| Description |
For structural elucidation of metabolite signals, MS/MS spectral tag (MS2T) libraries were constructed (Matsuda et al., 2011). The extracts of 14–15 cultivars were mixed and utilized for MS/MS spectra data acquisition. Analyses were repeated for 12 mixtures using automated data acquisition methods as previously described (Matsuda et al., 2009). Each MS2T entry was assigned a unique code, OSAXXpXXXXX, indicating the library name (OSAXXp) and entry number. MS2T libraries containing 164 051 entries were constructed. MS2Ts were added to metabolite signals, from which the structure of each metabolite signal was elucidated by searching the ReSpect (RIKEN MS/MS spectra database for phytochemicals) (Sawada et al., 2012), MassBank (Horai et al., 2010), KNApSAcK (Afendi et al., 2012), and PRIMe standard compound database (Sakurai et al., 2013). The two spectra were considered to be similar when the similarity score of the ReSpect search was greater than 0.6. Thresholds were set at m/z Δ0.05 and 0.15 min, respectively, for the molecular formula search on the KNApSAcK database and comparison of retention times. Based on the criteria proposed by the metabolome standard initiative (MSI) (Sumner et al., 2007), metabolite signals were ‘characterized’ when parts of a structure were deduced from mass data. Metabolite signals were‘annotated’ when a common metabolite was observed in the outputs from both the ReSpect and KNApSAcK searches. Metabolite signals were considered to be ‘identified’ when three distinct pieces of information, including the MS/MS spectra, exact mass number, and chromatographic behavior, were matched to identical metabolites (Table S4). Data obtained in this study are available on the PRIMe website (http://prime.psc.riken.jp/) (Sakurai et al., 2013). Isolation and structural determination of rice flavones has been reported previously (Yang et al., 2014). |