Capturing the Polarization Effect in Amino Acid Ionic Liquids

Amino acid ionic liquids (AAILs) have emerged as promising candidates for green chemistry applications due to their biocompatibility, biodegradability, and functional versatility. In this study, we develop and test polarizable force fields (FFs) with Drude oscillators (Drude) for AAILs, specifically [Cho][Ala], [Cho][Gly], [Cho][Pro], and [Cho][Ser], based on the nonpolarizable OPLS-AA, using quantum chemistry, symmetry-adapted perturbation theory (SAPT), and ab initio molecular dynamics (AIMD). Our work addresses the critical need to accurately capture polarization effects and hydrogen bond (h-bond) interactions, which are essential in determining the structure and dynamics of AAILs. Through extensive molecular dynamics (MD) simulations, we demonstrate that incorporating polarization effects significantly alters the structural and dynamical properties of AAILs, which cannot be captured by nonpolarizable FFs with unit ion charges (FixQ) nor scaled charges (ScaleQ). We also reveal the slow and heterogeneous dynamics of AAILs, extending over tens of nanoseconds, a behavior uncommon in traditional ILs. Additionally, we find that the ScaleQ model fails to capture polarization effects with the mean-field approximation and significantly underestimates h-bond interactions. This work represents a significant advance in the MD simulation of biocompatible ionic liquids, providing a reliable foundation for the understanding, design, and application of AAILs in green chemistry.