The concentration dependence of the sedimentation rate of amylose, cellulose, and dextran in aqueous solution

✍ Scribed by Hans Elmgren

• Publisher: Elsevier Science
• Year: 1987
• Tongue: English
• Weight: 954 KB
• Volume: 160
• Category: Article
• ISSN: 0008-6215
• DOI: 10.1016/0008-6215(87)80314-2

✦ Synopsis

The polymers amylose, cellulose, and dextran are compared with regard to their conformation. The comparison is based on a combination of sedimentation and electron-spin resonance or fluorecence-depolarization data according to a theory developed for the sedimentation of polymer coils. The comparison shows that the amylose coils are built up from heavy, compact segments (presumably helices). Cellulose coils consist of heavy, expanded segments. Dextran coils are flexible. These findings are not new. The advance made with the theory presented is that it gives a new way to determine the chain stiffness. Another advance is that it makes the extrapolation for s,, more precise; it is demonstrated for the case of cellulose in the solvent Cadoxen. A new picture of the molecular mass dependence on s,, is obtained thereby. From measured relations between the solvent mobility and the concentration, it is possible to calculate how the solvent mobility changes with the distance from a polymer segment (the mobility profile). A method for such calculations is described. The mobility profiles in the systems studied are shown, together with that of polystyrene in a theta-solvent. THEORY According to classical sedimentation theory', the relation between the sedimentation coefficient s and the friction coefficient f of a sedimenting species is where the index zero refers to zero concentration conditions. The relation is based on the balance of the driving force (which is proportional to the effective mass) and the friction force (which is the friction coef&ient times the sedimehtation rate of solute through the solvent). The observed (normalized) sedimentation rate s is measured with the cell as reference. The sedimenting volume forces solvent up-wards2. The speed which gives rise to the friction force is thus higher than s, except at zero concentration.