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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2018

Month Degree Awarded

September

First Advisor

Harry Bermudez

Second Advisor

David Hoagland

Abstract

Protein solubility in neat ionic liquids (ILs), was assessed using a variety of model proteins and different classes of ILs. The results indicate that not only can neat ILs solubilize proteins, they do so quite regularly. Generally, ILs that are more hydrophilic or miscible with water exhibit a larger solubility range; while those ILs that are more hydrophobic rarely solubilize proteins. Long alkyl chains on the cation or anion of the IL tend to lower solubility while highly denaturing anions like thiocyanate, [SCN]- or acetate, [OAc]- solubilize all model proteins tested (though they are likely denatured in the process).

A model protein lysozyme was tested in three ILs, 1-methyl-3-ethylimidazolium ethylsulfate, [C2MIM][EtSO4], tributylethylphosphonium diethylphosphate, [C2,4,4,4P][Et2PO4], and 1-methyl-3-ethylimidazolium diethylphosphate, [C2MIM][Et2PO4], with the last being a combination of the previous two. By dynamic light scattering (DLS) and fourier transform infrared spectroscopy (FTIR), it was determined that lysozyme retains its native size and secondary structure in [C2,4,4,4P][Et2PO4] but is denatured in [C2MIM][EtSO4]. Results for [C2MIM][Et2PO4] indicate that lysozyme retains its native size but not its native secondary structure. These results indicate that there is a tradeoff between solubility and structure and the interactions responsible for denaturation are linked to those responsible for solubility.

To elucidate the water-IL phase behavior of lysozyme and the previously discussed ILs, turbidity of these mixtures was assessed. A mixing dependent phase behavior was found for both of the imidazolium ILs. When lysozyme was first dissolved in a mixture of water or a premixed mixture of water and IL, no insolubility region was discovered. However, for all three IL cases, when lysozyme was first dissolved in neat IL, an insolubility region existed. From circular dichroism (CD) measurements, the aggregation observed is due to a loss of helical structure and an increase β-sheet conformation. In other words, lysozyme exposes hydrophobic residues, which in the presence of water, cause aggregation. Attempts to circumvent this insolubility region using a nonionic surfactant, PEG, temperature, and dialysis were all unsuccessful.

To understand why denaturation is occurring, the poly(amino acids), poly(l-lysine) and poly(l-glutamic acid) were studied in water-[C2,4,4,4P][Et2PO4] mixtures at varying temperature and pH. Poly(l-lysine) indicated peculiar conformational behavior and solubility, including precipitating out of solution at neutral pHs and adopting a coiled conformation at low IL concentrations at pH > 10. Poly(l-glutamic acid) behaved in IL similar to the way it behaved in inorganic salts, but at high IL concentration, stabilized helix formation was noted at both high pH (pH > 6) and low pH (pH < 4). Building upon these results, protein solubility may be partially understood. For [C2,4,4,4P][Et2PO4], a protein with more negative residues (lower pI) is more likely to be soluble. Conversely, a protein with more positive residues (higher pI) is more likely to be insoluble.

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