Über die Konfiguration der Amylosemoleküle in wäßriger Lösung

Using geometric criteria to study helix-like structures produced in molecular dynamics simulations of single amylose chains in water

Amylose is a linear polymer chain of α-d-glucose units connected through α(1 → 4) glycosidic bonds. Experimental studies show that in non-polar solvents, single amylose chains form helical structures containing precise H-bond patterns. However, both experimental and computational studies indicate that these perfectly H-bonded helices are not stable in pure water. Nevertheless, amylose chains are observed to form helix-like structures in molecular dynamics (MD) simulations that exhibit imperfect H-bond patterns. In this paper, we study the structure of amylose chains in water using MD simulations to identify and characterize these “imperfect” helical structures. To this end we devise geometry-based criteria to define imperfect helical structures in amylose chains. Using this approach, the propensity of amylose chains to form these structures is quantified as a function of chain length and solvent temperature. This analysis also uncovers both short and long time helix-breaking mechanisms such as band-flips and kinks in the chain. This geometric approach to defining imperfect helices thus allows us to give new insight into the secondary structure of single amylose chains in spite of imperfect H-bond patterns.

We introduce a geometrical approach to capture and study helix-like structures in MD simulations of single amylose chains in water.

[Kinetic studies of the (1 linked to 4)-alpha-D-glucopyranosyltransferase reaction catalyzed by cyclodextrin glycosyltransferase, particularly the cyclization with amylose, amylopectin and total starch as substrate]

The time course of the (1 leads to 4)-alpha-D-glucopyranosyltransfer reactions catalyzed by the cyclodextrin glycosyltransferase ((1 leads to 4)-alpha-D-glucan: [(1 leads to 4)-alpha-D-glucopyranosyl]transferase (cyclizing), EC 2.4.1.19, CGT) from Klebsiella pneumoniae was studied with several commercial amyloses, potato starch, and amylopectin, respectively. Amyloses were poor substrates for the cyclization reaction. In the initial phase of the transfer reactions, the CGT catalyzed a rapid shortening of the amylose chains. The rate of this shortening reaction was significantly accelerated by addition of maltooligosaccharides. Maximum rate of cyclohexaamylose formation was reached with amylose chains sufficiently short (less than Glc100) for the cyclization reaction. Cyclohexaamylose was formed with maximum rate from amyloses containing amylopectin impurities in the initial phase of the transfer reactions, suggesting that the non-reducing ends of the outer amylopectin chains serve as acceptors for the disproportionation of the amylose. Accordingly, water-soluble, high-molecular-weight products containing higher percentages of lengthened outer-chains were obtained from potato starch or amylopectin. In the course of the transfer reactions, only traces of smaller maltooligosaccharides were detected chromatographically.

[Kinetic studies of the (1 linked to 4)-alpha-D-glucopyranosyltransferase reaction catalyzed by cyclodextrin glycosyltransferase, particularly the cyclization with (1 linked to 4)-alpha-D-glucopyranosyl chains (average polymerization of 16) as substrate]

The transfer reactions, particularly the cyclization reaction, catalyzed by the cyclodextrin glycosyltransferase ((1 leads to 4)-alpha-D-glucan:[(1 leads to 4)-alpha-D-glucopyranosyl]-transferase (cyclizing), EC 2.4.1.19; CGT) from Klebsiella pneumoniae M 5 al were studied with (1 leads to 4)-alpha-D-glucopyranosyl chains (d.p. 16). The initial rate of the cyclization reaction with substrate concentrations from 1 up to 16 mM indicated a V of 6.2 kat . kg-1 of protein and a molar catalytical activity of 421.6 kat . mol-1 of enzyme. Km was found to be 1.03 mM. In addition to the cyclization, CGT simultaneously catalyzed a disproportionation of the substrate, yielding shorter maltooligosaccharides and (1 leads to 4)-alpha-D-glucopyranosyl chains which were significantly longer than the substrate itself. Cyclohepta- and cycloocta-amylose were accumulated in the course of longer incubation. They arose mainly from coupling reactions with the initially formed cyclohexaamylose and corresponding disproportionation of these transfer products. The extremely low formation rates of the higher cyclodextrins point to a "mistake" of the enzyme, when cyclizing to cyclohepta- and cyclooctaamylose.

Ligand-induced structural changes in amylose partially complexed with iodine

The influence of complexing agents such as methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, cyclohexanol and 2-octanol on the formation of a blue coloured amylose - iodine complex (pH 4.8), under suboptimum concentrations of iodine and in the absence of potassium iodide, is studied by recording the absorbance at 640 nm. A drop in absorbance at 640 nm accompanied by a blue shift in the spectrum (640-580 nm) was observed at higher concentrations of the complexing agents. This behaviour of amylose partially complexed with iodine appears to be due to ligand-induced structural changes in the amylose chain. The fall in absorbance at 640 nm observed when the temperature of amylose - iodine complex in the presence of complexing agents is raised, and the subsequent regeneration of the absorbance on cooling, indicates the possible helix to random coil transition of the amylose chain in an aqueous system.