Theoretical study on isomerization of α-acids: A DFT calculation

Area of Science:

• Chemistry
• Biochemistry
• Chemical Engineering

Background:

• Alpha-acids in hops are key beer ingredients.
• Isomerization of alpha-acids to iso-alpha-acids generates beer's bitterness.
• Understanding this isomerization is vital for brewing science.

Purpose of the Study:

• To elucidate the isomerization mechanism of cohumulone, an alpha-acid.
• To investigate the influence of solvation and magnesium ions on the reaction.
• To verify computational findings with experimental data.

Main Methods:

• Density functional theory (DFT) calculations.
• Polarizable continuum model (PCM) and 3D-RISM integral equation theory for solvation.
• Nuclear magnetic resonance (NMR) chemical shift calculations.

Main Results:

• The calculated reaction pathway aligns with experimental observations.
• The activation free energy difference between cis and trans isomers closely matches experimental estimates.
• Computational NMR shifts suggest a different proton position for isocohumulone than previously proposed.
• Magnesium ions stabilize both activation and reaction free energies.
• Water solvation decreases the activation free energy.

Conclusions:

• Computational methods accurately model the alpha-acid isomerization process.
• Solvation effects, particularly from water and Mg2+, play a critical role in the reaction energetics.
• The study refines the understanding of isocohumulone structure and the factors influencing beer bitterness.