Further observations on the size, shape and hydration of kappa-carrageenan in dilute solution

Abstract

This study builds on previous studies of the important commercial polysaccharide kappa (κ)-carrageenan by confirming, using quite different methdology, the molecular weight and the asymmetric conformation of the κ-carrageenan molecule and also provides an estimate of its large capacity to imbibe water. Studies (sedimentation velocity and equilibrium analytical ultracentrifugation and viscometry) were performed on unfractionated material in a dilute solution based on a sodium phosphate/chloride buffer (pH 6.5, I = 0.10). Low-speed sedimentation equilibrium using Rayleigh interference optics and using three different methods of extrapolation procedure yields a concensus weight average molecular weight, M w , of (300 000 ± 40 000) g mol ; this (i) demonstrates the relation between ‘whole cell average’ and ‘point weight average’ molecular weight approaches and (ii) is consistent with other published values based on light scattering procedures. A single non-ideality virial coefficient was shown to be insufficient to explain the concentration dependence behaviour of the apparent weight average molecular weight, M w, app . Sedimentation velocity yields a sedimentation coefficient, s 20, w of (4.19 ± 0.20) S and a sedimentation concentration regression coefficient, k s of (591 ± 40) ml/g; low-shear viscometry yielded an intrinsic viscosity [η]of (630 ± 60)ml/g and a Huggins constant K η of ~0.36. From these data, the hydration independent Wales/van Holde ratio ( k s [η] ) of ~0.9 is consistent with an extended conformation and making the crude approximation of a rigid structure, corresponds to an equivalent hydrodynamic prolate ellipsoid of aspect ratio ~15:1. These data also yield a frictional ratio f f o of ~7.6 which is consistent with a large hydration (~50 g water per g of dry polysaccharide, corresponding to a molecular expansion of ~100×), consistent with one of κ-carrageenans key functional properties in foods as a high water binder. No further comment is made about the order-disorder transition claimed for these molecules.