Structural features of naegeli amylodextrin as indicated by enzymic degradation

Contributions of Dexter French (1918-1981) to cycloamylose/cyclodextrin and starch science

Professor Dexter French (1918-1981) was an American chemist and biochemist at Iowa State College (University in 1959). He devoted his career to advance knowledge of polysaccharides and oligosaccharides, in particular starch, cyclodextrins, and enzymes. Cyclodextrins are oligosaccharides obtained from starch and are typically cage molecules with a hydrophobic cavity that can encapsulate other compounds nowadays the basis for many industrial applications. Since the 1960s, he has been recognized as an outstanding authority in the field of starches and cyclodextrins and has inspired researchers in laboratories around the world. This review, on the fortieth anniversary of his death, commemorates his remarkable contribution to starch and cyclodextrin chemistry. Firstly, we give an overview of his personal life and career. Secondly, we highlight some of the results on starch and cyclodextrins from Professor French and his group. A third part discusses his impact on the modern chemistry of cyclodextrins and starch.

Structural Basis for the Slow Digestion Property of Native Cereal Starches

Native cereal starches are ideal slowly digestible starches (SDS), and the structural basis for their slow digestion property was investigated. The shape, size, surface pores and channels, and degree of crystallinity of starch granules were not related to the proportion of SDS, while semicrystalline structure was critical to the slow digestion property as evidenced by loss of SDS after cooking. The high proportion of SDS in cereal starches, as compared to potato starch, was related to their A-type crystalline structure with a lower degree of perfection as indicated by a higher amount of shortest A chains with a degree of polymerization (DP) of 5-10. The A-type amorphous lamellae, an important component of crystalline regions of native cereal starches, also affect the amount of SDS as shown by a reduction of SDS in lintnerized maize starches. These observations demonstrate that the supramolecular A-type crystalline structure, including the distribution and perfection of crystalline regions (both crystalline and amorphous lamellae), determines the slow digestion property of native cereal starches.

Starch biosynthesis: the primer nonreducing-end mechanism versus the nonprimer reducing-end two-site insertion mechanism

Two mechanisms are recognized for polysaccharide chain elongation: (a) the nonreducing-end, primer-dependent mechanism and (b) the reducing-end, two-site insertion mechanism. We recently demonstrated the latter mechanism for starch biosynthesis by pulsing starch granules with ADP-[14C]Glc and chasing with ADPGlc for eight varieties of starch granules. Others have reported the addition of glucose from ADPGlc to the nonreducing ends of maltose, maltotriose, and maltopentaose and a branched maltopentasaccharide. It was concluded that starch chains are biosynthesized by the addition of glucose to the nonreducing ends of maltodextrin primers. In this study, we reinvestigated the maltodextrin reactions by reacting three kinds of starch granules from maize, wheat, and rice with ADP-[14C]Glc in the absence and presence of maltose (G2), maltotriose (G3), and maltodextrin (d.p.12) and found that they inhibited starch biosynthesis rather than stimulating it, as would be expected for primers. The major product in the presence of G2 was G3 with decreasing amounts of G4-G9 and the major products in the presence of G3 was G4 and G5, with decreasing amounts of G6-G9. It was concluded that maltodextrins are acceptors rather than primers. This was confirmed by pulsing the starch granules with ADP-[14C]Glc and chasing with G2, G3, and G6, which gave release of 14C-label from the pulsed granules in the absence of ADPGlc, further demonstrating that maltodextrins are acceptors that inhibit starch biosynthesis by releasing glucose from starch synthase, rather than acting as primers and stimulating biosynthesis.

Amylopectin Molecular Structure Reflected in Macromolecular Organization of Granular Starch

For lintners with negligible amylose retrogradation, crystallinity related inversely to starch amylose content and, irrespective of starch source, incomplete removal of amorphous material was shown. The latter was more pronounced for B-type than for A-type starches. The two predominant lintner populations, with modal degrees of polymerization (DP) of 13-15 and 23-27, were best resolved for amylose-deficient and A-type starches. Results indicate a more specific hydrolysis of amorphous lamellae in such starches. Small-angle X-ray scattering showed a more intense 9-nm scattering peak for native amylose-deficient A-type starches than for their regular or B-type analogues. The experimental evidence indicates a lower contrasting density within the "crystalline" shells of the latter starches. A higher density in the amorphous lamellae, envisaged by the lamellar helical model, explains the relative acid resistance of linear amylopectin chains with DP > 20, observed in lintners of B-type starches. Because amylopectin chain length distributions were similar for regular and amylose-deficient starches of the same crystal type, we deduce that the more dense (and ordered) packing of double helices into lamellar structures in amylose-deficient starches is due to a different amylopectin branching pattern.

