Two-Photon Excited Fluorescence and Molecular Reorientations in Liquid Solutions

Theoretical expressions are derived that relate the two-photon excited fluorescence depolarisation experiments to the molecular symmetry and the rotational motions of fluorescent molecules. Diffusive rotational motions in liquid solvents are considered, as well as the influence of fast unresolved motions (e.g. librations). The results obtained are compared with one-photon excited fluorescence depolarisation experiments. The derived theoretical expressions can be applied for detailed analyses of the molecular rotation in solvent. Several of the results are useful for determining and assigning the components of two-photon absorption tensors.     

Monte Carlo simulations of equilibrium solubilities and structure of water in n-alkanes and polyethylene

Gibbs ensemble Monte Carlo methods based on a force field that combines the simple point charge [Berendsen et al., in Intermolecular Forces, edited by Pullman (Reidel, Dordrecht, 1981), p. 331] and transferable potentials for phase equilibria [Martin and Siepmann, J. Phys. Chem. B 102, 2659 (1998)] models were used to study the equilibrium properties of binary systems consisting of water and n-alkanes with chain lengths from hexane to hexadecane. In addition, systems where extended linear alkane chains (up to 300 carbon units long) were used to represent amorphous polyethylene were simulated in the presence of water using a connectivity altering osmotic Gibbs ensemble. In these simulations the equilibrium between a liquid water phase and a polymer phase into which water was inserted was studied. The predicted solubilities, which were determined between 350 and 550 K, are in good agreement with experiment, where experimental results are available, and the density of water molecules in the hydrocarbons is approximately 63% as high as in saturated water vapor under the same conditions. At the lower temperatures most of the water exists as monomers; increasing the temperature leads to an increase in the density of water in the alkane phase and hence in the fraction of molecules that participate in clusters. Dimers are the most prevalent clusters in all hydrocarbons and at all temperatures studied, and the fraction of clusters of given size decrease with increasing cluster size. A large fraction of trimers, tetramers, and pentamers, which are the cluster sizes for which topologies have been studied, are cyclic at low temperatures, but at higher temperatures linear structures predominate. The same properties are observed for pure water vapor clusters in equilibrium with the liquid phase, showing that the cluster topologies are not significantly affected by the surrounding hydrocarbon.     

pH dependency of ligand binding to cellobiohydrolase 1 (Cel7A). Affinity, selectivity and inhibition for designed propranolol analogues

The affinity and enantioselectivity have been determined for designed propranolol derivatives as ligands for Cel7A by capillary electrophoresis (CE) at pH 7.0. These results have been compared to measurements at pH 5.0. In agreement with previous studies, the affinity increased at the higher pH. However, the affinity was not as dependent of the ligand structure at pH 7.0 as at pH 5.0, and the selectivity was generally decreased. Instead, at pH 7.0, the changes in binding were mainly dependent on the presence of additional dihydroxyl groups, indicating an increased importance of the electrostatic interactions. To evaluate the pH dependent variations in binding, changes in both the ligand and in the enzyme had to be taken into account. To ensure that the ligands had the same charge in all measurements, pKa-values of all compounds were determined. The ligand-protein interaction has also been studied by inhibition experiments at both pHs to evaluate the specific binding to the active site when competing with the substrate p-nitrophenyl lactoside (pNPL). With support of docking computations we propose a hypothesis on the effect of the ligand structure and pH dependency of the binding and selectivity of amino alcohols to Cel7A.     

Wittig reactions in water media employing stabilized ylides with aldehydes. Synthesis of alpha,beta-unsaturated esters from mixing aldehydes, alpha-bromoesters, and Ph3P in aqueous NaHCO3

Water is demonstrated to be an effective medium for the Wittig reaction over a wide range of stabilized ylides and aldehydes. Despite sometimes poor solubility of the reactants, good chemical yields normally ranging from 80 to 98% and high E-selectivities (up to 99%) are achieved, and the rate of the reactions in water is unexpectedly accelerated. The efficiency of water as a medium in the Wittig reaction is compared to conventional organic solvents ranging from carbon tetrachloride to methanol. The aqueous Wittig reaction works best when large hydrophobic entities are present, such as aromatic, heterocyclic aromatic carboxaldehydes, and long-chain aliphatic aldehydes with triphenylphosphoranes. The E/Z-isomeric ratio of the Wittig products appears dependent on the electron-accepting/donating capacity and the location of the substituents present in the aromatic ring. The effect of additives, such as benzoic acid, LiCl, and sodium dodecyl sulfate (SDS), on the Wittig reaction has been explored. The Wittig reaction can also be conducted in the presence of acidic entities, such as phenols and carboxylic acids. In addition, large alpha-substituents in the aliphatic aldehydes do not jeopardize the reaction. It is also demonstrated that hydrates of aldehydes can be used directly in the aqueous Wittig reaction as substrates. The scope of the aqueous Wittig reaction is extended to 24 examples of one-pot mixtures of Ph3P, alpha-bromoesters, and aldehydes in sodium bicarbonate solution (at 20 degrees C for 40 min to 3 h) to provide Wittig products of up to 99% yield and up to 98% E-selectivity. Since water is inexpensive, extremely easy to handle, and represents no environmental concerns, it should be considered a possible medium for new organic reactions.     

