Identification of phenylpropanoids in methyl jasmonate treated Brassica rapa leaves using two-dimensional nuclear magnetic resonance spectroscopy

Metabolic analysis showed a clear increase in phenylpropanoid levels in Brassica rapa leaves after treatment with methyl jasmonate. A fraction of phenylpropanoids was prepared by Diaion HP-20 and Sephadex LH-20 column chromatography after MeOH-water extraction. Even with these purification steps, isolation of each phenylpropanoid for structure elucidation is not easy due to the low levels in the plants (ca. 0.004%). A mixture was analyzed without further purification using HPLC-electrospray ionization mass spectrometry and NMR spectroscopy. Based on the NMR results including 1H NMR, J-resolved, correlated spectroscopy (COSY), heteronuclear single quantum coherence (HSQC), and heteronuclear multiple bond correlation (HMBC) spectra, both 1H and 13C resonances of the compounds were obtained. Using these NMR data, five phenylpropanoids conjugated with malate were identified: 5-hydroxyferuloyl-, caffeoyl-, coumaroyl-, feruloyl-, and sinapoyl malate. Of the compounds, 5-hydroxyferuloyl malate is a new phenylpropanoid. In addition to the five constitutive phenylpropanoids bearing trans-configuration, their cis forms, which are believed to be artifacts formed in the course of extraction steps, were also identified in the fraction.