Unfolding mechanism of lysozyme in various urea solutions: Insights from fluorescence spectroscopy

• Description of fluorescence quenching effect with dynamic contact concentration.
• Except slight conformational variation, lysozyme cannot be unfolded by urea solely.
• Emergence of intermediate state for lysozyme by a combination of urea and HCl.

Fluorescence spectroscopic technique is very popular in exploring the folding/unfolding process of proteins. In this paper, unfolding process of hen egg-white lysozyme was investigated in various denaturing solutions. Firstly, polymer solution theory was employed to comprehend the dependence of fluorescence quenching effect on protein concentration, and dynamic contact concentration was suggested as a critical value for related fluorescence experiment. Secondly, it was found that urea alone could not completely unfold lysozyme but did when together with DTT or HCl. Lysozyme was destabilized in concentrated urea solution, but still could maintain its spatial structure. Phase diagram of fluorescence intensities revealed that HCl could enhance the denaturing capacity of urea, resulting in the emergence of intermediate state in the thermodynamic unfolding process of lysozyme.

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