Structural and spectroscopic analysis of proline-based deep eutectic solvents and their interactions with alpha-lactalbumin protein

Abstract

This work represents the computational approaches using molecular dynamic (MD) simulations and quantum calculations to elucidate the proline-based Deep Eutectic Solvents (DES) L-Proline: Urea and D-Proline: Urea in 1:1 molar ratios. The most stable DES clusters are established by an MD simulation study followed by a quantum calculation with the B3LYP-D3/6-311G+(d,p) level of theory. By observing the orientation and vibrational spectra, the responsible H-bonds of 1:1 L-/D-Proline: Urea are NHL-Pro … HNUrea, COUrea…HCL-Pro, OHL-Pro … OCUrea and COUrea … HNL-Pro, probed by NBO charge transfer, HOMO-LUMO, and electrostatic potential map analyses. The changes in Gibb’s free energy (ΔG), electronic energy (ΔE), and enthalpy (ΔH) give evidence of the spontaneity of DES formation. Calculated IR and vibrational circular dichroism (VCD) signatures give the characteristics spectra that support the hydrogen bond formation of DES cluster conformers. In L-/D-Proline: Urea DES, the selected protein, alpha-lactalbumin gets stabilized. After 20% and 40% water insertion into DES-protein, this stabilizing gets deteriorated. In 20% water, the protein destabilizes most because the water molecules lodge in the DES microstructures producing urea-(water)cluster-proline or urea-proline-(water)cluster while 40% of water cleave the DES forming urea-(water)cluster-proline or urea-proline-(water)cluster by which DES-protein system stabilize and thus, water molecules dominate the DES-protein interactions. KEYWORDS: Deep eutectic solvents, Molecular dynamics simulation, Density functional theory, Vibrational circular dichroism, Non-covalent interaction.