Influence of the protonation degree on the self‐association properties of adenosine 5′‐triphosphate (ATP)

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Abstract
The concentration dependence of the chemical shifts for the protons H‐2, H‐8 and H‐1′ of ATP has been measured in D2O at 27°C under several degrees of protonation in the pD range from 1.5 to 8.4. The results at pD > 4.5 are consistent with the isodesmic model of indefinite noncooperative stacking, while those at pD < 4.5 indicate a preference for the formation of dimeric stacks. The stacking tendency follows the series, ATP4− (K= 1.3 M−1) < D(ATP)3− (2.1 M−1) < 1:1 ratio of D(ATP)3−/D2(ATP)2− (6.0 M−1) ≪ D2(ATP)2−(∼ 200M−1) ≫ D3(ATP)− (K≃ 17 M−1) (for reasons of comparison all constants are expressed in the isodesmic model). These results are compared with previous data for adenosine [Ado (K= 15 M−1) > 1:1 ratio of Ado/D(Ado)+ (6.0 M−1) > D(Ado)+ (0.9 M−1)] and AMP [AMP2− (K = 2.1 M−1) < D(AMP)− (3.4 M−1) < 1:1 ratio of D(AMP)−/D2(AMP)± (5.6 M−1) > D2(AMP)± (∼ 2M−1) > D3(AMP)+ (K ≤ 1 M−1)] to facilitate the interpretation of the results for the ATP systems. Stack formation of H2(ATP)2− is clearly favored by additional ionic interactions; this is confirmed by measuring via potentiometric pH titrations the acidity constants of H2(ATP)2− in solutions containing different concentrations of ATP. It is suggested that in the [H2(ATP)]4−2 dimer intermolecular ion pairs (and hydrogen bonds) are formed between the H + (N‐1) site of one H2(ATP)2− and the γ‐P(OH)(O)−2 group of the other; in this way (a) the stack is further stabilized, and (b) the positive charges at the adenine residues are compensated (otherwise repulsion would occur as is evident from the adenosine systems). A detailed structure for the [H2(ATP)]4−2‐dimer is proposed and some implications of the described stacking properties of ATP for biological systems are indicated.

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