Recent developments in our understanding of amylopectin structure

Structural characterization of oligosaccharides using MALDI-TOF/TOF tandem mass spectrometry

Extensive cross-ring fragmentation ions, which are very informative of the linkages of the monosaccharide residues constituting these molecules, were readily observed in the MALDI-TOF/TOF/MS/MS spectra of oligosaccharides. These ions, in some cases, were more intense than the commonly observed Y and B ions. The A-type ions observed for the simple oligosaccharides allowed the distinction between alpha(1-4)- and alpha(1-6)-linked isobaric structures. The distinction was based not merely on the differences in the type of ions formed, but also on the ion intensities. For example, both alpha(1-4)- and alpha(1-6)-linked isobaric structures produce ions resulting from the loss of approximately 120 m/z units, but with different intensities, as a result of the fact that they correspond to two different ions (i.e., 0,4A- and 2,4A-ions), requiring different energies to be formed. Abundant A- and X-type ions were also observed for high-mannose N-glycans, allowing the determination of linkages. In addition, the high resolution furnished by MALDI-TOF/TOF allowed determination of certain ions that were commonly overlooked by MALDI-TOF or MALDI-magnetic sector instruments as a result of their lower resolution. Moreover, as a result of the fact that MS/MS spectra for parent ions and all fragment ions are acquired under the same experimental conditions, accurate determination of the molar ratios of isomeric glycans in a mixture analyzed simultaneously by MALDI-TOF/TOF tandem MS becomes possible.

The molecular deposition of transgenically modified starch in the starch granule as imaged by functional microscopy

The molecular deposition of starch extracted from normal plants and transgenically modified potato lines was investigated using a combination of light microscopy, environmental scanning electron microscopy (ESEM) and confocal laser scanning microscopy (CLSM). ESEM permitted the detailed (10 nm) topographical analysis of starch granules in their hydrated state. CLSM could reveal internal molar deposition patterns of starch molecules. This was achieved by equimolar labelling of each starch molecule using the aminofluorophore 8-amino-1,3,6-pyrenetrisulfonic acid (APTS). Starch extracted from tubers with low amylose contents (suppressed granule bound starch synthase, GBSS) showed very little APTS fluorescence and starch granules with low molecular weight amylopectin and/or high amylose contents showed high fluorescence. Growth ring structures were sharper in granules with normal or high amylose contents. High amylose granules showed a relatively even distribution in fluorescence while normal and low amylose granules had an intense fluorescence in the hilum indicating a high concentration of amylose in the centre of the granule. Antisense of the starch phosphorylating enzyme (GWD) resulted in low molecular weight amylopectin and small fissures in the granules. Starch granules with suppressed starch branching enzyme (SBE) had severe cracks and rough surfaces. Relationships between starch molecular structure, nano-scale crystalline arrangements and topographical-morphological features were estimated and discussed.

Development and characterization of aqueous amylose-rich maize starch dispersion for film formation

Development and characterization of amylose-rich starch dispersion for film forming was performed. The influence of dispersion preparation temperature on amylose-rich maize starch (Hylon VII) film formation, and the physical properties of the films were investigated. The film-forming ability of the dispersions was evaluated with free films plasticized with glycerol and sorbitol, and the films were prepared at an elevated temperature (70 degrees C) by a casting technique. The solid-state and particle properties of dispersions were investigated by means of X-ray diffraction (XRD), Fourier transform near infrared (FT-NIR) spectroscopy and laser diffraction particle size analysis. Free films were characterized with respect to their appearance, by FT-NIR spectroscopy, and by XRD. Mechanical stress-strain properties were also studied. Increasing the temperature of dispersion preparation results in higher crystallinity, thus affecting the film forming ability. Mechanically strong and elastic films can be formed from amylose-rich starch dispersion formed at 40 degrees C. The more crystalline precipitate complex (obtained at 80 degrees C) and the entirely amorphous gel (obtained at 10 degrees C) formed non-continuous and cloudy films. The better film-forming properties of the dispersion formed at 40 degrees C are probably due to the highly amorphous structure and smaller particle size. The study shows the possibility of using ambient tempered amylose-starch dispersion for film forming.

