Changes In Molecular Weight Distribution Research Articles

Direct HPILC analysis of cellulose depolymerisation in ionic liquids

Changes of average molecular weight and molecular weight distribution of cellulose in a polar ionic liquid (IL) were analysed with high performance liquid chromatography using a polar IL as an eluent (HPILC). 1-Ethyl-3-methylimidazolium methylphosphonate was used as the polar IL. As a model of partly depolymerised cellulose, a mixed sample composed of cellulose and cello-oligosaccharides (glucose, cellobiose, cellotetraose, and cellohexaose) was evaluated to test the resolution of the HPILC. In the resulting chromatograms, the corresponding peaks for each saccharide were found. Hydrolysed cellulose catalysed by a cellulase mixture in water was then prepared and dried. Then, this was dissolved in the polar IL to analyse its molecular weight distribution. The molecular weight distribution changed depending on the enzymatic reaction time. The peak for cellulose was found to decrease with the increase of the peak for cellobiose, and subsequently the peak for cellobiose decreased with the increase of that for glucose. In addition, cellulose oligomers except for cellobiose were scarcely observed, showing the catalytic feature of cellulase. Depolymerisation of cellulose in the polar IL was also carried out using ultrasonication. The peak for cellulose in the HPILC profiles shifted to a higher retention volume side and broadened with the sonication time, strongly suggesting random depolymerisation of cellulose. Thus, HPILC was confirmed to be effective for the dynamic analysis of cellulose depolymerisation.

Derivatization-free gel permeation chromatography elucidates enzymatic cellulose hydrolysis

BackgroundThe analysis of cellulose molecular weight distributions by gel permeation chromatography (GPC) is a powerful tool to obtain detailed information on enzymatic cellulose hydrolysis, supporting the development of economically viable biorefinery processes. Unfortunately, due to work and time consuming sample preparation, the measurement of cellulose molecular weight distributions has a limited applicability until now.ResultsIn this work we present a new method to analyze cellulose molecular weight distributions that does not require any prior cellulose swelling, activation, or derivatization. The cellulose samples were directly dissolved in dimethylformamide (DMF) containing 10-20% (v/v) 1-ethyl-3-methylimidazolium acetate (EMIM Ac) for 60 minutes, thereby reducing the sample preparation time from several days to a few hours. The samples were filtrated 0.2 μm to avoid column blocking, separated at 0.5 mL/min using hydrophilic separation media and were detected using differential refractive index/multi angle laser light scattering (dRI/MALLS). The applicability of this method was evaluated for the three cellulose types Avicel, α-cellulose and Sigmacell. Afterwards, this method was used to measure the changes in molecular weight distributions during the enzymatic hydrolysis of the different untreated and ionic liquid pretreated cellulose substrates. The molecular weight distributions showed a stronger shift to smaller molecular weights during enzymatic hydrolysis using a commercial cellulase preparation for cellulose with lower crystallinity. This was even more pronounced for ionic liquid-pretreated cellulose.ConclusionsIn conclusion, this strongly simplified GPC method for cellulose molecular weight distribution allowed for the first time to demonstrate the influence of cellulose properties and pretreatment on the mode of enzymatic hydrolysis.

Changes in Muscle Protein Fractions of Grass Carp (<i>Ctenopharyngodon idellus</i>) during Early Storage Period

Muscle proteins degradation is one of the important manifestations relating to corruption of freshwater fishes during storage. In this paper, the changes in molecular weight distribution of grass carp muscle proteins during storage at nomal temperature were analyzed by SDS-PAGE and SEC-HPLC measurements. The results showed that the fish muscle proteins were always subject to changes during the whole process of storage, but significant changes of fish muscle proteins took place after 24 hours of storage. Particularly, the high molecular weight proteins were almost completely degraded after storage 24 hours, and the low molecular weight proteins were relatively stable but still undertaken more than 50% of degradation. Our results provide some new evidence for the change law of fish muscle proteins during early storage period.

