Effects of calcium, pH, and blockiness on kinetic rheological behavior and microstructure of HM pectin gels

The kinetic behavior during gel formation and the microstructure of 0.75% high methoxyl (HM) pectin gels in 60% sucrose have been investigated by oscillatory measurements and transmission electron microscopy for three comparable citrus pectin samples differing in their degree of blockiness (DB). Ca2+ addition at pH 3.0 resulted in faster gel formation and a lower storage modulus after 3 h for gels of the blockwise pectin A. For gels of the randomly esterified pectin B, the Ca2+ addition resulted in faster gel formation and a higher storage modulus at pH 3.0. At pH 3.5, both pectins A and B were reinforced by the addition of Ca2+. In the absence of Ca2+, the shortest gelation time was obtained for the sample with the highest DB. Microstructural characterization of the gel network, 4 and 20 h after gel preparation, showed no visible changes on a nanometer scale. The microstructure of pectins A and B without Ca2+ was similar, whereas the presence of Ca2+ in pectin A resulted in an inhomogeneous structure.     

Degree of blockiness of amide groups as indicator for difference in physical behavior of amidated pectins

Thickening and gelling properties of commercial amidated pectins depend on the degree of amidation and methyl-esterification, but also the distribution of these groups is of great importance. Methods have been developed during the last few years to determine the distribution of methyl esters over the pectic backbone. We applied the strategies developed for the analysis of high methyl-esterified pectins for studying the distribution of amide groups in amidated pectins. Low methyl-esterified amidated (LMA) pectins were digested before and after removal of methyl esters by an endo-polygalacturonase to determine the degree of blockiness of the substituents. The nature of the substituents (amide groups compared to methyl esters) did not modify the behavior of the enzyme. Oligomers released were separated by using high-performance anion exchange chromatography and pulsed amperometric detection (HPAEC-PAD) at pH 5. Fractions collected after on-line desalting were identified by using MALDI-TOF mass spectrometry. Oligomers were found to elute from the column as a function of their total charge. For the same overall charge and size, oligomers with methyl esters eluted before oligomers with amide groups. Both amide groups and methyl esters of the LMA pectins studied were found to be semirandomly distributed over the pectic backbone, but this may vary according to the amidation process used.     

Intramolecular distribution of carboxyl groups in low methoxyl pectins — A review

Low methoxyl pectins (LMP) have been manufactured since the 1940s primarily for use as gelling agents. At the time, it was noted that low methoxyl pectates (LMPs) prepared by different methods had different gelling properties and this was attributed to the way in which the free carboxyl groups were distributed along the chain following deesterification. Various workers have shown that LMP prepared by enzymic deesterification is more heterogeneous with respect to the degree of esterification (DE) than LMPs of the same DE prepared by acid deesterification. This, together with enzyme studies, has been taken as a sign that pectinesterase works in a sequential fashion and, similarly, acid deesterification is a random process.More recently, the preparation of crude enzyme-deesterified LMP, which is used as a thickener in canned goods, has been described. Although enzyme-deesterified LMPs appear to form weak gels with calcium ions at room temperature (acid-deesterified LMP/calcium gels are reported to be stronger) they are superior when incorporated into canned goods which receive a severe heat treatment.This review briefly describes the preparation of LMP and that work on LMP which provides information about the distribution of carboxyl groups in the pectate molecule. Since it is established that this distribution affects the gelling properties of the pectin a sound understanding of the chemical aspects may assist in understanding the mechanism of gelation.     

