Electrophilic substitution at saturated carbon. L. Isoinversion without an available conducted tour pathway for proton transfer

Kinetic Spectrophotometric Method for the 1,4-Diionic Organophosphorus Formation in the Presence of Meldrum's Acid: Stopped-Flow Approach

The kinetics of the reaction between triphenylphosphine (TPP) and dimethyl acetylenedicarboxylate (DMAD) in the presence of Meldrum’s acid (MA) for the generation of the 1,4-diionic organophosphorus compound has been investigated using the stopped-flow and UV-VIS spectrophotometry techniques. The first step of the reaction between TPP and DMAD for the generation of (I₁) in ethanol was followed by the stopped-flow apparatus. This step was recognized as a fast step. The reaction between the intermediate (I₁) and MA showed first-order kinetics, and it was followed by the UV-VIS spectrophotometry technique. The activation parameters for the slow step of the proposed mechanism were determined using two linearized forms of the Eyring equation. From the temperature, concentration and solvent studies, the activation energy (Ea = 20.16 kJ·mol⁻¹) and the related activation parameters (ΔG^‡ = 71.17 ± 0.015 kJ·mol⁻¹, ΔS^‡ = −185.49 ± 0.026 J·mol⁻¹ and ΔH^‡ =17.72 ± 0.007 kJ·mol⁻¹) were calculated. The experimental data indicated that the reaction was zero-order in MA and second-order overall. The proposed mechanism was confirmed with the observed kinetic data obtained from the UV-VIS and stopped-flow techniques.

Slow proton transfer from the hydrogen-labelled carboxylic acid side chain (Glu-165) of triosephosphate isomerase to imidazole buffer in D2O

The catalytic base at the active site of triosephosphate isomerase (TIM) was labelled with -H by abstraction of a proton from substrate d-glyceraldehyde 3-phosphate to form an enzyme-bound enediol(ate) in D2O solvent. The partitioning of this labelled enzyme between intramolecular transfer of -H to form dihydroxyacetone phosphate (DHAP), and irreversible exchange with -D from solvent was examined by determining the yields of H- and D-labelled products by 1H NMR spectroscopy. The yield of hydrogen-labelled product DHAP remains constant as the concentration of the basic form of imidazole buffer is increased from 0.014 to 0.56 M. This shows that the active site of free TIM, which has an open conformation needed to allow substrate binding, adopts a closed conformation at the enediolate-complex intermediate where the catalytic side chain is sequestered from interaction with imidazole dissolved in D2O.