Sodium ι-Carrageenan: A Paradigm of Polymorphism and Pseudopolymorphism

Abstract

X-ray fiber diffraction patterns of the sodium salt of ι-carrageenan correspond to three (I, II, and III) distinct packing arrangements of half-staggered, 3-fold, double helices having nearly the same helical repeat (c-axis 13.0−13.2 A). While two of the three forms crystallize in trigonal nets with a = 24.02 for I and 21.8 A for III, II favors an orthogonal net with a = 13.70 and b = 20.08 A. Detailed structure analysis of I has shown that three helices in the unit cell, each pair at 13.9 A apart, interact only through their 2-sulfate and 4-sulfate groups with the aid of sodium ions and ordered water molecules [Janaswamy, S.; Chandrasekaran, R. Carbohydr. Res. 2001, 335, 181−194]. The ability of sodium to induce two other crystalline structures, in which the helix−helix separations are considerably shorter (12.2 in II and 12.6 A in III), suggests that interhelical interactions vary significantly among the polymorphs. These observations have ramifications to the practical applications of ι-carrageenan in ...