Gelation properties of extruded lemon cell walls and their water-soluble pectins

The amount of water-soluble pectins was largely increased after extrusion-cooking of lemon fibres. These pectins showed the ability to form a gel in the presence of sucrose and at acidic pH. The gels obtained with the water-extracted pectins after extrusion-cooking and with pectins acid-extracted on a laboratory scale were softer than those prepared with commercial citrus pectins. The water-extracted pectins after extrusion-cooking and the pectins acid-extracted on a laboratory scale contained long neutral side-chains and required a higher sucrose concentration to gel than the commercial citrus pectins. The extruded lemon fibres showed the ability to form gels in the presence of sucrose and at acidic pH. The gels obtained with the extruded fibres containing some water-solube pectins of high molecular weight were stronger than those obtained with the extruded fibres containing higher amounts of more depolymerised water-soluble pectins. The extruded fibres containing 12.5–14.9% of water-soluble pectins of high molecular weight (intrinsic viscosity: 413–504 mL/g) were those showing the better gelation properties.     

Functional characterization of the gels prepared with pectin methylesterase (PME)-treated pectins

High- and low-methoxyl pectins were treated with pectin methylesterase (PME) and the functional properties of the resulting pectin gels were characterized. The degree of esterification of high- and low-methoxyl pectins decreased from 74.5% to 6.3% and 40.0% to 6.5%, respectively while not changing their molecular weight. Also, the addition of glucono-delta-lactone (GDL) dramatically affected the gel strength and the pH reduction by the GDL led to the increased syneresis of the pectin gels, which was also observed in the PME-treated samples. When flavor compounds were incorporated into the pectin gels, the flavor release from the gels increased with decreasing the degree of esterification due to increased hydrophilic properties.     

Chemically methylated and reduced pectins: preparation,characterisation by 1H NMR spectroscopy,enzymatic degradation,and gelling properties

The gelling properties of pectins are known to be closely related to the degree of methylation (DM) and the distribution of the ester groups. In order to investigate this dependency, a natural citrus pectin (DM 64%) has been methylated to pectins with higher DM or saponified to achieve pectins with lower DM. A simple method for determination of DM by 1H NMR spectroscopy is presented. New modified pectins have been prepared by treatment of pectins having different DM with NaBH(4) to reduce selectively the methyl esters to primary alcohols in the presence of free acids. The degree of reduction (DR) and the DM of the remaining carboxylic acids could likewise be determined by 1H NMR spectroscopy. The new reduced pectins are recognized by the pectin degrading enzymes polygalacturonase PGI and PGII as well as by pectin lyase, all from Aspergillus niger, but the enzymes exhibit lower specific activities as compared with unmodified pectin. The new reduced pectins exhibit high gelling properties.     

Evaluation of a method for determining the free carboxyl group distribution in pectins

The distribution of free carboxyl groups in pectins has been investigated by a method that involves blocking the free carboxyl groups by glycolation, and hydrolysis of the methylesterified regions with a mixture of pectic enzymes. The hydrolysis products are separated from the glycolated regions on an ion exchange column and after deglycolation the oligomer size distribution is obtained by Sephacryl S-200 chromatography.The method was applied to five pectins with degrees of esterification in the range 5–70%. For two of the samples (an enzyme and an alkali de-esterified low methoxyl pectin) the degree of hydrolysis was significantly lower than would be predicted from the initial degree of esterification and thus for these materials the values obtained for the carboxyl group block sizes were considered to be a maximum rather than an accurate estimate.All the samples investigated had a significant proportion of free carboxyl regions with a degree of polymerisation greater than 10. With the possible exception of the pectate (degree of esterification 5%) none of the samples had a random distribution of carboxyl groups. This was considered to be a reflection of the distribution in the native pectin rather than indicating that chemical de-esterification was non-random. The large free carboxyl group block sizes was consistent with the egg-box model for low methoxyl pectin gelation. Larger blocks were found in the enzyme de-esterified pectin compared with the alkali and acid de-esterified material.     

