The behavior of macromolecules dissolved in ionic liquids
McCarty, Taylor A.
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With the growing emphasis on the environment and the need to find environmentally friendly solvent systems, ionic liquids (ILs) have become a focus of increased investigation. ILs have been implemented as solvent systems in chemical reactions, separations, extractions, electroanalytical applications and chemical sensing, amongst many others. Moreover, owing to their unique properties, tunability, and possible advantages, ILs are being used to replace conventional molecular liquids as solvents for polymerization reactions and biocatalysis. Although research using ILs in polymer systems is still in its infancy, implementing ILs as solvents for polymerization processes have afforded some marked advantages such as increased molecular weight and narrower polydispersity in comparison to organic solvents. Several researchers have also shown that biocatalytic reactions can be successfully carried out in IL-based solvent systems, often with improved activity, enantioselectivity, reusability, and/or operational stability. Unfortunately, researchers lack a fundamental understanding of how these macromolecules behave on a molecular-level when dissolved in an IL. All that is known is that the reaction is capable of producing the desired product(s) often from a trial and error approach. This dissertation addresses this issue by investigating how flexible macromolecules, namely polymers and proteins, behave on a molecular-level when dissolved in IL-based solvent systems. Toward this end, the behavior and dynamics of the pyrenyl tail segments of three different flexible solutes; 1,3-bis-(1-pyrenyl) propane (BPP), 1,10-bis-(1-pyrenyl) decane (BPD), and pyrene-labeled polystyrene (Py-PS-Py, M n = 1100, M w /M n = 1.20) are explored when they are dissolved in the IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C 4 mpy][Tf 2 N]) as a function of temperature. Additionally, the behavior and dynamics of a well-known protein, human serum albumin (HSA) that has been site-selectively labeled with an environmentally sensitive probe molecule, acrylodan (Ac), are investigated when the labeled protein is "dissolved" in IL/H 2 O mixtures as a function of temperature. The ILs that are used in this current work are 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 4 mim][Tf 2 N]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C 4 mim][BF 4 ]), and 1-butyl-3-methylimidazolium hexafluorophosphate ([C 4 mim][PF 6 ]). The results of this research show that chain length plays an important role in determining the overall dynamics of a flexible solute when dissolved in an IL-based solvent system. In addition, changes in temperature alter the flexible molecule's dynamics significantly with the IL becoming a better solvent for the flexible molecules at higher temperatures. These results indicate that the dynamics of a flexible molecule can be tuned by changing the system temperature and/or chain length of the molecule. The results also demonstrate that a model protein system (HSA-Ac) behaves much differently when dissolved in IL/H 2 O mixtures in comparison to aqueous buffer. Specifically, the results indicate that the Ac residue is completely decoupled from the overall HSA molecule and the rotational dynamics are dominated by segmental and/or local motion in all IL/H 2 O mixtures studied. These results are consistent with domain I of HSA being decoupled from the rest of the protein.