Structure and conformation of red radish (Raphanus sativus L.) anthocyanins and their effect on color and pigment stability Public Deposited

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  • Red radish (Raphanus sativus L.) anthocyanins were extracted from liquid nitrogen powdered epidermal tissue using acetone and chloroform and isolated using C-18 resin. Chemical structure and conformation of major pigments were elucidated by 1-D and 2-D NMR, Mass Spectroscopy, HPLC, and spectral analyses. Two novel di-acylated anthocyanins, pelargonidin 3-0-[2-0-(β-D-glucopyranosyl)-(6-0- trans-p-coumaroyl- β-D-glucopyranoside)]-5-0-(6-0-malonyl-β-D-glucopyranoside)and pelargonidin 3-0-[2-0-(β-D-glucopyranosyl)-(6-0-trans-feruloyl-β-D-glucopyranoside)]- 5-0-(6-0-malonyl-β-D-glucopyranoside) represented 70% of total pigment. Two mono-acylated anthocyanins (20%) were pelargonidin 3-0-[2-0-(β-D-glucopyranosyl)-(6-0-trans-p-coumaroyl-β-D-glucopyranoside)]-5-0-(β-D-glucopyranoside) and pelargonidin 3-0-[2-0-(β-D-glucopyranosyl)-(6-0-trans- feruloyl-β-D-glucopyranoside)]-5-0-(β-D-glucopyranoside). NOESY revealed folding of the molecule. Electrospray (ES-MS) and tandem mass spectroscopy (MS-MS) were tested as tools for anthocyanin characterization. Anthocyanins were semi-purified using C-18 resin, washed with acidified water and ethyl acetate, and recovered with acidified methanol. Samples were injected into a mass spectrometer in aqueous or methanolic solutions. Charged character of anthocyanins favored the fast and effective detection of intact molecular ions requiring minimal sample preparation and with little interference. MS-MS provided clear and characteristic fragmentation patterns. Qualitative and quantitative anthocyanin pigment content of radish cultivars grown at 2 locations (Corvallis and Hermiston, OR) and harvested at 2 maturity stages, were evaluated. Pigment content depended on cultivar, root weight and location, with higher amounts obtained at Hermiston. Spring cultivars (n=22) had pigmentation in the skin, ranging from 39.3 to 185 mg anthocyanin/100g skin. Red-fleshed Winter cultivars (n=5) possessed from 12.2 to 53 mg anthocyanin/100g root. The major pigments were pelargonidin-3-sophoroside-5-glucoside, mono- or di-acylated with cinnamic and malonic acids; individual proportions varied among cultivars. Estimated pigment yields ranged from 1.3 to 15.8 kg/ha. Acylated and non-aeylated pelargonidin were isolated using semi-preparative HPLC to evaluate the effect of glycosylation and acylation in spectral characteristics, molar absorptivity and color. Molar absorptivity ranged from 15,600 to 39,590 for pelargonidin-3-glucoside and pg-3-rutinoside-5-glucoside acylated with p-coumaric acid, respectively. An hypsochromic shift on λmax was observed in presence of glycosylation. Pelargonidin-3,5-diglucoside and 3,5-triglucoside showed yellow-orange hue angle (>40°) in pH 1.0 buffer, higher than other pg-derivatives. Cinnamic acid acylation caused a bathochromic shift. Malonic acid acylation had little effect on color and no effect on λmax. The solvent system affected molar absorptivity and visual color characteristic of the pigments. Acylation increased pigment resistance to acid hydrolysis.
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