Computation of hydration free energies using a parameterized continuum model: Study of equilibrium geometries and reactive processes in water solution

A parameterized self-consistent reaction field model allowing computation of the total free energy of hydration of organic molecules at the ab initio level is presented. The approach uses electrostatic plus polarization energies calculated with the help of a continuum model. The remaining solvation free energy terms are obtained by a simple formula based on atomic parameters and atomic accessible surface areas (ASAs), which are determined with the ASA analytical algorithm. Analytical derivatives of the atomic surfaces areas have been implemented. The atomic parameters have been obtained by a linear regression fit of the calculated and experimental free energies of solution in water for a set of 35 molecules, leading to a standard deviation of 0.75 kcal/mol. Effects of nonelectrostatic terms on solute geometries, association energies, and activation barriers are illustrated. © 1996 by John Wiley & Sons, Inc.     

Ruminant pregastric lipases: experimental evidence of their potential as industrial catalysts in food technology

The historical importance of pregastric enzymes in cheese-making is reviewed and the potential for extending their use is discussed in terms of requiring an understanding of their physicochemical parameters. Commericial extracts from the tongues and epiglotti of suckling lambs and calves and adult goats have been processed to yield partially purified samples of the primary pregastric lipase (PGL). The N-terminal sequence and molecular weight of lamb PGL have been determined.

The activity of lamb and goat PGLs against tributyrin has been determined over a range of pH and temperature values. Optimum conditions were pH 6.4, 43°C, and pH 6.0, 52°C, for lamb and goat PGL respectively. The possible influence of the development of a ruminant multi-chambered stomach on the difference in optimal temperature is discussed. A lengthening of the carboxylic acid chain of homoacid triglycerides causes a decrease in hydrolytic activity of lamb PGL but in all cases only a single free fatty acid was released. Against a series of 4-nitrophenylalkanoate esters, maximum activity was observed against the decanoate ester but, in contrast to hydrolysis of the acetate ester which exhibited full Michaelis-Menten kinetics with increasing substrate concentration, activity against the decanoate ester was restricted to the monomeric substrate. Taurocholate inhibits the activity of lamb PGL at concentrations >8 mM. Values of pK₂ equal to 6.69 and 7.92 respectively have been determined for lamb PGL.

Attempts to interesterify coconut oil and cocoa butter, and tributyrin and tricaprylin, catalysed by calf PGL were unsuccessful, although positive results obtained using Candida cylindracea encourage further investigation of alternative methods for immobilizing the PGL. Finally, anhydrous milk fat has been hydrolysed by calf, lamb and goat PGLs and the differences in relative amounts of released free fatty acids have been used to explain the differences in taste which arise when Parmesan cheese is produced using different sources of PGL.     

Transition metal binding properties of the redox-active 1,4,7,10,13,16-hexa(ferrocenylmethyl)-1,4,7,10,13,16-hexaazacyclooctadecane and its electrochemical behaviour in a non-aqueous solvent

Solution studies to elucidate the coordination behaviour and the electrochemical response of the ferrocene-functionalized polyazamacrocycle 1,4,7,10,13,16-hexa(ferrocenylmethyl)-1,4,7,10,13,16-hexaazacyclooctadecane (L¹) by potentiometric methods and electrochemical techniques have been carried out. Potentiometric methods in the presence of Cd²⁺, Hg²⁺, Pb²⁺ and Zn²⁺ were carried out in 1,4-dioxane/water (70:30 v/v, 25°C, 0.1 mol dm⁻³ KNO₃). Electrochemical studies were carried out in acetonitrile/dichloromethane (50:50 v/v, 25°C, 0.1 mol dm⁻³ TBAClO₄) in the presence of transition metal ions and anions.     

Hypotheses sur la formation par pyrolyse du premier intermediaire a silicium digonal

The synthesis of 2,8-dimethyl 3,9-dichloro 6-sila[5.5] spiro undecatetra 2.4,8,10-ene is described. The thermal decomposition of the adduct formed by the reaction of this product with methyl acetylene dicarboxylate is studied. From the characterization of the copyrolysis products, two possible mechanisms are proposed which imply the formation of digonal silicon such as 2-silaallene and 2-silaketene.     

1,3-Diyl trapping reactions. Fundamental investigations with application to the synthesis of linearly fused tricyclopentanoids

A comparison of the inter- and intramolecular diyl trapping routes to linearly fused tricyclopentanoids is presented. In addition, several of the factors which are responsible for the stereoselectivity which is associated with the intramolecular process are examined and it is concluded that conformational rather than electronic (secondary orbital) factors play the dominant role. It is shown that gem methyl groups located on the acyclic chain which joins the diyl and diylophile (in reference to 32 and 35, but not to 47) have no practical effect upon the outcome of the trapping reaction. The intramolecular process is stereospecific with respect to diylophile geometry, and highly stereoselective with respect to the ring junction stereochemistry. Finally, an abortive attempt to synthesize the marine natural product Δ⁹⁽¹²)-capnellene (19) as well as a successful synthesis of the mold metabolite d,l- hirsutene (18) is presented.     

Chemistry of dithiocarbamate derivatives of amino acids. Part II. Complexes of nickel(II) with derivatives of branched and cyclic amino acids

Summary Barium salts of the dithiocarbamate derivates of the amino acids: DL-valine, L-valine, DL-leucine, DL-isoleucine and L-proline have been synthesized. The anions have been used to obtain the corresponding nickel(II) complexes in acid form. The complexes are diamagnetic, and coordination takes place in a near-square planar geometry around the nickel(II) ion through the sulphur atoms of the dithiocarbamate moiety, the structure being confirmed by i.r.,¹H n.m.r., u.v.-vis spectros copies and by chemical analysis.No significant difference has been found between the physicochemical properties (i.r. and 'H n.m.r. spectra and magnetic properties) of the complexes prepared from the dithiocarbamate derivatives of DL- and L-valine, but d-d bands in the u.v.-vis spectra show a slight different intensity.     

Halonickel(I) complexes

Summary The reduction of nickel(II) halides with NaBH₄ in the presence of different ligands, L=PPh₃, AsPh₃, SbPh₃, has been studied. With a molar ratio L/Ni=3, new complexes NiX(SbPh₃)₃, X=Cl, Br, I, were obtained. With a molar ratio L/Ni=2, dimeric species [NiXL₂]₂, X=Cl, Br, I; L=PPh₃, AsPh₃, SbPh₃, were isolated. They are unstable and decompose easily in the solid and rapidly in solution, so that pure samples were only identified for X=Cl, L=PPh₃, AsPh₃, SbPh₃; X=Br, L=PPh₃ and X=I, L=PPh₃. With a molar ratio L/Ni=1, complexes [NiXL]_n (probably polymeric) were obtained. They are very unstable and pure samples could only be isolated when X=Cl, L=PPh₃. Impure substances containing variable amounts of decomposition products were obtained in all the remaining cases. The chemical and structural behaviour of these complexes is discussed.