Structural features of the neutral sugar side chains of apple pectic substances

The ‘hairy regions’ of apple pectic substances (fragments of the rhamnogalacturonan chains carrying the neutral sugar side chains) have been subjected to enzymic, acidic and alkaline degradation. The results show that the hairy regions consist of rhamnogalacturonan fragments carrying arabinogalactans and galacturonan fragments carrying single unit xylose side chains. A separate population of molecules is present consisting of galacturonan main chains and side chains of 1,3/1,6-linked galactans. The structural relations of pectic substances from different plant species are discussed.     

Distribution of methoxyl groups in apple pectic substances

The distribution of methoxyl groups in apple pectic substances was investigated by means of fractionation on ion-exchange and gel-filtration columns and by means of degradation of pectin fractions by pectin lyase and pectate lyase. Pectin fragments thus obtained were fractionated by gel-permeation chromatography and high-pressure liquid chromatography. It was concluded that a heterogeneous intermolecular distribution of the methoxyl groups exists with peaks at degrees of esterification of about 50%, 70% and 95%. The intramolecular distribution of the methoxyl groups cannot be distinguished from a random distribution. Since plant pectin esterases cause a blockwise de-esterification, it is unlikely that the biosynthesis of apple pectic substances passes through a stage of 100% esterification after which partial de-esterification by pectin esterase occurs.     

Changes in the structure of apple pectic substances during ripening and storage

During ripening, the degree of polymerization, the degree of esterification, the neutral sugar content and the neutral sugar composition of extractable apple pectic substances did not change. Some xylose and glucose containing polysaccharides can be extracted from the ripe cell walls suggesting that changes in the hemicelluloses take place. In senescent apples, significant changes in the structure of apple pectic substances could be observed. The degree of polymerization of both the galacturonan chains and the arabinogalactan side chains decreased. The amount of water-extractable pectin molecules carrying 1,3/1,6-linked galactans increased. The degree of esterification and the distribution of the methoxyl groups in the apple pectic substances did not change very much.     

Labeling and chemistry of grapefruit pectic substances

The labeling of a number of polysaccharides found in grapefruit (Citrus paradisi) was achieved by feeding labeled myo-inositol to ripening grapefruit through their cut fruit stem, and allowing 4 days for the metabolism of label. The pectic polysaccharides were isolated by successive extraction of the labeled grapefruit with 80% ethanol, chloroform-methanol (1:1) and finally with 0.2 M Na₂ EDTA to solubilize pectic polysaccharides. The incorporation of label from myo-inositol into galacturonosyl, arabinosyl, xylosyl and galactosyl residues of pectic polysaccharides via myo-inositol oxidation pathway was demonstrated. Ion exchange chromatography of these labeled pectic polysaccharides using DE-52 cellulose resulted in the elution of eight totally or partially resolved polysaccharides with increasing salt concentration. The results suggest that, like other plant tissues, the myo-inositol oxidation pathway is also operative in ripening grapefruit and this metabolic pathway could be successfully utilized to achieve labeling of a number of pectic polysaccharides.     

Microcalorimetric studies on the lyophilic properties of pectic substances

The exothermic effects observed on wetting pectins with water and aliphatic alcohols were studied using a microcalorimeter.The heat released on wetting 1 g pectin with water was found to be 171 ± 7·5 J g^?1. It was experimentally established that 1 g of dry pectin exothermically bonded up to 0·57 g of water.By using the Gibbs-Helmholtz-Young equation which relates the heat released by wetting to the area of the wetted surface, it was estimated that the surface accessible to water in 1 g of pectin was 1·46 × 10³ m² g^?1. The heat of hydration was independent of the degree of esterification of the pectin. The experimental results revealed that there were about six molecules of energetically bonded water per monomer unit of pectin.A specific interaction between methanol and the methoxyl groups of pectin was observed on wetting pectins with methanol and dependence was established between the released heat and the degree of esterification. No similar dependence was reported for the remaining aliphatic alcohols.     

In vitro interactions between polyamines and pectic substances

Putrescine, spermidine and spermine induce a decrease in the pH value of 1 mM polygalacturonic acid or pectin solutions; spermidine and spermine also cause the precipitation of the polymers. The association constants between polyamines and polygalacturonic acid were in the order of 10(5) for putrescine and spermidine, and 10(6) for spermine. The number of galacturonic units per binding sites are proportional to the number of positive charges on the polyamine molecule. Low affinity binding sites appear at high polyamine concentrations. Calcium ions seem to compete weakly with spermine by lowering the association constant 4- to 6-fold. Two natural pectins tested, showed that methylation of the carboxylic groups influences only the number of galacturonic units per site but not the association constant.     

Light-scattering studies of pectic substances in aqueous solution

The behaviour of pectic substances in aqueous solution has been studied by the light-scattering technique. Pectic substances were found to be present as high molecular aggregates having strong, intermolecular interactions, and their behaviour depends upon the nature of the carbohydrate chain. Additives can cause dissociation of the polysaccharide aggregates, resulting, in some cases, in particles having similar weight-average ( _w) and number-average ( _n) molecular weights.     