Sulfhydryl groups of the uncoupling protein of brown adipose tissue mitochondria. Distinction between sulfhydryl groups of the H+ channel and the nucleotide binding site

Mersalyl, 5,5'-dithio-bis(2-nitrobenzoate) (Nbs2) and fluorescent Thiolyte DB react with SH groups in the H+ channel (SHc) of the uncoupling protein of brown adipose tissue mitochondria, as inferred from their inhibition of H+ transport. Cl- transport by the uncoupling protein was unaffected. Using these modifiers and N-ethylmaleimide (MalNEt), distinct SH groups (SHB) in the purine nucleotide binding site were identified. Nbs2 reacts more readily with the SHB than with the SHc groups, but mersalyl and Thiolyte DB are more reactive with the SHc groups. MalNEt reacts exclusively with the SHB. GDP inhibition is fully prevented after sufficient modification of the SHB. Pretreatment with p-diazobenzenesulfonate (N2PhSO2) suppresses only 20-25% of fluorescence of Thiolyte-DB-labeled uncoupling protein on SDS/PAGE gels, while MalNEt suppresses 66% and Nbs2 80-90%. Since N2PhSO2 also affects the GDP binding site, these results demonstrate that the N2PhSO2-reactive residue is not identical with the SHB.     

Effects of some oxidising agents,especially ammonium peroxysulfate,on sugar-beet pectins

Sugar-beet pectins contain feruloyl groups linked to the side chains of the rhamnogalacturonan backbone. Coupling reactions may cross-link these macro-molecules. Of several oxidising agents, only hydrogen peroxide-peroxidase and ammonium peroxysulfate were effective, indicating the involvement of free radicals. The effects of peroxysulfate and pectin concentrations, temperature, and the presence of some additives have been investigated mainly by viscometry and chromatography on Sepharose CL-2B. Depending on the concentration of the pectin, the reaction may be used to obtain either water-soluble products of increased molecular weight or gels.     

Characterization of hop pectins shows the presence of an arabinogalactan-protein

Hop pectins were extracted from spent hops using acid extraction conditions and were characterized chemically. The acid extraction of spent hops resulted in a yield of 2%, containing 59% of polysaccharides. The hop pectins under investigation had a relatively high molecular weight and an intrinsic viscosity comparable to that of commercially available apple and citrus pectins. The low degree of methyl esterification of these pectins implicates that they are mainly suitable for use in calcium gels. The degree of acetylation and the neutral sugar content were relatively high.

A high molecular weight fraction which contained arabinogalactan-proteins was shown to be present in the hop pectin extract after preparative size-exclusion chromatography. Additionally, a fraction with a lower molecular weight was present containing mainly homogalacturonans. The arabinogalactans in the high molecular weight population consisted of (1→3)- and (1→3,6)-linked galactans highly branched with arabinose and galactose side-chains. The protein part of the arabinogalactan-protein (13%) was found to be rich in cystein, threonin, serinin, alanin, and hydroxyprolin. The molecular weight distribution of the hop pectin after degradation with the enzymes endopolygalacturonase plus pectin methyl esterase suggested that the arabinogalactan-protein present in the hop pectin extract was linked to the pectin and that the arabinogalactan-protein itself had a fairly low molecular weight.     

Analysis of different de-esterification mechanisms for pectin by enzymatic fingerprinting using endopectin lyase and endopolygalacturonase II from A. niger

A series of pectins with different distribution patterns of methyl ester groups was produced by treatment with either plant (p-PME) or fungal pectin methyl esterases (f-PME) and compared with those obtained by base catalysed de-esterification. The products generated by digestion of these pectins with either endopectin lyase (PL) or endopolygalacturonase II (PG II) from Aspergillus niger were analysed using matrix assisted laser desorption ionisation mass spectrometry (MALDIMS) and high-performance anion-exchange chromatography with pulsed amperometric or UV detection (HPAEC-PAD/UV). Time course analysis using MALDIMS was used to identify the most preferred substrate for each enzyme. For PL, this was shown to be fully methyl esterified HG whereas for PG II, long regions of HG without any methyl esterification, as produced by p-PME was the optimal substrate. The blockwise de-esterification caused by p-PME treatment gave a decrease of partly methylated oligomers in PL fingerprints, which did not effect the relative composition of partly methylated oligomers. PG II fingerprints showed a constant increase of monomers and oligomers without any methyl ester groups with decreasing degree of esterification (DE), but almost no change in the concentration of partly methylated compounds. PL fingerprints of f-PME and chemically treated pectins showed decreasing amounts of partly methyl esterified oligomers with decreasing DE, together with a relative shift towards longer oligomers. PG II fingerprints were characterised by an increase of partly methylated and not methylated oligomers with decreasing DE. But differences were also seen between these two forms of homogenous de-esterification. Introduction of a certain pattern of methyl ester distribution caused by selective removal of certain methyl ester groups by f-PME is the most reasonable explanation for the detected differences.     

