Site-Resolved Measurement of Microsecond-to-Millisecond Conformational-Exchange Processes in Proteins by Solid-State NMR Spectroscopy

We demonstrate that conformational exchange processes in proteins on microsecond-to-millisecond time scales can be detected and quantified by solid-state NMR spectroscopy. We show two independent approaches that measure the effect of conformational exchange on transverse relaxation parameters, namely Carr-Purcell-Meiboom-Gill relaxation-dispersion experiments and measurement of differential multiple-quantum coherence decay. Long coherence lifetimes, as required for these experiments, are achieved by the use of highly deuterated samples and fast magic-angle spinning. The usefulness of the approaches is demonstrated by application to microcrystalline ubiquitin. We detect a conformational exchange process in a region of the protein for which dynamics have also been observed in solution. Interestingly, quantitative analysis of the data reveals that the exchange process is more than 1 order of magnitude slower than in solution, and this points to the impact of the crystalline environment on free energy barriers.     

Nonradiative deactivation of europium(III) luminescence as a detection scheme for moisture

The nonradiative deactivation of the excited state of europium(III)-based phosphors via OH-vibrations in the presence of water allows the detection of moisture in nitrogen at concentration levels down to at least 25 ppm (0.09% RH).     

Homology modelling of the human adenosine A2B receptor based on X-ray structures of bovine rhodopsin, the β2-adrenergic receptor and the human adenosine A2A receptor

A three-dimensional model of the human adenosine A_2B receptor was generated by means of homology modelling, using the crystal structures of bovine rhodopsin, the β₂-adrenergic receptor, and the human adenosine A_2A receptor as templates. In order to compare the three resulting models, the binding modes of the adenosine A_2B receptor antagonists theophylline, ZM241385, MRS1706, and PSB601 were investigated. The A_2A-based model was much better able to stabilize the ligands in the binding site than the other models reflecting the high degree of similarity between A_2A and A_2B receptors: while the A_2B receptor shares about 21% of the residues with rhodopsin, and 31% with the β₂-adrenergic receptor, it is 56% identical to the adenosine A_2A receptor. The A_2A-based model was used for further studies. The model included the transmembrane domains, the extracellular and the intracellular hydrophilic loops as well as the terminal domains. In order to validate the usefulness of this model, a docking analysis of several selective and nonselective agonists and antagonists was carried out including a study of binding affinities and selectivities of these ligands with respect to the adenosine A_2A and A_2B receptors. A common binding site is proposed for antagonists and agonists based on homology modelling combined with site-directed mutagenesis and a comparison between experimental and calculated affinity data. The new, validated A_2B receptor model may serve as a basis for developing more potent and selective drugs.     

Photolithographically patterned enzyme membranes for the detection of pesticides and copper(II) based on enzyme inhibition

Summary A non-aqueous and an aqueous photopolymer system with an enzyme are used to prepare photolithographically patterned enzyme membranes for amperometric (thinfilm platinum electrode) and potentiometric (ISFET) sensors based on enzyme inhibition. Flow methods for enzyme inhibition tests are described. The decrease in enzyme (AChE) activity after incubation in a solution of dichlorvos as inhibitor is detected amperometrically. The enzyme urease is immobilized onto the pH-sensitive gate area of an ISFET. Such a biosensor is able to detect copper-(II) in water in the ppm-range without preconcentration.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

Total Synthesis with a Chirogenic Opening Move Demonstrated on Steroids with Estrane or 18a-Homoestrane Skeleton

A concept of first choice for the synthesis of the title compounds had been proposed by Dane in the late 1930s. It was soon turned down, because the opening move–a chirogenic Diels-Alder reaction – did not work. With Lewis acids as mediators, however, a successful start has been achieved now. With Ti complexes of chelating ligands (Seebach's TADDOLs (= α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5-dimethanols)), enantioselective formation of the desired adducts does occur. Efficient total syntheses of 2 and 3a have been accomplished.     

Mansfield's imprimitivity theorem for arbitrary closed subgroups

Let be a nondegenerate coaction of on a -algebra , and let be a closed subgroup of . The dual action is proper and saturated in the sense of Rieffel, and the generalised fixed-point algebra is the crossed product of by the homogeneous space . The resulting Morita equivalence is a version of Mansfield's imprimitivity theorem which requires neither amenability nor normality of .

     

Determining the relative sign and size of scalar and residual dipolar couplings in homonuclear two-spin systems

Based on the sign and amplitude of TOCSY transfer functions, it is possible to determine the relative sign and size of scalar and residual dipolar couplings in homonuclear spin systems consisting of two spins 1/2. The efficiency of different mixing sequences and different transfer functions is examined both theoretically and experimentally.     

Effects of sonication on the size and crystallinity of stable zwitterionic organic nanoparticles formed by reprecipitation in water

Nanoparticles of a novel organic zwitterionic Meisenheimer complex, N',N' ',N' '-tri(isopropyl)-4-oxo-6-(isopropyliminio)-2-s-(2H)triazinespiro-1'-2',4',6'-trinitrocyclohexadienylide, were synthesized by reprecipitation in water under different conditions. While reprecipitation alone resulted in a suspension of amorphous particles that fell out of solution within hours, sonication for different periods of time resulted in the formation of crystalline particles that were stable in solution over the course of weeks. The disk-shaped particles had an average diameter of 140 nm and a thickness of 70 nm. Comparison of the optical spectroscopy of these particles with the monomer indicates that they possess delocalized excitonic states and enhanced radiative decay rates. The use of zwitterionic molecules in conjunction with sonication provides a way to exert some level of control over particle size and morphology, as well as increased colloidal stability.     

Structure ofN,N′-diphenyl-N-(2-pyridmyl)urea hemihydrate: A crystallographic and spectroscopic investigation

N,N-diphenyl-N-(2-pyridinyl)urea hemihydrate (C₁₈H₁₅N₃O·1/2H₂O) has been studied by crystallographic and NMR-spectroscopic methods. The crystals are monoclinic,I2/a with unit-cell parameters at 86K,a=17.820(1),b=14.857(1),c=11.299(1) Å,=99.17(1)° andZ=8. The structure was solved by direct methods which involved the use of seventeenmeasured triplet phases in the starting set. This appears to be the first centrosymmetric structure solution being based extensively on physically acquired X-ray phases. Refinement was by full-matrix least-squares involving 3843F _o and yieldedRR _w=0.042. The shape of the molecules can be described approximately by two, nearly orthogonal planes, one containing pyridine, the urea group and the NH-bonded phenyl ring, the other containing the second phenyl ring. Both the X-ray and the NMR studies show that there is an intramolecular H-bond between the urea N and the pyridine N atoms. When pH is lowered, this H-bond is weakened, and eventually breaks due to protonation of the pyridine ring. The X-ray study indicates that about 20% of the pyridine rings in the crystal are protonated. The structure details of the rings, in particular the valency angles, have been analyzed in relation to the orientation of the rings relative to the main molecular plane, and the electronic effects of the substituants.