Imidodiphosphate and pyrophosphate: possible biological significance of similar structures.

The structure of sodium imidodiphosphate has been determined by single crystal x-ray diffraction. The P-N-P bond angle (127.2 degrees) and P-N bond distance (1.68 angstroms) are remarkably similar to newly refined values for the P-O-P bond angle (128.6 degrees) and the bridging P-O bond distance (1.63 angstroms) of sodium pyrophosphate. This close similarity may explain why P-N-P linkages in algal "polyphosphates" escaped detection until recently and why adenosine triphosphate analogs with this linkage mimic adenosine triphosphate so closely.