Proton transfer between tryptophan and ionic liquid solvents studied with molecular dynamics simulations.

Area of Science:

• Computational chemistry
• Physical chemistry
• Biophysics

Background:

• Ionic liquids (ILs) are novel solvents with unique properties.
• Understanding proton transfer in ILs is crucial for applications involving biomolecules.
• Tryptophan's protonation state is sensitive to its environment.

Purpose of the Study:

• To computationally determine pK(a) values for proton transfer from charged tryptophan (HTrp(+)) to ILs BMIM-PF6 and BMIM-BF4.
• To investigate the influence of IL solvation on proton transfer reactions.
• To elucidate the factors governing the pH memory effect in protein-IL systems.

Main Methods:

• Molecular dynamics simulations with empirical force fields.
• Combined molecular mechanical and quantum mechanical (QM/MM) calculations.
• Calculation of reaction free energies and pK(a) values.

Main Results:

• pK(a) values of 16.5 (BMIM-BF4) and 21.5 (BMIM-PF6) were computed for HTrp(+) deprotonation.
• Proton transfer is highly unfavorable in these ILs compared to aqueous solutions.
• Dissociation of protonated anions (PF6(-), BF4(-)) into HF and BF3/PF5 was observed.
• Low proton acceptance of PF6(-) contributes to the higher pK(a) in BMIM-PF6.

Conclusions:

• Computational pK(a) determination in ILs is feasible.
• Solvation effects in ILs significantly impact proton transfer reactions.
• The annihilation of ions impedes tryptophan deprotonation in ILs.
• Results enhance understanding of protein behavior and pH memory in ILs.