Radiation processing of thermoplastic starch by blending aromatic additives: Effect of blend composition and radiation parameters

This paper reports on the effects of electron beam (EB) irradiation on poly α-1,4-glucose oligomers (maltodextrins) in the presence of water and of various aromatic additives, as model blends for gaining a better understanding at a molecular level the modifications occurring in amorphous starch–lignin blends submitted to ionizing irradiation for improving the properties of this type of bio-based thermoplastic material. A series of aromatic compounds, namely p-methoxy benzyl alcohol, benzene dimethanol, cinnamyl alcohol and some related carboxylic acids namely cinnamic acid, coumaric acid, and ferulic acid, was thus studied for assessing the ability of each additive to counteract chain scission of the polysaccharide and induce interchain covalent linkages. Gel formation in EB-irradiated blends comprising of maltodextrin was shown to be dependent on three main factors: the type of aromatic additive, presence of glycerol, and irradiation dose. The chain scission versus grafting phenomenon as a function of blend composition and dose were studied using Size Exclusion Chromatography by determining the changes in molecular weight distribution (MWD) from Refractive Index (RI) chromatograms and the presence of aromatic grafts onto the maltodextrin chains from UV chromatograms. The occurrence of crosslinking was quantified by gel fraction measurements allowing for ranking the cross-linking efficiency of the additives. When applying the method to destructurized starch blends, gel formation was also shown to be strongly affected by the moisture content of the sample submitted to irradiation. The results demonstrate the possibility to tune the reactivity of tailored blend for minimizing chain degradation and control the degree of cross-linking.

Wash–dry cycle induced changes in low‐ordered parts of regenerated cellulosic fibers

AbstractThree regenerated cellulosic fiber types: lyocell, viscose, and modal were subjected to repetitive wet–dry treatments. Simulated treatments showed reorganization of the internal fiber structure which could be determined by accessibility studies. The reduction in liquid water retention capacity was found to be greater for lyocell than that for modal and viscose, sorption of iodine, and water vapor reduced for all studied fibers. The wet–dry treatment did not have influence on chemical reactivity of cellulosic fibers characterized by complexation of iron under highly alkaline conditions. The effect of wet–dry treatment on the tenacity, elongation at break, abrasion resistance, and molecular weight distribution of fibers was also explored in this study. The reduced strength in treated specimens was not accompanied by changes in molecular weight distributions. Based on these results, the changes observed in wet–dry‐treated specimens were observed mainly owing to polymer reorganization in amorphous parts of the fibers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Quantifying Extrusion Modification of LDPE with a Microcompounder

Abstract Processibility of LDPE is known to be sensitive to processing history. Significant changes in processibility and product properties can occur through repeated extrusion without detectable changes in molecular weight and molecular weight distribution. Such changes of LDPE rheology are simulated with DSM micro-compounder and characterized with the small angle oscillatory tests and the SER uni-axial extensional flow with a K-BKZ analysis. Both the linear and the nonlinear rheology data suggest the major mechanism in the extrusion with the DSM micro-compounder is the reduction of the effects of long chain branching. The linear rheology analysis, along with GPC and NMR characterization confirms that the changes in the extrusion are not due to chemical crosslinking, though an increase in backbone molecular weight at extreme conditions is perceivable through the minor increase in molecular weight distribution, the increase in zero-shear viscosity and the decrease of melt index. The extensional rheology results, on the other hand, reveal the depression of the strain hardening with prolonged extrusion at 190 degree C or higher extrusion temperature. The stress growth data were evaluated with a K-BKZ constitutive with exponential damping function. The depression of strain hardening is well characterized with the strain hardening factor and the non-linear exponent beta as function of the extrusion conditions. The damping factor β is observed to be especially effective as the parameter to follow the depression of the strain hardening due to microcompounder extrusion. The microcompounder extrusion and the SER analysis with the K-BKZ analysis prove to be a new useful tool for simulating the shear modification of the LDPE.

Controlled peroxide-induced degradation of polypropylene in a twin-screw extruder: Change of molecular weight distribution under conditions controlled by micromixing

