Collins K D
Department of Biochemistry and Molecular Biology, University of Maryland Medical School, Baltimore 21201-1503, USA.
Biophys J. 1997 Jan;72(1):65-76. doi: 10.1016/S0006-3495(97)78647-8.
Small ions of high charge density (kosmotropes) bind water molecules strongly, whereas large monovalent ions of low charge density (chaotropes) bind water molecules weakly relative to the strength of water-water interactions in bulk solution. The standard heat of solution of a crystalline alkali halide is shown here to be negative (exothermic) only when one ion is a kosmotrope and the ion of opposite charge is a chaotrope; this standard heat of solution is known to become proportionally more positive as the difference between the absolute heats of hydration of the corresponding gaseous anion and cation decreases. This suggests that inner sphere ion pairs are preferentially formed between oppositely charged ions with matching absolute enthalpies of hydration, and that biological organization arises from the noncovalent association of moieties with matching absolute free energies of solution, except where free energy is expended to keep them apart. The major intracellular anions (phosphates and carboxylates) are kosmotropes, whereas the major intracellular monovalent cations (K+; arg, his, and lys side chains) are chaotropes; together they form highly soluble, solvent-separated ion pairs that keep the contents of the cell in solution.
高电荷密度的小离子（促溶剂）能强烈结合水分子，而低电荷密度的大单价离子（离液剂）相对于本体溶液中水分子间相互作用的强度而言，与水分子的结合较弱。这里表明，只有当一个离子是促溶剂而带相反电荷的离子是离液剂时，结晶碱金属卤化物的标准溶解热才为负（放热）；已知随着相应气态阴离子和阳离子的绝对水合热之差减小，该标准溶解热会按比例变得更正。这表明内界离子对优先在具有匹配绝对水合焓的带相反电荷的离子之间形成，并且生物组织源于具有匹配绝对溶解自由能的部分的非共价缔合，除非消耗自由能使它们分开。主要的细胞内阴离子（磷酸盐和羧酸盐）是促溶剂，而主要的细胞内单价阳离子（K⁺；精氨酸、组氨酸和赖氨酸侧链）是离液剂；它们共同形成高度可溶的、溶剂分隔的离子对，使细胞内容物保持在溶液中。