A novel amylose corn-starch dispersion as an aqueous film coating for tablets

A novel aqueous coating dispersion of amylose-rich corn starch (Hylon VII) was evaluated in an aqueous film-coating process of tablets using an instrumented laboratory-scale pan-coating apparatus. The influence of two independent process variables, the coating temperature and the atomizing air pressure, on the properties of the coated tablets were investigated. The preuse stability of aqueous coating dispersion (i.e., amylose corn-starch precipitate) was studied using a powder X-ray diffraction (XRD) technique. The crystallinity of amylose starch in the coating dispersion was found to increase slightly during 9 months of storage (in a refrigerator 6 +/- 2 degrees C). The film coatings of an aqueous amylose-rich starch dispersion were successfully applied onto tablets without any significant drawbacks, such as nozzle blockage or related problems. It was found that the temperature in the coating pan had a significant influence on the film surface roughness, mechanical strength, and drug release in vitro. When the lowest coating temperature (30 degrees C) was used, rougher film coatings were obtained due to overwetting. At higher temperatures (up to 50-60 degrees C), lower surface roughness and higher mechanical strength values for the coated tablets were obtained. With the present amylose starch dispersion, the atomizing air pressure had a minor influence on the quality of the coating. Under appropriate coating conditions, a smooth tablet film coating was produced with this new, natural, and inexpensive amylose starch dispersion.

Analysis of the retrogradation of low starch concentration gels using differential scanning calorimetry, rheology, and nuclear magnetic resonance spectroscopy

The retrogradation of 5, 10, 15, and 25% corn starch gels was measured using differential scanning calorimetry (DSC), rheology, and an array of NMR spectroscopy techniques. During the initial (<24 h) stage of retrogradation, an increase in G' corresponding to an increase in the number of solid protons participating in cross-relaxation (M(B)(0) was observed for all four concentrations studied. During the latter (>24 h) stage of retrogradation, amylopectin recrystallization becomes the dominant process as measured by an increase in deltaH(r) for the 25% starch gel, which corresponded to a further increase in. A decrease in the molecular mobility of the liquid component was observed by decreases in (17)O T(2), (1)H D(0), and T(2A). The value for T(2B) (the solid transverse relaxation time) did not change with concentration or time indicating that the mobility of the solid component does not change over time despite the conversion of the highly mobile starch fraction to the less mobile solid state during retrogradation.

Food-derived carbohydrates--structural complexity and functional diversity

Carbohydrates are biomolecules abundantly available in nature. They are found in bewildering types ranging from simple sugars through oligo- and polysaccharides to glycoconjugates and saccharide complexes, each exhibiting characteristic bio-physiological and/or nutritional functions both in in vivo and in vitro systems. For example, their presence or inclusion in food dictates the texture (body) and gives desirable customer appeal (satisfaction), or their inclusion in the diet offers beneficial effects of great therapeutic value. Thus, carbohydrates are integrally involved in a multitude of biological functions such as regulation of the immune system, cellular signaling (communication), cell malignancy, antiinfection responses, host-pathogen interactions, etc. If starch is considered the major energy storage carbohydrate, the gums/mucilages and other non-starch carbohydrates are of structural significance. The most investigated properties of starch are its gelatinization and melting behavior, especially during food processing. This has led to the development of the food polymer science approach, which has enabled a new interpretive and experimental frame work for the study of the plasticizing influence of simple molecules such as water, sugars, etc. on food systems that are kinetically constrained. Starch, although considered fully digestible, has been challenged, and starch is found to be partly indigestible in the GI tract of humans. This fraction of starch-resisting digestion in vivo is known as resistant starch (RS). The latter, due to its excellent fermentative capacity in the gut, especially yielding butyric acid is considered a new tool for the creation of fiber-rich foods, which are of nutraceutical importance. By a careful control of the processing conditions the content of RS, a man-made fiber, can be increased to as high as 30%. Arabinoxylans are the major endospermic cell wall polysaccharides of cereals. In wheat they are found complexed with ferulic acid esters, which after oxidative coupling in vivo mediated by H2O2 and peroxidases or even by photochemical means give cross linked diferuloyl derivatives. The latter confer strength and extensibility to the cell wall and offer resistance for digestibility by ruminants. They also help blocking of the ingress of pathogens. The ester bound ferulic acid after oxidation in vivo generates reactive oxygen species that contribute to the fragmentation of non-starch polysaccharides (hemicelluloses), and thereby reduces the product viscosity, a property seen during long-term storage of rice. In plant tissues, the arabinogalactans are implicated in such diverse functions as cell-cell adhesion, nutrition of growing pollen tubes, response to microbial infections, and also as markers of identity expressed in the terminal sequences of saccharide chains.