Controlled degradation of polypropylene (PP) by peroxide was carried out in a laboratory twin-screw extruder ZSK 18 and the change in Molecular Weight Distribution (MWD) was measured using Size Exclusion Chromatography–Differential Viscosimetry (SEC–DV). The MWD results were compared to MWD predictions from a kinetic model developed and validated in earlier work ( Iedema et al., 2001) assuming ideal mixing. Clear deviations were observed – the measured MWD was broader – that could only be explained by unaccounted heterogeneity in the extruder. Incorporating the relatively narrow Residence Time Distribution (RTD) in the twin-screw extruder did not lead to MWD broadening. In contrast, the exponential RTD of a Continuously Stirred Tank Reactor (CSTR) yielded a MWD widening that was too extreme. A new micromixing model, based on the striation thinning model by Ottino (1980), was constructed partly based on Monte Carlo sampling using a monomer scission probability ( Tobita, 1996). This model was adapted to the geometry of the extruder entrance and the peroxide feed practice consisting of introducing a few per thousand peroxide-rich PP particles among pure PP particles. This micromixing model indeed allowed obtaining very good matches between measured and modeled MWD. Under different experimental conditions with respect to initial PP quality and amount of peroxide added, with a constant value for the striation thinning parameter the errors between measured and predicted MWD were around 5%.

Thermal degradation behavior of polypropylene in the melt state: molecular weight distribution changes and chain scission mechanism

In order to clarify the effect of thermal degradation on the structure of polypropylene materials, we investigated the changes in molecular weight distribution. The samples of polypropylene were degraded iso-thermally at 190 °C at different time intervals. The molecular weight distribution was significantly changed with thermo-degradation time, and the carbonyl index increased drastically for 40 min degraded samples, where the molecular weight distribution started splitting into two peaks. The results imply that heterogeneous degradation proceeded in this system. Sequentially, the weight distribution of the oligomeric products observed was discussed on the basis of chain scissions; these results indicate that there are some kinetically favored scissions occurring near the oxygen-centered radicals.

Aggregation behavior of wheat gluten during carboxylic acid deamidation upon hydrothermal treatment

Changes of aggregation behavior of wheat gluten during carboxylic acid deamidation upon hydrothermal treatment were investigated to test the influences of deamidation on the aggregation extent of wheat gluten. Hydrothermal treatment induced that the size of soluble wheat gluten aggregate progressively increased by cross-linking of gliadins and slowly cleaved glutenins. But significant changes in molecular weight distribution, solubility under six denaturing agents’ treatment and Zeta potential of wheat gluten aggregates were observed at 6 min heating time and distinct shift of intra-/inter-molecular interactions of wheat gluten aggregates occurred before and after 6 min heating treatment respectively. Moreover, as heating time increased, the island-like aggregates decreased markedly and the striped aggregates increased notably. To explain the aggregation behavior in this case, we postulated that the extent of aggregation of wheat gluten depended on the balance between intra-/inter-molecular electrostatic repulsion, the non-covalent and disulfide bonds formation in the system. Hence, a scheme was drawn, which appeared to be the mechanism responsible for the aggregation of wheat gluten through thermal cross-linking and opening up of the network structure of wheat gluten aggregates by deamidation.

Tailoring MWD of Bimodal Polyethylene in the Presence of Ziegler‐Natta Catalyst

AbstractSummary: Linear poly (ethylene‐co‐1‐butene) was produced through two‐step polymerization in one reactor using a Ziegler‐Natta catalyst, where in the first step, low molecular weight homopolymer of ethylene in the presence of hydrogen and in the next step, high molecular weight copolymer of ethylene with 1‐butene in the absence of hydrogen were produced. Molecular weight distribution of bimodal polyethylene was tailored through adjustment of polymerization time of each stage and hydrogen concentration of the first stage. Increasing hydrogen concentration shifted the molecular weight distribution curve to the lower molecular weights and broadened molecular weight distribution while interestingly increased high molecular weight incorporation of copolymer produced in the second stage due to increasing of reaction rate in the second step. To achieve bimodal molecular weight distribution, the polymerization times of the first and the second steps, which are highly dependent on the amount of hydrogen, were adjusted properly. The effects of the mentioned parameters on the processability as well as rheological properties of some samples were investigated. The rheological results showed shear thinning behavior of all specimens and confirmed the changes in molecular weight and molecular weight distribution. It was also demonstrated that the melt miscibility between low molecular weight and high molecular weight fractions improved with increasing of chains having very low molecular weight.

