PHYSICOCHEMICAL STUDIES ON XYLINAN (ACETAN) .3. HYDRODYNAMIC CHARACTERIZATION BY ANALYTICAL ULTRACENTRIFUGATION AND DYNAMIC LIGHT-SCATTERING

A laboratory-made sample of the polysaccharide xylinan (acetan) has be en further characterized with respect to (i) purity, (ii) molar mass a nd polydispersity, and (iii) guess conformation by a combination of hy drodynamic measurements (sedimentation velocity and equilibrium analyt ical ultracentrifugation, viscometry, and dynamic light scattering) in aqueous NaCl (I = 0.10 mol . L(-1)). Sedimentation velocity diagrams recorded using Schlieren optics revealed highly pure material sediment ing as a single boundary [s(20,w)(o) = 95 +/- 0.7) S; k(s) = (273 +/- 112) mL/g]. The hypersharp nature of these boundaries is symptomatic o f a polydisperse and highly nonideal (in the thermodynamic sense) syst em. Low speed sedimentation equilibrium in the analytical ultracentrif uge using Rayleigh interference optics and two different types of extr apolation procedure (involving point and whole-cell molar masses) gave a weight average molar mass M(w), of(2.5 +/- 0.5) x 10(-6) g . mol(-1 ) and also a second virial coefficient, B = (2.8 +/- 0.7) x 10(-4) mL . mol . g(-2), both values in good agreement with those from light sca ttering-based procedures (Part II of this series). A dynamic Zimm plot from dynamic light scattering measurements gave a z-average translati onal diffusion coefficient D-20,w(o) = (3.02 +/- 0.05) x 10(-8) cm(2) . s(-1) and the concentration-dependence parameter k(D) = (370 +/- 15) mL/g. Combination of s(20,w)(o) with D-20,w(o) via the Svedberg equat ion gave another estimate for M(w) of approximate to 2.4 x 10(6) g/mol , again in good agreement. Both The Wales-van Holde ratio (ks/[eta]) a pproximate to 0.4 (with [eta] = (760 +/- 77) mL/g) and the rho-paramet er (ratio of the radius of gyration from static light scattering to th e hydrodynamic radius from dynamic light scattering) as rho > 2.0 all indicate an extended conformation for the macromolecules in solution. These findings, plus Rinde-type simulations of the sedimentation equil ibrium data are all consistent with the interpretation in terms of a u nimodal wormlike coil model performed earlier. (C) 1996 John Wiley & S ons, Inc.