抄録
The presence of big solutes generates the regions from which the solvent molecules are excluded. When big solutes touch each other, the excluded regions overlap and the volume available to the solvent molecules increases by this amount. The touching leads to a gain in the translational entropy of the solvent and a decrease in the system free energy. A three-dimensional integral equation theory is solved on a cubic grid to calculate the interaction entropically induced between big bodies with high asphericity immersed in dense small spheres. The pattern of the aggregation of big bodies, which is strongly dependent on the big-body shapes, is discussed. Cases where the big-body shapes are changeable are also analyzed. It is shown that even the presence of a single big rigid disc is capable of changing a number of big spheres into discs with the same shape, which stack together to form a cylindrical aggregate. The fully extended polypeptide chain has a long spherocylindrical shape with small diameter and is capable of forming β-strands by folding the chain back and forth, leading to the formation of a unit with a disc-like shape. However, an isolated protein molecule is not likely to be transformed into such a unit. Partially unfolded molecules aggregate, and the transformation proceeds in an aggregate. Once a sufficiently large, rigid aggregate is formed, even folded molecules are sequentially incorporated in the aggregate. The aggregate continues to elongate with the diameter unchanged. The long cylinders thus constructed are driven to aggregate laterally or to be twisted together, leading to the amyloid fibril formation. The formation could be categorized as an ordered arrangement of protein molecules occurring to maximize the total volume available to the thermal motion of the solvent molecules.
