We have extended our technology for studying macromolecular structure in solution under weakly aligning conditions. New developments increase both the size of systems that can be studied, and the accuracy that can be obtained. Rhodopsin-containing membranes contain sufficient residual diamagnetic susceptibility anisotropy that they align spontaneously in a sufficiently strong magnetic field. Undeca-peptide analogs of the C-terminus of the gamma-subunit of transducin are known to bind weakly but specifically to photo-activated rhodopsin. The orientation of the peptide occurring upon binding to rhodopsin, following photo-activation, was used by us to provide precise structural and orientational information on the peptide in the bound state, and thereby on the orientation of the entire G protein in the receptor-bound state. A prerequisite for such studies is a relatively low affinity of the peptide for the membrane-anchored receptor. For water-soluble proteins and nucleic acids, weak alignment allows very accurate measurement of internuclear dipolar couplings, thereby providing extremely precise information on local as well as global structure. For a small, 56-residue domain, we used this technology to study planarity of peptide groups and found that the out-of-plane position of the amide hydrogen has a root-mean-square value of less than about """"""""4"""""""", much smaller than predicted on the basis of literature theoretical calculations, but in good agreement with neutron diffraction results. The in-plane deviation from the line bisecting the C?-N-Ca angle is less than """"""""2"""""""". For CO-ligated adult human hemoglobin, analogous technology revealed that in solution, at 100 mM ionic strength, the quaternary structure is about half-way intermediate between the so-called R and R2 states. Application to nucleic acids shows that accurate measurement of the bending of the helical axis is possible with the weak alignment technology. We also have succeeded in measurement of interactions over much larger distances, up to 12 ?, than previously possible by NMR.
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