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Crystal Complex (crystal + complex)
Selected AbstractsAsymmetric Reduction of Activated Alkenes by Pentaerythritol Tetranitrate Reductase: Specificity and Control of Stereochemical Outcome by Reaction OptimisationADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 17 2009Anna Fryszkowska Abstract We show that pentaerythritol tetranitrate reductase (PETNR), a member of the ,ene' reductase old yellow enzyme family, catalyses the asymmetric reduction of a variety of industrially relevant activated ,,,-unsaturated alkenes including enones, enals, maleimides and nitroalkenes. We have rationalised the broad substrate specificity and stereochemical outcome of these reductions by reference to molecular models of enzyme-substrate complexes based on the crystal complex of the PETNR with 2-cyclohexenone 4a. The optical purity of products is variable (49,99% ee), depending on the substrate type and nature of substituents. Generally, high enantioselectivity was observed for reaction products with stereogenic centres at C, (>99% ee). However, for the substrates existing in two isomeric forms (e.g., citral 11a or nitroalkenes 18,19a), an enantiodivergent course of the reduction of E/Z -forms may lead to lower enantiopurities of the products. We also demonstrate that the poor optical purity obtained for products with stereogenic centres at C, is due to non-enzymatic racemisation. In reactions with ketoisophorone 3a we show that product racemisation is prevented through reaction optimisation, specifically by shortening reaction time and through control of solution pH. We suggest this as a general strategy for improved recovery of optically pure products with other biocatalytic conversions where there is potential for product racemisation. [source] Recurring main-chain anion-binding motifs in short polypeptides: nestsACTA CRYSTALLOGRAPHICA SECTION D, Issue 11 2004E. James Milner-White A novel tripeptide motif called a nest has recently been described in proteins with the function of binding anionic, or partially anionic, atoms such as carbonyl O atoms. In the present work, a search for nests in small polypeptides stored in the Cambridge Structural Database is reported. 37 unique examples were found: over half form part of hydrogen-bond arrangements resembling those in proteins, such as Schellman/paperclip loop motifs, various types of ,-turn and Asx-turns or Ser/Thr-turns, while a third are in novel situations, some involving binding to anionic groups from other molecules within the crystal complex. An example is the antibiotic vancomycin, which incorporates a prominent nest forming part of a peptide-binding site. This nest binds the carboxylate of the C-terminal d -alanine of the bacterial cell-wall precursor peptide, thereby inhibiting the final step of bacterial cell-wall synthesis. As in proteins, a number of nests occur in short peptides with an alternating glycine/l -amino-acid sequence but, uniquely to non-ribosomally synthesized short peptides, several nests within them are constructed from alternating d - and l -amino acids, and such sequences seem to specially favour nests. [source] Synthesis of Substituted Indenyl Lanthanide Chloride and Molecular Structure of [(C5H9C9H6) 2Yb (,-Cl) 2Li (Et2O)2]CHINESE JOURNAL OF CHEMISTRY, Issue 2 2003Cui Dong-Mei Abstract Reaction of anhydrous ytterbium trichlorides with 2 equiv. of cyclopentylindenyl lithium in THF solution, followed by removal of the solvent and crystallization of the product from diethyl ether, affords a crystal complex of the composition (C5H9C9H6)2Yb (,-Cl)2Li(Et2O)2. Crystallographic analysis shows that the ytterbium coordinated by two cyclopentylindenyl rings and lithium surrounded by two ether molecules are bridged by the two chlorine atoms and Yb, Li and two chlorine atoms form a plane. [source] Cross-linking of protein crystals as an aid in the generation of binary protein,ligand crystal complexes, exemplified by the human PDE10a,papaverine structureACTA CRYSTALLOGRAPHICA SECTION D, Issue 8 2009Ole Andreas Andersen Protein crystallography has proven to be an effective method of obtaining high-resolution structures of protein,ligand complexes. However, in certain cases only apoprotein structures are readily available and the generation of crystal complexes is more problematic. Some crystallographic systems are not amenable to soaking of ligands owing to crystal-packing effects and many protein,ligand complexes do not crystallize under the same conditions as used for the apoprotein. Using crystals of human phosphodiesterase 10a (hPDE10a) as an example of such a challenging crystallographic system, the structure of the complex with papaverine was obtained to 2.8,Å resolution using protein crystals cross-linked by glutaraldehyde prior to soaking of the ligand. Inspection of the electron-density maps suggested that the correct mode of binding was obtained in one of the two monomers in the asymmetric unit and inspection of crystal-packing contacts explained why cocrystallization experiments and soaking of crystals that were not cross-linked were unsuccessful. [source] | ||||||||||