Chromatographic and enzymatic strategies to reveal differences between amidated pectins on a molecular level

The intermolecular distributions of amide groups within two commercial LMA pectins was studied after removal of the methyl esters followed by fractionation of the different populations by anion exchange chromatography. The populations obtained had almost equal degrees of amidation while the values of the degree of blockiness were not the same, indicating also a different intramolecular distribution of the substituents considered as semirandom. Populations from the methyl-esterified amidated pectins showed a rather random distribution for almost all populations. A striking difference between these different populations was that, despite the same level of substitution, the ratio between amide groups and methyl esters varied significantly, indicating a heterogeneous amidation process.     

Immunocytochemical localization of pectins in the maturing anther of Allium cepa L.

Distribution of pectins in cell walls of maturing anther of Allium cepa L. was investigated. The monoclonal antibodies against defined epitopes of pectin were used: JIM5 recognizing unesterified pectin and JIM7 recognizing esterified pectin. It has been found that the cell walls of all anther tissues mainly contain esterified pectins. In the somatic tissues only small amounts of unesterified pectins are present in the cell wall junctions and adjacent middle lamellae and in the cell walls of the connective tissue. Thickening of the epiderm cell walls and growth of trabeculae in endothecium are completed through deposition of esterified pectins. In the cell walls of the middle layer and tapetum, unesterified pectins have been found only prior to their disintegration. The primary wall of microsporocytes is made up mainly of esterified pectins. Unesterified pectins occur outside microsporocytes only prior to the callose isolation stage. The presence of esterified pectins has also been detected on the surface of the callose wall surrounding dividing microsporocytes. Lysis of those pectins takes place after microsporogenesis, simultaneously with the lysis of the callosic walls. Before these processes pectins are unesterified. In the sporoderm of pollen grains mainly esterified pectins occur. They have been localized in the intine and aperture. The level of unesterified pectins in the intine is markedly lower.     

Polypotency of the immunomodulatory effect of pectins

Pectins are the major component of plant cell walls, and they display diverse biological activities including immunomodulation. The pectin macromolecule contains fragments of linear and branched regions of polysaccharides such as homogalacturonan, rhamnogalacturonan-I, xylogalacturonan, and apiogalacturonan. These structural features determine the effect of pectins on the immune system. The backbones of pectic macromolecules have immunosuppressive activity. Pectins containing greater than 80% galacturonic acid residues were found to decrease macrophage activity and inhibit the delayed-type hypersensitivity reaction. Branched galacturonan fragments result in a biphasic immunomodulatory action. The branched region of pectins mediates both increased phagocytosis and antibody production. The fine structure of the galactan, arabinan, and apiogalacturonan side chains determines the stimulating interaction between pectin and immune cells. This review summarizes data regarding the relationship between the structure and immunomodulatory activity of pectins isolated from the plants of the European north of Russia and elucidates the concept of polypotency of pectins in native plant cell walls to both stimulate and suppress the immune response. The possible mechanisms of the immunostimulatory and anti-inflammatory effects of pectins are also discussed.     

