Many oligopeptides with two or more peptide units fold into the so-called secondary structures, which give several rules and accurate parameters of protein foldings. β-turn: Type I is frequently found in the oligopeptides having three or more peptide units. The NCαC' angles at the second site are 113-116°. The mean length of the intramolecular 4→1 hydrogen bonds is 3.03Å. Val and Ile having two Cγ atoms are not usually accomodated at the third site. β-sheet structure: Several peptides show the (ideally) parallel β-sheet structures, while at present only tri (L-Ala) crystallizes in an infinitely extended antiparallel β-sheet structure. In some peptide crystals two molecules are linked to a dimer by a pair of hydrogen bonds of the antiparallel β-structure type. Several peptides crystallize in “twisted” β-sheets, and their conformational angles are close to those of the collagen group peptides. poly (L-proline) II helix: Tri- and tetra-prolines are in a poly (L-proline) II helix with the trans peptide bonds. Several others containing Pro and Gly have similar conformations. Cis peptide bond: Cis prolines have been found in Z-GIy-Pro-Leu and a few cyclic oligopeptides. The bond angles around the N atom are : C'NCα (cis) = C'NCδ (trans) =126°, C'NCδ (cis) =C'NCα (traps) =121°, and CαNCγ (trans, cis) =113°.
The word ‘tropochemical cell twinning’ was given to such a twinning on cell scale that yields, with concomitant changes in composition, a chemical series of structures. A review has been presented on this sort of cell twinning which is here reworded as ‘TCT mechanism’. Examples of the PbS-Bi2S3 system, pinakiolite-orthopinakiolite series, and enstatite IV series have shown a common feature of the mechanism that it is closely related to the generation of defect structures or deviation from ideal stoichiometry; a great majority of so called ‘extended defect structures’ may be systematically discussed along this mechanism. A brief survey has been given together on the historical development on the concept of cell twinning which originated with Teiichi Ito, M. J. A., as early as 1935.
Model building of crystals and molecules, both small and large, is necessary for a better understanding of the structures in connexion with their physical and chemical properties or biological functions. Interactive three-dimensional computer graphics provides a valuable tool for the accurate and rapid construction, manipulation, measurement and display of the models. Programme systems, TSD : MODL for large molecular complexes and TSD : XTAL for crystal structures, have been developed for a mini-computer graphic satellite system [NOVA 3, 32 Kw, disk cartridge 1.2 Mw, Tektronix 4010-1, and TI ASR 733] . An effective new algorism was applied successfully for local conformational change and for approach between two molecules with various orientations in attempts to examine intermolecular interactions. Basic plans and techniques for programming are described and examples of some applications are given.