Intertwined Analytical, Experimental and Theoretical Studies on the Formation and Structure of a Copper Dienolate.

The reaction of a silyl dienolate, a Cu(II) salt and TBAT yielding the corresponding copper dienolate is addressed. A combined NMR and cyclic voltammetry analysis first highlight the role of TBAT in the Cu(II) to Cu(I) reduction and the structure of the precatalytic species. From these first results a second set of NMR and theoretical studies enable the determination of the structure and the mechanism of formation of the copper dienolate catalytic species. Finally, we showed that that the copper catalyst promote the E/Z s-cis/s-trans equilibration of the silyl dienolate precursor through a copper dienolate intermediate. All of these results unveil some peculiarities of the catalytic and asymmetric vinylogous Mukaiyama reaction.

Complexes of a Zn-metalloenzyme binding site with hydroxamate-containing ligands. A case for detailed benchmarkings of polarizable molecular mechanics/dynamics potentials when the experimental binding structure is unknown.

Zn-metalloproteins are a major class of targets for drug design. They constitute a demanding testing ground for polarizable molecular mechanics/dynamics aimed at extending the realm of quantum chemistry (QC) to very long-duration molecular dynamics (MD). The reliability of such procedures needs to be demonstrated upon comparing the relative stabilities of competing candidate complexes of inhibitors with the recognition site stabilized in the course of MD. This could be necessary when no information is available regarding the experimental structure of the inhibitor-protein complex. Thus, this study bears on the phosphomannose isomerase (PMI) enzyme, considered as a potential therapeutic target for the treatment of several bacterial and parasitic diseases. We consider its complexes with 5-phospho-d-arabinonohydroxamate and three analog ligands differing by the number and location of their hydroxyl groups. We evaluate the energy accuracy expectable from a polarizable molecular mechanics procedure, SIBFA. This is done by comparisons with ab initio quantum-chemistry (QC) calculations in the following cases: (a) the complexes of the four ligands in three distinct structures extracted from the entire PMI-ligand energy-minimized structures, and totaling up to 264 atoms; (b) the solvation energies of several energy-minimized complexes of each ligand with a shell of 64 water molecules; (c) the conformational energy differences of each ligand in different conformations characterized in the course of energy-minimizations; and (d) the continuum solvation energies of the ligands in different conformations. The agreements with the QC results appear convincing. On these bases, we discuss the prospects of applying the procedure to ligand-macromolecule recognition problems. © 2016 Wiley Periodicals, Inc.

Bridging organometallics and quantum chemical topology: Understanding electronic relocalisation during palladium-catalyzed reductive elimination.

This article proposes to bridge two fields, namely organometallics and quantum chemical topology. To do so, Palladium-catalyzed reductive elimination is studied. Such reaction is a classical elementary step in organometallic chemistry, where the directionality of electrons delocalization is not well understood. New computational evidences highlighting the accepted mechanism are proposed following a strategy coupling quantum theory of atoms in molecules and electron localization function topological analyses and enabling an extended quantification of donated/back-donated electrons fluxes along reaction paths going beyond the usual Dewar-Chatt-Duncanson model. Indeed, if the ligands coordination mode (phosphine, carbene) is commonly described as dative, it appears that ligands lone pairs stay centered on ligands as electrons are shared between metal and ligand with strong delocalization toward the latter. Overall, through strong trans effects coming from the carbon involved in the reductive elimination, palladium delocalizes its valence electrons not only toward phosphines but interestingly also toward the carbene. As back-donation increases during reductive elimination, one of the reaction key components is the palladium ligands ability to accept electrons. The rationalization of such electronic phenomena gives new directions for the design of palladium-catalyzed systems.

The reaction mechanism of type I phosphomannose isomerases: new information from inhibition and polarizable molecular mechanics studies.

