We report on the rotational reorientation dynamics associated with loop I of domain I within a large multidomain protein (human serum albumin, HSA) when it is dissolved in binary mixtures of ionic liquid (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C(4)mim][Tf2N]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF4]), or 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][PF6])) and distilled deionized water (ddH(2)O) as a function of temperature and water loading. In IL/2% ddH(2)O (v/v) mixtures, loop I of domain I is more significantly denatured:in comparison to the protein dissolved in aqueous solutions containing strong chemical denaturants (e.g., 8 M guanidine HCl (Gu center dot HCl) or urea). As water loading increases, there is evidence for progressive refolding of loop I of domain I followed by recoupling with domains I, II, and III in the [C(4)mim][BF4]/ddH(2)O mixtures at 20 degrees C. Above 30% (v/v) water, where domain I appears refolded, the Ac reporter molecule's serniangle steadily decreases from 35 degrees to 20 degrees with increasing water loading. From the perspective of domain I in HSA, this behavior is similar to the effects of dilution from 4 to 0 M Gu center dot HCl in aqueous solution. Overall, these results lend insight into the tangle of biocatalytic and structural/dynamical mechanisms that enzymes may undergo in ionic liquid-based systems. It will be particularly motivating to extend this work to include enzyme-attuned ionic liquids shown to improve biocatalytic performance beyond that possible in the native (predominantly aqueous) setting.