IONIZATION BEHAVIOR OF HOP COMPOUNDS AND HOP-DERIVED COMPOUNDS

The ionization behaviour of colupulone, (-)-humulone, trans-isohumulone and trans-humulinic acid was investigated using a number of techniques. When a potentiometric technique was employed, precise estimates of pKa could not be obtained for any of the compounds, since a drift in the observed pKa values occurred during the titrations. Approximate, or equilibrium pKa values for the most acidic function of each of the compounds were: colupulone, 6.1; (-)-humulone, 5.0; trans-isohumulone, 3.1; trans-humulinic acid, 2.7. Evidence from spectroscopic measurements (UV, H-1-NMR, C-13-NMR), conductiometric experiments and measurements of the pH dependency of solvent partition behaviour and aqueous solubility of the compounds is consistent with the possibility that, in aqueous solution, compounds such as trans-isohumulone are present as covalent hydrates. It is likely that reversible hydration occurs at one or more carbonyl groups. As a consequence of this behaviour, no single pKa value is sufficient to describe the behaviour of any one hop compound or hop-derived compound. The potential significance of this phenomenon is discussed.