Type I phosphomannose isomerases (PMIs) are zinc-dependent metalloenzymes involved in the reversible isomerization of D-mannose 6-phosphate (M6P) and D-fructose 6-phosphate (F6P). 5-Phospho-D-arabinonohydroxamic acid (5PAH), an inhibitor endowed with nanomolar affinity for yeast (Type I) and Pseudomonas aeruginosa (Type II) PMIs (Roux et al., Biochemistry 2004; 43:2926-2934), strongly inhibits human (Type I) PMI (for which we report an improved expression and purification procedure), as well as Escherichia coli (Type I) PMI. Its K(i) value of 41 nM for human PMI is the lowest value ever reported for an inhibitor of PMI. 5-Phospho-D-arabinonhydrazide, a neutral analogue of the reaction intermediate 1,2-cis-enediol, is about 15 times less efficient at inhibiting both enzymes, in accord with the anionic nature of the postulated high-energy reaction intermediate. Using the polarizable molecular mechanics, sum of interactions between fragments ab initio computed (SIBFA) procedure, computed structures of the complexes between Candida albicans (Type I) PMI and the cyclic substrate ß-D-mannopyranose 6-phosphate (ß-M6P) and between the enzyme and the high-energy intermediate analogue inhibitor 5PAH are reported. Their analysis allows us to identify clearly the nature of each individual active site amino acid and to formulate a hypothesis for the overall mechanism of the reaction catalyzed by Type I PMIs, that is, the ring-opening and isomerization steps, respectively. Following enzyme-catalyzed ring-opening of ß-M6P by zinc-coordinated water and Gln111 ligands, Lys136 is identified as the probable catalytic base involved in proton transfer between the two carbon atoms C1 and C2 of the substrate D-mannose 6-phosphate.

Polarizable water molecules in ligand-macromolecule recognition. Impact on the relative affinities of competing pyrrolopyrimidine inhibitors for FAK kinase.

Using polarizable molecular mechanics (PMM), we have compared the complexation energies of the focal adhesion kinase (FAK) kinase by five inhibitors in the pyrrolopyrimidine series. These inhibitors only differ by the substitution position of a carboxylate group on their benzene or pyridine rings, and/or the length of the connecting (CH2)(n) chain (n = 0-2) while their inhibitory properties vary from micromolar to nanomolar. Energy balances in which solvation/desolvation effects are computed by a continuum reaction field procedure failed to rank the inhibitors according to their inhibitory potencies. In marked contrast, including energy-minimizing in the protein-inhibitor binding site limited numbers of structural water molecules, namely five to seven, ranked these energy balances conforming to the experimental ordering. The polarization energy contribution was the most critical energy contribution that stabilized the best-bound inhibitor over the others. These results imply that (a) upon docking charged inhibitors into the active site of kinases such as FAK, the presence of a limited number of structured water molecules is critical to enable meaningful relative energy balances and (b) accounting for an explicit polarization contribution within DeltaE is indispensable.

Synthesis and evaluation of non-hydrolyzable D-mannose 6-phosphate surrogates reveal 6-deoxy-6-dicarboxymethyl-D-mannose as a new strong inhibitor of phosphomannose isomerases.

Non-hydrolyzable d-mannose 6-phosphate analogues in which the phosphate group was replaced by a phosphonomethyl, a dicarboxymethyl, or a carboxymethyl group were synthesized and kinetically evaluated as substrate analogues acting as potential inhibitors of type I phosphomannose isomerases (PMIs) from Saccharomyces cerevisiae and Escherichia coli. While 6-deoxy-6-phosphonomethyl-d-mannose and 6-deoxy-6-carboxymethyl-D-mannose did not inhibit the enzymes significantly, 6-deoxy-6-dicarboxymethyl-D-mannose appeared as a new strong competitive inhibitor of both S. cerevisiae and E. coli PMIs with K(m)/K(i) ratios of 28 and 8, respectively. We thus report the first malonate-based inhibitor of an aldose-ketose isomerase to date. Phosphonomethyl mimics of the 1,2-cis-enediolate high-energy intermediate postulated for the isomerization reaction catalyzed by PMIs were also synthesized but behave as poor inhibitors of PMIs. A polarizable molecular mechanics (SIBFA) study was performed on the complexes of d-mannose 6-phosphate and two of its analogues with PMI from Candida albicans, an enzyme involved in yeast infection homologous to S. cerevisiae and E. coli PMIs. It shows that effective binding to the catalytic site occurs with retention of the Zn(II)-bound water molecule. Thus the binding of the hydroxyl group on C1 of the ligand to Zn(II) should be water-mediated. The kinetic study reported here also suggests the dianionic character of the phosphate surrogate as a likely essential parameter for strong binding of the inhibitor to the enzyme active site.

