A comparative spectroscopic and computational insight on the interaction of short and long chain pyrrolidinium based ionic liquids with bovine serum albumin

Abstract

In this study, bovine serum albumin (BSA), a model transport protein, is shown to interact with two pyrrolidinium based ionic liquids (ILs), namely, 1-hexyl-1-methyl pyrrolidinium bromide $\left[\text{Pyr}{C}_6\right]B{r}^{-}$and 1-dodecyl-1-methyl pyrrolidinium bromide $\left[\text{Pyr}{C}_{12}\right]B{r}^{-}$. The various spectroscopic techniques explored in the analysis are UV–visible spectroscopy, steady state fluorescence spectroscopy, synchronous fluorescence spectroscopy, 3-Dimensional emission spectroscopy, time resolved fluorescence spectroscopy, steady state fluorescence anisotropy, site marker experiment, circular dichroism, along with molecular docking as the computational investigative studies. The results from UV–vis spectroscopy suggest the presence of static quenching in case of both the ILs with BSA which was further confirmed by fluorescence as well as time-resolved fluorescence spectroscopic results. Moreover, UV–vis spectroscopic studies were used for calculating the binding constants ($K_a)$ at 298 K and 308 K for BSA’s interaction with both ILs. The thermodynamic parameters, such as $\Delta G$, $\Delta H$ and $\Delta S$, were calculated using $K_a$. Also, Stern Volmer constants $\left(K_{\mathrm{sv}}\right)$ for both the ILs were calculated. The higher value of the binding constant was observed when BSA interacted with the long alkyl-chained IL. CD spectroscopy results suggested the stabilization of the secondary structures in presence of both the ILs but more enhanced in case of the long chain IL. The site marker experiment indicated towards the involvement of Site I in the interaction of both the ILs with BSA. This was supported by molecular docking, where a higher value of binding energy was obtained for BSA- $\left[\text{Pyr}{C}_{12}\right]B{r}^{-}$ interaction. Overall, the study could provide structural insights into future studies on using ILs as therapeutic agents.