PubMed · 2928028
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I A Abu-Arafeh, C Stuart, P J Smail. 1989. Fictitious crystalluria.. https://pubmed.ncbi.nlm.nih.gov/2928028/
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In isotropic solution, internuclear dipolar couplings average to zero as a result of rotational diffusion. By dissolving macromolecules in a dilute aqueous nematic discotic liquid-crystalline medium containing widely spaced magnetically oriented particles, a tunable degree of solute alignment with the magnetic field can be created while retaining the high resolution and sensitivity of the regular isotropic nuclear magnetic resonance (NMR) spectrum. Dipolar couplings between 1H-1H, 1H-13C, 1H-15N, and 13C-13C pairs in such an oriented macromolecule no longer average to zero, and are readily measured. Distances and angles derived from dipolar couplings in human ubiquitin are in excellent agreement with its crystal structure. The approach promises to improve the accuracy of structures determined by NMR, and extend the size limit.
A novel application of electron paramagnetic resonance (EPR) is reported to gain three dimensional structural information on cofactors in proteins. The method is applied here to determine the unknown position of the electron acceptor QK, a phylloquinone (vitamin K1), in the electron transfer chain in photosystem I of oxygenic photosynthesis. The unusual electron spin echo (out-of-phase echo) observed for the light induced radical pair P700.+QK.- in PS I allows the measurement of the dipolar coupling between the two radical pair spins which yields directly the distance between these two radicals. Full advantage of the information in the out-of-phase echo modulation can be taken if measurements using single crystals are performed. With such samples, the orientation of the principal axis of the dipolar interaction, i.e., the axis connecting P700.+QK.-, can be determined with respect to the crystal axes system. An angle of theta = (27 +/- 5)degrees between the dipolar coupling axis and the crystallographic c-axis has been derived from the modulation of the out-of-phase echo. Furthermore, the projection of the dipolar axis into the crystallographic a,b-plane, is found to be parallel to the a-axis. The results allow for the determination of two possible locations of QK within the electron transfer chain of photosystem I. These two positions are related to each other by the pseudo C2 symmetry of the chlorophyll cofactors.
Spermine phosphate hexahydrate crystallizes in space group P2(1)/a with unit-cell dimensions a = 7.931 (1), b = 23.158 (5), c = 6.856 (2) A, and beta = 113.44 (2) degrees at 125 K with unit-cell contents [(C10H30N4)2(4+)(HPO4)4(2-).12H2O]. The packing of spermines and monohydrogen phosphates in this crystal structure has features which may be relevant to the binding of spermine to DNA. Another important structural feature is the presence of channels containing water that is hydrogen bonded as in ice-Ih with disordered protons. The channels occur between sheets of spermine long chains and are also bordered by hydrogen-bonded monohydrogen phosphate chains. The hydrogen-bonding scheme of these water chains proposed on the basis of an earlier X-ray study is now confirmed. Nuclear positions, anisotropic mean-square (m.s.) displacements, an overall scale factor and two extinction parameters (rho and g) were refined using full-matrix least-squares giving values of R(F0(2)) = 0.09, Rw(F0(2)) = 0.11 and S = 1.02. Thermal vibrational analysis revealed that the backbone of the spermine cation can be described as a single rigid segment with a substantial libration of 27 deg2 around the spermine molecular long axis.