Tablet film-coating with amylose-rich maize starch

The purpose of the present study was to investigate an aqueous-based amylose-rich maize starch (Hylon VII) film-coating process of tablets performed by a side-vented pan coating system. Three formulation or process parameters of potential importance, including the plasticizer concentration (X(1)), the temperature of coating pan (X(2)) and the spray rate of the coating solution (X(3)), were evaluated using a central composite face-centred experimental design. Only a few process-related limitations associated with the aqueous film coating of Hylon VII were observed, and, in general, the coated tablets were of fairly good quality. At low spray rates, the temperature of the coating pan did not affect the roughness of the coated tablets. At higher spray rates, higher temperature gave smoother films. As regards surface quality and smoothness, a plasticizer concentration (i.e., a 1:1 mixture of sorbitol and glycerol) of approximately 65% of the polymer weight, seems to be suitable for the present formulations. The dissolution of all Hylon VII-coated tablets in an acidic medium was rapid, more than 75% of the drug (theophylline) dissolved within 15 min. On the basis of the present results, it can be concluded that amylose-rich maize starch (Hylon VII) may be considered as an aqueous film-coating agent to be used for pharmaceutical purposes and in established film-coating processes.

Intermediary glucan structures formed during starch granule biosynthesis are enriched in short side chains, a dynamic pulse labeling approach

The formation of intermediary glucans, mature starch, and phytoglycogen was studied using leaves of Arabidopsis thaliana wild type and dbe mutant, which lacks plastidic isoamylase (Zeeman, S. C., Umemoto, T., Lue, W. L., Au-Yeung, P., Martin, C., Smith, A. M., and Chen, J. (1998) Plant Cell 10, 1699-1711). A new approach to the study of starch biosynthesis was developed based on "very short pulse" labeling of leaf starch through photosynthetic fixation of (14)CO(2). This allowed selective analysis of the structure of starch formed within a 30-s period. This time frame is shorter than the period required for the formation of a single crystalline amylopectin lamella and consequently permits a direct analysis of intermediary structures during granule formation. Analysis of chain length distribution showed that the most recently formed outer layer of the granules has a structure different from the mature starch. The outer layer is enriched in short chains that are 6-11 glucose residues long. Side chains with 6 glucose residues are the shortest abundant chains formed, and they are formed exclusively by transfer from donor chains of 12 glucose residues or longer. The labeling pattern shows that chain transfer resulting in branching is a rapid and efficient process, and the preferential labeling of shorter chains in the intermediary granule bound glucan is suggested to be a direct consequence of efficient branching. Although similar, the short chain intermediary structure is not identical to phytoglycogen, which is an even more highly branched molecule with very few longer chains (more than 40 glucose residues). Pulse and chase labeling profiles for the dbe mutant showed that the final structure is more highly branched than the intermediary structures, which implies that branching of phytoglycogen occurs over a longer time period than branching of starch.

Molecular characteristics of amylose and starch in dimethyl sulfoxide

The aim of this work was the molecular characterization of starch polysaccharides to determine solution structure. Studies of amylose and potato starches of different origins were carried out by the static light scattering, dynamic light scattering, and HPSEC-MALLS methods. Molecular parameters such as Mw, Rg, A2, molar mass distribution, Dz, Rh, the structure-dependent rho-parameter, and osmotic modulus for amylose were determined. The Mw of amylose was found to be in the range from 1 x 10(5) to 1 x 10(6) g mol-1. The Mw of potato starches was much higher, that is, in the range of 23-37 x 10(6) g mol-1. The Rg of the amylose samples was in the range of 24-71 nm, and that of the potato starches was between 130 and 150 nm. The intensity-time correlation function showed one diffusive relaxation motion for amylose as well as for starch. The diffusion coefficients of the amylose prepared from starch by several methods were in the range of 2.7-9.1 x 10(-8) cm2 s-1, and those of the starches were 1 magnitude lower between 4.8 and 6.7 x 10(-9) cm2 s-1. The rho-parameter of amylose was calculated as having values between 1.5 and 2.2, and that of starches was calculated to be an average value of 0.62. The assumed solution behavior of amylose in dimethyl sulfoxide corresponds to that of a flexible chain, while the behavior of starch more closely resembles that of a spherelike structure.

