A 31P and 1H NMR relaxometric study of the interaction between adenosine triphosphate (ATP) and paramagnetic ions (Gd 3+ and Mn 2+)

Abstract

In the ATP-Gd complex, the predominant mechanism responsible for 31P longitudinal and transverse paramagnetic relaxation is dipolar interaction. At room temperature and for a ratio [ATP]/[Gd 3+] ≫ 1, the complex has a 2:1 structure characterized by (i) a slow exchange between free and bound phosphates, (ii) identical distances between the phosphates of ATP and the ion, and (iii) a rotational correlation time ( τ R) constant for ATP concentrations ranging from 10 to 100 mM. The complex has two water molecules in the first coordination sphere. The exchange process between ATP and Gd 3+ is only influenced by the ATP concentration. At 310 K, τ m is 58.4 ± 1.1 μs for [ATP] = 10 mM, and 101.5 ± 1 μs for [ATP] = 100 mM. In all ATP-Mn complexes, longitudinal paramagnetic relaxation is dominated by a dipolar mechanism whereas transverse relaxation proceeds mainly from scalar interaction. Consequently at 4.7 T and 310 K, linewidths of 31P peaks are considerably broadened at each ATP concentration. The type of ATP-Mn complex is dependent on the ATP concentration. At a high ATP concentration (50 to 100 mM) and [ATP]/[Mn 2+] ≫ 1.2:1 or larger complexes exist that are characterized by (i) a relatively slow exchange between free and bound phosphates at temperatures lower than 295 K, (ii) long τ R, and (iii) shorter metal-phosphorus distances for β and γ phosphates than for the α phosphate. At a low concentration of ATP (5 mM), τ R is smaller and the Mn-P distance r α is 0.31 nm whereas r β and r γ are 0.30 nm. A 1:1 complex seems thus to be favored. One and two water molecules would be participating in the innersphere proton relaxation mechanism in the 2:1 and the 1:1 complexes respectively.