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.

The present studies have shown that (13)C=O, (13)C(α) and (13)C(β) 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 (13)C 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 (13)C probes. This prompts us to suggest that efforts to determine the extent of hairpin folding from (13)C shifts should be based exclusively on the observation at the cross-strand H-bonded sites. Furthermore, the statistics suggest the (13)C' and (13)C(β) CSDs 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 (13)C shifts in β-proteins indicates that some of the currently employed random coil values need to be adjusted, particularly for ionization-induced effects.