Relationship between nuclear magnetic resonance chemical shift and protein secondary structure.

The parameters for HN chemical shift calculations of proteins have been determined using data from high-resolution crystal structures of 15 proteins. Employing these chemical shift calculations for HN protons, the observed secondary structure chemical shift trends of HN protons, i.e., upfield shifts on helix formation and downfield shifts on β-sheet formation, are discussed. Our calculations suggest that the main reason for the difference in NH chemical shifts in helices and sheets is not an effect from the directly hydrogen-bonded carbonyl, which gives rise to downfield shifts in both cases, but arises from an additional upfield shift predicted in helices and originating in residues i-2 and i-3. The calculations also explain the well-known relationship between amide proton shifts and hydrogen-bond lengths. In addition, the HN chemical shifts of the distorted amphipathic helices of the GCN4 leucine zipper are calculated and used to characterise the solution structure of the helices. By comparing the calculated and experimental shifts, it is shown that in general the agreement is good between residues 15 and 28. The most interesting observation is that in the N-terminal half of the zipper, although both calculated and experimental shifts show clear periodicity, they are no longer in phase. This suggests that for the N-terminal half, in the true average solution structure the period of the helix coil is longer by roughly one residue compared to the NMR structures.

The relationship between amide proton chemical shifts and secondary structure in proteins.

Magnetic Shielding and Susceptibility Anisotropies

A complete analysis of the proton NMR spectra of the six stable steroisomers of retinals has been undertaken. 11-cis-Retinal, the prosthetic group of the visual pigment rhodopsin, relieves steric strain between 10 H and 13 CH3 groups by a skew geometry around diene unit C11=C12–C13=C14.

220 MHz proton nuclear magnetic resonance spectra of retinals.

The PMR analysis of non-conjugated alkenoic and alkynoic acids and esters.

NMR—the chemical shift—IV: The anisotropies of the carbonyl group

The chemical shift—I

The chemical shift—II: The anisotropies of the carboncarbon double bond

The chemical shift—III: The NMR spectra of some diterpenes

The calculation of chemical shift values by the method used in parts I–V (1–5) has been extended to a derivation of the shielding effects of the ethylene–ketal and −thioketal groups. For these studies ketal and thioketal derivatives of monoketoandrostanes were prepared. The chemical shifts of the C-18 and -19 methyl protons in these compounds are reported for the solvents CDCl3, CCl4 and benzene.Representing both groups by point dipoles, values for the anisotropies and for K, a parameter descriptive of the electric field effect, were derived for various, coincidental, locations of the magnetic and electric dipoles along the symmetry axes of the two groups.

W.G. Craig

Papers

NMR—the chemical shift—IV: The anisotropies of the carbonyl group

The chemical shift—I

The chemical shift—II: The anisotropies of the carboncarbon double bond

The chemical shift—III: The NMR spectra of some diterpenes

Chemical Shift. VI. The Long-range Shielding Effects of Ethylene–Ketal and −Thioketal Groups