Solution properties of dextran in water and in formamide

We investigate here solution properties of the common polysaccharide dextran (with various molecular weight fractions: T70, T110, T500, and T2000) in aqueous and in formamide solutions at 20°C and 40°C, as obtained from viscosity measurements. Dextran behaves as a random coil in water and formamide solutions on the basis of the Mark-Houwink-Sakurada constant being in the range 0.5< α < 0.8 and the molecular weight dependence of the hydrodynamic coil radius, R _coil ∼ M _W ^1/2. Dextran segments interact more favorably with the formamide solvent molecules than with water, as attested by higher intrinsic viscosity, R _coil, and hydrodynamic expansion factor α _η values in formamide compared to aqueous solutions. This can be attributed to formamide offering more readily available sites (C=O and NH ₃) to form hydrogen bonds with dextran. Apparently the stronger cohesive forces (reflected in higher surface tension σ and solubility parameter δ values) between water molecules make it harder for dextran to break them, thus the dextran coils swell less in this medium. As the temperature increases from 20°C to 40°C, dextran is driven toward theta conditions in both water and formamide solutions. Upon heating, the polymer segments interact less with the solvent molecules and adopt a less expanded conformation in the solution; the solvent quality decreases, and the coils contract.