β-Sheet 13C Structuring Shifts Appear Only at the H-Bonded Sites of Hairpins
journal contributionposted on 09.02.2011 by Irene Shu, James M. Stewart, Michele Scian, Brandon L. Kier, Niels H. Andersen
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The 13C chemical shifts measured for designed β-hairpins indicate that the structuring shifts (upfield for Cα and C′, downfield for Cβ) previously reported as diagnostic for β-structuring in proteins appear only at the H-bonded strand residues. The resulting periodicity of structuring shift magnitudes is not, however, a consequence of H-bonding status; rather, it reflects a previously unrecognized alternation in the backbone torsion angles of β-strands. This feature of hairpins is also likely to be present in proteins. The study provides reference values for the expectation shifts for 13C sites in β-structures that should prove useful in the characterization of the folding equilibria of β-sheet models.