Secondary-structure dependent chemical shifts in proteins

Chemical shift data have been collected on eight proteins that have the same conformation in solution as in their crystal structures. Ring-current shifts have been calculated and subtracted from the exerimentally measured shifts, to leave shifts that depend only on local conformation. Overall, the shifts show an approximately normal distribution with no appreciable skewness, thus confirming that ring-current shifts have the overall effect of skewing the distribution to high field. In helices, NH and C(alpha)H have a high significant tendency to resonate to high field, whereas they resonate to low field in beta-sheets. Side-chain protons resonate slightly to high field in beta-sheets. Chemical shift distributions are narrowest for side-chain protons, and widest for amide protons. When only slowly exchanging amide protons are considered, the high field shift for amide protons in helices is more pronounced, but there is only a small difference in sheets. C(alpha)H signals at the N-terminal end of helices tend to resonate to higher field than those at the C-terminal end, whereas for NH signals it is the C-terminal end that resonates to higher field. There is no significant effect of position within the helix on side-chain signals, implying that the helix dipole has little effect on shifts within the helix.