RESUMEN
Model A-type amylose single crystals were prepared by recrystallizing dextrins from acid-hydrolyzed native starch and narrow fractions of short chains of enzymatically synthesized amylose, in dilute water/acetone solutions. In most cases, spindle-shaped crystals with a sharp, round or flat apical end were formed, organized in rosettes or fan-like assemblies. The morphology and crystal size were shown to strongly depend on the average degree of polymerization (DP), distribution width (DW), and degree of branching of the chains. The largest and most clearly faceted single crystals were prepared using fractions of synthetic amylose. Typically, 5-10 µm long crystals were obtained from fractions with 17 ≤ DP ≤ 20 and DW ≤ 8. Chains with DP > 40 and a high polydispersity formed ill-defined networks of smaller crystallites. Fractions of branched and more polydisperse limit dextrins yielded crystals smaller than those obtained from narrow fractions of synthetic amylose. The morphological analysis of faceted single crystals combined with electron diffraction data confirmed that the double helices were oriented along the long dimension of the crystal and packed into lamellae with a parallelogram cross section defined by the a and b directions of the monoclinic unit cell of A-amylose. The lamellae are stacked along the c-axis that is oriented parallel but opposite to the growth direction of the crystal.
Asunto(s)
Amilosa/química , Tamaño de la Partícula , Difracción de Polvo , Soluciones , Propiedades de SuperficieRESUMEN
Recombinant expression of the dextransucrase dsrS gene by Escherichia coli was optimized to produce 5850 U L(-1) (culture) of DSR-S, corresponding to a 30-fold increase compared with previous studies. Rational deletions of the signal peptide, the beginning of the variable region and the last four repeats of the C-terminal end caused no loss of activity. This new variant successfully purified was remarkably stable. With a k(cat) of 584 s(-1), it is the most efficient recombinant glucansucrase described to date. The synthesized polymer possesses more than 95% of alpha-1,6 links, like the dextran produced by the native enzyme, and innovative gel properties were obtained.