Structural Destabilization of Azurin by Imidazolium Chloride Ionic Liquids in Aqueous Solution

Arusha Acharyya, David Digiuseppi, Brittany L. Stinger, Reinhard Schweitzer-Stenner, Timothy D. Vaden

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10 Scopus citations


Alkyl imidazolium chloride ionic liquids (ILs) have been used for numerous biochemical applications. Their hydrophobicity can be tuned by changing the alkyl chain length, and longer-chain ILs can form micelles in aqueous solution. We have investigated the effects of imidazolium chloride ILs on the structure and stability of azurin, which is a very stable Cu2+ redox protein with both α-helix and β-sheet domains. Temperature-dependent infrared (IR) and vibrational circular dichroism spectroscopy can provide secondary-structure-specific information about how the protein is affected, and temperature-jump transient IR measurements can quantify the IL-influenced unfolding dynamics. Using these techniques, we can quantify how azurin is destabilized by 1.0 M ILs in aqueous solution. The shorter, less hydrophobic ILs, 1-butyl-3-methylimidazolium chloride and 1-hexyl-3-methylimidazolium chloride likely interact with the α-helix domain and decrease protein melting temperature from 82 °C without IL to 55 °C and disturb the overall tertiary structure, resulting in a looser, more open shape. Thermodynamic analysis indicates that protein destabilization is due to increased unfolding entropy. 1-Octyl-3-methylimidazolium chloride [OMIM]Cl, which forms micelles in solution that may partially solvate the protein, has a more significant destabilizing effect, resulting in a melting temperature of 35 °C, larger unfolding entropy, and relaxation kinetics several orders of magnitude faster than with unperturbed azurin. The temperature-independence of the relaxation time constant suggests that in the presence of [OMIM]Cl, the protein folding potential energy surface has become very smooth.

Original languageEnglish (US)
Pages (from-to)6933-6945
Number of pages13
JournalJournal of Physical Chemistry B
Issue number32
StatePublished - Aug 15 2019

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry


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