Resolution of (±)-Tetrahydrocohumulones and Properties of Their Isomerized Derivatives

Abstract

Resolution of a racemic mixture of (±)-tetrahydrocohumulones (±-THCO) was achieved in gram quantities by dynamic crystallization with (1S,2R)-(–)-1-amino-2-indanol. Both (+)- and (–)-THCO were isomerized to produce a mixture of cis/trans-tetrahydroisocohumulones (THICO) of identical molecular configurations but with opposite optical rotations. The chiral recognition of novel (–)-cis- and (+)-trans-THICO (II), natural (+)-cis- and (–)-trans-THICO (I), and racemic (±)-cis- and (±)-trans-THICO (III) on sensory bitter perception (sensing with G protein-couple receptors), foamability (interaction with foam proteins), and antimicrobial activity were also assessed. Sensory analysis indicated that the bitter intensity of the three pair molecules is affected by stereochemistry: II > III > I. III was perceived as smoother, cleaner, and milder than I and II. No apparent differences were detected during the foamability assessment of beers spiked with the three optical isomers. The antimicrobial effects of I, II, and III were assessed by minimum inhibitory concentration (MIC) and bacterial zone of inhibition (BZI) tests against Pediococcus damnosus and Lactobacillus brevis. Both I and II showed similar MIC results against P. damnosus. BZI diameters of I, II, and III against both microorganisms were consistent, suggesting that both enantiomers and the racemic mixtures exhibit a similar level of antibacterial effectiveness.