Structure studies continued

Research in halogenophosphines during this year s coverage has in the main been directed at either new applications or a better understanding of known reactions. In the phosphorane field, structural studies continue to dominate, although a further decline is apparent in the number of papers devoted to M.O. calculations. The surprise of the year must be Becker s phospha-alkene (65). 

Thiosemicarbazone (RNH-CS-NH-N=CR/R//, tsc) complexes of cobalt(III) have been extensively studied since the early 1980s and continue to attract attention, gaining particularly from an interest in their biological activity and potential cytotoxicity. A truly extensive range of tsc compounds has now been reported, although structural definition of their complexes widely relied on basic analytical and spectroscopic techniques up to the late 1980s, when X-ray crystal structural studies of tsc compounds became more common. A review of thiosemicarbazone and S-alkyldithiocarbazate complexes has appeared.1053... 

This chapter is concerned with the application of liquid state methods to the behavior of polymers at surfaces. The focus is on computer simulation and liquid state theories for the structure of continuous-space or off-lattice models of polymers near surfaces. The first computer simulations of off-lattice models of polymers at surfaces appeared in the late 1980s, and the first theory was reported in 1991. Since then there have been many theoretical and simulation studies on a number of polymer models using a variety of techniques. This chapter does not address or discuss the considerable body of literature on the adsorption of a single chain to a surface, the scaling behavior of polymers confined to narrow spaces, or self-consistent field theories and simulations of lattice models of polymers. The interested reader is instead guided to review articles [9-11] and books [12-15] that cover these topics. 

Many proteins will not yield crystals in initial crystal trials. Unfortunately, crystallization is still a trial-and-error procedure, with no real way of predicting success or failure. If you fail to get crystals in your initial trials, it need not be the end of your structural studies (although it could be ). Many of the targets for neuroscientists are going to be membrane-bound or membrane-spanning proteins, and these are notoriously difficult to crystallize. Techniques are continuously being developed and refined to improve our ability to crystallize difficult protein examples. 

Fig. 3. DNA sequence used in Oak Ridge NCP structural studies. The DNA sequence of the human a-satellite sequence (A) and the a-satellite palindrome (B) for which it was a model. The palindrome is one of 24 potential phasing sequences taken from the human a-satellite DNA repeats. The center of the palindrome sequence is marked by a vertical bar. The a-satellite sequence continues to serve as the starting point for high-resolution NCP structures.

Iminoboranes (continued) in transition metal complexes, 31 165-167 X-ray structural studies, 31 135 Iminofluorosulfinamide ions, 19 206-208 Iminofluorosulfonylamide ions, 19 200 Iminolithium complexes, solid-state structures, 37 67-75 dimers, 37 68-70 trimers, 37 68-70... 

Synthesis as a means of unequivocal structure elucidation. Synthetic studies continued... 

The continued development of high-resolution NMR spectroscopy indicates the value of these instruments and the importance of this technology in future chemical research. Proton f H) detected NMR spectra are the easiest to obtain and can be acquired with just a few micrograms of sample. Since carbon forms the backbone of organic structures, carbon (13C) NMR is also important in structural studies. For this reason, H NMR, 13C NMR, and 2D techniques involving these two nuclei are widely used in the characterization of unknown natural products. 

Jhe distribution of hydrogen types in coals continues to be a subject of considerable interest in coal structure studies. Published data indicate that the fraction of aromatic hydrogens usually increases with increasing rank, but the absolute values depend on the specific analytical method used (7). Hydrogen type analysis of a single coal based on the application of NMR spectroscopy to the soluble fraction from depolymerization with phenol-BFa has been reported by us (3). The conversion of coal to soluble fragments in substantial yields under very mild conditions permits a reliable determination of the hydrogen types by NMR analysis, and these results can be extrapolated to the parent coal with considerable confidence. 

Many other herbicides and compounds having growth-alteration properties are known, and new ones appear annually from commercial screening. It has been quite evident so far that no correlation has been demonstrated between chemical structure or physical properties and biological activity. The empirical search continues for new structures having biological properties, and basic research studies continue with newer instrumental techniques for investigating the mechanisms of activity. 

Recent nuclear-structure studies of proton-rich and neutron-rich nuclei, performed at the GSI on-line mass separator, are surveyed and an outlook is given towards a continuation of such investigations with heavy-ion beams of higher energies. 

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