Laurent T C, Sundelöf L O, Wik K O, Wärmegård B
Eur J Biochem. 1976 Sep;68(1):95-102. doi: 10.1111/j.1432-1033.1976.tb10767.x.
A free-diffusion method has been developed for the determination of the intradiffusion coefficient ('self-diffusion coefficient') of a polymer in highly concentrated solutions. A fraction of the polymer is labelled with a small amount of light-absorbing substituent. The diffusion of this labelled species, present in low concentration, is followed in the presence of a high concentration of unlabelled material with the aid of absorption optics in the analytical ultracentrifuge. The diffusion proceeds over a boundary at which the difference in concentration of unlabelled material is varied. The average concentration of total polymer and the concentration of the labelled material are, however, constant. From theoretical considerations it is shown that by extrapolation of the diffusion coefficient so obtained to zero concentration difference of total material, the intradiffusion coefficient of the polymer at that concentration is obtained. The procedure also permits the ordinary translational diffusion coefficient to be estimated. The method has been applied to two dextran fractions with weight-average molecular weights of 19000 and 150000, which were labelled with fluorescein groups. As expected, the intradiffusion coefficient decreases with increasing polymer concentration, the decrease being more pronounced for the high-molecular-weight material. This decrease in the diffusion rate of dextran is, however, less than the corresponding decrease in the sedimentation rate which proteins with similar hydrodynamic parameters experience in dextran solutions. This agrees with the hypothesis that flexible linear polymers move through a network as chains rather than as hydrodynamic spheres. By combining measurements of the ordinary diffusion coefficient and the intradiffusion coefficient, it is possible to calculate the thermodynamic properties (as expressed by the virial expansion) of the system. This method is of particular importance in studies on concentrated solutions of high-molecular-weight polymers.
已开发出一种自由扩散法，用于测定聚合物在高浓度溶液中的内扩散系数（“自扩散系数”）。聚合物的一部分用少量吸光取代基进行标记。借助分析超速离心机中的吸收光学装置，在高浓度未标记物质存在的情况下，跟踪这种低浓度存在的标记物质的扩散。扩散在未标记物质浓度有差异的边界上进行。然而，总聚合物的平均浓度和标记物质的浓度是恒定的。从理论考虑可知，通过将如此获得的扩散系数外推至总物质浓度差为零，可得到该浓度下聚合物的内扩散系数。该方法还允许估算普通平移扩散系数。该方法已应用于两种重均分子量分别为19000和150000的葡聚糖级分，它们用荧光素基团进行了标记。正如预期的那样，内扩散系数随聚合物浓度增加而降低，高分子量物质的降低更为明显。然而，葡聚糖扩散速率的这种降低小于具有相似流体动力学参数的蛋白质在葡聚糖溶液中沉降速率的相应降低。这与柔性线性聚合物以链状而非流体动力学球体的形式在网络中移动的假设相符。通过结合普通扩散系数和内扩散系数的测量，可以计算该体系的热力学性质（由维里展开式表示）。该方法在高分子量聚合物浓溶液的研究中尤为重要。
