Biomedical Laboratory Science and Technology, Faculty of Health and Society, Malmö University, SE-205 06 Malmö, Sweden.
Langmuir. 2010 Mar 16;26(6):3918-22. doi: 10.1021/la903210e.
Water sorption isotherms of proteins are usually interpreted with such models as BET or GAB that imply the formation of multilayers at solid-gas interface. However, this approach is not applicable to globular proteins such as humid lysozyme where a solid-gas interface does not exist. Another popular approach is the D'Arcy-Watt model, where besides the formation of multilayers the heterogeneity of energies of sorption sites of proteins is taken into account. Here we present sorption calorimetric data on the hydration of lysozyme that confirms the existence of the heterogeneity. The magnitude of the heterogeneity is, however, lower than one can expect on the basis of the existence of a solid-gas interface. Moreover, the calorimetric data show a strong enthalpy-entropy compensation that leads to almost constant effective free energy of hydration in the activity range normally used for fitting the data to sorption models. This allows the use of the Langmuir equation for the fitting of the initial part of the sorption isotherm of lysozyme. Assuming the formation of a monolayer of water at the protein-protein interface, one can estimate the size of the lysozyme molecules from the sorption isotherm. The result of this estimation is in good agreement with the structural data on lysozyme, which supports the presented approach.
蛋白质的水吸附等温线通常用 BET 或 GAB 等模型来解释，这些模型暗示在固-气界面形成多层。然而，这种方法不适用于球形蛋白质，如湿润溶菌酶，因为不存在固-气界面。另一种流行的方法是 D'Arcy-Watt 模型，该模型除了考虑多层的形成外，还考虑了蛋白质吸附位能的非均质性。在这里，我们提出了溶菌酶水合作用的吸附量热数据，该数据证实了非均质性的存在。然而，非均质性的大小低于基于固-气界面存在的预期值。此外，量热数据显示出强烈的焓熵补偿，导致在通常用于将数据拟合到吸附模型的活性范围内，水合作用的有效自由能几乎保持不变。这允许使用 Langmuir 方程来拟合溶菌酶吸附等温线的初始部分。假设在蛋白质-蛋白质界面形成一层水，可以根据吸附等温线估计溶菌酶分子的大小。该估计的结果与溶菌酶的结构数据非常吻合，这支持了所提出的方法。
