FIG. 7.

MALDI-TOF/TOF MS/MS analysis. Shown are the enzymatic product Gal₃Ara₁-C₈H₁₇ obtained from using the synthetic acceptor β-d-Galf-(1→5)-β-d-Galf-(1→6)-β-d-Galf-octyl (A) and β-d-Galf-(1→6)-β-d-Galf-(1→5)-β-d-Galf-octyl (B). The [M + Na]⁺ molecular ions afforded by the permethylated sample at m/z 939 (Fig. 6) were selected for high-energy CID MS/MS. Assignment of the key fragment ions were as schematically illustrated. In addition to the well-established A, X, C, and Y ions, the G and E ions were named by Spina et al. and have been described in full in the context of furanoses (18, 32). (Note that the ^1,4X ions give linkage information only pertaining to the C-1 position. The ^0,2X ions provide information about whether C-2 contains a glycosidic bond. ^0,3A and ^2,4A ions provide the C-5 and C-3 linkage information. C and Y ions, as well as the E and G ions, represent concerted elimination of substituents around the ring.) The two specific ion series corresponding to E − 30 mass units and C − 16 mass units (m/z 447 and 607, respectively) were likewise identified previously for the arabinan and not further drawn out here. The concerted cleavages corresponding to loss of both C-5 and C-6 substituents on the Galf have not been reported before and were noted to produce a pair of ions differing in having either unsaturated or saturated bonds (e.g., m/z 675/673 in panel A and 513/515 in panel B). The ^1,4X ions at m/z 385 in panel A and 545 in panel B coincide, respectively, with the E ions at m/z 415 and 575 − 30 mass units and thus may not be taken as sequence-specific ions to indicate presence of isomeric products. In contrast, other specific ions as described in the text are present only in either and not both spectra. The ion at m/z 505 in panel A may be assigned as an ^O,3A ion, which is indicative of possible presence of Araf on the nonreducing terminal Gal₂, as in panel B, but other supporting ions that are specific to this isomeric structure are lacking.