β-Cyclodextrin Improves Solubility and Enzymatic C-Glucosylation of the Flavonoid Phloretin

Nothofagin is a prominent bioactive ingredient of rooibos tea. We recently reported its synthesis through a glucosyltransferase cascade reaction involving 3′-C-β-D-glucosylation of the dihydrochalcone phloretin from uridine 5′-diphosphate (UDP)-glucose and in situ formation of UDP-glucose from sucrose and catalytic amounts of UDP. Here we show that the limitation in process efficiency caused by the vanishingly low water solubility of phloretin – a major problem for biocatalytic modifications of hydrophobic natural products in general – was overcome effectively using phloretin inclusion complexation with β-cyclodextrin. Unlike operating in a two-phase system containing uncomplexed insoluble phloretin or using organic cosolvents, the addition of β-cyclodextrin inclusion complexes was well tolerated regarding enzyme activity and stability. Besides enhancing the effective phloretin concentration in water (∼0.2 mM) to about 50 mM, inclusion complexation offered the additional advantage of overcoming the complex inhibition/inactivation effect of the free/microaggregated dihydrochalcone acceptor. Thus oversaturated phloretin solution was transformed in a single batch reaction in excellent conversion (99% in solution; 88% overall) and isolated yield (78%; 17.0 g L−1). The UDP-glucose was regenerated up to ∼90 times and the nothofagin space-time yield of 2.4 mM h−1 presented an eight-fold improvement compared to a reference reaction using 20% DMSO (dimethyl sulfoxide) and requiring controlled phloretin feed. We thus demonstrate the high potential of inclusion complexation by cyclodextrins for boosting the glycosylation of hydrophobic flavonoid-like natural products.