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 α2-8 sialic acid (SiA) in H2O and that H-bonds define its structure. A 15N,13C α2-8 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 `5N and C8 for residues R-I- R-III of the tetramer, indicating H-bonds between the `5N's and the 08's of these residues. Additional J couplings between the 15N's and C2's of the adjacent residues confirm the putative H-bonds. NH groups showing this long-range correlation also experience slower 1H/2H 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 (24 helix) and 4 residues/turn (14 helix). To discriminate between models, we resorted to 1H,1H NOEs. The 24 helical model is in better agreement with the experimental data. We provide direct evidence of H- bonding for (SiA)4and show how H-bonds can be a determining factor for shaping its 3D structure. [ABSTRACT FROM AUTHOR]