Kinetically stable complexes in water: the role of hydration and hydrophobicity

We describe here the synthesis and characterization of a molecular receptor that forms kinetically and thermodynamically stable host-guest complexes in water. This cavitand-based host is composed of a preorganized aromatic pocket whose rim is decorated with four negatively charged carboxylate groups. (1)H NMR and isothermal titration calorimetry have been used to characterize the behavior of the resulting complexes in response to changes in pH, buffer identity, and salt concentration and in the presence of sodium dodecyl sulfate micelles.     

N-(2,6-Dihalobenzoyl)-N′-pyrimidinylharnstoffe

N-(2,6-Dichlorobenzoyl)- and N-(2,6-difluorobenzoyl)-N-pyrimidinylureas3b–3n have been synthesized by reaction of the corresponding aminopyrimidine derivatives with 2,6-dichlorobenzoyl isocyanate or 2,6-difluorobenzoyl isocyanate. The insecticidal activity of compounds3b–3n has been evaluated.

     

Zur Chemie des Bis(methylsulfonyl)-amins (Dimesylamins). III. Onium-Salze des Dimesylamins

Investigations on Bis(methylsulfonyl)-amine (Dimesylamine). III. Onium Salts of Dimesylamine Thirty-five salts of the strong acid HN(SO₂Me)₂ containing onium cations are reported. These include higher quaternary ammonium salts which show excellent solubility in solvents of low polarity, thus being particularly useful for preparing solutions of “naked” (MeSO₂)₂N^? or corresponding ion-pairs. Pyrolysis of the quaternary ammonium salts at temperatures of 200—250°C produces tertiary amines and N-alkyl-dimesylamines by (N → N′)-alkyl transfer. Alkene formation is not observed.     

Studies on the enzymatic hydrolysis of polyesters I. Low molecular mass model esters and aliphatic polyesters

The bio-catalysed cleavage of ester bonds in low molecular mass model esters and aliphatic polyesters was studied in detail with the aim to gain improved information about the underlying mechanism and the parameters controlling polyester degradation. Among various hydrolytic enzymes the lipase of Pseudomonas species (PsL) was chosen for the investigations. In the heterogeneous phase system the specific hydrolysis rate of the esters was constant as long as free substrate surface was available. In addition to aliphatic low molecular mass model esters, also cycloaliphatic and aromatic esters were cleaved by PsL, indicating that a steric hindrance of the enzymatic ester cleavage is not the predominant controlling factor in polyester degradation. However, the cleavage rates of the aliphatic model esters are larger by more than an order of magnitude. For aliphatic polyesters the temperature difference between the melting point of the polymer and the temperature where degradation takes place (ΔT_mt), turned out to be the primary controlling parameter for polyester degradation with the lipase. Only if ΔT_mt<30 °C, a measurable enzymatic degradation rate is found. ΔT_mt can be regarded as a measure of the mobility of the polyesters chains in the crystalline domains, necessary for the access of the esters to the active site of the lipase. Though aliphatic homopolyesters are seemingly very similar with regard to their chemical structure and reactivity of the ester bonds, their enzymatic degradation rates still differ significantly even at the same ΔT_mt. These differences have obviously to be attributed to small changes in the chemical structure, as, for instance, the C number of the aliphatic diacid.     

A water sluice is generated in the active site of bovine lens leucine aminopeptidase

Nature has provided the binuclear zinc based active site of bovine lens leucine aminopeptidase (blLAP) with two water channels: one for substrate docking and a much smaller one (function unknown) above Zn1. In addition, Zn1 possesses an unusual pentacoordinate geometry with a loosely bound carbonyl ligand (Ala333). Extensive DFT calculations on a model of the active site provide first mechanistic implications for these structural features. The weakly bound carbonyl ligand is capable of functioning as a "traffic cop" to direct water molecules coming from the small channel into the heart of the active site. A water sluice is thus generated that is capable of repeatedly providing a series of nucleophilic active "Zn-OH" functionalities.     

Interrelation between side chain crystallization and dynamic glass transitions in higher poly(n-alkyl methacrylates)

Calorimetric and dielectric results for crystallizable poly(n-alkyl methacrylates) (PnAMA) with C=12, 16 and 18 alkyl carbons per side chain are presented. Degree of crystallization D_cal and melting peak temperature T_M are estimated from conventional DSC measurements. For poly(n-hexadecyl methacrylate) (C=16) the influence of isothermal crystallization is studied by DSC as well as TMDSC. Changes in dielectric relaxation strength Δε and α peak shape during crystallization are investigated. Effects of side chain crystallization on the complex dynamics of PnAMA are discussed. The results are related to the relaxation behavior of lower nanophase-separated PnAMA with two co-existing glass transitions, the conventional glass transition (a or α) and the polyethylene-like glass transition (α_PE) within alkyl nanodomains formed by aggregated alkyl rests. It is shown that amorphous as well as semicrystalline PnAMA can be understood as nanophase-separated polymers with alkyl nanodomains having a typical dimension of 1-2 nm. The results are compared with the predictions of simple morphological pictures for side chain polymers. X-ray scattering data for the amorphous and semicrystalline PnAMA are included in the discussion. Common aspects of nanophase-separated systems in both states as well as differences caused by crystallization are discussed. Indications for the existence of rigid amorphous regions are compiled. Different approaches to explain a similar increase of T_g(α_PE)—the glass temperature of the amorphous alkyl nanodomains—and T_M—the melting temperature of crystalline alkyl nanodomains—with side chain length are considered. Pros and cons of both approaches, based on increasing order within the alkyl nanodomains and confinement effects in nanophase-separated systems, are discussed. Main trends concerning crystallization and cooperative dynamics are compared with those in other systems with self-assembled nanometer confinements like microphase-separated blockcopolymers or semicrystalline main chain polymers.     

Influence of pH*-value of methanolic electrolytes on electroosmotic flow in hydrophilic coated capillaries

The dependency of EOF on the H+-concentration and the related so called pH* value of methanolic electrolytes has been examined with poly(ethylene glycol) (PEG), poly(vinyl alcohol) (PVA) and uncoated capillaries. These results were compared with the pH dependency of EOF of these capillaries using aqueous buffers. In uncoated capillaries the dependency of EOF on the pH(*)-value is very similar for aqueous and methanolic electrolytes. The EOF increases with increasing H+-concentration and pH-hysteresis is observed. In PVA coated capillaries the EOF is strongly reduced over wide pH* or pH ranges for both methanolic electrolytes and aqueous buffers. The EOF in PEG coated capillaries is surprisingly directed to the anode with methanolic electrolytes whereas a reduced cathodic EOF is observed in aqueous electrolytes. The anodic EOF of PEG-coated capillaries in methanolic electrolytes is independent of the pH*-value. The usefulness of PEG- and PVA-coated capillaries for adjusting the EOF in non-aqueous electrolytes for the analysis of isomeric organic acids was demonstrated.