FIG 2.

Q-TOF MS/MS spectra of U1 identified as O-methyl-dihexosyl diacylglycerol (A) and U2 identified as monohexuronosyl diacylglycerol (B). Each spectrum shows one abundant representative molecular species. The ions were selected as ammonium adducts with the calculated masses 1,004.7240 u (U1) and 842.6353 u (U2) and fragmented. The fragments with m/z 631.5640 or 631.5630 represent DAG with 18:1-Me and/or 19:0c fatty acids as a protonated form after loss of H₂O. 18:1-Me and 19:0c fatty acids are isobaric and thus not distinguishable with respect to their masses. The neutral loss of 373.1578 u (1,004.7218 u minus 631.5640 u) is derived from dihexose with an O-methyl group (as an ammonia adduct); the neutral loss of 211.0760 u (842.6393 u minus 631.5630 u) is derived from hexuronic acid (as an ammonia adduct). The ions m/z 353.3041 and 353.3017 represent MAG-18:1-Me and/or MAG-19:0c (each as a protonated form after loss of H₂O). COO-R1 and COO-R2 represent 18:1-Me and/or 19:0c fatty acyl residues bound to the glycerol backbone. The head group structures of U1 and U2 were further determined by NMR, with U1 identified as a methylated form of DGD (Me-DGD) with the O-methyl group linked to the C-3 position of the innermost galactose and U2 as α-GlcAD, as illustrated in the figure.