Timasheff S N
Department of Biochemistry, Brandeis University, 415 South Street, MS 009, Waltham, MA 02254-9110, USA.
Proc Natl Acad Sci U S A. 1998 Jun 23;95(13):7363-7. doi: 10.1073/pnas.95.13.7363.
In the practice of "osmotic stress," the effect of excluded cosolvents on a biochemical equilibrium is interpreted as the number of water molecules participating in the reaction. This action is attributed to lowering of solvent water activity by the cosolvent. This concept of osmotic stress in disperse solution is erroneous: (i) A cosolvent cannot be both excluded and inert, i.e., noninteracting, because exclusion requires a positive free energy change; (ii) a decrease in water activity alone by addition of solute cannot affect an equilibrium when the reacting surface is in contact with the solvent; and (iii) osmotic stress in disperse solution is a restricted case of preferential interactions; the reaction is driven by the free energy of cosolvent exclusion, and the derived number of water molecules is solely a measure of the mutual perturbations of the chemical potentials of the cosolvent and the protein.
在“渗透应激”的实践中，被排除的共溶剂对生化平衡的影响被解释为参与反应的水分子数量。这种作用归因于共溶剂降低了溶剂水的活度。分散溶液中这种渗透应激的概念是错误的：（i）共溶剂不能既被排除又呈惰性，即不相互作用，因为排除需要正的自由能变化；（ii）当反应表面与溶剂接触时，仅通过添加溶质降低水的活度并不能影响平衡；（iii）分散溶液中的渗透应激是优先相互作用的一种受限情况；反应是由共溶剂排除的自由能驱动的，而推导得出的水分子数量仅仅是共溶剂和蛋白质化学势相互扰动的一种度量。
