Proton magnetic resonance spectroscopic analysis of diadenosine 5',5"'-polyphosphates

Certain diadenosine 5',5"'-polyphosphates are potent inhibitors of ADP stimulated platelet aggregation, acting possibly via competitive ADP-receptor binding. 1H NMR studies of a series of such compounds where the number of phosphate groups between adenosine groups was varied from 2 to 6 were performed to analyze possible preferred solution conformations and to define structure-activity relations. Relative to mononucleotides ADP and ATP, chemical shifts of adenosine proton resonances in diadenosine polyphosphate analogs are upfield shifted suggesting base stacking. This effect is greatest for AP2A and AP3A. Coupling constants of ribose ring proton resonances support the idea of an anti-base-ribose ring conformation, and 3JH5'-P values suggest a preferred gauche H-C-O-P structure. In all cases, NMR parameters for AP2A are near-limiting values for a static base stacked conformation. Increasing the number of phosphate groups between adenosine moieties tends to weaken this interaction.