Xxvi. the Nature of the Pectic Substances of Flax

As the result of recent researches our knowledge of the pectic substances of plants has become more systematic, and it is proposed in the present paper to adhere to the nomenclature adopted by the American Pectin Symposium of 1925. Thus we are now acquainted with protopectin, the naturally occurring pectic substance; pectin, the substance extracted from the plant by chemical or other means with the minimum of chemical degradation; and pectic acid, a derived product. For the purposes of the present communication it is necessary to refer briefly to some recent investigations, commencing with the isolation of pectic acid [Clayson, Norris and Schryver, 1921]. It was shown that pectic acid could be prepared from a large variety of plant sources, that it appeared to be of constant, or very nearly constant, composition without reference to the particular source, and was, in fact, a definite chemical entity. Important researches, notably by Ehrlich [1917, 1924], culminated in the suggestion of a ring formula for pectic acid by Nanji, Paton and Ling [1925]. According to this suggestion, pectic acid is the basal substance to which the other pectic substances may be referred, and the molecule is a closed six-membered ring, whose members represent one molecule of anhydro-arabinose, one molecule of anhydro-galactose and four molecules of galacturonic anhydride, the four carboxyl groups of the last being free. Pectin, or the "pectinogen" of Schryver and Haynes [1916] and Norris and Schryver [1925], contains, in addition, methyl alcohol to a varying extent, in ester combination with the free carboxyl groups of the original pectic acid. No serious objections to this suggested formula have been advanced since its publication, nor has the uniform nature of pectic acid derived from different sources been disputed. Only in one case has there been any indication of a departure from this generalisation, that of the pectic substance of flax. Ehrlich and Schubert [1926] suggested the presence of xylose in addition to the usual constituents of the molecule, but this has not been confirmed. In a recent paper on the subject, Henderson [1928] casts some doubt on the validity of the ring formula in the case of flax pectin, and rejects it in favour of a simpler galactose-tetragalacturbnic acid formula: (C6H10O5 4C6H806 H2-)-; arabinose is regarded as an adventitious adsorbed impurity.