Photolysis of hop-derived trans-iso-alpha-acids and trans-tetrahydroiso-alpha-acids: product identification in relation to the lightstruck flavour of beer.

The photolysis of hop-derived trans-iso-alpha-acids (2a-c; naturally occurring bitter compounds present in beer) and of trans-tetrahydroiso-alpha-acids (5a-c; semi-synthetic advanced hop products) was investigated at 300 nm in methanol. The complex photoreaction mixtures were separated by high-performance liquid chromatography (HPLC) using diode array detection and the major photoreaction products were identified by HPLC-mass spectroscopy. The main part of the mixture consisted of compounds, which originated from recombination of radicals derived from Norrish Type I photocleavage of the acyloin moiety in both trans-iso-alpha-acids and trans-tetrahydroiso-alpha-acids. The results confirm the intermediacy of radicals that were previously identified by time-resolved electron paramagnetic resonance and they bear relevance to the formation of the lightstruck flavour that is generated when beer is exposed to light. Additionally, new photoproducts were found that are formed by photochemical reactions hitherto undiscovered in hop chemistry, including photoenolization of trans-isohumulone (2a) leading to trans-alloisohumulone (13a) and a retro oxa-di-pi-methane rearrangement in trans-isohumulone (2a) and trans-tetrahydroiso-alpha-acids to humulone (1a) and tetrahydro-alpha-acids (23a-b), respectively.