On-line Abstract Book

Ab Initio Study on the Fragmentation Mechanisms of Glycopeptides in Mass Spectrometry. III.

Takae Takeuchi^1,2, Sayaka Akiuchi¹, Rie Kiyama¹, Michiko Tajiri^4,5, Yoshinao Wada^3,4,5

Correspondence address: Takae Takeuchi, Nara Women's University, Department of Chemistry, Faculty of Science, Kitauoyanishi-machi, Nara, 630-8506 Japan.

Keywords: Calculations, Ab Initio; Collision-Induced Dissociation (CID); Fragmentation Mechanism/Pathways; Oligosaccharides.

Novel aspect: A new strategy of predicting fragmentation processes of glycopeptide glycan was developed based on MO methods.

 

Deciphering the complexity of carbohydrate structures is an exciting challenge for MS, since the CID fragmentation patterns differ according to the glycoforms, and can be used to differentiate their subtle structural differences. While most of the studies on this issue have been just empirical, recent progress in computer technology has enabled us to quantitatively predict the geometry of such medium-sized molecular species.

We reported the preliminary calculations for protonated aspartoylglycosylamines and singly or doubly sodiated sialylgalactopyranoses using ab initio and semiempirical methods in the 53rd ASMS Conference.¹ In this study, to establish a theoretical basis of glycan analysis, fragmentation mechanisms of aspartoylglycosylamine [GlcNAcAsn], sialylgalactopyranose [Neu5AcGal] and Sialyllactose cations were investigated and clarified by using ab initio DFT calculations. Potential energy surfaces for their possible fragmentation processes were calculated at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) level. Optimized geometries of possible isomers and conformers of [GlcNAcAsn+H]⁺, [Neu5AcGal+Na]⁺, [Neu5AcGal-H+2Na]⁺, intermediates, transition states, and fragments were calculated. The semiempirical PM3 and PM5 MO calculations were also performed for the fragmentation of the above-mentioned cations in order to clarify its validity of application to a medium size of glycans.

The calculated energy barrier for N-glycosidic cleavages of protonated aspartoylglycosylamine, which is accompanied by H transfer from C(2) in sugar ring to N linking between glucopyranose and aspart-4-oyl group (1,3-hydrogen shift), was estimated to be small. The calculated potential energy curves showed that ^0,2X type of cross-ring cleavage of [GlcNAcAsn+H]⁺ can also occur via rearrangement after a protonation of N-acetyl group. The calculated results were qualitatively in good agreement with the MALDI CID MS/MS of the corresponding glycopeptides.^2-4

In calculated five conformers of protonated sialyllactose with the lowest energy, sialyl group was relatively close to the lactose part, and N-acetyl group was near CH₂OH group in glucose part. In Na⁺-adducted sialyllactose, Na⁺ is easy to interact with the other functional groups in the molecule. In consequence of the interaction, Na⁺ combines with more than one functional groups to form more stable conformations. This result suggested that Na⁺-adducted sialyllactoses are stable and it is not easy that fragmentations from the Na⁺-adducted sialyllactoses occur.

1. T. Takeuchi, S. Akiuchi and R. Kiyama, The 53rd ASMS Conference on Mass Spectrometry, San Antonio (2005).
2. Y. Wada, M. Tajiri, S. Yoshida, Anal. Chem. 76, 6560 (2004).
3. C. E. Seggern, P. E. Zarek and R. J. Cotter, Anal. Chem. 75, 6523 (2003).
4. T. Yamagaki, J. Mass Spectrom. Soc. Jpn. 52, 72 (2004).