Interplay of hierarchical dynamics and their microscopic structures of polyampholyte gels and proteins

Polyampholyte gel is a perfect physics model to mimic condensed state of proteins. We have studied the hierarchical dynamics of polyampholyte gels by dynamic light scattering. In addition to the normal gel mode, which indicates the gel elasticity, we also discovered a new mode with a stretched exponential decay with the stretched exponent β = 1/3, and a diffusive exponential decay, which indicates the coupled motion between counterion and the polyampholyte backbone. After dialysis to low salt concentration, the coupled motion of the counterion will go away, so that there are only two modes. Combined with a newly developed theory, we attribute this stretched exponential mode to hierarchical dynamics of the segments between two crosslinking junctions, whose segmental distribution obeys Poisson distribution. As salt concentration inside the gel increases, β decreases from 0.38 to 0.33, which is consistent with theoretical results. The gel with the molar charge ratio R=1, which is at the charge balance point, has the highest value β = 0.38. As long as R deviates further away from the charge balance point from either side, the β values decrease. When the gel is 100% positive charged, their dynamic light scattering results will go back to that of the normal polyelectrolyte gels.

聚两性电解质凝胶是模拟蛋白质凝聚态的理想物理模型。我们通过动态光散射研究了聚两性电解质凝胶的分级动力学。除了表明凝胶弹性的正常凝胶模式外，我们还发现了一种新的模式，其具有拉伸指数衰减（拉伸指数β = 1/3）以及扩散指数衰减，这表明了反离子与聚两性电解质主链之间的耦合运动。在透析至低盐浓度后，反离子的耦合运动将会消失，因此仅存在两种模式。结合一种新发展的理论，我们将这种拉伸指数模式归因于两个交联节点之间链段的分级动力学，其链段分布服从泊松分布。随着凝胶内部盐浓度的增加，β从0.38降低至0.33，这与理论结果相符。摩尔电荷比R = 1（处于电荷平衡点）的凝胶具有最高的β值（0.38）。只要R从任一侧进一步偏离电荷平衡点，β值就会降低。当凝胶100%带正电时，其动态光散射结果将回到普通聚电解质凝胶的结果。