Purification and characterization of the maize amyloplast stromal 112-kDa starch phosphorylase

A plastidic 112-kDa starch phosphorylase (SP) has been identified in the amyloplast stromal fraction of maize. This starch phosphorylase was purified 310-fold from maize endosperm and characterized with respect to its enzymological and kinetic properties. The purification procedure included ammonium sulfate fractionation, Sephacryl 300 HR chromatography, affinity starch adsorption, Q-Sepharose, and Mono Q chromatography. The procedure resulted in a nearly homogeneous enzyme preparation as determined by native and SDS-polyacrylamide gel electrophoresis. Anti-SP antibodies recognized the purified 112-kDa SP enzyme and N-terminal amino acid sequence analysis confirmed that the purified enzyme is the amyloplast stromal 112-kDa SP. Analysis of the purified enzyme by Superose 6 gel filtration chromatography indicated that the native enzyme consisted of two identical subunits. The pH optimum for the enzyme was 6.0 in the synthetic direction and 5.5 in the phosphorolytic direction. SP activity was inhibited by thioreactive agents, diethyl pyrocarbonate, phenylglyoxal, and ADP-glucose. The activation energies for the synthetic and phosphorolytic reactions were 11.1 and 16.9 kcal/mol, respectively, and the enzyme was thermally labile above 50 degrees C. Results of kinetic experiments indicated that the enzyme catalyzes its reaction via a sequential Bi Bi mechanism. The Km value for amylopectin was eight-fold lower than that of glycogen. A kinetic analysis indicated that the phosphorolytic reaction was favored over the synthetic reaction when malto-oligosaccharides (4 to 7 units) were used as substrates. The specificity constants (Vmax/Km) of the enzyme measured in either the synthetic or the phosphorolytic directions increased with increasing chain length.

The effect of water content on the ordered/disordered structures in starches

(13)C cross-polarization magic angle spinning NMR has been used to study the ordered and disordered structures of starches with different water contents. The amorphous regions of starch have been shown to produce NMR patterns only if they are in a glassy state, the widths, positions, and areas of the peaks to some extent being dependent on the temperature and the water content of the starch. In the amorphous region, the peaks were all Gaussian in shape, while the peaks in the ordered regions had Lorentz profiles. Water contents in the range 10-50% did not influence the proportion of double helices in the starch. Decreasing the water content to 1-3%, however, resulted in a significant decrease in the proportion of double helices, the effect being greater in B- than in A-type starches. It is suggested that short-range order structures in starches (double helices) are stabilized by becoming part of long-range order structures (crystallites).

Characterisation of the substituent distribution in hydroxypropylated potato amylopectin starch

The distribution of substituents in hydroxypropylated potato amylopectin starch (amylose deficient) modified in a slurry of granular starch (HPPAPg) or in a polymer 'solution' of dissolved starch (HPPAPs), was investigated. The molar substitution (MS) was determined by three different methods: proton nuclear magnetic resonance (1H NMR) spectroscopy, gas-liquid chromatography (GLC) with mass spectrometry, and a colourimetric method. The MS values obtained by 1H NMR spectroscopy were higher than those obtained by GLC-mass spectrometry analysis and colourimetry. The relative ratio of 2-, 3-, and 6-substitution, as well as un-, mono-, and disubstitution in the anhydroglucose unit (AGU) were determined by GLC-mass spectrometry analysis. Results obtained showed no significant difference in molar distribution of hydroxypropyl groups in the AGU between the two derivatives. For analysis of the distribution pattern along the polymer chain, the starch derivatives were hydrolysed by enzymes with different selectivities. Debranching of the polymers indicated that more substituents were located in close vicinity to branching points in HPPAPg than in HPPAPs. Simultaneous alpha-amylase and amyloglucosidase hydrolysis of HPPAPg liberated more unsubstituted glucose units than the hydrolysis of HPPAPs, indicating a more heterogeneous distribution of substituents in HPPAPg.

