Polyphosphate macrocycles

Oxygen chelates such as those of edta and polyphosphates are of importance in analytical chemistry and in removing Ca ions from hard water. There is no unique. sequence of stabilities since these depend sensitively on a variety of factors where geometrical considerations are not important the smaller ions tend to form the stronger complexes but in polydentate macrocycles steric factors can be crucial. Thus dicyclohexyl-18-crown-6 (p. 96) forms much stronger complexes with Sr and Ba than with Ca (or the alkali metals) as shown in Fig. 5.6. Structural data are also available and an example of a solvated 8-coordinate Ca complex [(benzo-l5-crown-5)-Ca(NCS)2-MeOH] is shown in Fig. 5.7. The coordination polyhedron is not regular Ca lies above the mean plane of the 5 ether oxygens... 

Eq. (6) with oji = 2x. A rotational isomeric state model for polyphosphate chains (2 76,187) was used to compute values of < rj >o required by Eq. (6). Comparison of the experimental values with the theoretical values, as well as with the values for the corresponding cyclic siloxanes (R(CH3)SiO)x [see Section III.(A)5] stronj y supports the contention that polyphos diate chains in sodium phosphate melts adopt random-coil conformations. Further evidence for the structure of sodium phosphate melts would come from precise data for macrocyclic concentrations in quenched sodium phosphate ring-chain equilibrates, if these became available. 

Polyammonium macrocycles are successful mimics of a variety of phosphoryl transfer enzymes. These simple compounds are noted for their capability to bind to a number of anions, including the simple phosphate and polyphosphates as well as more complex nucleotides. "Large" polyazacycloalkanes in their protonated forms have been found to exhibit ATP binding as well as ATPase activity. A general accepted mechanism for ATP hydrolysis catalyzed by polyaza macrocycles involve the formation of a ATP-receptor complex, held by electrostatic interaction and hydrogen bonds, followed by the formation of a covalent intermediate macrocyclic-phosphoramidate species, which is subsequently hydrolyzed (see Figure 2)P... 

---