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International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 26.1, p. 766
Section 26.1.4.2. The crystal structure of GlcNAc
C. C. F. Blake,a‡ R. H. Fenn,a§ L. N. Johnson,a*¶ D. F. Koenig,a‡‡ G. A. Mair,a‡‡ A. C. T. North,a§§ J. W. H. Oldham,a¶¶ D. C. Phillips,a¶¶ R. J. Poljak,a‡‡‡ V. R. Sarmaa§§§ and C. A. Vernonb¶¶
a
Davy Faraday Research Laboratory, The Royal Institution, London W1X 4BS, England, and bDepartment of Chemistry, University College London, Gower Street, London WC1E 6BT, England |
During the first two years of her graduate work (1962–1964), LNJ determined the crystal structure of GlcNAc. β-D-N-Acetylglucosamine was crystallized from a methanol–water mixture, and diffraction data were recorded on film with a modified Weissenberg camera. Intensities were estimated visually from comparison with a scale of fixed-time exposures of the attenuated main beam. The structure was solved manually using a sharpened Patterson function and application of the minimum function of Buerger (1959)![[link]](/graphics/greenarr.gif)
. The structure revealed a standard glucopyranose ring in the chair conformation with the plane of the N-acetyl group normal to the ring (Johnson & Phillips, 1964; Johnson, 1966). The structure of glucosamine hydrochloride had been solved by Cox & Jeffrey in 1939 (Cox & Jeffrey, 1939), a remarkable early achievement.
By 1963, about ten glucopyranose structures were available, including that of cellobiose (Jacobson et al., 1961). An analysis by Ramachandran et al. (1963) showed that the glucopyranose ring is remarkably uniform in its conformation and may be regarded as a rigid structure.
The final difference-Fourier synthesis of β-GlcNAc revealed an additional peak adjacent to the C1 carbon atom in the position of a hydroxyl group in the α configuration. Peak heights suggested that there could be a mixture in the crystal of approximately 80% β-N-acetylglucosamine and 20% α-N-acetylglucosamine. Refinement indicated that a mixture of the two anomers could be accommodated in the crystal lattice.
Tests on the optical rotation of the crystalline sample after dissolution, compared with the starting material, also added support to the notion that the crystal contained a mixture of α and β anomers, probably as a result of mutarotation during crystallization. Although a later and more precise structure determination of GlcNAc indicated a lower proportion of the α anomer (Mo & Jensen, 1975), mixtures of α and β sugars in crystals have been observed for other compounds (Jeffrey, 1990). The consideration of the α and β anomers of GlcNAc meant that we were keenly aware of the importance of configuration at the C1 atom. This turned out to be essential when we were interpreting the results with lysozyme.
References
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