UV-Vis spectrophotometry was used to determine acid dissociation constant (pKa) values of the natural flavonoids phloretin, phlorizin, naringenin, and naringin, as well as 4′-hydroxychalcone, 4′-(dimethylamino)chalcone, and their cyclic analogues. Comparison of the results with those previously reported for the natural flavonoids showed the applied method is a relatively straightforward and easy-to-perform technique for the determination of pK a values of compounds with relatively low solubility. Comparative analysis of the pK a values of the synthetic chalcones showed a strong correlation between the degree of conjugation and the acid strength of the respective compounds with different geometry. Our results provide further evidence that modification of the three-dimensional structure of open-chain bioactive compounds is the method of choice to modify not only their stereochemistry but also their physicochemical properties.
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