The results of thermal studies of denaturation of hen egg white lysozyme (HEWL) in water and an aqueous solution of N-butylurea (BU) are presented. High-precision densimetric measurements were used to characterize and analyze the changes of the specific volume, v, during temperature elevation. The temperature of the midpoint of protein denaturation was also determined by nanoDSF technique (differential scanning fluorimetry). The densities of lysozyme solutions were measured at temperatures ranging from 298.15 to 353.15 K with an interval of 5 K at atmospheric pressure (0.1 MPa). The concentration of the protein covered the range from 2 to 20 mg per 1 ml of the solution. The optimal range of the concentration for the densimetric measurements was roughly estimated. In the transition region, the structural changes of the protein are accompanied by the biggest increase of ν values with temperature. Our measurements show that this effect can be monitored from volumetric data without precise determination of protein concentration. The results prove that a two-state model of denaturation could be used for data interpretation. Contrary to common misconception, the volumetric measurements suggest that the denatured protein does not necessarily need to be in a fully extended state. In this way, the ‘protein volume paradox’ could be explained. The surface area of the protein remains unchanged and thus the increase of the specific volume of the protein is relatively small. Additionally, the self-stabilizing effect of the protein in BU solution was reported. For the HEWL in pure water, this phenomenon was not observed.

中文翻译：

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在正丁脲存在下通过体积测量和 nanoDSF 技术监测溶菌酶热变性

介绍了鸡蛋清溶菌酶 (HEWL) 在水和正丁脲 (BU) 水溶液中变性的热研究结果。使用高精度密度测量来表征和分析温度升高期间比容 v 的变化。蛋白质变性的中点温度也通过nanoDSF技术（差示扫描荧光法）测定。溶菌酶溶液的密度在 298.15 至 353.15 K 的温度范围内以 5 K 的间隔在大气压 (0.1 MPa) 下测量。蛋白质的浓度范围为每 1 毫升溶液 2 至 20 毫克。粗略估计了密度测量的最佳浓度范围。在过渡区，蛋白质的结构变化伴随着 ν 值随温度的最大增加。我们的测量表明，这种影响可以从体积数据中监测，而无需精确测定蛋白质浓度。结果证明，变性的二态模型可用于数据解释。与常见的误解相反，体积测量表明变性蛋白质不一定需要处于完全伸展状态。通过这种方式，可以解释“蛋白质体积悖论”。蛋白质的表面积保持不变，因此蛋白质比体积的增加相对较小。此外，还报道了蛋白质在 BU 溶液中的自稳定作用。对于纯水中的 HEWL，没有观察到这种现象。