A crystallographic orbit has been defined as the set of all points which are equivalent under the symmetry operations of a space group. A non-characteristic crystallographic orbit is defined as a crystallographic orbit, the eigensymmetry group of which is higher than generating space group. The non-characteristic orbits, including extraordinary orbits, have been derived by Engel, Matsumoto, Steinmann and Wondratschek. The lists of them for all space groups have been published. The non-characteristic orbits are explained.
Periodical lattice modulation is one of the prominent properties of the one-dimensional metal systems. The typical examples of the modulated structures can be found in partially oxidized platinum complexes. Usually, the conductivity of one-dimensional metal system is considered to be limited because of the inherent 2kF-instabilities. Based on the simple tight-binding band analyses, the way to overcome the one-dimensionality of the molecular conductors is suggested.
As a structural basis for the specificity of protein-nucleic acid interactions, the elementary interactions between nucleic acid base (A, G, C, U or T) and amino-acid side chain (hydroxyl, imidazolyl, carboxyl, carbamoyl, phenyl, hydroxyphenyl or indolyl group) were extensively investigated by using model crystals that contain both components. Interaction patterns between complementary base pair and amino acid side chain were also examined with the ternary model systems newly designed. Several characteristics are observed in the model crystals ; for instance, stabilization of stacking between the bases and histidine by the protonation of the latter, common hydrogen bonding patterns between the bases and hydroxyl or carboxyl group by reversion of donor-acceptor relationship or ionization. UV, NMR and CNDO/2 calculation suggest energetistic aspects of these interactions. A structural constraint is derived between the hydrogen bonds and the secondary structural fitting of α-helical segment into the major groove of B-DNA. In terms of the elementary interactions, stereochemical interpretations are given for the binding of cro repressor to operator, enzymatic reaction of RNAase and initiation and elongation in the assembly of TMV.