Relationship between branching density and crystalline structure of A- and B-type maize mutant starches

Amylopectin from two double maize mutant starches of A-crystalline (wxdu) and B-crystalline type (aewx) was subjected successively to hydrolysis involving alpha and beta amylases, which isolated clusters and all branching zones of clusters (BZC). Enzymatic analysis together with ionic and size-exclusion chromatography revealed the structural features of the clusters and BZC and their role in starch crystallization. A-type clusters were larger (dp(n) > 80) and contained more (but shorter) chains than B-type clusters. The BZC of A-type starch was also larger, but with a shorter distance between the branching points than in B-type BZC. A-type clusters had a densely packed structure and B-type a poorly branched structure. Models for the structure of A- and B-type clusters are presented, and a hypothesis for the influence of cluster geometry on crystallization is proposed.

Starch granules: structure and biosynthesis

The emphasis of this review is on starch structure and its biosynthesis. Improvements in understanding have been brought about during the last decade through the development of new physicochemical and biological techniques, leading to real scientific progress. All this literature needs to be kept inside the general literature about biopolymers, despite some confusing results or discrepancies arising from the biological variability of starch. However, a coherent picture of starch over all the different structural levels can be presented, in order to obtain some generalizations about its structure. In this review we will focus first on our present understanding of the structures of amylose and amylopectin and their organization within the granule, and we will then give insights on the biosynthetic mechanisms explaining the biogenesis of starch in plants.

Characterization of SU1 isoamylase, a determinant of storage starch structure in maize

Function of the maize (Zea mays) gene sugary1 (su1) is required for normal starch biosynthesis in endosperm. Homozygous su1- mutant endosperms accumulate a highly branched polysaccharide, phytoglycogen, at the expense of the normal branched component of starch, amylopectin. These data suggest that both branched polysaccharides share a common precursor, and that the product of the su1 gene, designated SU1, participates in kernel starch biosynthesis. SU1 is similar in sequence to alpha-(1-->6) glucan hydrolases (starch-debranching enzymes [DBEs]). Specific antibodies were produced and used to demonstrate that SU1 is a 79-kD protein that accumulates in endosperm coincident with the time of starch biosynthesis. Nearly full-length SU1 was expressed in Escherichia coli and purified to apparent homogeneity. Two biochemical assays confirmed that SU1 hydrolyzes alpha-(1-->6) linkages in branched polysaccharides. Determination of the specific activity of SU1 toward various substrates enabled its classification as an isoamylase. Previous studies had shown, however, that su1- mutant endosperms are deficient in a different type of DBE, a pullulanase (or R enzyme). Immunoblot analyses revealed that both SU1 and a protein detected by antibodies specific for the rice (Oryza sativa) R enzyme are missing from su1- mutant kernels. These data support the hypothesis that DBEs are directly involved in starch biosynthesis.

Examination of number-average degree of polymerization and molar-based distribution of amylose by fluorescent labeling with 2-aminopyridine

Suitable conditions for the fluorescent labeling of the reducing residue of amylose with 2-aminopyridine were examined. Amylose of up to 38.5 nmol was labeled with a constant labeling efficiency. The same efficiencies were obtained for amyloses having a number-average degree of polymerization (dpn) of 521-4400. The analysis of labeled amylose on size-exclusion HPLC with refractive index and fluorescence detection enabled the determination of dpn and dp distribution on a molar basis. The analysis of eight amylose specimens from seven botanical sources (potato, sweet potato, barley, wheat, indica rice, japonica rice, and maize) gave dpn values in good agreement with those determined by a conventional colorimetric method. The molar-based distributions of these amyloses were characteristic of botanical source and revealed the presence of several molecular species with different dp not detectable in the distribution on a weight basis. Small amyloses with a dp less than 10(3) were predominant in the cereals while amyloses with a dp over 10(3) were predominant in the tubers, suggesting a difference in the biosynthetic process determining the dp distribution of amylose between cereals and tubers.