Investigation of the fine structure of alpha-dextrins derived from amylopectin and their relation to the structure of waxy-maize starch

Alpha-dextrins, obtained by fractional precipitation with methanol of the products of the action of Bacillus subtilis alpha-amylase on waxy-maize amylopectin, were debranched with isoamylase and the distributions of the unit chains were analysed by gel-permeation chromatography. The large alpha-dextrins still contained long B-chains after hydrolysis for 60 min, but these were absent from the small dextrins with chain numbers of approximately 11 or less. The small dextrins contained increased amounts of chains with lengths intermediate of those of the long B-chains and the main part of the short chains. After hydrolysis for 210 min, almost all of the long B-chains had disappeared and the chains with intermediate lengths had been shortened further. The distributions of the unit chains of the internal chains, obtained by debranching of the phosphorolysis (phi)-limit dextrins, gave similar results and showed that the ratio of A- to B-chains was unchanged during the alpha-amylolysis. Models for the fine structure of the intermediate alpha-dextrins are proposed.

A flow-calorimetric study of the binding of iodine to amylodextrin fractions

Acid hydrolyzates of waxy-maize starch were separated to give Fractions I, II, and III [T. Watanabe, and D. French, Carbohydr. Res., 84 (1980) 115-123]. Watanabe and French suggested that Fraction II, which contains approximately 25 D-glucose residues including an alpha-D-(1----6)-linked branch, has a double helical structure. In the present study, the thermodynamics of binding of iodine to Fractions II and III, and debranched Fraction II (Fraction II') was measured by isothermal-flow calorimetry. If four binding sites for Fraction II and two for Fractions II' and III are assumed, the standard free-energy changes, delta Gb0, for the binding of I2 are -18.5, -18.8, and -18.4 kJ X (mol I2)-1, and the enthalpy changes, delta Hb, are -28.4, -24.7, and -26.9 kJ X (mol I2)-1, respectively. The similarity of these values for the three fractions indicates that the conformation of Fraction II is essentially the same as those of Fractions II' and III, and that Fraction II, therefore, does not have a double helical structure in solution. The values for delta Gb0 are approximately 15 kJ X mol-1 less negative, and those for delta Hb approximately 40 kJ X mol-1 less negative than published values for the starch-I2 complex. These differences are due to the relatively very short D-glucose chains in the amylodextrin fractions employed in the present work.

13C-N.M.R. study of the conformation of helical complexes of amylodextrin and of amylose in solution

Amylose (average d.p. 1000) and amylodextrin (average d.p. 25) have identical 13C-n.m.r. spectra, except for some minor signals from the small amount of alpha-1----6 branch linkages present in amylodextrin. Amylodextrin can be obtained as stable solutions in much higher concentrations than amylose and so requires only 1/100th as many scans to obtain a spectrum comparable to that of amylose. 13C-N.m.r. spectroscopy has been used to study the formation of amylodextrin complexes with organic complexing agents in aqueous solution. A control study using dextran, which does not form helical complexes, showed that, when complexing agents are added, the signals from all of the carbons show a slight downfield shift due to a general solvent effect. In the case of amylodextrin, the addition of increasing concentrations of complexing agent also produced a downfield shift of the signals of all the carbons, but there was a greater shift of the signals for carbons 1 and 4 than for carbons 2, 3, and 6, indicating that something more than a solvent effect was occurring. The cycloamyloses (cyclic alpha-1----4 linked D-glucose oligosaccharides which may be considered as model for an amylose helix) in water have chemical shifts for carbons 1 and 4 that are comparable to those shown by the amylodextrin complexes. It is thus proposed that the formation of a helical complex with amylodextrin results in a change in the conformation of the glycosidic linkage, which is reflected by greater downfield shifts of the signals for carbons 1 and 4, relative to those for carbons 2, 3, and 6. It was observed that differences in the ratio of the downfield shifts of C-1 and C-4 of the different amylodextrin complexes indicate differences in the degree of compactness of the helical structures. A comparison of the 13C chemical shifts of methyl alpha-D-glucoside and methyl alpha-maltoside showed that, for a molecule as small as a disaccharide, there is a conformational change about the glycosidic linkage when complexing agents are added.