Yang P H, Rupley J A
Biochemistry. 1979 Jun 12;18(12):2654-61. doi: 10.1021/bi00579a035.
Calorimetric measurements of the heat capacity of the lysozyme-water system have been carried out over the full range of system composition at 25 degrees C. The partial specific heat capacity of the protein in dilute solution is 1.483 +/- 0.009 J K-1 g-1. The heat capacity of the dry protein is 1.26 +/- 0.01 J K-1 g-1. The system heat capacity responds linearly to change in composition from dilute solution to 0.38 g of water per g of protein (h) and is an irregular function at lower water content. The break in the heat capacity function at 0.38 h defines the amount of water needed to develop the equilibrium solution properties of lysozyme as being 300 molecules of water/protein molecule, just sufficient for monolayer coverage. The heat capacity behavior at low water content describes three hydration regions. The most tightly bound water (0-0.07 h), probably principally bound to charged groups, is characterized by a partial specific heat capacity of 2.3 J K-1 g-1, a value close to that for ice. A heat of reaction associated with proton redistribution is reflected in the heat capacity function for the low-hydration region. Between 0.07 and 0.25 h the heat capacity increases strongly, which is understood to reflect the growth of patches of water covering polar and adjacent nonpolar portions of the protein surface. The hydration shell is completed by condensation of solvent over the weak-interacting portions of the surface, in a process displaying a transition heat.
已在25摄氏度下对溶菌酶 - 水体系的热容进行了全体系组成范围内的量热测量。稀溶液中蛋白质的偏比热容量为1.483±0.009 J K⁻¹ g⁻¹。干燥蛋白质的热容为1.26±0.01 J K⁻¹ g⁻¹。体系热容从稀溶液到每克蛋白质含0.38克水（h）时，对组成变化呈线性响应，而在较低含水量时是不规则函数。热容函数在0.38 h处的断点表明，使溶菌酶呈现平衡溶液性质所需的水量为每个蛋白质分子300个水分子，刚好足以形成单层覆盖。低含水量时的热容行为描述了三个水合区域。结合最紧密的水（0 - 0.07 h），可能主要与带电基团结合，其偏比热容量为2.3 J K⁻¹ g⁻¹，该值接近冰的比热容量。与质子重新分布相关的反应热反映在低水合区域的热容函数中。在0.07至0.25 h之间，热容强烈增加，这被认为反映了覆盖蛋白质表面极性和相邻非极性部分的水斑的增长。通过溶剂在表面弱相互作用部分的凝聚完成水合壳，此过程显示出一个转变热。
