The measurement of anisotropic spin interactions, such as residual dipolar couplings, in partially ordered solutions can provide valuable information on biomolecular structure. While the information can be used to refine local structure, it can make a unique contribution in determining the relative orientation of remote parts of molecules, which are locally well structured, but poorly connected based on NOE data. Analysis of dipolar couplings in terms of Saupe order matrices provides a concise description of both orientation and motional properties of locally structured fragments in these cases. This paper demonstrates that by using singular value decomposition as a method for calculating the order matrices, principal frames and order parameters can be determined efficiently, even when a very limited set of experimental data is available. Analysis of 1H-15N dipolar couplings, measured in a two-domain fragment of the barley lectin protein, is used to illustrate the computational method.

The data most commonly available for the determination of macromolecular structures in solution 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 measuring anisotropic spin interactions, such as residual dipolar couplings, to obtain unique long-range structural information. In this work, the latter approach is taken to explore the relative domain orientation in a two-domain fragment from the protein barley lectin. An approach based on singular value decomposition as opposed to simulated annealing is used to directly determine order tensors for each domain from residual (15)N-(1)H dipolar couplings, and the limitations of the two approaches are discussed. Comparison of the order tensor principal axis frames as separately determined for each domain indicates that the two domains are not oriented as in the crystal structure of wheat germ agglutinin, a highly homologous protein ( approximately 95% sequence identical). Furthermore, differences in the order tensor values suggest that the two domains are not 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 association occurs between one domain and the lipid bicelles comprising the liquid crystal solvent.

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NMR relaxation parameters were measured for the peptide-plane carbonyl and nitrogen nuclei for the protein Escherichia coli flavodoxin. A poor correlation between the general order parameters of the C‘−Cα vector (Zeng, L.; Fischer, M. W. F.; Zuiderweg, E. R. P. J. Biomol. NMR 1996, 7, 157−162) and the N−NH vector was observed. We interpret this lack of correlation in this nearly spherical protein as evidence of local or semilocal anisotropic motion. A new experiment is introduced from which the cross-correlation between the carbonyl chemical shift anisotropy relaxation and carbonyl-Cα dipole−dipole relaxation is obtained. We show theoretically that the three relaxation measurements, reporting on the dynamics of the C‘−Cα vector, N−NH vector, and CSA tensor components behave differently under anisotropic motion. The cross-correlation order parameter formalism for dipolar cross-correlation spectral densities, as introduced by Daragan and Mayo (Daragan, V. A.; Mayo, K. H. J. Magn. Reson. B 1995, 107, 274−278), has been extended to include cross-correlations between nonaxial chemical shift anisotropy and dipole−dipole relaxation. By analyzing our experimental data with the theoretical models for anisotropic local motion, dynamic models were obtained for the peptide planes of 32 residues of E. coli flavodoxin.

Ballistospore discharge is a feature of 30000 species of mushrooms, basidiomycete yeasts and pathogenic rusts and smuts. The biomechanics of discharge may involve an abrupt change in the center of mass associated with the coalescence of Buller's drop and the spore. However this process occurs so rapidly that the launch of the ballistospore has never been visualized. Here we report ultra high-speed video recordings of the earliest events of spore dispersal using the yeast Itersonilia perplexans and the distantly related jelly fungus Auricularia auricula. Images taken at camera speeds of up to 100,000 frames/ s demonstrate that ballistospore discharge does involve the coalescence of Buller's drop and the spore. Recordings of I. perplexans demonstrate that although coalescence may result from the directed collapse of Buller's drop onto the spore, it also may involve the movement of the spore toward the drop. The release of surface tension at coalescence provides the energy and directional momentum to propel the drop and spore away from the fungus. Analyses show that ballistospores launch into the air at initial accelerations in excess of 10,000 g. There is no known analog of this micromechanical process in animals, plants or bacteria, but the recent development of a surface tension motor may mimic the fungal biology described here.