The anomeric effect is a chemical phenomenon1,2,3,4,5,6,7,8,9 that refers to an observed stabilization10 of six-membered carbohydrate rings when they contain an electronegative substituent at the C1 position of the ring. This stereoelectronic effect influences the three-dimensional shapes of many biological molecules. It can be manifested not only in this classical manner involving interaction of the endocyclic oxygen atom (O5) found in such sugars with the C1 substituent (endo-anomeric effect) but also through a corresponding interaction of the electronegative exocyclic substituent with O5 (exo-anomeric effect). However, the underlying physical origin(s) of this phenomenon is still not clear1,3,4,11,12,13,14. Here we show, using a combination of laser spectroscopy and computational analysis, that a truncated peptide motif can engage the two anomers of an isolated sugar in the gas phase, an environment lacking extraneous factors which could confound the analysis. (Anomers are isomers that differ in the orientation of the substituent at C1.) Complexes formed between the peptide and the α- or β-anomers of d-galactose are nearly identical structurally; however, the strength of the polarization of their interactions with the peptide differs greatly. Natural bond order calculations support this observation, and together they reveal the dominance of the exo- over the endo-anomeric effect. As interactions between oxygen atoms at positions C1 and C2 (O1 and O2, respectively) on the pyranose ring can alter the exo/endo ratio of a carbohydrate, our results suggest that it will be important to re-evaluate the influence, and biological effects, of substituents at position C2 in sugars.
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We thank the EPSRC for funding (GR/T26542), the Leverhulme Trust for the award of an Emeritus Fellowship (J.P.S.), the Spanish Ministry (MCINN) for a Juan de la Cierva contract (E.J.C.), the Royal Society for a USA/Canada Research Fellowship and Linacre College for a Junior Research Fellowship (T.D.V.), the STFC for the provision of equipment from the Laser Loan Pool, and the Oxford Supercomputing Centre for their services. B.G.D. is a Royal Society Wolfson Research Merit Award recipient and is supported by an EPSRC LSI Platform grant.
This file contains Supplementary Methods, Supplementary Figures 1-3 with legends, Supplementary Tables 1- 4 and additional references.
About this article
Journal of The American Society for Mass Spectrometry (2011)