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Threonine synthase catalyzes the most complex reaction among the pyridoxal-5'-phosphate (PLP)-dependent enzymes. The important step is the addition of a water molecule to the Cβ-Cα double bond of the PLP-α-aminocrotonate aldimine intermediate. Transaldimination of this intermediate with Lys61 as a side reaction to form α-ketobutyrate competes with the normal addition reaction. We previously found that the phosphate ion released from the O-phospho-l-homoserine substrate plays a critical role in specifically promoting the normal reaction. In order to elucidate the detailed mechanism of this "product-assisted catalysis", we performed comparative QM/MM calculations with an exhaustive search for the lowest-energy-barrier reaction pathways starting from PLP-α-aminocrotonate aldimine intermediate. Satisfactory agreements with the experiment were obtained for the free energy profile and the UV/vis spectra when the PLP pyridine N1 was unprotonated and the phosphate ion was monoprotonated. Contrary to an earlier proposal, the base that abstracts a proton from the attacking water was the ε-amino group of Lys61 rather than the phosphate ion. Nevertheless, the phosphate ion is important for stabilizing the transition state of the normal transaldimination to form l-threonine by making a hydrogen bond with the hydroxy group of the l-threonine moiety. The absence of this interaction may account for the higher energy barrier of the side reaction, and explains the mechanism of the reaction specificity afforded by the phosphate ion product. Additionally, a new mechanism, in which a proton temporarily resides at the phenolate O3' of PLP, was proposed for the transaldimination process, a prerequisite step for the catalysis of all the PLP enzymes.

The dissociation energy curves of low-lying spin-mixed states for Group 5 hydrides (VH, NbH, and TaH), as well as Group 3 hydrides (ScH, YH, and LaH), have been calculated by using both effective core potential (ECP) and all-electron (AE) approaches. The two approaches are based on the multiconfiguration self-consistent field (MCSCF) method, followed by second-order configuration interaction (SOCI) calculations: the first method employs an ECP basis set proposed by Stevens and co-workers (SBKJC) augmented by a set of polarization functions, and spin−orbit coupling effects are estimated with a one-electron approximation, using effective nuclear charges. The second method employs a double-ζ basis set developed by Huzinaga (MIDI) and three sets of p functions are added to both transition element and hydrogen and one set of f functions is also added to the transition element. The relativistic elimination of small components (RESC) scheme and full Breit−Pauli Hamiltonian are employed in the AE approaches to i...

The results of a theoretical study are presented on quantum control of a chiral exchange reaction of a polyatomic molecule by using infrared laser pulses. Difluorobenzo[c]phenanthrene was chosen to be the simplest model for its helical chirality exchange reaction. This molecule has two stable configurations: M and P forms. From the viewpoint of chemical reaction dynamics, isomerization is regarded as the movement of one of the two representative points that initially correspond to the two forms to the position of the other representative point, while the other representative point remains in its initial position. The ground-state potential energy surface and dipole moment functions required to control this reaction were evaluated at the MP2/6-31+G(d,p) and MP2/TZV+(d,p) levels of molecular orbital (MO) theory. An effective potential energy surface (PES) that is a function of twisting motion of the benzene rings and wagging motion of the CF(2) group was constructed on the basis of the MO results. An analytical expression for the effective PES and that for the dipole moment functions were prepared to make the isomerization control tractable. A quantum control method in a classical way was applied to the isomerization of preoriented difluorobenzo[c]phenanthrene in low temperature limits. The time evolution of the representative point of the M form and that of the P form are separately evaluated to determine the optimal laser fields. This means that the laser control produces pure helical enantiomers from a racemic mixture. Representative points are replaced by the corresponding nuclear wave packets in this treatment. The derived control laser field consists of two linearly polarized E(x)() and E(z)() components that are perpendicular to each other. These components are pi-phase-shifted when the representative point is in the transition-state regions. Under the irradiation of this laser pulse, one of the two representative points of the isomerization is transferred to the target position along the intrinsic reaction path between the enantiomers, while the other representative point remains in its initial potential well. This results in one-way isomerization control, that is, the M(P) to P(M) form. The isomerization is completed with yields of ca. 70% within a few picoseconds. Temporal behaviors of the nuclear wave packet whose center corresponds to the representative point are drawn to see how the desired chiral exchange reaction proceeds in the presence of the control field, while its reverse process is suppressed.