Transition state

About: Transition state is a research topic. Over the lifetime, 4978 publications have been published within this topic receiving 117965 citations. The topic is also known as: transition state of elementary reaction.

Transition state model and mechanism of nucleophilic aromatic substitution reactions catalyzed by human glutathione S-transferase M1a-1a.

Abstract: An active site His107 residue distinguishes human glutathione S-transferase hGSTM1-1 from other mammalian Mu-class GSTs. The crystal structure of hGSTM1a-1a with bound glutathione (GSH) was solved to 1.9 A resolution, and site-directed mutagenesis supports the conclusion that a proton transfer occurs in which bound water at the catalytic site acts as a primary proton acceptor from the GSH thiol group to transfer the proton to His107. The structure of the second substrate-binding site (H-site) was determined from hGSTM1a-1a complexed with 1-glutathionyl-2,4-dinitrobenzene (GS-DNB) formed by a reaction in the crystal between GSH and 1-chloro-2,4-dinitrobenzene (CDNB). In that structure, the GSH-binding site (G-site) is occupied by the GSH moiety of the product in the same configuration as that of the enzyme-GSH complex, and the dinitrobenzene ring is anchored between the side chains of Tyr6, Leu12, His107, Met108, and Tyr115. This orientation suggested a distinct transition state that was substantiated from the structure of hGSTM1a-1a complexed with transition state analogue 1-S-(glutathionyl)-2,4,6-trinitrocyclohexadienate (Meisenheimer complex). Kinetic data for GSTM1a-1a indicate that kcat(CDNB) for the reaction is more than 3 times greater than kcat(FDNB), even though the nonenzymatic second-order rate constant is more than 50-fold greater for 1-fluoro-2,4-dinitrobenzene (FDNB), and the product is the same for both substrates. In addition, Km(FDNB) is about 20 times less than Km(CDNB). The results are consistent with a mechanism in which the formation of the transition state is rate-limiting in the nucleophilic aromatic substitution reactions. Data obtained with active-site mutants support transition states in which Tyr115, Tyr6, and His107 side chains are involved in the stabilization of the Meisenheimer complex via interactions with the ortho nitro group of CDNB or FDNB and provide insight into the means by which GSTs adapt to accommodate different substrates.

Ab Initio MO Study of the Global Potential Energy Surface of C4H4 in Triplet Electronic State and the Reactions of C(3Pj) with C3H4 (Allene and Propyne) and C2(A3Πu) with C2H4(X1A1g

Abstract: The global potential energy surface of C4H4 in the lowest triplet electronic state have been studied at the G2M(RCC,MP2) level. Of 28 distinct isomers the most stable are aromatic cyclobutene q3 (3A1g,D4h) and linear butyne c1 (3E,D2d), and 66 transition states for various isomerization and dissociation pathways have been found. The information about the global PES is applied to describe the potential energy surfaces for the C(3Pj) + H2CCCH2, C(3Pj) + H3CCCH, and C2(3Πu) + C2H4 reactions, recently studied experimentally in crossed molecular beams. The reaction of the carbon atom with allene is shown to occur by a barrierless addition of C to the CC bond to yield the three-member ring structure t1 and/or to the central carbon atom of allene to form the branching structure b1 which isomerizes to t1 with a low barrier. t1 undergoes ring opening to c1 with a barrier of 9.4 kcal/mol, and the latter emits a H atom to give the major reaction product n-C4H3 with an exit barrier of 2.2 kcal/mol. The minor reaction...

On the way to biofuels from furan: Discriminating Diels-Alder and ring-opening mechanisms

Abstract: We performed kinetics experiments and quantum calculations to investigate the reaction of furan to benzofuran catalyzed by the acidic zeolite HZSM-5, which is a key step in the conversion of biomass to biofuels through catalytic fast pyrolysis. The reaction was studied experimentally by placing the zeolite in contact with solution-phase furan and detecting the benzofuran product over the temperature range 270–300 °C, yielding an apparent activation energy of 72 ± 3 kJ/mol. The reaction was modeled in gas and zeolite phases to determine the energetics of the following two competing pathways: a Diels–Alder mechanism often assumed in interpretations of experimental data and a ring-opening pathway predicted by the chemoinformatic software RING. Quantum calculations on the zeolite/guest system were performed using the ONIOM embedded cluster approach. We computed the energetics of reactants, products, and all intermediate steps. Locating relevant transition states fell beyond our computational resources because...