13C structuring shifts for the analysis of model β-hairpins and β-sheets in proteins: diagnostic shifts appear only at the cross-strand H-bonded residues
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The present studies have shown that 13C=O, 13Cα and 13Cβ of H-bonded strand residues in β-hairpins provide additional probes for quantitating the extent of folding in β-hairpins and other β-sheet models. Large differences in the structuring shifts (CSDs) of these 13C sites in H-bonded versus non-H-bonded sites are observed: the differences between H-bonded and non-H-bonded sites are greater than 1.2 ppm for all three 13C probes. This prompts us to suggest that efforts to determine the extent of hairpin folding from 13C shifts should be based exclusively on the observation at the cross-strand H-bonded sites. Furthermore, the statistics suggest the 13C′ and 13CβCSDs will provide the best differentiation with 100 %-folded CSD values approaching −2.6 and +3 ppm, respectively, for the H-bonded sites. These conclusions can be extended to edge-strands of protein β-sheets. Our survey of reported 13C shifts in β-proteins indicates that some of the currently employed random coil values need to be adjusted, particularly for ionization-induced effects.
Keywordsβ-hairpin β-sheet 13C chemical shift deviations Folding probes 13C statistical coil shifts
This work was supported by the National Science Foundation (grants CHE-0650318 and -1152218).
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