Galactans from Cryptonemia species . Part II: Studies on the system of galactans of Cryptonemia seminervis (Halymeniales) and on the structure of major fractions

Abstract

Cryptonemia seminervis biosynthesizes a family of d, l-hybrid galactans based on the classical 3-linked β- d-galactopyranosyl→4-linked α- d- and α- l-galactopyranosyl alternating sequence (A-units→B-units) with major amounts of α- d- and α- l-galactose and 3,6-anhydro- d- and l-galactose and lesser percentages of 3,6-anhydro-2- O-methyl- l-galactose, 2- O-methyl-, 4- O-methyl- and 6- O-methylgalactoses. The dispersion of structures in this family is based on five structural factors, namely: (a) the amount and position of substituent groups as sulfate (major), pyruvic acid ketals, methoxyl and glycosyl side-chain (4- O-methyl galactopyranosyl and/or xylosyl); (b) the ratio galactose/3,6-anhydrogalactose in the B-units; (c) the ratio d, l-galactoses and d, l-3,6-anhydrogalactoses also in the B-units, (d) the formation of diads and (e) the sequence of the diads in the linear backbone. Considering these variables it is not unexpected to find in the fractions studied at least 18 structural units producing highly complex structures. Structural studies carried out in two major fractions (S2S-3 and S2S-4) showed that these galactans were formed mainly by β- d-galactopyranosyl 2-sulfate (20 and 11.9 mol %), β- d-galactopyranosyl 2-sulfate 4,6- O-(1′-carboxyethylidene) (8.9 and 6.0 mol %) and β- d-galactopyranosyl 2,6-sulfate (5.4 and 18.6 mol %), together with 3,6-anhydro-α- l-galactopyranosyl (11.4 and 7.3 mol %) and 3,6-anhydro-α- l-galactopyranosyl 2-sulfate (4.9 and 15.4 mol %) and minor quantities of 12–15 other structural units. Preparative alkaline treatment carried out on fraction (S2S-3) produced a quantitative formation of 3,6-anhydro α- l-galactopyranosyl units from precursor units (α- l-galactose 6-sulfate and α- l-galactose 2,6-sulfate). Kinetic studies on this 3,6-anhydro cyclization show a rate constant of 5.2 × 10 4 s −1 indicating diads of the type G→L6S/2,6S. Data from chemical, spectroscopic and kinetic studies suggest that, in S2S-3, the agaran block in the d, l-hybrid galactan is composed of the following diads: G(6R)→L6S/2,6S and G2S(P)(2,6S)→LA(2S)(2R)(2M) and the carrageenan block of G2S(P)→D(2S)(2,3S)(3S)(3,6S) in a molar ratio of agaran to carrageenan structures of ∼2:1.