Conformational isomorphism

Polymorphs are classified according to the following terminologies. Concomitant polymorphs crystallize simultaneously from the same solvent and crystallization flask under identical crystal growth conditions. They may be viewed as supramolec-ular isomers in a chemical reaction. Conformational polymorphs occur for flexible molecules, i.e. these molecules can adopt more than one conformation under ambient conditions. When different conformers of the same molecule are present in the same crystal structure the situation represents conformational isomorphs. Conformational isomorphism, the existence of multiple conformations in the same crystal structure, is closely related to the presence of more than one molecule in the asymmetric unit, i.e. Z >1. The exact reasons why some crystals have Z > 1 are still not properly understood even as several research groups are working to seek answers to this enigma [9]. Pseudopolymorphism, [10] the occurrence of the same molecule with different solvent molecules in the crystal lattice or the same solvent in a different stoichiometry, is closely related to polymorphism. 

This way, applying a graph automorphisms to a POP with respect to Swill result in a POP with respect to the same selection S. We can introduce canonic forms for POPs with respect to Sand generate canonic representatives only, avoiding the generation of isomorphic conformations. 

Chains of identical chirality and conformation are isomorphous. Chains of opposite chirality but equivalent conformation are enantiomorphous. 

In the crystalline state these two forms are not isomorphous, and adding substrate to ConA crystals causes them to shatter.48 (This is a common observation and occurs even in hemoglobin crystals.) The differences between the conformers in the ConA part of the complex are not fully known, but there is considerable rearrangement of the protein. The rates of the reaction are fast, and so in solution the protein must be able to fluctuate readily between its different forms. Comparison with lysozyme (earlier in this article) shows that when lysozyme binds its inhibitors, a conformational change does occur, but it is not so gross that in the solid state shattering of crystals occurs. 

Isomorphism in macromolecular systems may be defined as the statistical substitution, within a single crystalline phase, between monomer units differing either in chemical structure or in conformation or in configuration. The distribution of the different monomer units needs not to be totally random. As we shall see, there are examples of isomorphous systems consisting of a mixture of different homopolymer chains. In these cases the randomness is confined to the macromolecules... 

The second requirement is perhaps best illustrated by the cases of isomorphism in isotactic vinyl copolymers (7, 2, 4, 5, 6) only if the helical conformations of the two crystalline homopolymers are not too different, a regular helical conformation is also possible for the copolymer chains. 

However, if we refer to isomorphism in a strict sense, an additional requirement must be met, i.e. the crystalline phases of the two homo-polymers must be analogous, either from the point of view of the chain conformation, and of the lattice symmetry and dimensions. It is in fact quite obvious that only in this case a single crystalline phase is possible, with small, continuous changes with changing composition. 

The vinyl fluoride/vinylidene fluoride and the vinyl fluoride/tetra-fluoro ethylene copolymer systems were also studied (21). In the first case isomorphism is observed in the whole range of compositions, while the distribution of the two types of units is random. The crystal structure is that of polyvinyl fluoride, which is virtually identical with one of the three known crystalline forms of polyvinylidene fluoride, and characterized by a planar zig-zag chain conformation. High degrees of crystallinities in the whole range of compositions are also observed in the second case. However, the crystal structure of the two pure homopolymers is not the same hence we are in the presence of isodimorphism. In any case, for vinyl fluoride contents ranging between 0 and 75 mole-% the structure observed is essentially that of polytetrafluoro ethylene in the crystalline... 

As to the system styrene/o-fluoro styrene (case a) a complete isomorphism is observed within the whole range of compositions. The macromolecules assume the conformation of a 3/1 helix, characteristic... 

As shown in Figure 42 for the Norrish II reactions of a simple ketone, 2-nonanone, not only do the shapes of the products differ from those of the reactant, but so do their molecular volumes [265]. Interestingly, the volume of the fragmentation products, 1-hexene and 2-hydroxypropene (which ketonizes to acetone), are closer in volume to 2-nonanone than is either of the cyclization products. They are also capable of occupying more efficiently the shape allocated by a stiff solvent matrix to a molecule of 2-nonanone in its extended conformation the cross-sectional diameter of either of the cyclobutanols is much larger than that of extended 2-nonanone or the fragmentation products when spaced end-on. Both of these considerations should favor fragmentation processes if isomorphous substitution for the precursor ketone in the reaction cavity is an important requirement for efficient conversion to photoproducts. 

P2i2i2i Z = 4 Dx = 1.68 R = 0.052 for 1,518 intensities. This structure is isomorphous with that of the chloro compound,41 reported next. The conformation of the two molecules is the same, and the molecular dimensions agree within experimental errors, except for the C-halogen bonds. 

Several diffraction criteria define a promising heavy-atom derivative. First, the derivative crystals must be isomorphic with native crystals. At the molecular level, this means that the heavy atom must not disturb crystal packing or the conformation of the protein. Unit-cell dimensions are quite sensitive to such disturbances, so heavy-atom derivatives whose unit-cell dimensions are the same as native crystals are probably isomorphous. The term isomorphous replacement comes from this criterion. 

It is generally recognized that in order to analyze the stereochemical features of real systems (molecules, or groups of molecules and reactions) it is useful to abstract those features which determine the properties of interest and to represent them in terms of a single model system. It is possible to perform an isomorphic mapping of the essential stereochemical features of each of the real systems onto this model, and an equivalence relation will thus exist between the real systems. We shall say that any two systems whose static stereochemistry (structure, conformation) and conformational dynamics may be represented by the same abstract model exhibit stereochemical correspondence. 

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