Quantum‐Chemical investigation of cation motion mechanism in modes of biological channels with phosphate groups

Abstract

Specimens of amorphous zirconium phosphates specifically formed by alkali metal cations were first studied as models of biological membranes. The mechanism of ion motion in zirconium phosphate ultrapores was investigated by the sCF MO LCAO method using a C N D 0 / 2 approximation. Differently arranged dihydrophosphate groups were considered as models of the sorbent channel surface. Analysis of the form of potential surfaces resulting from the motion of lithium, sodium, and potassium ions near the charged centers showed that the value and position of energy barriers essentially depend on the nature of cation and the size of the pore that it penetrated into. Optimum conditions for the selectivity absorption of the ions and their further advance along the ultrapore channel are created if the cation size corresponds to the ultrapore diameter.Recently, great attention has been drawn to the studies on the detailed mechanisms of the protoplasmic membranes, in particular, their ionic selectivity [ 1-51, In spite of numerous works in this field, however, the main problem is still how to choose sufficiently simple but adequate models of biological membranes.The present work is an attempt to employ specifically formed zirconium phosphate as such systems [6-91. Zirconium phosphate has been chosen not only because its reactive centers of ion exchange (the phosphate groups) are one of the main carriers of negative charges in biological systems but, mainly, because of the possibility of applying amorphous zirconium phosphate to obtain systems with a controlled ionic slectivity.The molecular structure of zirconium phosphate is that of an inorganic polymer with the phosphate groups bonded to zirconium atom chains (-Zr-O-Zr--)".*The porous structure of sorbents may be widely governed depending upon the preparation procedure. It is necessary to remember that in crystalline zirconium phosphate modifications in the diameters of the channels are strictly determined and are equal to 2.4-2.5 A [lo], while in amorphous specimens one can easily observe the transition porosity. The pore diameters are as high as 10-20 A and larger. With this objective however, the fact that amorphous zirconium phosphate has a considerable amount of pores having diameters of 3-4 8, is of particular concern. Moreover, it has been reported [7,8] that when * In crystalline zirconium phosphate these chains are aligned in such a way that zirconium atoms are located in the same plane, see Figure 1.