Morphological symmetry

S)-Serine forms tabular crystals with point symmetry 2hn (Figure 22a) the crystals affected by either (/ )- or (S)-thr exhibit reduced morphological symmetry 2 (the mirror plane is lost) and are enantiomorphous (Figure 22b, c). When (R,S)-thr is used as the additive, the morphological symmetry 2/m is left unchanged because the effects induced by each additive separately combine. The crystals turn into rhombs, with a clear increase in the areas of the 011 side faces relative to those of the pure crystals (Figure 22d) (45, 78). 

When platelike crystals of gly with well-developed 010 faces were partially dissolved in solvents containing variable amounts of resolved R a-amino acids, well-defined etch pits were formed only on the (010) face (Figure 30a). These pits exhibited twofold morphological symmetry with surface edges parallel to the a and c axes of the crystal. The enantiotopic (010) face dissolved smoothly (Figure 30b), exactly as it does when the crystal is dissolved in a solution of pure gly. As expected, S amino acids induced etch pits on the (010) face. Racemic... 

The morphological symmetry differences between the acetanilide and p-chloroacetanilide crystals originate from their internal structures. The acetanilide molecules appear in pairs and the two molecules in each pair are related by an inversion center. On the other hand, the p-chloroacetanilide molecules are all aligned in one direction. The molecular arrangements in the two crystals are shown in Figure 2-33. 

Hessel (1796-1872) determines the finite number of morphological symmetry types a three-dimensional crystal can have, as a whole, to be 32. 

The morphological symmetry differences between the acetanilide and / -chloroacetanilide crystals originate from their internal structures. The acet-... 

How, if at all, does the chirality of molecules determine the morphological symmetry or asymmetry of organisms This dates almost to the first appearance of life on earth, established from both body fossils and trace fossils. Chirality is heritable, and genes have been identified that are related to lack of symmetry, but there is as yet very limited evidence for any strong connection with molecular chirality. 

PET fibers in final form are semi-crystalline polymeric objects of an axial orientation of structural elements, characterized by the rotational symmetry of their location in relation to the geometrical axis of the fiber. The semi-crystalline character manifests itself in the occurrence of three qualitatively different polymeric phases crystalline phase, intermediate phase (the so-called mes-ophase), and amorphous phase. When considering the fine structure, attention should be paid to its three fundamental aspects morphological structure, in other words, super- or suprastructure microstructure and preferred orientation. 

FIG. 2 Schematic drawing of different S-layer lattice types detected on prokaryotes. The regular arrays exhibit either oblique (pi, p2), square (p4), or hexagonal lattice symmetry (p3, p6). The morphological units are composed of one, two, three, four, or six identical subunits. (Modified from Ref. 59.)... 

Virus symmetry The nucleocapsids of viruses are constructed in highly symmetrical ways. Symmetry refers to the way in which the protein morphological units are arranged in the virus shell. When a symmetrical structure is rotated around an axis, the same form is seen again after a certain number of degrees of rotation. Two kinds of symmetry are recognized in viruses which correspond to the two primary shapes, rod and spherical. Rod-shaped viruses have helical symmetry and spherical viruses have icosahedral symmetry. 

The case of isotactic polypropylene (iPP) presents some differences with respect to those just discussed. While both sPP and PET adopt in their mesophases disordered, extended, essentially non-helical conformations, iPP is characterized by a unique, relatively well ordered, stable chain structure with three-fold helical symmetry [18,19,36]. More accurately we can state that an iPP chain segment can exist in the mesophase either as a left handed or as the enantiomeric right-handed three-fold helix. The two are isoener-getic and will be able to interconvert only through a rather complex, cooperative process. From a morphological point of view Geil has reported that thin films of mesomorphic iPP quenched from the melt to 0 °C consist of... 

---