Functional importance of coacervation to convert calcium polyphosphate nanoparticles into the physiologically active state

Inorganic Polyphosphate: Coacervate Formation and Functional Significance in Nanomedical Applications

“…Turbidimetric measurements showed that Ca-polyP coacervate formation at pH 7 is a slow process compared to nanoparticle formation at pH 10. 104 The formation of the respective products could be confirmed not only by SEM but also by FTIR measurements. It was found that the coacervate and the nanoparticles of Ca-polyP show a characteristic difference in their FTIR spectra, which allows a distinction between the two Ca-polyP phases.…”

“…If, however, the Na-polyP solution is dropped into the CaCl 2 solution, a coacervate is obtained at both pH values. 104 Under these conditions, the ν as (PO 3 ) 2− signal does not disappear as it does during the formation of the nanoparticles. This finding is explained by an immediate binding of Ca 2+ ions to polyP when dropping the Na-polyP into a CaCl 2 solution containing excess amounts of the divalent cation (2.5-fold on the stoichiometric basis of Ca: P).…”

“…Surprisingly, these studies showed different changes in the distribution of polyP and calcium ions during the reaction. 104 Under conditions that support coacervate formation, these components show a more or less random arrangement pattern, while nanoparticle formation is characterized by clustering of the anionic polyP molecules accompanied by partial separation of the Ca 2+ cations. These results made it possible to propose a model of Ca-polyP coacervate and nanoparticle formation that also takes into account ion–dipole interactions in addition to electrostatic interactions.…”

“…These results made it possible to propose a model of Ca-polyP coacervate and nanoparticle formation that also takes into account ion–dipole interactions in addition to electrostatic interactions. 104 It is proposed that at pH 7 the initially randomly organized Ca 2+ and polyP ions, surrounded with a hydrate shell, assemble in a time-dependent manner into concentric ring-like structures with a polyP core and outer Ca 2+ ions forming liquid droplets that coalesce, to form a water-rich coacervate. At pH 10, compartmentalization takes place and significantly denser structures form, 104 caused by the higher negative charge and Ca 2+ binding affinity of the almost completely ionized polyP molecule, which also applies to the second, less acidic OH of its P i end groups.…”

“…The ν as (PO 2 ) − signal at a wavenumber of 1255 cm −1 , which forms a distinct peak in the spectrum of the Ca-polyP coacervate, is absent and remains only as a shoulder in the spectrum of the Ca-polyP nanoparticles. 104 This signal has been attributed to the asymmetric stretching vibrations of the internal (PO 2 ) − units of the polyP chain. This change in the FTIR spectrum is believed to be caused by the binding of Ca 2+ at the internal P i residues of the polyP chain.…”