Influence of Linkage Stereochemistry and Protecting Groups on Glycosidic Bond Stability of Sodium Cationized Glycosyl Phosphates

Abstract

Energy-resolved collision-induced dissociation (ER-CID) experiments of sodium cationized glycosyl phosphate complexes, [GP +Na], are performed to elucidate the effects of linkage stereochemistry (α versus β), the geometry of the leaving groups (1,2-cis versus 1,2-trans), and protecting groups (cyclic versus non-cyclic) on the stability of the glycosyl phosphate linkage via survival yield analyses. A four parameter logistic dynamic fitting model is used to determine CID values, which correspond to the level of rf excitation required to produce 50% dissociation of the precursor ion complexes. Present results suggest that dissociation of 1,2-trans [GP +Na] occurs via a McLafferty-type rearrangement that is facilitated by a syn orientation of the leaving groups, whereas dissociation of 1,2-cis [GP+Na] is more energetic as it involves the formation of an oxocarbenium ion intermediate. Thus, the C1-C2 configuration plays a major role in determining the stability/reactivity of glycosyl phosphate stereoisomers. For 1,2-cis anomers, the cyclic protecting groups at the C4 and C6 positions stabilize the glycosidic bond, whereas for 1,2-trans anomers, the cyclic protecting groups at the C4 and C6 positions tend to activate the glycosidic bond. The C3 O-benzyl (3 BnO) substituent is key to determining whether the sugar or phosphate moiety retains the sodium cation upon CID. For 1,2-cis anomers, the 3 BnO substituent weakens the glycosidic bond, whereas for 1,2-trans anomers, the 3 BnO substituent stabilizes the glycosidic bond. The C2 O-benzyl substituent does not significantly impact the glycosidic bond stability regardless of its orientation. Graphical abstract ᅟ.

Advancing Solutions to the Carbohydrate Sequencing Challenge

“…Combining these strategies generates complementary information, increasing the confidence and level of structure assignment. Mass spectra are sensitive to differences in precursor structure such that diagnostic product ions can be generated even from isomeric precursors. ,− Ashwood et al recently developed a workflow that resulted in product ions diagnostic for sialylation linkage (α2,3 versus α2,6) and arm position for native reduced N - and O -glycans . Additionally, the ratio of certain fragment ions will likely differ between isomers, facilitating differentiation. , In fact, Pett et al recently discovered that the intensities of specific product ions generated by HCD of glycopeptides differed between α2,3 versus α2,6-linked sialic acids .…”

Direct Experimental Characterization of Glycosyl Cations by Infrared Ion Spectroscopy

“…Tandem mass spectrometry (MS/MS) can be employed to generate glycosyl cations in complete isolation, and their gas-phase fragmentation has been extensively studied. 10 − 14 Recently, the use of MS in combination with infrared (IR) ion spectroscopy has emerged as a powerful method to determine the gas-phase structures of molecular ions in MS/MS 15 − 20 and to probe glycan structure. 21 − 26 …”

“…However, the harsh conditions used to generate the oxocarbenium ions are not compatible with all types of protecting groups commonly used in oligosaccharide synthesis. Tandem mass spectrometry (MS/MS) can be employed to generate glycosyl cations in complete isolation, and their gas-phase fragmentation has been extensively studied. − Recently, the use of MS in combination with infrared (IR) ion spectroscopy has emerged as a powerful method to determine the gas-phase structures of molecular ions in MS/MS − and to probe glycan structure. − …”

Characterization of Elusive Reaction Intermediates Using Infrared Ion Spectroscopy: Application to the Experimental Characterization of Glycosyl Cations

“…More recently, Rodgers et al studied the influence of the anomeric configuration and protecting group pattern on the stability of the glycosidic bond. 20 To this end, the sodium adducts of eight glycosyl phosphates were examined via survival yield analysis based on their CID fragmentation toward glycosyl cations ( Scheme 2 D). They found that the relative C1–C2 stereochemistry is a major factor affecting the stability of the glycosidic bond.…”