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Protein Thermal Aggregation Involves Distinct Regions: Sequential Events in the Heat-Induced Unfolding and Aggregation of Hemoglobin

Yong-Bin Yan ^* , Qi Wang ^*, Hua-Wei He ^* and Hai-Meng Zhou ^* 

^* Department of Biological Sciences and Biotechnology, State Key Laboratory of Biomembrane and Membrane Biotechnology, and Protein Science Laboratory of the Ministry of Education, Tsinghua University, Beijing 100084, China

Correspondence: Address reprint requests to Dr. Yong-Bin Yan, NMR Laboratory, Dept. of Biological Sciences and Biotechnology, Tsinghua University, Beijing 100084, China. Tel.: +86-10-6278-3477; Fax: +86-10-6277-1597; E-mail: ybyan{at}mail.tsinghua.edu.cn.

Protein thermal aggregation plays a crucial role in protein science and engineering. Despite its biological importance, little is known about the mechanism and pathway(s) involved in the formation of aggregates. In this report, the sequential events occurring during thermal unfolding and aggregation process of hemoglobin were studied by two-dimensional infrared correlation spectroscopy. Analysis of the infrared spectra recorded at different temperatures suggested that hemoglobin denatured by a two-stage thermal transition. At the initial structural perturbation stage (30–44°C), the fast red shift of the band from -helix indicated that the native helical structures became more and more solvent-exposed as temperature increased. At the thermal unfolding stage (44–54°C), the unfolding of solvent-exposed helical structures dominated the transition and was supposed to be responsible to the start of aggregation. At the thermal aggregation stage (54–70°C), the transition was dominated by the formation of aggregates and the further unfolding of the buried structures. A close inspection of the sequential events occurring at different stages suggested that protein thermal aggregation involves distinct regions.

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