Janc Tadeja, Vlachy Vojko, Lukšič Miha
University of Ljubljana, Faculty of Chemistry and Chemical Technology, Večna pot 113, SI-1000 Ljubljana, Slovenia.
J Mol Liq. 2018 Nov 15;270:74-80. doi: 10.1016/j.molliq.2017.10.105. Epub 2017 Oct 28.
Isothermal titration calorimetry was used to determine the temperature and salt concentration dependence of the enthalpy of mixing, Δ , of bovine serum albumin (BSA) in aqueous buffer solutions with several low molecular weight salts. Three buffers were used: acetate (H = 4.0), MOPS (7.2), and borate (9.2). Since the isoionic point of BSA is at I ≈ 4.7, the net charge of BSA in acetate buffer was positive (≈ +20), while in the other two buffer solutions it was negative (≈ -15 in MOPS and ≈ -25 in borate). The majority of the recorded heat effects were exothermic, while only at H = 9.2 a weak endothermic effect upon mixing BSA with LiCl, NaCl, and KCl was observed. For all buffer solutions the absolute values of Δ of sodium salts followed the order: NaCl < NaBr < NaNO < NaI < NaSCN, which is the reverse Hofmeister series for anions. The magnitude of the effects was the largest in acetate buffer and decreased with an increasing H value of the solution. While the effect of varying the anion of the added salts was strongly pronounced at all H values, the effect of the cation (LiCl, NaCl, KCl, RbCl and CsCl salts) was weak. The most interesting feature of the results obtained for H > I was the fact that Δ were considerably more sensitive to the anion (co-ion to the net BSA charge) than to the cation species. This indicated that anions interacted quite strongly with the BSA even at H values where the net charge of the protein was negative. We showed that Δ at high addition of salts correlated well with the enthalpy of hydration of the corresponding salt anion. This finding suggested, consistently with some previous studies, that a part of the exothermic contribution to Δ originated from the hydration changes upon the protein-salt interaction. Theoretical analysis, based on the primitive model of highly asymmetric electrolyte solutions solved within the mean spherical approximation, was used to estimate Coulomb effects upon mixing.
等温滴定量热法用于测定牛血清白蛋白（BSA）在含有几种低分子量盐的水性缓冲溶液中的混合焓Δ随温度和盐浓度的变化。使用了三种缓冲液：醋酸盐（pH = 4.0）、MOPS（pH = 7.2）和硼酸盐（pH = 9.2）。由于BSA的等离子点约为I≈4.7，因此在醋酸盐缓冲液中BSA的净电荷为正（≈ +20），而在其他两种缓冲溶液中为负（在MOPS中≈ -15，在硼酸盐中≈ -25）。记录的大多数热效应是放热的，而仅在pH = 9.2时，观察到将BSA与LiCl、NaCl和KCl混合时存在微弱的吸热效应。对于所有缓冲溶液，钠盐的Δ绝对值遵循以下顺序：NaCl < NaBr < NaNO₃ < NaI < NaSCN，这是阴离子的反向霍夫迈斯特序列。效应的大小在醋酸盐缓冲液中最大，并随着溶液pH值的增加而减小。虽然在所有pH值下添加盐的阴离子变化的影响都很明显，但阳离子（LiCl、NaCl、KCl、RbCl和CsCl盐）的影响较弱。对于pH > I获得的结果中最有趣的特征是，Δ对阴离子（与BSA净电荷的共离子）的敏感性远高于对阳离子种类的敏感性。这表明即使在蛋白质净电荷为负的pH值下，阴离子与BSA的相互作用也很强。我们表明，在高盐添加量下的Δ与相应盐阴离子的水合焓密切相关。这一发现与之前的一些研究一致，表明对Δ的部分放热贡献源于蛋白质 - 盐相互作用时的水合变化。基于在平均球近似内求解的高度不对称电解质溶液的原始模型进行理论分析，以估计混合时的库仑效应。
