Many membrane proteins undergo relatively rapid rotational diffusion (Tau = 1-10 musec) in membrane systems. This rotational motion can lead to partial narrowing of carbon 13 and nitrogen 15 nuclear magnetic resonance spectra of the membrane protein. In principle, the motionally narrowed spectra contain sufficient information to deduce the orientation of the rotation axis relative to the molecular frame for ech of many independent positions in the protein. This in turn can allow the structure of elected regions ouf theprotein to be determined with resolution comparable to that of X-ray crystallography in favorable cases. This proposal addresses one approach to extracting this information. The approach involves the production of peptide bonds which contain both a carbon 13 carbonyl group and a nitrogen 15 amino group label. Using biosynthetic incorporation of the two labels with different amino acids, a unique dipeptide sequence can be labeled which will usually occur at a single position in proteins of moderate size. Thus single residue resolution can be obtained. In addition, at last three perameters cans be measured cabon 13 shift, nitrogen 15 shift and C-N dipolar coupling) which can fix the orientation of the rotation axis in the peptide plane. By labeling sequential positions along the amino acid sequence, the relative orientations of adjacent peptide planes can be established, allowing the Ramachandran angles between the planes to be measured with precision of a few degrees in favorable cases. A series of experiments are proposed to establish the variation in carbon 13 and nitrogen 15 amide chemical shift tensors with microenvironment using a protein with a known crystal structure, T4 lysozyme. The use of the proposed approach to probe the structure in the membrane of M13 coat protein and the light harvesting proteins of photosynthetic bacteria is suggested.
