Effect of Sunlight-Induced Isomerisation on the Biotransformation of 4′-Hydroxychalcones by Yarrowia lipolytica KCh 71

This study investigates the impact of light exposure on the biotransformation of chalcones in yeast cultures. 4'-Hydroxychalcones, with a hydroxyl group in the A-ring, are characteristic substrates efficiently converted into 4'-hydroxydihydrochalcones-compounds naturally occurring in medicinal plants such as <i>Glycyrrhiza glabra</i> (licorice), <i>Stevia rebaudiana</i>, and <i>Angelica keiskei</i> (ashitaba). These compounds are valued for their bioactivity and are relevant to natural product research. In this research, we present the outcomes of the selective microbial reduction of chalcones to dihydrochalcones using the yeast <i>Yarrowia lipolytica</i> KCh 71, cultivated under both light and dark conditions. The aim was to determine whether light exposure affects the efficiency or selectivity of the transformation. Furthermore, the effect of substrate photoisomerisation induced by light was investigated, as the <i>trans-cis</i> isomerisation of chalcones may affect their availability and affinity toward enzymatic systems. The resulting metabolites were analysed using chromatographic and spectroscopic methods. No significant differences in transformation efficiency were observed between light and dark conditions. In all tested conditions, the 4'-hydroxydihydrochalcones were obtained with high yield, typically exceeding 90% conversion. Additionally, the selective bioreduction of the α,β-unsaturated bond in selected 4'-hydroxychalcones by the studied yeast culture is an exceptionally efficient process. The primary factor influencing the reaction rate is the structure of the substrate, particularly the number and distribution of methoxyl groups on the B-ring. In addition, we establish biocatalytic access to three target dimethoxy dihydrochalcones-4'-hydroxy-2,4-dimethoxydihydrochalcone (<b>5a</b>), 4'-hydroxy-2,5-dimethoxydihydrochalcone (<b>6a</b>), and 4'-hydroxy-3,5-dimethoxydihydrochalcone (<b>7a</b>)-under mild conditions using <i>Yarrowia lipolytica</i> KCh 71. Under preparative-scale conditions (7-day incubation), a minor additional product (≤10%) was detected only for the 4'-hydroxy-2,5-dimethoxydihydrochalcone transformation and identified as 4',5-dihydroxy-2-methoxydihydrochalcone (<b>6b</b>); no such side reaction was observed in short-term experiments.