1H and 31P NMR study of speciation in systems containing ADP, Al3+, and fluoride

It has been proposed that AlF4- can serve as a tetrahedral pseudophosphate bound to guanosine diphosphate (GDP) [or other nucleoside diphosphates (NDP)] in G-protein systems. In a previous paper [D. J. Nelson and R. B. Martin, J. Inorg. Biochem. 43, 37 (1991)], 19F and 1H NMR were used to analyze the ternary system Al(3+)-NDP-F- in aqueous solutions. Ternary complexes (NDP)AlFx (with x = 1-3) were identified, but no (NDP)AlF4 was found. In this paper, the equilibrium constants for ternary complex formation that were obtained in the previous paper were further tested in a more extensive 1H and 31P NMR study of speciation in systems that contained Al3+, F-, and adenosine 5'-diphosphate (ADP). The results of the study are in general support of previously derived constants for ternary complexes and also provide support for the existence at relatively high ADP concentration (approximately 10 mM) of a base-stacked intermolecular dihydroxy-di-Al3+ bridged ADP dimeric structure at an ADP to Al3+ molar ratio of 1:1. 31P NMR of the dimer reveals that each of the two Al3+ ions is bidentately coordinated to the alpha and beta phosphates of a single (but different) ADP molecule. Evidence is also presented for the existence at relatively low ADP concentration (approximately 0.5 mM) of a monomeric species in which a single Al3+ ion is coordinated to alpha and beta phosphates of a single ADP molecule. 1H NMR of the monomeric species reveals the expected "wrong-way chemical shift" of the adenine C8 proton upon Al3+ ion complexation to the phosphate chain.