Distinguishing isomeric pyridylaminated high-mannose (Man7) oligosaccharides based on energy-resolved mass spectra

Selective fragmentation of the N-glycan moiety and protein backbone of ribonuclease B on an Orbitrap Fusion Lumos Tribrid mass spectrometer

RATIONALE

The functional study and application of an intact glycoprotein require the structural characterization of both the protein backbone and the glycan moiety; the former has been successfully demonstrated with selective fragmentation of the protein backbone in CID and ExD; whether the latter can be achieved with selective fragmentation of the glycan moiety remains to be explored.

METHODS

RNase B solution was electrosprayed and its intact glycoforms of GlcNAc₂ Man_n (n = 5-9) with the highest abundance (charge state z = 16) were isolated individually and fragmented using CID, ETD, HCD, ETciD, and EThcD on the Orbitrap Fusion Lumos Tribrid mass spectrometer; the dissociation parameters were optimized for selective fragmentation of the N-glycan moiety and protein backbone as well as high sequence coverage. The obtained spectra were interpreted using the protein and N-glycan database search engines ProteinGoggle and GlySeeker, respectively.

RESULTS

With exploration of different dissociation parameters for all the five methods, selective fragmentation of the N-glycan moiety (the protein backbone staying intact) was observed in both HCD and EThcD at low collisional energies, but only a few matched product ions were observed; more comprehensive fragmentation was observed at high collisional energies (the protein backbone lost). Selective protein backbone fragmentation was observed in all the five dissociation methods.

CONCLUSIONS

For comprehensive structural characterization of intact N-glycoproteins using tandem mass spectrometry, the composition and topology of the N-glycan moiety can be identified using HCD and EThcD complementarily at low and high energies; while the amino acid sequence and glycosite can be identified using CID, ETD, HCD, ETciD, and EThcD with their optimal dissociation parameters.

Mannose7 glycan isomer characterization by IMS-MS/MS analysis

The isomers of the Man(7)GlcNAc(2) glycan obtained from bovine ribonuclease B have been characterized by ion mobility spectrometry-tandem mass spectrometry (IMS-MS/MS). In these experiments, [Man7 + 2Na](2+) precursors having different mobilities are selected by ion mobility spectrometry and analyzed by MS/MS techniques in an ion trap. The fragmentation spectra obtained for various precursor ions are specific, suggesting the isolation or enrichment of different glycan isomers. One fragment ion with a mass-to-charge ratio (m/z) of 903.8 is found to correspond to the loss of an internal mannose residue of a specific isomer. Extracted fragment ion drift time distributions (XFIDTDs) yield distinctive precursor ion drift time profiles indicating the existence of four separate isomers as proposed previously.

Multi-stage mass spectrometric information obtained by deconvolution of energy-resolved spectra acquired by triple-quadrupole mass spectrometry

Triple-quadrupole mass spectrometry (TQ-MS) provides the capability to carry out collision-induced dissociation (CID) and it offers advantages in quantification when connected with high-performance liquid chromatography through an electrospray ionization interface. However, although TQ-MS provides information on partial structures through the analysis of product ions obtained by CID experiments, the method only provides single-stage CID experiments, which limits the detailed structural information that can be obtained. Herein, a method of overcoming this limitation of TQ-MS is described. A spectrum obtained by energy-resolved mass spectrometry (ERMS) was used to deconvolute the fragmentation process, with a Galili-antigenic trisaccharide derivative being used as an example. A replot of the ERMS data showing the ratios of the product ions to the precursor ion resulted in a descriptive graph. Analysis of the sum of the ratios of individual product ions to the precursor ion at specific CID energies revealed that the members of a series of product ions were related to each other. The obtained relationships and the m/z values of the product ions provided information on the fragmentation process taking place during the dissociation, indicating that the ERMS spectrum obtained by TQ-MS contained equivalent information to that obtainable by multi-stage MS/MS (MS(n); n≥2). This method may allow users of triple-quadrupole mass spectrometers to obtain MS(n)-type information by performing a single ERMS experiment, which is even advantageous over quadrupole ion trap (QIT)-MS/MS because CID experiments on individual first-generation product ions are not required.

Analysis of behavior of sodiated sugar hemiacetals under low-energy collision-induced dissociation conditions and application to investigating mutarotation and mechanism of a glycosidase

Analysis of anomericity is one of the most important issues in the structure elucidation of carbohydrates. Mass spectrometry (MS)-based methods are of particular interest and important to address the issue related to resolving anomericity of monosaccharide units in a glycan. However, direct analysis of hemiacetals has not been possible by MS because of the nonavailability of information regarding the gas-phase behavior of such ion species. We addressed this issue by using stage-discriminated energy-resolved mass spectrometry (ERMS) at the stages of MS(n) and MS(n+1) and showed that such analysis can be made. This was achieved by proving that individual anomers can be identified and that the equilibrium of sodium adducted ion species of alpha- and beta-anomers can be negated in the gas phase under collision-induced dissociation (CID) conditions. On the basis of these results, we could 1) observe the mutarotation of lactose and 2) speculate the hydrolysis mechanism of endo-glycosylceramidase by using mass spectrometry.

Mechanism of a gas-phase dissociation reaction of 4-aminobutyl glycosides under CID MS/MS conditions

The assembly of monosaccharides during the synthetic process of glycan structures is responsible for the diversity of this family of molecules. Because of the complexity of the glycan structure, synthesis of oligosaccharides and structural analysis have been difficult tasks. During efforts to develop glycosides carrying an aglycon that can be used in both functional and structural investigations, we found that 4-aminobutyl glycosides fulfill these criteria. We also observed that the glycosidic linkage underwent an interesting dissociation reaction under collision-induced MS/MS, and that the reaction product is very useful in structural investigation based on mass spectrometry, especially since it provides information regarding anomeric configurations. Despite its importance, the reaction mechanism of the dissociation is not fully understood. For this reason, we studied the mechanism by synthesizing possible products and used them in detailed analyses based on energy-resolved mass spectrometry where the energy dependence of the dissociation reaction was analyzed under collision-induced dissociation conditions. As a result of spectral match with one of synthesized reference compounds, it was suggested that the dissociation reaction to generate a C-ion species and a pyrrolidine took place through a five-membered transition state in two-step reaction sequence.

High-yielding and controlled dissociation of glycosides producing B- and C-ion species under collision-induced dissociation MS/MS conditions and use in structural determination

Collision-induced dissociation (CID) in mass spectrometry is a powerful technique with which to understand gas-phase chemical reactions. A mass spectrometer is used to carry out the reaction, isolation, and analysis. On the other hand, structural analysis of glycan structures is of extreme importance in the analysis of biomolecules, such as glycoproteins and glycolipids. In the analysis of glycan structures based on CID, certain ion species, including B-/Y-, C-/Z-, and A-/X-ions, are produced. Among these ions, we are interested in C-ion species that carry a glycosyl oxygen atom at the anomeric center and that possibly provide information regarding anomeric configuration. A method for generating C-ion species when necessary is thus considered to be important; however, none is currently available. In this study, synthetic glycosides carrying a series of aglycons were analyzed with the aim of identifying suitable glycosides with which to produce C-ions to be used in the structural determination of oligosaccharides. The results showed a 4-aminobutyl group was an excellent candidate. Furthermore, the use of C-ion species obtained in this manner in the structural characterization of a ganglioside, GM3, is described. The type of glycoside is believed to be valuable not only in structural analysis but also in biological investigation, because of the existing amino functionality that has been proven to be useful by enabling the generation of conjugates with other molecules and materials.