Comparison of advanced oxidation processes for the removal of natural organic matter

This study examined the impact of UV, ozone (O 3), advanced oxidation processes (AOPs) including O 3/UV, H 2O 2/UV H 2O 2/O 3 in the change of molecular weight distribution (MWD) and disinfection by-product formation potential (DBPFP). Bench-scale experiments were conducted with surface river water and changes in the UV absorbance at 254 nm (UV 254), total organic carbon (TOC), trihalomethane and haloacetic acid formation potential (THMFP, HAAFP) and MWD of the raw and oxidized water were analyzed to evaluate treatment performance. Combination of O 3 and UV with H 2O 2 was found to result in more TOC and UV 254 reduction than the individual processes. The O 3/UV process was found to be the most effective AOP for NOM reduction, with TOC and UV 254 reduced by 31 and 88%, respectively. Application of O 3/UV and H 2O 2/UV treatments to the source waters organics with 190–1500 Da molecular weight resulted in the near complete alteration of the molecular weight of NOM from >900 Da to <300 Da H 2O 2/UV was found to be the most effective treatment for the reduction of THM and HAA formation under uniform formation conditions. These results could hold particular significance for drinking water utilities with low alkalinity source waters that are investigating AOPs, as there are limited published studies that have evaluated the treatment efficacy of five different oxidation processes in parallel.

Cell wall modifications during softening in melting type peach “Akatsuki” and non-melting type peach “Mochizuki”

Cell wall polysaccharides were sequentially extracted from the melting peach cultivar “Akatsuki” [Prunus persica L. Batsh] and the non-melting peach cultivar “Mochizuki” with 1,2-cyclohexanediaminetetraacetic acid (CDTA), Na2CO3 and NaOH solutions. Residues remaining after solvent extraction were treated with cellulase, and the released soluble pectic polysaccharides were referred to as the cellulosic residue extract (CE). The abundance of pectic polysaccharides present in the CDTA-soluble extract increased during ripening of “Akatsuki” fruit accompanied by a decrease of these cell-wall polymers in the Na2CO3-soluble and CE extracts. Size-exclusion chromatography (SEC) of pectic polysaccharides in the CDTA, Na2CO3 and CE extracts from ripening “Akatsuki” fruit revealed the existence of neutral sugar rich polyuronides and homogalacturonan-like polysaccharide fractions. These fractions did not show obvious changes in molecular weight distribution except that a homogalacturonan-like pectic polysaccharide in the CDTA-soluble extract shifted to a lower molecular weight. A relatively small amount of pectic polysaccharides were solubilized in non-melting “Mochizuki” fruit during ripening compared to melting “Akatsuki” fruit. The molecular weight distribution of cell-wall polymers in all extracts of “Mochizuki” peach did not change during ripening. Homogalacturonan-like polysaccharides observed in the CDTA-soluble fraction in melting “Akatsuki” were not found in the non-melting “Mochizuki”, but the high molecular weight pectic polymers and the relatively small amount of neutral sugar rich polymers distributed in a relatively wide range of molecular weights increased steadily in the CDTA-soluble extract of “Mochizuki” during ripening. These results suggest that pectic polysaccharides might be solubilized during peach fruit ripening without substantial de-polymerization. In melting type peach fruit that increases endo-polygalacturonase (endo-PG) activity during ripening, the homogalacturonan regions of pectic macromolecules might be hydrolyzed and de-polymerized only after or accompanied by the solubilization of pectic polysaccharides. Polysaccharides in the CE were characterized by a significantly higher amount of neutral sugars compared to other polyuronide fractions, and loss of these pectic polysaccharides during ripening occurred in both melting and non-melting peaches. We also investigated the roles of endo-PG and other cell wall modifying enzymes and the significance of loosing CE polysaccharides in the solubilization of polyuronides during peach fruit ripening.

Structure, mechanical properties and degradation behaviors of the electrospun fibrous blends of PHBHHx/PDLLA

Blends of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) and poly( d, l-lactic acid) (PDLLA) with different ratios were fabricated into fibrous membranes by electrospinning processes. Suggested by DSC, WAXD, and SAXS results, the molecular chains of PHBHHx and PDLLA were partially mixed in the amorphous phase, PDLLA didn’t affect the growth of PHBHHx crystalline phase, and PDLLA was excluded from PHBHHx lamella stacks, i.e. in form of interstack segregation, in the blend fibrous matrix. The mechanical properties of the electrospun fibrous membranes depended on the orientation of fibers in the membranes. The electrospun membranes had higher elongation; furthermore, the tensile strength and modulus of the fibers within the membranes were higher than the corresponding cast membranes. As the content of PDLLA increased, the electrospun fibrous membranes of the blends showed higher elongation and lower tensile modulus due to the decreased number of lamellae. According to the change of molecular weight distribution, both PHBHHx and PDLLA portions in the electrospun blend membranes followed bulk erosion and PDLLA degraded faster than PHBHHx during the degradation process. The morphology change of the electrospun fibrous blends during the hydrolytic degradation indicated that the degradation behaviors were strongly influenced by the miscibility and the structural phase segregation of PHBHHx/PDLLA blend in the electrospun fibers.

