JLN_A) <i>Aspergillus nidulans</i> FCYB (GI: 169798762), and PLUTO homologs from <i>Oryza sativa</i> (Os02g44680), <i>Zea mays</i> (Zm362848), <i>Vitis vinifera</i> (GSVIVT01033705001), <i>Populus trichocarpa</i> (Pt0006S12110), <i>Brachypodium distachyon</i> (Bradi3g51350) were aligned with ClustalW (<a href="http://www.ebi.ac.uk" target="_blank">www.ebi.ac.uk</a>). The residues are shown in white, gray or black color according to their conservation mode (black for highly conserved residues). Residues marked with asterisks were mutated in the course of this work. Residues marked with red boxes are directly involved in PLUTO substrate binding. Blue boxes indicate two Trp residues which are highly conserved among NCS1 proteins and probably exhibit a function in stabilizing the protein-substrate complex with weak pi-stacking interactions. (B) PLUTO homology model with marked TMs. A three-dimensional model of PLUTO was built using I-TASSER server based on structural information of MHP1. The TMs are marked in colors according to the alignment in Figure 1A. The N-terminus of PLUTO was removed for better visualization.</p