Molecular structure and conformational analysis of chiral alcohols. Application to the enantioselectivity study of lipases

The molecular structures of the chiral compounds 1-phenylethanol, 2-hexanol and 1-phenylethanol acetate have been studied theoretically by ab initio methods. Conformational analysis and electronic structure studies have been carried out with these molecules at STO-3G^* and 6-31G^* basis sets. For the study of the interaction of lipases with substrates, a simplified model of the tetrahedral intermediate has been calculated at the 6-31G^*//4-31G^* level. Molecular mechanics simulations of the interaction of these compounds with the lipases of Candida rugosa, Pseudomonas cepacia and Rhizomucor miehei have been used to study the enantioselectivity of these lipases in the transesterification reaction of the chiral alcohols. The theoretical results have been compared with experimental data and good agreement was observed. It can be concluded that the enantioselectivity of these lipases is controlled by the formation of a tetrahedral intermediate, whereas Michaelis complex formation has a much lower significance.     

DFT research on the dehydroxylation reaction of pyrophyllite 2. Characterization of reactants, intermediates, and transition states along the reaction path

We delineate the dehydroxylation reaction of pyrophyllite in detail by localizing the complete reaction path on the free energy surface obtained previously by Car-Parrinello molecular dynamics and the implemented metadynamics algorithm ( Molina-Montes et al. J. Phys. Chem. B 2008, 112, 7051 ). All intermediates were identified, and a transition state search was also undertaken with the PRFO algorithm. The characterization of this reaction and the atomic rearrangement in the intermediates and products at quantum mechanical level were performed for the two reaction paths found previously: (i) direct dehydroxylation through the octahedral hole (cross mechanism) or between contiguous hydroxyl groups (on-site mechanism) and (ii) two-step dehydroxylation assisted by apical oxygens for each of the two steps. New intermediates were found and determined structurally. The structural variations found for all intermediates and transition states are in agreement with experimental results. The formation of these structures indicates that the dehydroxylation process is much more complex than a first-order reaction and can explain the wide range of temperatures for completing the reaction, and these results can be extrapolated to the dehydroxylation of other dioctahedral 2:1 phyllosilicates.     

Further calculations on the internal rotation spectrum of phenol

The influence of a small deformation of C?O angle in phenol (tilt), into the rotational far-infrared (FIR ) spectrum is analyzed using several approaches. In all of them, the CNDO /2 method is used to determine the potential energy functions. In a first step, the C? O bond and the rotation axis are both supposed to coincide with the C₂ symmetry axis of the phenyl group. With this assumption the torsional frequencies are calculated in both the symmetric and asymmetric rotor approximations. In a second step, the tilt of the C? OH bond is determined theoretically and found to be ?3°, measured from the C₂ symmetry axis, the C? OH bond crossing this axis, Using this second geometry, and taking as the rotation axis the C₂ axis, the torsional frequencies are again determined in both approximations. An improvement of the calculated transition energies is encountered at each stage of the calculation, when compared with experimental data. Finally the importance of the introduction of a tilt into the FIR torsional frequency calculations is discussed.