INTERACTION OF ALUMINUM(III) WITH BIOLOGI CALLY IMPORTANT PHOSPHATES

In order to obtain information about the Al(III)-phosphate interaction , pH-metric measurements were carried out to assess the complex-formin g properties of aluminium(III) with organic phosphates and phosphonate s. Special attention was paid to nucleoside 5'-monophosphates. The Al( III)-orthophosphate system is difficult to characterize due to AlPO4 p recipitation. To overcome this problem that makes data available in th e literature ambiguous, the logarithms of the stability constants of t he 1:1 Al(III) complexes of ligands with the same donor groups (lg K-1 :1) were plotted against the basicities of the ligands (lg K-PO3H-) Th e fact that a straight line was obtained indicates that 1:1 complexes of the same bonding mode are formed with organic phosphates, phosphona tes and nucleoside 5'-monophosphates. The difference between phosphate and phosphonate groups does not affect the interaction. The correspon ding complex of cytidine 5'-monophosphate proved to be of relatively g reat stability, however, it is only the apparent result of two simulta neous processes. In the case of adenosine 5'-monophosphate, the stabil ity of the 1:1 complex is too great to be accounted for by merely the Al(III)-phosphate interaction. The increase in stability is possibly c aused by coordination of the N7 atom and by stacking interaction. The formation constant for the AlHPO4+ species was determined by using the parameters of the log K-PO3H- -log K-1:1 line. Direct pH-metric titra tions were performed in order to characterize the stoichiometries and stabilities of the other complexes formed in the Al(III)-orthophosphat e system, too. It was established that the Al(III) complexes formed wi th pyrophosphate and triphosphate are of the same type whereas those o f orthophosphate have different structures. The results of pH-metric s tudies were supported by P-31 NMR measurements.