Galacturonic acid esters

Scheme 13 Difference in reactivity between a- and (3-galacturonic acid ester C-4-OH acceptors...

Pectin belongs to a family of plant polysaccharides in which the polymer backbone consists of (1— 4)-linked a-D-galacturonic acid repeating-units. Often, (1— 2)-linked a-L-rhamnose residues interrupt the regular polygalacturonate sequence. The high viscosity and gelling properties of pectins are exploited by the food and pharmaceutical industries. X-Ray studies on sodium pectate, calcium pectate, pectic acid, and pectinic acid (methyl ester of pectic acid) have disclosed their structural details. 

The pyranose structure of D-galacturonic acid, which lactonizes with great difficulty, has been confirmed by Levene and Kreider66 from a study of the methylated ester of the uronoside. Methyl a-D-galacturonoside (XXXIII) was methylated with silver oxide and methyl iodide and there was obtained methyl 2,3,4-trimethyl-a-D-galacturonoside methyl ester (XXXVI) which on oxidation was converted to d,l(2,3,4-trimethylmucic) acid (XXXVII) and h-arabo-trimethoxyglutaric acid (XXXVIII). 

Fig. 1. — Effect47 of Tomato Pectinesterase on the Activity of Pectate Lyase Acting on Highly Esterified Pectin. [Reaction mixtures contained 0.5% of D-galacturonan (broken line) or poly(D-galacturonic acid methyl ester) methyl glycoside (solid lines), 5 mM CaCl2, 33 mM phosphate buflFer (pH 7.0), and 0.25 unit of lyase in a final volume of 3.0 ml. In addition, the reaction mixture represented by the middle curve contained 0.031 unit of tomato pectinesterase in a final volume of 3.3 ml.] Reprinted, with permission, from M. Lee and J. D. Macmillan, Biochemistry, 9, 1930-1934 (1970). Copyright by the American Chemical Society.

Polymers of uronic acids are encountered in nature in structures known as pectins, which are essentially chains of D-galacturonic acid residues linked al -> 4, though some of the carboxyl groups are present as methyl esters. These materials are present in the cell walls of fruit, and the property that aqueous solutions under acid conditions form gels is the basis of jam making. 

Galacturonic acid is the major constituent of all natural pectins. Pectins also contain varying quantities of neutral sugars, principally arabi-nose, galactose and rhamnose (5). The carboxyl function of the galactur-onosyl residues may be present as a methyl ester, acid or salt. 

Citrus pectin ( Polygalacturonic Acid Methyl Ester from Citrus Fruits, Grade I ) was obtained from the Sigma Chemical Company. It had a galacturonic acid content of 89% and a degree of esterification of 57. Separate aqueous solutions of citrus pectin were freeze-dried and air-dried in deionized water. These samples were replicated with 9.8A Pt/C and backed with 148A of carbon. The replicas for these samples were picked up without a carbon support film (38). 

---