Molecular Weight Distributions and Linkages in Lignocellulosic Materials Derivatized from Ionic Liquid Media

A novel and reproducible method is described for accurately determining the molecular weight distribution by size exclusion chromatography (SEC) of whole lignocellulosic materials. This approach offers the opportunity to compare the molecular weight distributions of intact milled woods and its component fractions, lignins and holocelluloses, all from the same source, thus highlighting the potential of the technique and the contributions of the individual components to the chromatogram. The method is based on the dissolution of the ball-milled samples in the ionic liquid 1-allyl-3-methylimidazolium chloride ([amim]Cl). Under these homogeneous ionic liquid media, a derivatization reaction was performed with benzoyl chloride in the presence of pyridine. The thoroughly benzoylated wood with its associated carbohydrate and lignin components was found to be completely soluble in the THF SEC eluent with marked UV detector sensitivity. This methodology, when applied to the individually isolated holocellulose and lignin (enzymatic mild acidolysis lignin; EMAL) materials from Norway spruce ( Eucalyptus grandis ) wood and corn stover, offered a better understanding as to the possible ways the lignin and the carbohydrates may interact within these three different species. Finally, the applicability of the methodology is shown for a series of pure cellulosic samples under intense mechanical defibration conditions, offering a visualization of the molecular weight distribution changes induced during the production of nanofibrillated cellulose.

Increase in 4-Coumaryl Alcohol Units during Lignification in Alfalfa (Medicago sativa) Alters the Extractability and Molecular Weight of Lignin

The lignin content of biomass can impact the ease and cost of biomass processing. Lignin reduction through breeding and genetic modification therefore has potential to reduce costs in biomass-processing industries (e.g. pulp and paper, forage, and lignocellulosic ethanol). We investigated compositional changes in two low-lignin alfalfa (Medicago sativa) lines with antisense down-regulation of p-coumarate 3-hydroxylase (C3H) or hydroxycinnamoyl-CoA:shikimate hydroxycinnamoyltransferase (HCT). We investigated whether the difference in reactivity during lignification of 4-coumaryl alcohol (H) monomers versus the naturally dominant sinapyl alcohol and coniferyl alcohol lignin monomers alters the lignin structure. Sequential base extraction readily reduced the H monomer content of the transgenic lines, leaving a residual lignin greatly enriched in H subunits; the extraction profile highlighted the difference between the control and transgenic lines. Gel permeation chromatography of isolated ball-milled lignin indicated significant changes in the weight average molecular weight distribution of the control versus transgenic lines (CTR1a, 6000; C3H4a, 5500; C3H9a, 4000; and HCT30a, 4000).

Flow field‐flow fractionation and multiangle light scattering for ultrahigh molecular weight sodium hyaluronate characterization

This review describes the utility of flow field-flow fractionation coupled with multiangle light scattering and differential refractive index (FlFFF-MALS-DRI) detection methods for the separation of ultrahigh molecular weight sodium hyaluronate (NaHA) materials and for the characterization of molecular weight distribution as well as structural determination. The sodium salt of hyaluronic acid (HA), NaHA, is a water-soluble polysaccharide with a broad range of molecular weights (10(5) -10(8) ) found in various naturally occurring fluids and tissues. Basic principles of FlFFF-MALS using field programming for the separation of the degraded products of NaHA prepared by treating raw materials with depolymerization or degradation processes such as membrane filtration, enzymatic degradation, ultrasonic degradation, alkaline reaction, irradiation by γ-rays, and thermal treatment for the development of pharmaceutical applications are introduced. Changes in molecular weight distribution and conformation of NaHA materials due to external stimuli are also discussed.

