Solvation power of HFIP for the hydrophilic and the hydrophobic moieties of l-leucine studied by MD, IR, and NMR techniques

Abstract

Abstract The solvation properties of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) in its aqueous binary solvents have been evaluated using l -leucine (Leu) as a probe molecule by both theoretical and experimental approaches. Thus, molecular dynamics (MD) simulations, infrared (IR) and 1 H and 13 C NMR spectroscopic experiments have been conducted on Leu-HFIP-water solutions with varying the HFIP mole fraction from x HFIP = 0 to 1. Spatial distribution functions (SDFs) obtained from the MD simulations showed that the Leu carboxylate group is gradually solvated by HFIP molecules with increasing x HFIP , instead of water molecules. Thus, the replacement of water by HFIP for the carboxylate group progresses as the x HFIP rises. This was proved by the C O stretching vibrations and the 13 C chemical shift of the carboxylate group. On the contrary, the Leu aminium group is preferentially solvated by water below x HFIP = 0.7. However, HFIP molecules begin to solvate the aminium group above this mole fraction. The trifluoromethyl groups of HFIP significantly enclose the alkyl group of Leu even at the low x HFIP . By surrounding the Leu alkyl group with the HFIP trifluoromethyl groups, the C H stretching vibrations of Leu blueshift with increasing x HFIP . Simultaneously, the hydrogen and carbon atoms of the Leu alkyl group are electronically shielded as the x HFIP rises. The MD pair correlation function g ( r ) HCLeu-FHFIP showed the H F interactions between the Leu alkyl hydrogen and the HFIP fluorine atoms with the distance of 2.75 A. Consequently, the present results proved that the HFIP trifluoromethyl groups may interact with the Leu alkyl group through the blueshift hydrogen bonds of C H ⋯ F C.