A Rare Benzothiazole Glucoside as a Derivative of 'Albedo Bluing' Substance in Citrus Fruit and Its Antioxidant Activity

'Albedo bluing' of fruits occurs in many varieties of citrus, resulting in a significant reduction in their commercial value. We first presented a breakthrough method for successfully extracting and purifying the 'albedo bluing' substance (ABS) from citrus fruits, resulting in the attainment of highly purified ABS. Then, HPLC and UPLC-QTOF-MS were used to prove that ABS in the fruits of three citrus varieties (Citrus reticulate Blanco cv. 'Gonggan', 'Orah', and 'Mashuiju') are identical. However, the chemical structure of ABS remains elusive for many reasons. Fortunately, a more stable derivative of ABS (ABS-D) was successfully obtained. Through various analytical techniques such as HRESIMS, 1D and 2D NMR, and chemical shift calculation, ABS-D was identified as 2,4-dihydroxy-6-(β-D-glucopyranosyloxy)phenyl(5,6-dihydroxy-7-(β-D-glucopyranosyloxy)benzo[d]thiazol-2-yl)methanone, indicating that both ABS and its derivative belong to a rare category of benzothiazole glucosides. Furthermore, both ABS and ABS-D demonstrated potent antioxidant abilities. These findings lay the groundwork for further elucidating the chemical structure of ABS and the causative mechanism of the 'albedo bluing' phenomenon in citrus fruits.