Stacked and H-Bonded Cytosine Dimers. Analysis of the Intermolecular Interaction Energies by Parallel Quantum Chemistry and Polarizable Molecular Mechanics.

Until now, atomistic simulations of DNA and RNA and their complexes have been executed using well calibrated but conceptually simple pair-additive empirical potentials (force fields). Although such simulations provided many valuable results, it is well established that simple force fields also introduce errors into the description, underlying the need for development of alternative anisotropic, polarizable molecular mechanics (APMM) potentials. One of the most abundant forces in all kinds of nucleic acids topologies is base stacking. Intra- and interstrand stacking is assumed to be the most essential factor affecting local conformational variations of B-DNA. However, stacking also contributes to formation of all kinds of noncanonical nucleic acids structures, such as quadruplexes or folded RNAs. The present study focuses on 14 stacked cytosine (Cyt) dimers and the doubly H-bonded dimer. We evaluate the extent to which an APMM procedure, SIBFA, could account quantitatively for the results of high-level quantum chemistry (QC) on the total interaction energies, and the individual energy contributions and their nonisotropic behaviors. Good agreements are found at both uncorrelated HF and correlated DFT and CCSD(T) levels. Resorting in SIBFA to distributed QC multipoles and to an explicit representation of the lone pairs is essential to respectively account for the anisotropies of the Coulomb and of the exchange-repulsion QC contributions.

Conformational analysis of a polyconjugated protein-binding ligand by joint quantum chemistry and polarizable molecular mechanics. Addressing the issues of anisotropy, conjugation, polarization, and multipole transferability.

We investigate the conformational properties of a potent inhibitor of neuropilin-1, a protein involved in cancer processes and macular degeneration. This inhibitor consists of four aromatic/conjugated fragments: a benzimidazole, a methylbenzene, a carboxythiourea, and a benzene-linker dioxane, and these fragments are all linked together by conjugated bonds. The calculations use the SIBFA polarizable molecular mechanics procedure. Prior to docking simulations, it is essential to ensure that variations in the ligand conformational energy upon rotations around its six main-chain torsional bonds are correctly represented (as compared to high-level ab initio quantum chemistry, QC). This is done in two successive calibration stages and one validation stage. In the latter, the minima identified following independent stepwise variations of each of the six main-chain torsion angles are used as starting points for energy minimization of all the torsion angles simultaneously. Single-point QC calculations of the minimized structures are then done to compare their relative energies ΔE conf to the SIBFA ones. We compare three different methods of deriving the multipoles and polarizabilities of the central, most critical moiety of the inhibitor: carboxythiourea (CTU). The representation that gives the best agreement with QC is the one that includes the effects of the mutual polarization energy E pol between the amide and thioamide moieties. This again highlights the critical role of this contribution. The implications and perspectives of these findings are discussed.

Polarizable water networks in ligand-metalloprotein recognition. Impact on the relative complexation energies of Zn-dependent phosphomannose isomerase with D-mannose 6-phosphate surrogates.

