Abstract
The active site of histidine decarboxylase (HDC) has been modeled with both ab initio (MP2/6-31G(d)) and DFT (BH&HLYP/6-311G(d,p)) calculations. The results clearly point out the role of zwitterionic transition structures and the importance of hydrogen bonding interactions in enzymatic decarboxylation. A comparison between the gas-phase decarboxylation of aminoformylacetic acid (H(CO)CH(NH2)COOH) and the corresponding process in solution according to the supermolecule model approach with six water molecules is provided. This study analyzes the role of the proton distribution in lowering the reaction barrier in an intermediate Schiff base (H2CNCH2COOH) and its transition structure for decarboxylation (ΔE⧧ = 29.8 kcal mol-1 at the MP2/6-31G(d) level of theory). Electronic features displayed by the intermediate imine are analyzed by making use of models of increased complexity. The iminium ion functionality has been established to be the dominant factor in lowering the barrier for the decarboxylation of the ...
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