Structural characterization and identification of oleanane-type triterpene saponins in Glycyrrhiza uralensis Fischer by rapid-resolution liquid chromatography coupled with time-of-flight mass spectrometry

Oleanane-type triterpene saponins (OTS) are major active ingredients in Glycyrrhiza uralensis. In this work, a rapid-resolution liquid chromatography with time-of-flight mass spectrometry (RRLC/TOF-MS) method has been developed to characterize and identify OTS from G. uralensis. The major diagnostic ions and fragmentation pathways from thirteen OTS have been characterized for the first time. At a low fragmentor voltage of 120 V in positive ion mode, the precursor ion [M + H](+) or/and [M + Na](+) was obtained for accurate determination of molecular formula. When the fragmentor voltage was increased to 425 V, abundant characteristic fragment ions were observed for structural characterization. Neutral losses of sugar moieties, such as glucuronic acid (GlcA, 176 Da), glucose (Glc, 162 Da) and rhamnose (Rha, 146 Da), were commonly observed in the MS spectra for prediction of the sugar number and sequences. Other typical losses included AcOH (60 Da), CH(2)O (30 Da), 2 × H(2)O (2 × 18 Da) and HCOOH (46 Da) from [Aglycone + H-H(2)O](+) (named [B](+)), corresponding to the presence of a C(22)-acetyl group, C(24)-hydroxyl group, C(22)-hydroxyl group or C(30)-carboxyl group on the aglycone moiety, respectively. In particular, characteristic ring cleavages of the aglycone moieties on A- and B-rings were observed. Based on the fragmentation patterns of reference compounds, nineteen OTS have been identified in an extract of G. uralensis, thirteen of which were unambiguously identified and the other six were tentatively assigned.