What molecule am I?
D-Galacturonic acid (GalA) is an oxidized form of the monosaccharide D-galactose1, a component of the disaccharide lactose. Its structure is sometimes displayed in an open, linear form, with a carboxylic acid group on one end of the chain and an aldehyde on the other; but it is more frequently depicted as a closed aldopyranose. This cyclic form can take two configurations: α-D-galacturonic acid, with the hydroxyl adjacent to the ring oxygen in the axial position (shown here), or the β-epimer with the same hydroxyl in the equatorial position.
Why is GalA significant? It is the primary building block and structure-giving element of pectin2 and other biopolymers found throughout the plant kingdom. The polymeric GalA chains in pectin are connected by α-1,4 glycosidic bonds; some of its carboxyl groups are in the form of methyl esters. Pectin molecules have atomic masses of 200,000 g/mol or higher.
According to the Merck Index, pectin is found in the cell walls of all plant tissues and functions as an intercellular connecting material. It is especially abundant (≈30 wt%) in citrus rinds; but it is also commercially sourced from apples, spinach, sugarbeets, and other fruits and vegetables (see photo). Its main use is as a gelling or filling agent in foods such as jellies, jams, desserts, and candies and as a stabilizer in juices and milk-based drinks.
Henri Braconnot at the Royal Academic Society of Nancy (France) first described pectin in 1824. The primary source of commercial GalA is the hydrolysis of pectin, a process that was first reported by Felix Ehrlich3 at the University of Breslau (Germany; now the University of Wrocław [Poland]) in 1917. The process was refined in 2004 by Tetsuya Miyazawa and Toshitaka Funazukuri* at Chuo University (Tokyo). Starting with poly(galacturonic acid) and water with no additives, they used a semibatch flow reactor (220 °C, 10 MPa pressure, 2 min heating time) to obtain a 79% yield of water-soluble products, which were then enzymatically hydrolyzed to GalA and its dimer and trimer.
An internet search showed that worldwide production and consumption of GalA are steadily increasing, but specific numbers are available only in expensive market research reports.
1. CAS Reg. No. 59-23-4.
2. CAS Reg. No. 9000-69-5.
3. Ehrlich was a prominent biochemist. Well before his work on the structure of pectin, he discovered the amino acid isoleucine as a student in 1903.
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