Michel Loos

Distributed polarizabilities using the topological theory of atoms in molecules

Distributed atom-atom multipolar polarizabilities have been calcd. at the coupled perturbed Hartree-Fock level, using Baders topol. theory to partition the mol. charge d. into at. domains. The proposed scheme applies without difficulty to mols. of arbitrary shape and symmetry and maintains a remarkable stability of the individual at. polarizability components with respect to basis set extension, exemplified by the mols. CO, H2O, NH3 and BF3.

HYBRID CLASSICAL QUANTUM FORCE FIELD FOR MODELING VERY LARGE MOLECULES

A coherent computational scheme on a very large molecule in which the subsystem that undergoes the most important electronic changes is treated by a semiempirical quantum chemical method, though the rest of the molecule is described by a classical force field, has been proposed recently. The continuity between the two subsystems is obtained by a strictly localized bond orbital, which is assumed to have transferable properties determined on model molecules. The computation of the forces acting on the atoms is now operating, giving rise to a hybrid classical quantum force field (CQFF) which allows full energy minimization and modeling chemical changes in large biomolecules. As an illustrative example, we study the short hydrogen bonds and the proton-exchange process in the histidine-aspartic acid system of the catalytic triad of human neutrophil elastase. The CQFF approach reproduces the crystallographic data quite well, in opposition to a classical force field. The method also offers the possibility of switching off the electrostatic interaction between the quantum and the classical subsystems, allowing us to analyze the various components of the perturbation exerted by the macromolecule in the reactive part. Molecular dynamics confirm a fast proton exchange between the three possible energy wells. The method appears to be quite powerful and applicable to other cases of chemical interest such as surface reactivity of nonmetallic solids.

Theoretical investigation of the monomer reactivity in polyindole derivatives

The possibility to yield polymers was examined on the basis of quantum-chemical calculations for various substituted indole monomers. Such theoretical studies showed that the monomer reactivity has a significant influence on the electropolymerization reaction. Therefore, theoretical calculations appeared to be very useful to predict the processes involved in the electrosynthesis.

Theoretical study of indole polymerization

Abstract The processes of the polymerization of indole have been studied using a theoretical approach based on Hartree–Fock calculations. The description of the polymerization reaction using both thermodynamical and kinetic controls yields very useful data concerning the regiospecificity of the coupling reaction as well as the final structure of the polymeric chain. The regular character of the 2,3 structure is thus evidenced by the theoretical data contrary to the experimental results which were unable to precise if such a 2,3 structure was regular, statistical or at random.

Conformationally induced double degenerate uneven sulfuranes

Abstract In the case of constitutionally symmetric model sulfuranes, conformationally induced, doubly-degenerate molecular structures were predicted particularly for HS-SH2-SH and to some degree for +H2S-SH2-SH+2. The predicted difference between the two S-S bond lengths, obtained at the SCF level, using a 4–31G∗ basis set, were not large but significant: 0.0116 ± 0.00005 A and 0.0135 ± 0.00005 A for those two compounds, respectively. The role of electron correlation was studied using the Moller-Plesset perturbation theory.

Breaking and making of the S–S linkage via nucleophilic substitution.: An ab initio study

Abstract The energetics of nucleophilic opening and closing of protonated disulphide bridges Download : Download full-size image has been studied for the case of two sulphur-containing (H2S and CH3SH) nucleophiles. The energetics of this reaction was compared to that of the nucleophilic opening and closing of disulphide bridges with thiolate ions: Download : Download full-size image In the former case a not so good nucleophile is combined with an excellent leaving group, and in the latter case a superior nucleophile is combined with a relatively poor leaving group. The barrier heights for these SN2-type reactions were found to be comparable as calculated by ab initio MO methods. In addition, two oxygen-containing nucleophiles (H2O, HCONH–CH[CH2OH]–CONH2) have also been studied in the following reaction: Download : Download full-size image The water molecule is not nucleophilic enough to open the disulphide bridge. However, in a serine residue, in which the oxygen of the side-chain OH group is activated through backbone/side-chain hydrogen bonding, the hydroxy group is sufficiently nucleophilic to open the disulphide bridge, just like the sulphur-containing nucleophiles mentioned above.

A Quantum Chemical Approach to Reactions in Biomolecules

In order to overcome the limitations of conventional molecular mechanics and quantum mechanics studies of model systems, we recently proposed a coherent computational scheme, for very large molecules, in which the subsystem that undergoes the most important electronic changes is treated by a semi-empirical quantum chemical method, though the rest of the molecule is described by a classical force field. The continuity between the two subsystems is obtained by a strictly localized bond orbital, which is assumed to have transferable properties determined on model molecules. The computation of the forces acting on the atoms is now operative, giving rise to a hybrid Classical Quantum Force Field (CQFF) which allows full energy minimization and the modelling of chemical changes in large biomolecules.As illustrative examples we present the proton exchange process in the histidine–aspartic acid system of the catalytic triad of human neutrophil elastase and the inhibition of the charge relay system in the trypsin-BPTI complex. In contrast to a classical force field, the CQFF approach reproduces the crystallographic data quite well. The method also offers the possibility of switching off the electrostatic interaction between the quantum and the classical subsystems allowing us to analyze the various components of the perturbation exerted by the macromolecule in the reactive part. Molecular dynamics confirms a fast proton exchange between the three possible energy wells in HNE. We also explain the inhibition of trypsin by BPTI by a perturbation of the catalytic triad geometry of trypsin in the presence of BPTI.

Constitutionally Symmetric, Structurally Uneven Sulfuranes. A Theoretical Study

Abstract Uneven structures may be induced, conformationnally, i n constitutionally symmetric sulfuranes. In the case of HS-S(H)2-SH the two different S-S bond lengths obtained [l] are shown below.

Topomerization, tautomerization and deprotonation of S2H3⊕

Abstract The stereochemistry of H 2 S ⊕ -SH has been studied at the SCF level using a 4-31G ∗ basis set. In addition to the topomerization (the change on the rotation-inversion surface) the tautomerization (the transfer of H ⊕ from one sulfur to the other) and the deprotonation (including the transfer of H ⊕ from SzH ⊕ 3 to H 2 O) have been studied.

Ab initio geometry optimisation of the C1S(H)OHCl uneven sulfurane with the inclusion of electron correlation

Abstract The unevenness of the SCl bonds in the ClS(H)OHCl molecule was studied using a series of basis sets, at the Hartree-Fock and post Hartree-Fock levels of theories. The results show that the unevenness exists at all computed molecular structures even at as high a level of theory as CCSD(T)/6-31 1G(d). The OHCl and the OCl through space interactions stabilise the asymmetric, uneven structure relative to symmetric Cs conformations.