Major Phosphate Crystal Structures

The main building block of orthophosphate crystals is the PO4 polyhedron. It is possible to construct more complex phosphate structures using this unit. 

The radius of is 0.035 nm, while that of is 0.14 nm. Thus, their ratio is 0.25. According to Table 8.1, this gives a coordination number of 4 and hence the structure is a tetrahedron. 

Acid phosphates of alkali metals, used in the acid-base reaction in forming a CBPC are represented by the formula A2H2P04 n H2O, where A represents a monovalent element, such as Na, K, and Cs, from Group la of the Periodic table. The symbol A can also represent (NH4) , the ammonium ion. The integer n denotes the number of water molecules of hydration that are bonded by the van der Waal forces in the crystal structure. 

Some alkali orthophosphates have been well studied. The most important among these are KH2PO4 and NH4H2PO4. In the KH2PO4 structure, hydrogen bond links the PO4 tetrahedron to four others in a continuous three-dimensional network, while the K ion is coordinated 8-fold by oxygen atoms. In the case of NH4H2PO4, the structure is similar, but a system of N-H-0 bonds exists instead of coordination of the K ion. These bonds are mostly ionic, and hence, acid phosphates are soluble and used as such in the acid-base reaction to form CBPCs. 

Next to the acidic alkali phosphates, alkaline earth orthophosphates are of interest, because they represent many CBPC products, such as CaH2P04-H20 and MgH2P04 2H20. They are also the result of an acid-base reaction between an acid phosphate and an oxide containing an alkaline earth metal, such as Ca and Mg. They exhibit the same structure 

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In the crystal structure [711], a dimes formed by two N-terminal 434 repressor fragments is bound to the 20 base pairs DNA duplex so that the complex has overall 2-fold rotational symmetry. The polypeptide chain is folded into five a-helices HI to H5, with helices H2 and H3 forming the helix/turn/helix motif (Fig. 20.16). Helices H3 and H3 of the 434 repressor dimer insert into two successive major grooves of the operator DNA whereas the N-termini of the flanking helices H2, H4 and H2 H4 contact the sugar-phosphate backbones. 

Fig. 1. X-ray crystal structure of the DNA duplex dKS -CGtCAAATTTGCG-S llj. From Coll et al. (49). The closer proximity of phosphates in the minor groove compared to the major groove provides a highly anionic template for the binding of cations.

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