Monte Carlo study of molecular weight distribution changes induced by degradation of ion beam irradiated polymers

In this work we study a polymeric material that degrades upon irradiation due to the energy inhomogeneously deposited by heavy ion beams. Ion beam irradiation of polymers generates rather different effects than those induced by “classical” low ionizing particles such as electrons or gamma rays. This is due to the high electronic stopping power and the inhomogeneous distribution of deposited energy. This energy is transferred to the material within a small volume along the ion path forming the so called “nuclear track” or “latent track”. The track size primarily depends on the ion velocity, and it is determined by the secondary electrons (delta rays) generated along the ion trajectory. By means of Monte Carlo simulations we first obtained equilibrated polymer configurations using a coarse-grained model, and then investigated the spatially inhomogeneous chain scission process due to the passage of the ions. The number average molecular weight, weight average molecular weight and the polydispersity were calculated as a function of track radius, scission probability within the ion track and irradiation fluence. Finally we compared our results with a numerical implementation of a model for random homogeneous degradation.

Heterogeneous continuous kinetics modeling of PET depolymerization in supercritical methanol

The kinetics of PET depolymerization in supercritical methanol was investigated. A continuous kinetics model was developed to analyze PET decomposition behavior. This model includes molecular weight distribution (MWD) changes in the polymer by random and specific scissions and secondary reactions of monomer components for complex macromolecular reactions. The changes of MWD and monomers as a function of time were simulated by continuous kinetics. Reactions in two phases, polymer melt phase and supercritical phase, were considered. By comparing simulated and experimental results, values of the rate constants were determined. These results indicated that random scission proceeds predominantly in the heterogeneous phase during the initial stage of PET depolymerization in supercritical methanol and specific scission proceeds predominantly in the homogeneous phase during the final stage. It was also shown that mass transfer influences the depolymerization of PET.

Gamma polymorph and branching formation as inductors of resistance to electron beam irradiation in metallocene isotactic polypropylene

The influence of a wide dose range of electron beam (EB) irradiation is analyzed on films obtained from metallocene isotactic polypropylene (iPP) under two different thermal processing conditions. A greater irradiation resistance is observed in those films with content in γ crystallites higher than in α monoclinic entities in terms of melting temperature variation with irradiation. This change is on the order of 0.5 °C per 100 kGy in slowly crystallized films while a value of 1.5 °C per 100 kGy is found in quenched specimens. On the other hand, dual studies on films and pellets have proved that branching takes place in this metallocene iPP. The scission processes that occur during irradiation cannot account by themselves for the observed changes in molecular weight distribution as demonstrated by simulation. The observed changes in the rheological behaviour of the irradiated iPP corroborate that increasing degrees of branching are generated by the irradiation process.

Characterization of 4-α-glucanotransferase from Synechocystis sp. PCC 6803 and its application to various corn starches

A putative 4-alpha-glucanotransferase (alphaGTase) gene from Synechocystis sp. PCC 6803 was identified being composed of 1505 nucleotides, and the overexpressed protein was purified with an affinity chromatography. The recombinant alphaGTase had about 57kDa of molecular mass when judged by SDS-PAGE analysis. The optimum reaction condition of the alphaGTase was shown to be pH 7 at 45 degrees C in 50mm phosphate buffer. This enzyme displayed transglycosylating activity on various maltooligosaccharides, of which the smallest donor and acceptor molecules were determined to be maltose and glucose, respectively. Various corn starches consisting of different proportions of amylopectin and amylose were incubated with the recombinant alphaGTase. The change in molecular weight distribution of alphaGTase-modified starch was analyzed by HPSEC. The reaction pattern of alphaGTase showed substantial decrease in amylopectin and increase in the peak corresponding to cycloamylose (CA). The production yield of CA tended to increase from 5 to 30% along with the increase in the apparent amylose content in corn starch, which suggested that linear amylose chain would be preferred to produce CA in the alphaGTase treatment. The detectable minimum degree of polymerization (DP) of CA was shown to be 22 by MALDI-TOF-MS analysis. As another action mode of alphaGTase, the rearrangement of amylopectin branch-chain distribution occurred without hydrolysis to small oligosaccharides. After isoamylolysis, alphaGTase-treated starch displayed the increase in DP 4-9 and longer than DP 21 when the relative proportion of branch chains in amylopectin was determined by HPAEC.