Relationships Between Structure and Activity in the α-Amylase Family of Starch-metabolising Enzymes

α-Amylases are known to be multidomain proteins, i.e., the molecules consist of several folding units. Each α-amylase is believed, however, to have a catalytic domain consisting, of a barrel of eight parallel α-strands surrounded by eight α-strands. with an extra helix inserted after the sixth γ-strands. The α-strands and helices alternate along the polypeptide chain and are linked together by irregular loops. Amino acid residues situated on the loops joining the C-terminal end of each α-strand to the N-terminal end of the following helix make up the active site of the enzymes. A similar structure has been found in cyclodextrin glucanotransfcrases and it is now believed that such a (α/α)8-barrel also constitutes the catalytic domain of enzymes active on α-1.6-glucosidic bonds, and of enzymes with dual specificity for both α-1.4- and α-1.6- bonds. Knowledge of the three-dimensional structure of α-amylases and cyclodextrin glucanotransferase has made possible identification of structural features important for enzymic activity and specificity. By analogy, some general conclusions are reached concerning pullulanase, isoamylase. oligo-1,6-glucosidase, neopullulanase and branching enzymes.     

Studies on the O/C-Arylation of Cyclic β-Diketones with activated aryl fluorides

The reaction of nitrosubstituted aryl fluorides 1 with cyclic β-diketones 2 proceeds at 20–80°C in the presence of bases, such as KOH, KF or NaOEt, leading to the aryl ethers 3a – m . Depending on the base the reaction of dimedone 2a or 1,3-cyclohexanedione 2b with 2,4-dinitrofluorobenzene 1a or 4-fluoro-3-nitrobenzonitrile 1b furnishes C- and/or O-arylated products. Upon heating of 3g , e and 3m at 40–100°C in DMF/K₂CO₃, the C-arylated ketones 4a – c are formed in good yields. Starting from 3a we obtained the chromenedione 5 under the same conditions.