Introduction Potassium adenosine diphosphate dihydrate, K +.C10 HNNSOIOP~-. 2 H 2 0 , crystallizes in the orthorhombic space group P2~2~2 with four molecules in a unit cell of dimensions a -28.491 (6), b -10.446 (3) and c = 6.316 (1) A. The structure was solved by direct methods and refined to an R index of 0.062 (R w = 0.076) on 1384 intensities. The mean e.s.d.'s in bond lengths and angles are 0.013 A and 0.7 ° respectively. The crystal structure has revealed that the adenosine diphosphate exists as a zwitterion with N(1) of the base protonated and the pyrophosphate group dinegatively charged (one negative charge on each phosphate) and has provided details of the molecular conformation and chirality of the chelate ring. The two water molecules in the asymmetric unit are distributed over three positions, two on the same diad axis and one in a general position. The molecule is folded into one of the preferred compact conformations, viz. sugar pucker 2E (P--163.3 °, r m = 38.3°), the exocyclic C(4 ' ) -C(5 ' ) bond torsion angle ~ gauche + (57.8 °) and the glycosyl torsion angle X anti (32.4°), with an anionic oxygen of the o-phosphate and the hydroxyl oxygen of the fl-phosphate ligated to the K + ion. A second anionic oxygen of the fl-phosphate forms a slightly weaker coordination to the metal; thus, the metal ion is engaged in an tt,fl, fl chelate complex. In addition, the metal ion is coordinated to the base N(3), the ribose 0(2'), the anionic phosphate oxygen O(13) of neighboring nucleotides, and to one of the water molecules on the diad axis. Thus, the K + ion is surrounded by seven possible ligands. The chirality of the chelate ring corresponds to the A diastereomer. The N(1) and N(6) sites and N(6) and N(7) sites of the adenine base form pairs of hydrogen bonds with the oxygens of the ~-phosphate groups of neighboring molecules. There are no basestacking interactions observed in the structure. *Presented at the American Crystallographic Association Winter Meeting at Honolulu, Hawaii, March 26-30, 1979, Abstract C5. t To whom correspondence should be addressed. X-ray crystallographic studies have shown that the free acids of three of the five common nucleotides of DNA and RNA involving the bases adenine, cytosine and guanine exist as zwitterions (pH range 2 to 4.5; Ts'o, 1974) with the bases protonated at N(1), N(3) and N(7) respectively neutralized by a net negative charge on the phosphate group (Sundaralingam & Prusiner, 1978). Here we report the structure of the potassium salt of ADP and show that the base is protonated at N(1) with a negative charge on each of the aand fl-phosphate groups. The net negative charge on the ADP molecule is neutralized by the metal ion. KADP.H20 is isostructural with RbADP.2HzO (Muller & DeLuke, 1971) and RbADP.HzO (Viswamitra, Hosur, Shakked & Kennard, 1976). Although RbADP.2HzO was reported to be a trihydrate, it actually corresponds to a dihydrate since the two water molecules are distributed over three sites: one in a general position and the other two on a diad axis as in KADP.2H20. In RbADP.HzO, the water molecules on the diad axis were not located. In both RbADP structures the protonation of the adenine base was not reported. Similarly, no protonation of adenine was reported in NazATP (Kennard, Isaacs, Motherwell, Coppola, Wampler, Larson & Watson, 1971; Larson, 1978).