Micro-heterogeneity of pectins and calcium distribution in the epidermal and cortical parenchyma cell walls of flax hypocotyl

Summary Pectic polysaccharides are major components of the plant cell wall matrix and are known to perform many important functions for the plant. In the course of our studies on the putative role of pectic polysaccharides in the control of cell elongation, we have examined the distribution of polygalacturonans in the epidermal and cortical parenchyma cell walls of flax seedling hypocotyls. Pectic components have been detected with (1) the nickel (Ni²⁺) staining method to visualize polygalacturonates, (2) monoclonal antibodies specific to low (JIM5) and highly methylesterified (JIM7) pectins and (3) a combination of subtractive treatment and PATAg (periodic acid-thiocarbohydrazide-silver proteinate) staining. In parallel, calcium (Ca²⁺) distribution has been imaged using SIMS microscopy (secondary ion mass spectrometry) on cryo-prepared samples and TEM (transmission electron microscopy) after precipitation of calcium with potassium pyroantimonate. Our results show that, at the tissular level, polygalacturonans are mainly located in the epidermal cell walls, as revealed by the Ni²⁺ staining and immunofluorescence microscopy with JIM5 and JIM7 antibodies. In parallel, Ca²⁺ distribution points to a higher content of this cation in the epidermal walls compared to cortical parenchyma walls. At the ultrastructural level, immunogold labeling with JIM5 and JIM7 antibodies shows a differential distribution of pectic polysaccharides within cell walls of both tissues. The acidic polygalacturonans (recognized by JIM5) held through calcium bridges are mainly found in the outer part of the external wall of epidermal cells. In contrast, the labeling of methylesterified pectins with JIM7 is slightly higher in the inner part than in the outer part of the wall. In the cortical parenchyma cells, acidic pectins are restricted to the cell junctions and the wall areas in contact with the air-spaces, whereas methylesterified pectins are evenly distributed all over the wall. In addition, the pyroantimonate precipitation method reveals a clear difference in the Ca²⁺ distribution in the epidermal wall, suggesting that this cation is more tightly bound to acidic pectins in the outer part than in the inner part of that wall. Our findings show that the distribution of pectic polysaccharides and the nature of their linkages differ not only between tissues, but also within a single wall of a given cell in flax hypocotyls. The differential distribution of pectins and Ca²⁺ in the external epidermal wall suggests a specific control of the demethylation of pectins and a central role for Ca²⁺ in this regulation.Abbreviations Cdta diamino-1,2-cyclohexane tetra-acetic acid - PATAg periodic acid-thiocarbohydrazide-silver proteinate - PGA polygalacturonic acid - PME pectin methylesterase - RG I rhamnogalacturonan I - SIMS secondary ion mass spectrometry - TEM transmission electron microscopy     

Chain-chain interactions for methyl polygalacturonate: models for high methyl-esterified pectin junction zones

The ability of pectins to form gels in the presence of calcium is well-known, and it implies the interaction of carboxylate groups and bivalent ions. However, even when most of the galacturonic units are methyl esterified, pectins are able to form gels but only under certain experimental conditions. In this case, hydrogen bonding and hydrophobic interactions are believed to be responsible for gel formation, and it is likely, as in the other mechanisms of polysaccharide gel formation, that stable junction zones consist of cooperatively ordered chains linked together throughout nonbonded interactions to provide a three-dimensional network. To investigate the junction zones in HM-pectin gels, we investigated, by molecular modeling, all of the ways to associate two, and then three, fully methyl-esterified galacturonic acid chains. Two models are obtained: the first one is based on a packing of parallel chains; it agrees with the hypothetical model derived from fiber diffraction study; the second one displays an antiparallel orientation of the chains; it presents a better arrangement of the chains and, theoretically, a much lower potential energy. In both cases, all of the favorable associations occur within a network of hydrogen bonds and of hydrophobic contacts.     

Structure of citrus pectins and viscometric study of their solution properties

Citrus pectins with degrees of methylation between 30 and 72% were carefully characterized in order to determine their charge density and molecular weight distribution, the content in galacturonic acid and in neutral sugars, the degree of methylation and acetylation. Using enzymic degradation it has been found that pectin molecules consist mainly of long homogalacturonan regions with some regions of neutral sugars as side chains attached on rhamnose residues. The viscometric behaviour of the different samples indicates that 0.1 M NaCl, at 25 degrees C, is a good solvent of sodium pectinates. From the evolution of the Huggins parameter, it appears that pectins with 50% of methylated galacturonic groups exhibit a maximum flexibility. A Mark-Houwink exponent of 0.8 has been found in good agreement with theoretical predictions for flexible polymers in a good solvent.     

