Selective binding and dynamics of imidazole alkyl sulfate ionic liquids with human serum albumin and collagen – a detailed NMR investigation

The interaction of ionic liquid (IL) with protein is now becoming important as it stabilizes the protein due to the selective cation–anion combination of the IL. The binding and dynamics of the green solvents such as imidazole alkyl sulfate based ILs, viz., 1-butyl-3-methylimidazolium alkyl [where alkyl = hydrogen, methyl, octyl and dodecyl] sulfate, with two distinct model proteins, namely human serum albumin (HSA) and collagen in aqueous solution, have been investigated with the aid of solution nuclear magnetic resonance (NMR). Interactions of ILs with HSA and collagen have been probed at the atomistic level through NMR determined parameters, such as 1H line-shapes, selective and non-selective spin–lattice relaxation times (T1SEL & T1NS) and spin–spin relaxation times (T2). Furthermore, saturation transfer difference (STD) NMR has been used to monitor the spatial proximities of ILs with HSA and collagen. The results indicate that despite the type of protein (HSA or collagen), STD NMR of protein–IL mixtures exhibits responses only from the anionic part of the selected ILs. Also, a combination of T1SEL and T1NS measurements indicates the genuine protein–IL interaction. Furthermore, it was observed that the global binding affinity between IL and proteins is enhanced with an increase in alkyl chain length of the anionic portion of the IL. The present study thus highlights the role of the anionic part of ILs in the interaction with the selected proteins. The outcome of the present study provides an opportunity to design new ILs with a judicious choice of anionic and cationic parts for targeted functionalities.