FCS Study of the Structural Stability of Lysozyme in the Presence of Morpholinium Salts

Ability of the ionic liquids to alter the structural stability of proteins in aqueous solution is a topic of considerable interest in modern bioscientific research because of possible applications of these substances in protein purification and as refolding agents. A few early studies involving the imidazolium ionic liquids have demonstrated their role as both denaturants and refolding agents. As the influence of an ionic liquid on a given protein depends on the identity of both species, it is necessary to extend the studies to a wider number of ionic liquids and proteins to obtain insight into the mechanism of interaction between the two and to arrive at a comprehensive picture. It is in this context that we have studied the effect of two morpholinium salts, [Mor1,2]­[Br] and [Mor1,4]­[Br], differing in the alkyl chain length of cation, on chicken egg white lysozyme in its native and chemically denatured states employing primarily the fluorescence correlation spectroscopy (FCS) technique. Fluorescence signal of Alexa488-labeled lysozyme (A488-Lysz) has been used to determine the changes in hydrodynamic radius of protein in the presence of additives. The results reveal a conformational dynamics of lysozyme with a time constant of 56 ± 10 μs in its native state. It is observed, when in its native state, both the morpholinium salts induce structural changes of lysozyme. However, when in its unfolded state, [Mor1,4]­[Br] at low concentration compacts the protein, but at higher concentration, it stabilizes the unfolded state, unlike [Mor1,2]­[Br], which compacts lysozyme at both low and high concentrations. A comparison of the effect of these salts and arginine, a protein stabilizer, on lysozyme indicates that [Mor1,2]­[Br] is a superior compacting agent for the unfolded state of the protein compared to arginine.