| Abstract
| - The data most commonly available for the determination of macromolecular structures insolution are NOE based distance estimates and spin−spin coupling constant based dihedral angle estimates.This information is, unfortunately, inherently short-range in nature. Thus, for many multidomain proteins,little information is available to accurately position weakly interacting domains with respect to each other.Recent studies of proteins aligned in dilute liquid crystalline solvents have shown the utility of measuringanisotropic spin interactions, such as residual dipolar couplings, to obtain unique long-range structuralinformation. In this work, the latter approach is taken to explore the relative domain orientation in atwo-domain fragment from the protein barley lectin. An approach based on singular value decompositionas opposed to simulated annealing is used to directly determine order tensors for each domain from residual15N−1H dipolar couplings, and the limitations of the two approaches are discussed. Comparison of theorder tensor principal axis frames as separately determined for each domain indicates that the two domainsare not oriented as in the crystal structure of wheat germ agglutinin, a highly homologous protein (∼95%sequence identical). Furthermore, differences in the order tensor values suggest that the two domains arenot statically positioned but are experiencing different reorientational dynamics and, to a large degree,may be considered to reorient independently. Data are also presented that suggest that a specific associationoccurs between one domain and the lipid bicelles comprising the liquid crystal solvent.
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