The system of galactans from Cryptonemia crenulata (Halymeniaceae, Halymeniales) and the structure of two major fractions. Kinetic studies on the alkaline cyclization of the unusual diad G2S→D(L)6S

Abstract

Cryptonemia crenulata biosynthesizes a family of dl-hybrid galactans that are based on the classical 3-linked β- d-galactopyranosyl→4-linked α-galactopyranosyl alternating sequence (A-units→B-units). The dispersion of structures in these galactans is based on four factors, namely: (a) the amount and position of substituent groups as sulfate (major), pyruvic acid ketals, methoxyl and side substituents of β- d-xylose and/or β- d-galactose; (b) the ratio galactose/3,6-anhydrogalactose in the B-units; (c) the ratio d-/ l-galactoses and 3,6-anhydrogalactoses also in the B-units and (d) the sequence of the diads in the linear backbone. Alkali treatment carried out on the major fraction produced a nearly quantitative formation of 3,6-anhydrogalactose units from precursor units (α-galactose 6-sulfate (major) and α-galactose 2,6-sulfate, minor). Kinetic studies show a rate constant, for the diad G2S-D(L) 6-S, of 1.7 × 10 4 s −1 indicating a reaction faster than in λ-carrageenans but slower than in porphyrans.