Using polarizable molecular mechanics, a recent study [de Courcy et al. J. Am. Chem. Soc., 2010, 132, 3312] has compared the relative energy balances of five competing inhibitors of the FAK kinase. It showed that the inclusion of structural water molecules was indispensable for an ordering consistent with the experimental one. This approach is now extended to compare the binding affinities of four active site ligands to the Type I Zn-metalloenzyme phosphomannose isomerase (PMI) from Candida albicans. The first three ones are the PMI substrate ß-D-mannopyranose 6-phosphate (ß-M6P) and two isomers, α-D-mannopyranose 6-phosphate (α-M6P) and ß-D-glucopyranose 6-phosphate (ß-G6P). They have a dianionic 6-phosphate substituent and differ by the relative configuration of the two carbon atoms C1 and C2 of the pyranose ring. The fourth ligand, namely 6-deoxy-6-dicarboxymethyl-ß-D-mannopyranose (ß-6DCM), is a substrate analogue that has the ß-M6P phosphate replaced by the nonhydrolyzable phosphate surrogate malonate. In the energy-minimized structures of all four complexes, one of the ligand hydroxyl groups binds Zn(II) through a water molecule, and the dianionic moiety binds simultaneously to Arg304 and Lys310 at the entrance of the cavity. Comparative energy-balances were performed in which solvation of the complexes and desolvation of PMI and of the ligands are computed using the Langlet-Claverie continuum reaction field procedure. They resulted into a more favorable balance in favor of ß-M6P than α-M6P and ß-G6P, consistent with the experimental results that show ß-M6P to act as a PMI substrate, while α-M6P and ß-G6P are inactive or at best weak inhibitors. However, these energy balances indicated the malonate ligand ß-6DCM to have a much lesser favorable relative complexation energy than the substrate ß-M6P, while it has an experimental 10-fold higher affinity than it on Type I PMI from Saccharomyces cerevisiae. The energy calculations were validated by comparison with parallel ab initio quantum chemistry on model binding sites extracted from the energy-minimized PMI-inhibitor complexes. We sought to improve the models upon including explicit water molecules solvating the dianionic moieties in their ionic bonds with the Arg304 and Lys310 side-chains. Energy-minimization resulted in the formation of three networks of structured waters. The first water of each network binds to one of the three accessible anionic oxygens. The networks extend to PMI residues (Asp17, Glu48, Asp300) remote from the ligand binding site. The final comparative energy balances also took into account ligand desolvation in a box of 64 waters. They now resulted into a large preference in favor of ß-6DCM over ß-M6P. The means to further augment the present model upon including entropy effects and sampling were discussed. Nevertheless a clear-cut conclusion emerging from this as well as our previous study on FAK kinase is that both polarization and charge-transfer contributions are critical elements of the energy balances.

Energy Analysis of Zn Polycoordination in a Metalloprotein Environment and of the Role of a Neighboring Aromatic Residue. What Is the Impact of Polarization?

We analyze the intermolecular interaction energies stabilizing the complex of ethanol in the binding site of alcohol dehydrogenase Zn-metalloenzyme (ADH). In this site Zn(II) is ligated by two cysteine and one imidazole residue and by the ethanol substrate. Ethanol is stacked over a phenylalanine residue. The system has been studied by means of SIBFA (Sum of Interactions Between Fragments Ab initio computed) polarizable molecular mechanics (PMM) supplemented by quantum chemical (QC) computations at various levels of theory. The nonadditivities of the QC interaction energies can be traced back by energy-decomposition analyses and are essentially due to polarization, charge-transfer, and electron correlation energies. These contributions can be reproduced by PMM computations. Interestingly, the polarization energy associated with the presence of the benzene ring in the ADH complex is canceled due to many-body/nonadditivity effects. Therefore this ring does not contribute to stabilization prior to including electron correlation/dispersion effects in the QC calculations or in the absence of the PMM dispersion energy contribution. When these effects are taken into account, the stabilization it contributes is in the 3-9 kcal/mol range, reflecting the need for an accurate reproduction of all components of the interaction energy by PMM.