Analysis of the Substituent Distribution in Carboxymethyl-1,4-glucans on Different Structural Levels—An Ongoing Challenge

Carboxymethylglucans (CMGs) are widely used semisynthetic polyelectrolytes, e.g., for pharmaceuticals. They are produced in heterogeneous processes on activated starch granules or cellulose fibers. In contrast to neutral ether derivatives, a lower DS in the range, commonly between 0.6 and 1.2, is sufficient to achieve the water solubility of CM cellulose. The high proportion of unsubstituted domains, which could aggregate and therefore only swell and form gel particles but do not dissolve, places higher demands on the statistical distribution of the substituents. The knowledge of regioselectivity, essential for the interpretation of higher structural-level data, can be obtained by various methods, preferentially by CE/UV after hydrolysis. To study the distribution of substituents at the polymer level by mass spectrometric (MS) analysis, partial random depolymerization is required. Due to the ionic character and acid functionality, all the attempts of the direct depolymerization of CMG and further sample preparation suffered from bias, side reactions, and multiple ion formation in MS. Finally, the transformation of CMGs to the corresponding hydroxyethylglucans (HEGs) by the reduction of the esterified carboxy groups with LiAlH₄ opened the window for quantitative oligomer MS analysis. While the CM amyloses were reduced quantitatively, the transformation of the CMC was only about 60% but without the formation of mixed CM/HE ethers.