Isotypism

The crystal structures of two compounds are isotypic if their atoms are distributed in a like manner and if they have the same symmetry. One of them can be generated from the other if atoms of an element are substituted by atoms of another element without changing their positions in the crystal structure. The absolute values of the lattice dimensions and the interatomic distances may differ, and small variations are permitted for the atomic coordinates. The angles between the crystallographic axes and the relative lattice dimensions (axes ratios) must be similar. Two isotypic structures exhibit a one-to-one relation for all atomic positions and have coincident geometric conditions. If, in addition, the chemical bonding conditions are also similar, then the structures also are crystal-chemical isotypic. 

The ability of two compounds which have isotypic structures to form mixed crystals, i.e. when the exchange process of the atoms can actually be performed continuously, has been termed isomorphism. However, because this term is also used for some other phenomena, it has been recommended that its use be discontinued in this context. 

Two structures are homeotypic if they are similar, but fail to fulfill the aforementioned conditions for isotypism because of different symmetry, because corresponding atomic positions are occupied by several different kinds of atoms (substitution derivatives) or because the geometric conditions differ (different axes ratios, angles, or atomic coordinates). An example of substitution derivatives is C (diamond)-ZnS (zinc blende)-Cu3SbS4 (famatinite). The most appropriate method to work out the relations between homeotypic structures takes advantage of their symmetry relations (cf. Chapter 18). 

If two ionic compounds have the same structure type, but in such a way that the cationic positions of one compound are taken by the anions of the other and vice versa ( exchange of cations and anions ), then they sometimes are called antitypes . For example in Li20 the Li+ ions occupy the same positions as the F ions in CaF2, while the O2- ions take the same positions as the Ca2+ ions Li2Q crystallizes in the anti-CaF2 type . 

1 Calculate effective coordination numbers (e.c.n.) with the formula given on page 3 for  

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The basic structure of all immunoglobulin (Ig) molecules comprises two identical light chains and two identical heavy chains linked together by disulfide bonds (Figure IS.2a). There are two different classes, or isotypes, of light chains, X and k, but there is no known functional distinction between them. Heavy chains, by contrast, have five different isotypes that divide the immunoglobulins into different functional classes IgG, IgM, IgA, IgD, and IgE, each with different effector properties in the elimination of antigen... 

In addition to the heteronuclear clusters considered in the preceding paragraphs. As, Sb and Bi also form homonuclear clusters. We have already seen that alkaline earth phosphides M3P14 contain the [Pv] cluster isoelectronic and isostructural with P4S3, and the analogous clusters [Asy] " and [Sbv] have also been synthesized. Thus, when As was heated with metallic Ba at 800° C, black lustrous prisms of BasAs were obtained, isotypic with Ba3Pi4 these contained the [Asv] anion with dimensions as shown in Fig. 13.25(a).Again,... 

The other tetrahalides can all readily be made by direct reactions of the elements. Crystalline SeCU, TeCU and -SeBr4 are isotypic and the structural unit is a cubane-like tetramer of the same general type as [Me3Pt(/Z3-Cl)]4 (p. 1168). This is illustrated schematically for TeCU in Fig. 16.13d each Te is displaced outwards along a threefold axis and thus has a distorted octahedral environment. This can be visualized as resulting from repulsions due to the Te lone-pairs directed towards the cube centre and, in the limit, would result in the separation into... 

Reprinted from Journal of High Resolution Chromatography, 15, S. Nitz et ah, Multidimensional gas chi omatography-isotype ratio mass spectrometry (MDGC-IRMS). Part A system description and technical requirements, pp. 387-391, 1992, with peraiission from Wiley-VCH. 

Prasher, D. C., McCann, R. O., Longiaru, M., and Cormier, M. J. (1987). Sequence comparisons of complememtary DNAs encoding aequorin isotypes. Biochemistry 26 1326-1332. 

Ranganathan S, Benetatos CA, Colamsso PJ et al (1998) Altered beta-tubulin isotype expression in paclitaxel-resistant human prostate carcinoma cells. Br J Cancer 77 562-566... 

Two isotypes of SUR have been cloned, SUR1 and SUR2. In addition, two splicing variants of SUR2, distinguished by 42 C-terminal amino acids, have been... 

For some of the GLUT isotypes, the phenotype of null mutants is known. Haploinsufficiency of the GLUT1 in... 

Insulin Analogs. At present, the only known ligands of the insulin receptors are insulin isotypes from different species and a number of synthetic analogs... 

The tissue-specific patterns of expression of the PPAR. isotypes suggested that these proteins have distinct physiological roles, and this was further supported when each was specifically disrupted in mouse gene knockout models. 

The expression of all three PPAR isotypes peaks in the rat central nervous system between days 13.5-18.5 of gestation, and while expression of both PPARa and PPARy decline post-natally, expression of PPARS remains high (except for the retina, where all three isoforms are expressed in the adult rodent). An important role for PPARS in CNS development is underscored by the occurrence of defective myelination in the PPARS-null mouse. 

Epidermal Maturation and Wound Healing. All three PPAR isotypes are expressed during epidermal maturation each isotype has a specific pattern of expression in regard to development and the various layers of the epidermis. An important role for PPARS in the development and/or maintenance of normal skin health is indicated by the presence of defective wound healing in the PPARS-null mouse. 

Table 4 summarizes the physiological functions of the PPAR isotypes and their synthetic ligands. 

PPAR isotype Tissues of highest expression Primary physiological effects Endogenous ligands Synthetic ligands Associated disease state/ indication... 

It is an autoantibody whose autoantigen is the Fc portion of IgG. Rheumatoid factors may be of any immunoglobulin isotype but it is IgM rheumatoid factor that is commonly measured in rheumatoid arthritis. Classification criteria for rheumatoid arthritis include only one serological test, namely rheumatoid factor. However, it is not diagnostic test rather it may be confirmatory when a number of other clinical features are present. 

Cytokine receptors that couple to the JAK-STAT Pathway decode the signaling though hematopoietic cytokines (erythropoietin, thrombopoietin, colony-stimulating factors), prolactin, growth hormone, the a-, (3- and y- interferons, and a number of immunomodulatory interleukins [3], They form homodimetic or heterodimeric receptor complexes, which after ligandbinding recruit and activate isotypes of Janus kinases (JAKs). Activated JAKs in turn... 

Isomorphism. Like TiC, ZrC forms solid solutions with oxygen and nitrogen, which have a wide range of composition. The lattices of ZrC, ZrN, and ZrO are isotypical. ZrC forms solid solutions with the other monocarbides of Group IV and V. 

Sullivan, K.F. (1988). Structure and utilization of tubulin isotypes. Ann. Rev. Cell Biol. 4,687-716. 

For crystallographic data, see Table I. The compounds of the type CuTeX and CuTe X (X = Cl, Br, or I), respectively, are isotypic, and their crystal structures have been determined. Copper has the oxida-... 

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