High-resolution structure determinations

In the past few years, there have been advances in the detailed understanding of the crystalline structures through x-ray and neutron diffraction experiments. These advances were possible because synchrotron x-ray beams are much more powerful than ordinary laboratory beams, giving double or triple the amount of diffraction data. Neutron diffraction permits location of deuterium atoms so the hydrogen bonding systems can be worked out on samples that have had the hydroxyl hydrogen atoms substituted with deuterium atoms. Also, new methods have been used to prepare samples that are much more crystalline, also leading to more diffraction data. 

So far, highly detailed structures have been published for cellulose la (Nishiyama et al. 2003a), Ip (Nishiyama et al. 2002), mercerized II (Langan et al. 2001), and 

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Figure 12.3 Two-dimensional crystals of the protein bacteriorhodopsin were used to pioneer three-dimensional high-resolution structure determination from electron micrographs. An electron density map to 7 A resolution (a) was obtained and interpreted in terms of seven transmembrane helices (b).

Rosenbusch, j. P., Stability of membrane proteins Relevance for the selection of appropriate methods for high-resolution structure determinations. J. 

Our recent NMR study of the 24mer hairpin motif comprising largely the symmetrical internal loop revealed that a unique and stable structure is formed only in the presence of Mg2+ ions, which seem to bind specifically in the internal loop region. Unfortunately, this construct had a large propensity for duplex formation and we did not consider the NMR data of sufficient quality to allow a high resolution structure determination. This particular problem could be circumvented by making the transition from the kinetically favored hairpin... 

Structural studies on proteins became possible with the advent of multidimensional NMR techniques. These rely on the use of isotopic labeling with C, and N and techniques to provide a facile method for assigning all of the H resonances in a protein, which would otherwise be a difficult task. The measurement of nuclear Overhauser effect (NOE) intensities provide much of the distance information necessary to derive a structure, although additional chemical shift information is needed for a high-resolution structural determination. 

For even larger proteins, protein aggregates, protein complexes, or proteins embedded in a hpid bUayer, where the molecular tumbling is further reduced, solid-state NMR spectroscopy may become a viable alternative for stracture elucidation or even high-resolution structure determination. In the chapter Solid-State NMR Spectroscopy of Proteins , basic principles of biological solid-state NMR spectroscopy as well as fundamental techniques for isotope labeling, sample preparation, and some selected applications are reviewed. In addition, recent developments in polarization enhancement by DNP for solid-state NMR spectroscopy are outlined. 

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