This study builds on previous studies of the important commercial polysaccharide kappa (kappa)-carrageenan by confirming, using quite different methdology, the molecular weight and the asymmetric conformation of the kappa-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,M-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 [eta] of 630 +/- 60) ml/g and a Huggins constant K-eta of similar to 0.36. From these data, the hydration independent Wales/van Holde ratio (k(s)/[eta]) of similar to 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 similar to 15:1. These data also yield a frictional ratio f/f(o) of similar to 7.6 which is consistent with a large hydration (similar to 50 g water per g of dry polysaccharide, corresponding to a molecular expansion of similar to 100x), consistent with one of kappa-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. (C) 1997 Elsevier Science Ltd.
