How deprotonation of cohumulone and colupulone influence their UV/Vis and CD spectrum

Abstract

Cohumulone and colupulone are representatives of α- and β-acids, respectively. These compounds are important antimicrobial hop (Humulus lupulus) constituents, where cohumulone is an important source of the bitter taste of beer. In this study, we examined the pH dependence of UV/Vis spectra of both compounds while CD spectra of cohumulone were also measured at various wavelengths. This facilitated the examination of the protolytic equilibrium of both compounds, where the second pKa value of cohumulone was determined for the first time. Additionally, comparing experimental spectra with spectra calculated using time-dependent density functional theory (TD-DFT) enabled the determination of the most likely deprotonation positions and corresponding species most likely present in the aqueous solution at various pH values. Last but not least, comparing calculated and experimental CD spectra of cohumulone facilitated the determination of the absolute stereoconfiguration of cohumulone.