Structural Patterns in Nucleic Acids

Of the three major components of biological structure, the proteins, nucleic acids, and polysaccharides, least is known about the polysaccharides at the secondary and tertiary level of molecular structure. This is because the polysaccharides cannot be obtained in crystals which are large enough for single crystal X-ray or neutron structure analysis. What structural information there is comes from fiber X-ray diffraction patterns. However good these diffraction patterns are, the structures derived from them will always be model-dependent. This is because the number of variable atomic parameters which determine the diffraction intensities exceeds the number of observed intensities. 

Rigoutsos I, Riek P, Graham RM, Novotny J. Structural details (kinks and nonce conformations) in transmembrane helices are intrahelically determined and can be predicted by sequence pattern descriptors. Nucleic Acids Res 2003 31 4625-4631. 

What makes the thiated nucleobases so markedly different from the normal ones Put in the other words, what is a key effect of the thio substitution which, in turn, causes a variety of bizarre effects in the interaction of thiouracils with water, in proton migration, recognition patterns, biological and pharmacological activities, etc., etc. There are actually two key effects underlying the thio substitution and certainly inter-related. One is that such substitution dramatically changes the structural features of nucleic acids whereas the other is due to the drastic change of their functional ones expressed in terms of intermolecular interactions. 

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