Pectic acid
Main page |
| Other names/abbreviations: | Pectin |
| Material type: | Polymer |
| Material group: | Hetero polysaccharide, linear Pectic substance |
| Monomer(s): | L-arabinopyranose D-galactose D-galacturonic acid L-rhamnose |
| Links: | alpha-(1-2) alpha-(1-4) |
| Origin of the polymer: | Natural |
| CAS-Number: | 9000-69-5 |
Structure
Pectic substances comprise a group of plant polysaccarides in which D-galacturonic acid is the principal unit. The basic structure in pectin is (→4)-α-D-GalA-(1→) (pectic acid), with interruptions of (1→2)-α-L-rhamose units inserted in the chain (pectin). The substitution is typically around 10%. The backbone may be partially acetylated and may be further substituted with terminal xylose. The galacturonans can be methylated i.e. methyl esterified [7]. The degree of methylation (DM) is often used for destinction of commercial grades of pectin. The rheological properties of pectin is further controlled by the degree of esterification (DE) (methyl esters) [8].A part of the rhamnose units are branching points for neutral sugar side chains of L-arabinose and D-galactose [7]. The galactans are mainly linear chains of (→4)-α-D-Gal-(1→), whereas the arabinans are principally chains of (1→5)-linked-α-L-arabinofuranosyl residues with substituents at C(3) of additional units of the same sugar or of D-galactopyranose. In a few cases, well-defined L-arabino-D-galactans have been isolated [1]. The rhamnosyl substitution is thought to cluster in "hairy" regions, leaving "smooth" galacturonan sequences as the backbone [7].
X-ray difrraction patterns from polycrystalline fibers of sodium pectate show that the molecule forms 3-fold helices that pack in a rectangular unit cell of dimensions a= 8.39 Å, b= 14.27Å and c= 13.36 Å [5].
Chemical properties
Behavior in solution:The degree of methylation is devided in five destinct categories [2]:
1) 30 DM for low sugar gels.
2) 45 DM pectin for rapid-setting calcium-precipitable pectin suitable for high-sugar gels and emulsions.
3) 60 DM or slow-set pectin for high sugar gels and confectionary jellies.
4) 74 DM for typical rapid-set pectin for jams and jellies.
5) higher DMs for special-purpose applications.
Sometimes the following destinction is used [2]:
1) Rapid-set, 70% DE or higher.
2) Slow-set, 50-70% DE.
3) Low methoxy pectin 50% DE or lower.
The solubility decreases with the increase of lenght and the decrease of methoxyl groups [2].
The viscousity of pectin solutions depends on DM, concentration, pH, presence of salts and their concentration and temperature. No gel is formed above pH 3.6. Optimum performance for slow-setting pectins is mostly 2.8-3.2 and for rapid-setting pectins it is mostly pH 3.0-3.4. The effect of pH, however, is not entirely predictable. The effect of alkali metal ions is not predictable either. At low pH the presence of alkali metal ions will increase the viscousity, but at pH ~6 the viscousity is solely dependent on the lenght and shape of the chains. Calcium can increase viscousity of pectin even at pH 8.55. The viscousity increases at lowering the degree of methylation as calcium cross-link the pectin chains [2]. Experiments with calcium also suggests that gelling is initialized by binding of Ca++, followed by network formation through chain/chain association [7].
The gelling of pectin is removed if the pectin is converted into the ethyl- og 2-hydroxyethyl ester [6]. This show that there is a steric fit between methyl D-galactopyranosiduronate residues in the helix-forming junctions [6]. One actylation per eight residues is enough to diminish the gelling drastically [6].
An experiment on swelling of pectin gels as a function of the concentration of Ca++ show, that the neutral components of the network does not have a high affininty for water [7].
Complex formation: Calcium
The affinity for divalent ions like Ca++ increases with decreasing DE and increasing length of galacturonan backbone. High DE pectins have been observed to gel in the presence of Ca++, this is believed to be the an "eggbox" conformation in which Ca++-ions are enclosed [7].
Biological properties
Gastrointestinal tract:Pectins are degestible by humans [2].
Antitumor activity:
Inhibit tumor growth [3].
Molecular biology:
10-35% inhibition on the restriction enzyme HindIII at 10 μg polysaccharide / μg λ DNA, 75-95% inhibition at 100 μg polysaccharide / μg λ DNA, and 100% inhibition at 500 μg polysaccharide / μg λ DNA [4].
Physical properties
Appearance | ||
| Physical state @ 20°C: | Solid | |
| Color: | White to off-white | |
Occurence, isolation & synthesis
OccurenceWidespread in higher plants, most commonly as interrupted chains in rhamnogalacturonans [1].
References
1: H.F.Mark, N.M.Bikales, C.G.Overberger, G.Menges, J.I.Kroschwitz. Encyclopedia of Polymer Science and Engineering 2.ed.(1988) John Wiley & Sons
2: Glicksman,M. Gum Technology in the Food Industry
(1969) Academic Press
3: Taper,H.S., Delzenne,N.M., Roberfroid,M.B. Growth Inhibition of Transplantable Mouse Tumors by Non-Digestible Carbohydrates
Int. J. Cancer (1997) 71 1109-1112
4: Do,N., Adams,R.P. A simple technique for removing plant polysaccharide contaminants from DNA
Biotechniques (1991) 10 162-166
5: Millane,R.P., Arnott,S. Ordered water in hydrated solid-state polysaccharide systems
Adv. Exp. Med. Biol. (1991) 302 785-803
6: Rees,D.A. Structure, conformation, and mechanism in the formation of polysaccharide gels and networks
Adv. Carbohydr. Chem. Biochem. (1969) 24 267-332
7: Tibbits,C.W., MacDougall,A.J., Ring,S.G. Calcium binding and swelling behaviour of a high methoxyl pectin gel
Carbohydrate Research (1998) 310 (1-2) 101-107
8: Chandrasekaran,R. X-ray Diffraction of Food Polysaccharides
Adv. Food Nutr. Res. (1998) 42 131-210
© Michael Pilgaard
Created: March 22, 2008