Anthocyanins from the Red Juvenile Leaves of Loropetalum Chinense var. rubrum (Chinese Fringe Flower) Identification and pH Sensing Behaviors

In this study, anthocyanins were extracted from the frozen juvenile leaves of Loropetalum Chinense var. rubrum with high efficiency using ethanol containing 0.1% HCl as extractant. After being purified with AB-8 resin, this extract was investigated with HPLC-DAD-MS/MS determination, and six anthocyanins were identified according to chromatogram, MS data and ratios of absorbance at 400-460 nm to that at absorption maximum (~520 nm), A_400-460/A₅₂₀. Besides cyanidin 3-O-glucoside, peonidin 3-O-glucoside, malvidin 3-O-glucoside, the most abundant malvidin 3,5-O-diglucoside was confirmed through preparative HPLC separation and ¹ H NMR determination. However, two delphinidin- and petunidin-derived anthocyanins should be further clarified whether they are 3,5-O-diglucoside or 5-O-glucobioside. In addition, a novel anthocyanin, 2H-pyran[5,6]malvidin 3-O-glucobioside was also proposed yet need more data to confirm. Moreover, malvidin 3,5-O-diglucoside showed excellent pH sensing ability, displaying distinct color change from pink (pH 1.5) to colorlessness (pH 5.5), to blue (pH 6.8) and finally green (pH 8.8 to pH 10.5). All these should be helpful to evaluate this plant as natural pigment resource.