Polymorphism packing

Many protein crystals exist with more than one molecule per asymmetric unit. These molecules are sometimes related by noncrystallographic symmetry (pseudosymmetry), that is, additional symmetry (such as a twofold rotation axis) that is not part of the symmetry defined by the space group. This feature can be very useful in finding the molecular structure using rotation functions. Additionally, it is also possible for a compound to crystallize in different forms with different packing (polymorphism). If the molecular transforms of the components of the crystal are known, it is possible, by the methods described above, to determine their positions and orientations in the respective unit cells. 

A direct search was made for the spinel polymorph of Ni2Si04(cr) using high-pressure techniques. An olivine - spinel transition was discovered at 650°C and (18000 5000) bars. This result is in excellent agreement with the theoretical prediction. This enables the free energy of transition and the density of the closer packed polymorph to be foimd. From these data the transition pressure can be calculated. 

Different crystalline structures of one substance (single- or multicomponent) are named polymorphs [9, 10]. Polymorphism, which exists as a result of different crystal packing of rigid molecules, is called a packing polymorphism. Conformational polymorphism is a more common phenomenon for typically flexible drug-like molecules and results from crystallization of different conformers of the same molecule. At a given enviromnental conditions (temperature, humidity, pressure, etc.) only one solid form is thermodynamically stable (lowest free energy), while all other forms are considered metastable. 

Peptide crystals may possess different density, particularly favoured by size, shape or conformation of the molecules. Their geometry depends on the crystallization conditions Hke as solvent, pressure, temperature and crystaUization time, etc. Due to possible variations in the crystal packing and the existence of different conformers of the same molecule, the polymorphism could be called as the packing polymorphism or conformational polymorphism [150]. 

Keywords confomtation, packing, polymorphism, disorder, crystallographic density, isotactic, a-form, 3-form, y-form, mesophase, WAXS, WAXD, syndiotactic, orthorhombic form (high temperature, low temperature), triclinic form. 

I. Packing polymorphism is a redundant term. It has presumably been used to indicate different packing of the same compound, but the term polymorphism is exactly that. Bernstein has discussed the superfluous use of packing polymorphism and similarly redundant terms such as pharmaceutical polymorphism, synthon polymorphism, and structural poly-morphism. ... 

The existence of two modifications is of interest as neither anion nor cation displays conformational isomerism. The two forms arise, therefore, from crystal-packing polymorphism What is particularly surprising is that the melting points of the two forms differ by ca. 50°C. By a combined use of NMR spectroscopy, Raman spectroscopy and calorimetry, three modifications of [C4mim][PFg] have also been detected, each with a... 

One of the earliest studies in this area was concerned with a case of packing polymorphism, where Robertson initially reported the structure of resorcinol (1,3-dihydroxy-benzene).The crystal studied in this work corresponded to the form obtained at room temperature, and which was later termed the ot-form. However, it was subsequently learned that this a-form was in fact a metastable form, and that it could undergo a phase transformation into a denser crystalline modification (denoted as the (3-form) when heated at about 74 °C. The structure of this newer form was found to be completely different, and the crystallography permitted an explanation of the anomalous density change. 

---