Racemization at C-2 of naringin in sour oranges with increasing maturity determined by chiral high-performance liquid chromatography

Naringin is the major flavanone-7-O-glycoside of sour orange, and it is mainly responsible for the bitter taste of the fruit. The relative content of (2S)- and (2R)-naringin in the albedo of sour oranges during maturation in the entire season was determined by normal phase HPLC using Chiralcel OD-H as chiral stationary phase and n-hexane/ethanol doped with 0.5% of TFA as mobile phase. HPLC traces were complicated by the presence of (2S)-neohesperidin, and a software-guided analysis was developed to assess the relative amount of the C-2 diastereomers of naringin. A sigmoid curve was obtained showing variation from 94% of (2S)-naringin in very immature fruits to 69.7% in mature fruit samples. Spontaneous epimerization of (2S)-naringin in the ethanolic solutions of the albedo was noted after prolonged keeping for less acid samples. The separation of the C-2 diastereomers of other flavanoid-7-O-glycosides present in Citrus (neoeriocitrin, neohesperidin, and eriocitrin) was also obtained by chiral HPLC and afforded the identification, including stereochemistry, of the eluting peaks in the HPLC traces of the albedo extracts.