Hydrodynamic versus size exclusion chromatography characterization of alternan and comparison to off-line MALS

Alternan is an ultrahigh molar mass polysaccharide composed of alternating α-(1→3) and α-(1→6) repeat units and that also possesses long-chain branching. Its molar mass distribution (MMD) can extend into the hundreds of millions of grams per mole. Characterizing alternan by means of size exclusion chromatography (SEC) is a lengthy process and an incomplete one because even under the best possible experimental conditions, the polysaccharide appears to degrade during its passage through the SEC columns. As an alternative to SEC, we have investigated the use of hydrodynamic chromatography (HDC) as a possible characterization technique for alternan. Results from packed-column HDC with multiangle static light scattering (MALS) detection compare favorably to results from off-line MALS analysis, and HDC results are obtained in a fraction of the time needed for SEC. The largest molar mass alternan did appear to undergo some degradation during HDC as a result of interstitial stresses in the column bed. This could be remedied by the use of columns with larger packing particles. Evidence of long-chain branching in alternan is provided via a comparison of the intrinsic viscosities and viscometric radii of the polysaccharide to those of a pullulan standard and of a theoretical pullulan of molar mass equal to those of the alternans examined.