Evidence for Helical Structure in a Tetramer of α2-8 Sialic Acid: Unveiling a Structural Antigen

Characteristic H-bonding patterns define secondary structure in proteins and nucleic acids. We show that similar patterns apply for alpha 2-8 sialic acid (SiA) in H2O and that H-bonds define its structure. A N-15,C-13 alpha 2-8 SiA tetramer, (SiA)(4), was used as a model system for the polymer. At 263 K, we detected intra-residue through-H-bond J couplings between N-15 and C8 for residues R-I-R-III of the tetramer, indicating H-bonds between the N-15's and the O8's of these residues. Additional J couplings between the N-15's and C2's of the adjacent residues confirm the putative H-bonds. NH groups showing this long-range correlation also experience slower H-1/H-2 exchange. Additionally, detection of couplings between H7 and C2 for R-II and R-III implies that the conformations of the linkers between these residues are different than in the monomers. These structural elements are consistent with two left-handed helical models: 2 residues/turn (2(4) helix) and 4 residues/turn (1(4) helix). To discriminate between models, we resorted to H-1,H-1 NOEs. The 2(4) helical model is in better agreement with the experimental data. We provide direct evidence of H-bonding for (SiA)(4) and show how H-bonds can be a determining factor for shaping its 3D structure.