Electron Transfer Dissociation and Collision-Induced Dissociation of Underivatized Metallated Oligosaccharides

Abstract

Electron transfer dissociation (ETD) and collision-induced dissociation (CID) were used to investigate underivatized, metal-cationized oligosaccharides formed via electrospray ionization (ESI). Reducing and non-reducing sugars were studied including the tetrasaccharides maltotetraose, 3α,4β,3α-galactotetraose, stachyose, nystose, and a heptasaccharide, maltoheptaose. Univalent alkali, divalent alkaline earth, divalent and trivalent transition metal ions, and a boron group trivalent metal ion were adducted to the non-permethylated oligosaccharides. ESI generated [M + Met]⁺, [M + 2Met]²⁺, [M + Met]²⁺, [M + Met - H]⁺, and [M + Met - 2H]⁺ most intensely along with low intensity nitrate adducts, depending on the metal and sugar ionized. The ability of these metal ions to produce oligosaccharide adduct ions by ESI had the general trend: Ca(II) > Mg(II) > Ni(II) > Co(II) > Zn(II) > Cu(II) > Na(I) > K(I) > Al(III) ≈ Fe(III) ≈ Cr(III). Although trivalent metals were utilized, no triply charged ions were formed. Metal cations allowed for high ESI signal intensity without permethylation. ETD and CID on [M + Met]²⁺ produced various glycosidic and cross-ring cleavages, with ETD producing more cross-ring and internal ions, which are useful for structural analysis. Product ion intensities varied based on glycosidic-bond linkage and identity of monosaccharide sub-unit, and metal adducts. ETD and CID showed high fragmentation efficiency, often with complete precursor dissociation, depending on the identity of the adducted metal ion. Loss of water was occasionally observed, but elimination of small neutral molecules was not prevalent. For both ETD and CID, [M + Co]²⁺ produced the most uniform structurally informative dissociation with all oligosaccharides studied. The ETD and CID spectra were complementary. Graphical Abstract ᅟ.

Keywords: Carbohydrates; Collision-induced dissociation; Electron transfer dissociation; Metallated oligosacchardies; Oligosaccharides.

Figure 1

Oligosaccharides included in this study: (a) stachyose, (b) nystose, (c) 3α,4β,3α-galactotetraose, (d) maltotetraose, and (e) maltoheptaose.

Figure 2

ETD mass spectra of maltotetraose for (a) [M + 2Na]²⁺, (b) [M + Mg]²⁺, (c) [M + Cu]²⁺, and (d) [M + Co]²⁺. Colors used to illustrate the product ions are red for cross-ring cleavages, blue for glycosidic bond cleavages, and green for internal cleavages. Refer to Table 2 for identities of internal cleavages indicated by mass lost. Undissociated precursor ion is labeled in light blue with a large light blue diamond arrow head. Product ions involving solely neutral losses are labeled in black.

Figure 3

CID mass spectra of maltotetraose for (a) [M + 2Na]²⁺, (b) [M + Mg]²⁺, (c) [M + Cu]²⁺, and (d) [M + Co]²⁺. Refer to the Figure 2 caption for an explanation of the color codes.

Figure 4

Mass spectra from (a) ETD and (b) CID of [M + Co]²⁺ from maltotetraose with ¹⁸O isotopic labeling at the reducing end. Refer to the Figure 2 caption for an explanation of the color codes.

Figure 5

Mass spectra from (a) ETD and (b) CID of [M + Co]²⁺ from galactotetraose with ¹⁸O isotopic labeling at the reducing end. Refer to the Figure 2 caption for an explanation of the color codes.

Figure 6

Mass spectra from (a) ETD and (b) CID of [M + Co]²⁺ from stachyose. Refer to the Figure 2 caption for an explanation of the color codes.

Figure 7

Mass spectra from (a) ETD and (b) CID of [M + Co]²⁺ from nystose. Refer to the Figure 2 caption for an explanation of the color codes.

Figure 8

Mass spectra from (a) ETD and (b) CID of [M + Co]²⁺ from maltoheptaose with ¹⁸O isotopic labeling at the reducing end. Refer to the Figure 2 caption for an explanation of the color codes.