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

• Published in: Food chemistry 2021-12, Vol.364, p.130418-130418, Article 130418
• Main Authors: Kimura, Minami, Ito, Tadashi, Sato, Hirofumi, Higashi, Masahiro
• Format: Article
• Language: English
• Subjects: Cohumulone; iso-α-acid; Isomerization; Metal cation effect; Solvent effect; α-acid
• ISSN: 0308-8146; 1873-7072
• DOI: 10.1016/j.foodchem.2021.130418

[Display omitted] •The formation process of iso-α-acids, beer bitterness, was theoretically analyzed.•The activation free energy difference is attributed to the solvation effect.•The correct structures of the compounds causing bitterness in beer were determined.•The reaction mechanism and acceleration effect of magnesium cation were elucidated.•The solvation effect of water accelerates isomerization with magnesium cation. The α-acids contained in hops are one of the ingredients of beer. The isomerization of α-acids produces iso-α-acids, the main source of bitterness in beer. In this study, the isomerization mechanism of the α-acid, cohumulone, was elucidated by using density functional theory in conjunction with the polarizable continuum model or 3D-RISM integral equation theory of liquids. The calculated reaction diagram is consistent with experimental results; the activation free energy difference between the cis and trans isomers is in good agreement with the experimental estimate. The activation energy difference results from solvation energy. Additionally, a calculation of NMR chemical shifts showed that the proton position of isocohumulone is different from that proposed previously. The effect of Mg2+ cation on the isomerization was also investigated. Both the activation and reaction free energy are stabilized by the presence of Mg2+, which is consistent with experimental results. Water solvation reduces the activation free energy.