Short-term boron deprivation inhibits endocytosis of cell wall pectins in meristematic cells of maize and wheat root apices

By using immunofluorescence microscopy, we observed rapidly altered distribution patterns of cell wall pectins in meristematic cells of maize (Zea mays) and wheat (Triticum aestivum) root apices. This response was shown for homogalacturonan pectins characterized by a low level (up to 40%) of methylesterification and for rhamnogalacturonan II pectins cross-linked by a borate diol diester. Under boron deprivation, abundance of these pectins rapidly increased in cell walls, whereas their internalization was inhibited, as evidenced by a reduced and even blocked accumulation of these cell wall pectins within brefeldin A-induced compartments. In contrast, root cells of species sensitive to the boron deprivation, like zucchini (Cucurbita pepo) and alfalfa (Medicago sativa), do not internalize cell wall pectins into brefeldin A compartments and do not show accumulation of pectins in their cell walls under boron deprivation. For maize and wheat root apices, we favor an apoplastic target for the primary action of boron deprivation, which signals deeper into the cell via endocytosis-mediated pectin signaling along putative cell wall-plasma membrane-cytoskeleton continuum.     

Interchain heterogeneity of enzymatically deesterified lime pectins

Two series of pectins with different levels and patterns of methyl esterification were produced by treatment of a very highly methylated lime pectin with a fungus- or plant-pectin methylesterase. The interchain distribution of free carboxyl groups was investigated by size exclusion and ion exchange chromatography. "Homogeneous" populations with respect to molar mass or charge density were thereby obtained, and their composition, molar mass, and calcium binding properties were investigated. The composition varies from one size exclusion chromatography fraction to another, the highest molar mass fraction being richer in rhamnogalacturonic sequences and exhibiting a slightly higher degree of methylation (DM). Separation of pectins by ion exchange chromatography revealed a narrow charge density distribution for pectins deesterified by fungus-pectin methylesterase, in agreement with a multichain mechanism. Conversely, pectins deesterified by plant-pectin methylesterase exhibited a very large charge density distribution suggesting a processive mechanism. The interchain polydispersity with regard to DM was however shown to have no impact on calcium binding properties of the different fractions. The progressive dimerization through calcium ions with decreasing DM of pectins deesterified by plant-pectin methylesterase seems to be the result of a peculiar intrachain pattern of methyl esterification that can be attributed to a multiple attack mechanism.     

Nanostructure of native pectin sugar acid gels visualized by atomic force microscopy

Height and phase shift images of high methoxyl sugar acid gels (HMSAG) of pectin were obtained by atomic force microscopy in the tapping mode. Images revealed that pores in these gels were fluid and flattened out when measured as a function of time. These images revealed for the first time the structure of adsorbed sugar on pectin in the hydrated native gels and how the pectin framework is organized within these gels. Segmentation of images revealed that the underlying pectin framework contained combinations of rods, segmented rods, and kinked rods connected end to end and laterally. The open network of strands was similar to pectin aggregates from 5 mM NaCl solution imaged earlier by electron microscopy (Fishman et al., Arch. Biochem. Biophys. 1992, 294, 253). Area measurements revealed that the ratio of bound sugar to pectin was in excess of 100 to 1 (w/w). Furthermore, images indicated relatively small differences in the organization of native commercial citrus pectin, orange albedo pectin, and lime albedo pectin gels at optimal pH as determined in this study. The findings are consistent with earlier gel strength measurements of these gels. In addition, values of gel strength were consistent with values of molar mass and viscosity of the constituent pectins in that they increased in the same order. Finally, we demonstrated the advantage of simultaneous visualization of height and phase shift images for observing and quantitating the nanostructure of relatively soft gels which are fully hydrated with a buffer.