Isolation and characterization of cell-wall pectic substances from potato tuber

Exhaustive treatment of potato tuber tissues by purified endo-polygalacturonase from Aspergillus japonicus solubilized 95% of the total uronides of     

Enzymatic degradation and β-elimination of the pectic substances in cherry fruits

Pectic substances from cherry fruits (Prunus avium) were studied after extraction and purification. They were subjected to β-elimination by heat treatment and depolymerized by endopolygalacturonase from Aspergillus niger. The degraded pectins were fractionated by gel permeation chromatography (Sephadex G-100, Bio-gel P2). The results suggest that the pectic substances largely consist of an α-d-galacturonic backbone interspersed with occasional l-rhamnosyl residues. Neutral sugars as side-chains of varying lengths appear to be concentrated along certain regions of the polymer (‘hairy$\text{)}\text{?}$ in contrast to side-chain free (4mooth’) parts.     

Extraction and characterization of pectic substances from pulp of grape berries

Pectic substances were successively extracted from the alcohol-insoluble residue (AIR) of the pulp of grape berries, by water (WSP), oxalate (OXP), hot dilute HCl (HP) and cold dilute NaOH (OHP). Pectins (WSP, OXP, HP) were purified by ion-exchange chromatography (DEAE-Sephacel) or precipitation with cupric ions (OHP). Total pectic substances represent 20·8% (w/w) of the AIR, WSP and HP being the main components (6% and 12% of the AIR, respectively). An alternative to oxalate extraction of the water-insoluble pectin was extraction with NaCl solutions of increasing concentrations, which had released small amounts of pectins. Each of the fractions contained mainly galacturonic acid, arabinose and galactose, lower amounts of rhamnose and xylose, and minor amounts of glucose and mannose. Ion-exchange chromatography was performed on DEAE-Sephacel, M_w distribution was checked by gel permeation on Sepharose CL-2B, and M_v was determined as well as degrees of methylesterification (DM), acetylation (DA), and protein content.     

Pectic substances,pectic enzymes and haze formation in fruit juices

The pectic substances, located primarily in the middle lamella between cells in higher plant tissues, are complex polysaccharides. They include the negatively charged rhamnogalacturonans, and the neutral arabinogalactans I and II and l-arabinans. These polysaccharides add viscosity to juices but may also form hazes and precipitates and retard maximum recovery of juices from the fruit. The rhamnogalacturonans are degraded by the enzymes pectin methylesterase and polygalacturonase normally present in plant tissues and by these enzymes and pectate lyase in microbially derived commercial pectic enzymes added during processing. The presence of a?abinofuranosidase, which degrades l-arabinans, in commercial pectic enzyme preparations, can cause haze formation in juices such as apple and pear.     

Mango ripening--chemical and structural characterization of pectic and hemicellulosic polysaccharides

Ripening of mango is characterized by a gradual, but natural softening of the fruit, which is due to progressive depolymerization of pectic and hemicellulosic polysaccharides with significant loss of galactose, arabinose and mannose residues at the ripe stage. Structural characterization employing permethylation followed by GC-MS analysis, IR and ¹³C NMR measurements revealed the major CWS fractions of both unripe and ripe mangoes to be of variable molecular weights and having a 1,4-linked galactan/galacturonan backbone, which is occasionally involved in side chain branches consisting of single residues of galactose and arabinose or oligomeric 1,5-linked arabinofuranose residues linked through 1,3-linkages; whereas the major hemicellulosic fractions of unripe mango to be of xyloglucan-type having 1,4-linked glucan backbone with branching by non-reducing terminal arabinose and xylose residues.     

On the molecular weight of pectic substances and its relation to their gel strengths

A study has been made on the possibility of determining the molecular weight M_w, the second virial coefficient A₂ and the radius of gyration by measuring the light scattering of pectin aqueous solutions. The experimental data were processed on a computer by using an algorithm, accounting for the effect of microgel formations in the pectic solution. A tendency towards a decrease in M_w when decreasing the degree of esterification for a certain type of pectin was established and was confirmed by the change in the intrinsic viscosity [η]. This fact is explained by degradation during deesterification.

The dependence of the gel strength of the pectic preparations on their degree of esterification, molecular weight M_w and purity (p%) was also studied as well as the dependence of the gel strength on the intrinsic viscosity [η], the degree of esterification and the purity p%. The passive multifactor experiment method was used to obtain regression models which were tested for an optimum in the factors influencing the gel strength. The optimal value for the degree of esterification was obtained at 57–58% which confirmed the view that within this range the pectin macromolecules possess maximum conformational flexibility.     

Degradation of pectic substances in green flax fibre by Erwinia carotovora

In this paper, we report that Erwinia carotovora can perform controlled biological pectinolysis of green flax fibre. The degradation of pectic substances is considered a key step during this process and is estimated by determining the cation exchange capacity (CEC). A CEC characteristic of a ‘good retting degree’, is reached after 20 h incubation, whatever the tested temperature and initial pectate lyase activity of the bacterial culture. Similar results are obtained after only 4 h, at a temperature of 45°C and the pectate lyase activity is greater than 2 nmol ml^-1 min^-1. The linear relationship between the CEC and the weight loss of green fibre which is established, allows us to estimate an equivalent weight of 1000 g eq^-1 for the solubilised polymers.