Faculty of Chemistry and Chemical Technology, University of Ljubljana, SI-1000 Ljubljana, Slovenia.
J Phys Chem B. 2010 Apr 1;114(12):4313-9. doi: 10.1021/jp9071845.
It is well-known that the addition of salts influences the properties of proteins in solution. The essential nature of this phenomenon is far from being fully understood, partly due to the absence of the relevant thermodynamic information. To help fill this gap, in this work isothermal titration calorimetry (ITC) was employed to study the ion-lysozyme association in aqueous buffer solutions at pH = 4.0. ITC curves measured for NaCl, NaBr, NaI, NaNO3, NaSCN, KCl, CaCl2, and BaCl2 salts at three different temperatures were described by a model assuming two sets of independent binding sites on the lysozyme. The resulting thermodynamic parameters of binding of anions (counterions) to the first class of sites (N approximately 7) indicate that the binding constant (K approximately 102 M-1) increases in the order Cl- < Br- < I- < NO3- < SCN-. The anion-lysozyme association is entropy driven, accompanied by a small favorable enthalpy contribution and a positive change in heat capacity. It seems that the entropy and heat capacity increase is due to the water released upon binding, while the net exothermic effect originates from the anion-NH3+ pair formation. Moreover, the results reveal that the nature of the cation has little effect on the thermodynamics of the anion-lysozyme association under the given experimental conditions. Taken together, it seems that the observed thermodynamics of association is a result of a combination of both electrostatic and short-range interactions. The anion ordering reflects the strength of water mediated interactions between anions and lysozyme.
众所周知，盐的添加会影响蛋白质在溶液中的性质。这种现象的本质远未被完全理解，部分原因是缺乏相关的热力学信息。为了帮助填补这一空白，本工作采用等温热滴定法（ITC）研究了在 pH = 4.0 的水缓冲溶液中离子溶菌酶的缔合。在三个不同温度下测量的 NaCl、NaBr、NaI、NaNO3、NaSCN、KCl、CaCl2 和 BaCl2 盐的 ITC 曲线通过一个模型进行了描述，该模型假设溶菌酶上存在两组独立的结合位点。结合于第一类（N 约为 7）位点的阴离子（抗衡离子）的热力学参数表明，结合常数（K 约为 102 M-1）按 Cl-<Br-<I-<NO3-<SCN-的顺序增加。阴离子-溶菌酶的缔合是熵驱动的，伴随着较小的有利焓贡献和正的热容变化。似乎熵和热容的增加是由于结合时释放的水，而净放热效应源于阴离子-NH3+对的形成。此外，结果表明，在给定的实验条件下，阳离子的性质对阴离子-溶菌酶缔合的热力学影响很小。综上所述，观察到的缔合热力学似乎是静电和短程相互作用结合的结果。阴离子的有序性反映了阴离子与溶菌酶之间水介导相互作用的强度。