"Super armed" glycosyl donors: conformational arming of thioglycosides by silylation

Glycosyl donors protected with bulky silyl protective groups (tert-butyldimethylsilyl, TBS), on the 2-, 3-, and 4-OH groups were found to have superior reactivity compared with benzylated thioglucosides. The enhanced reactivity is explained by the stereoelectronic effects associated with the conformational change induced by the silylation. A TBS silylated thioglucoside donor has axial OR groups, whereas a benzylated thioglucoside has equatorial OR groups, leading to much more favorable charge-dipole interactions in the transition state. This concept could be used to create "super armed" glucosyl, mannosyl, rhamnosyl, and galactosyl donors, which could cross-couple with the armed acceptors, phenyl 2,3,4-tri-O-benzyl-beta-D-thioglucoside or phenyl 2,3,6-tri-O-benzyl-beta-D-thioglucoside, to give the corresponding armed disaccharides in good to excellent yields.     

O-O bond cleavage in dinuclear peroxo complexes of iron porphyrins: a quantum chemical study

To gain insight into the mechanisms of O2 activation and cleavage in metalloenzymes, biomimetic metal complexes have been constructed and experimentally characterized. One such model complex is the dinuclear peroxo complex of iron porphyrins observed at low temperature in a non-coordinating solvent. The present theoretical study examines the O-O bond cleavage in these complexes, experimentally observed to occur either at increased temperature or when a strongly coordinating base is added. Using hybrid density functional theory, it is shown that the O-O bond cleavage always occurs in a state where two low-spin irons (S = +/-1/2) are antiferromagnetically coupled to a diamagnetic state. This state is the ground state when the strong base is present and forms an axial ligand to the free iron positions. In contrast, without the axial ligands, the ground state of the dinuclear peroxo complex has two high-spin irons (S = +/-5/2) coupled antiferromagnetically. Thus, the activation barrier for O-O bond cleavage is higher without the base because it includes also the promotion energy from the ground state to the reacting state. It is further found that this excitation energy, going from 10 unpaired electrons in the high-spin case to 2 in the low-spin case, is unusually difficult to determine accurately from density functional theory because it is extremely sensitive to the amount of exact exchange included in the functional.     

First aminoacetone chelate: [Co(tren){NH2CH2C(O)CH3}]3+-a substrate binding and activation model for zinc(II)-dependent 5-aminolaevulinic acid dehydratase

The complex p-[Co(tren){NH(2)CH(2)C(O)CH(3)}](ClO(4))(3).H(2)O was produced stereoselectively from [Co(tren)(O(3)SCF(3))(2)]O(3)SCF(3) () and 2-(aminomethyl)-2-methyl-1,3-dioxolane. The structure of was determined by X-ray crystallography. The complex is the first aminoacetone chelate to be reported and the first structurally characterized example of a non-conjugated ketone moiety coordinated to cobalt(iii). The robust complex was stable to aquation in strong acid and behaved as an acid with pK(a) = 4.99(1) indicative of a strong activation of the aminoacetone ligand towards deprotonation. The complex constitutes a structural model for a proposed substrate binding mode relevant for substrate activation of the zinc(ii)-dependent enzyme 5-aminolaevulinic acid dehydratase.     

Pump-probe spectroscopy and circular dichroism of nanocrystalline benzophenone--towards absolute kinetic measurements in solid state photochemical reactions

Nanocrystals suspended in water can be used to record steady state and pump-probe absorption spectra, which should be useful for the study of excited states and reactive intermediates in the solid state.     

Improved and efficient synthesis of chiral N,P-ligands via cyclic sulfamidates for asymmetric addition of butyllithium to benzaldehyde

A robust and scaleable route to chiral 1-isopropylamino-2-(diphenylphosphino)ethanes is described via the ring-opening of chiral, cyclic sulfamidates with potassium diphenylphosphide (KPPh(2)). The novel protocol offers a robust access to gram quantities of chiral amino phosphinoethanes in high yields. The Li-amides of the chiral aminophosphines were evaluated as chiral ligands in the asymmetric addition of n-butyllithium (BuLi) to benzaldehyde, yielding 1-phenylpentanol up to 98% ee.