Radical formation in isohumulones
was investigated under different
types of stress, including temperature, transition metal ions, and
hydrogen peroxide. Including dihydroisohumulones and tetrahydroisohumulones,
as relevant analogues, allowed us to evaluate critical functionalities
in radical formation. Using
spin-trapping methodology with 5,5-dimethyl-1-pyrroline N-oxide and N-tert-butyl-α-phenylnitrone
as relevant traps, followed by simulation of corresponding spin adducts,
identification of incipient radicals was attempted. The isohexenoyl
side chain in isohumulones, but not present in dihydro- and tetrahydroisohumulones,
was most sensitive to radical formation. Kinetic profiles further
demonstrated that radical formation in this moiety was accelerated
in the presence of ferrous ions. Reactivity of parent six-membered-ring
humulones in radical formation was different, as scavenging of free
radical species was more important. Lupulones, despite similarity
with humulones, showed a different behavior with an obvious radical
decay pathway during ageing, mainly ascribed to radical formation
on the ring structure. Quantification of final spin adducts allowed
us to determine absolute importance of the different degradation pathways.
Eventually, mechanisms are presented explaining why isohumulones are
more prone to radical processes in (aut)oxidation and thermal decay
than close relatives such as dihydroisohumulones.