A P-31 AND H-1-NMR RELAXOMETRIC STUDY OF THE INTERACTION BETWEEN ADENOSINE-TRIPHOSPHATE (ATP) AND PARAMAGNETIC-IONS (GD3+ AND MN2+)

In the ATP-Gd complex, the predominant mechanism responsible for P-31 longitudinal and transverse paramagnetic relaxation is dipolar interac tion. At room temperature and for a ratio [ATP]/[Gd3+] much greater th an 1, the complex has a 2:1 structure characterized by (i) a slow exch ange between free and bound phosphates. (ii) identical distances betwe en the phosphates of ATP and the ion, and (iii) a rotational correlati on time (tau(R)) constant for ATP concentrations ranging from 10 to 10 0 mM. The complex has two water molecules in the first coordination sp here. The exchange process between ATP and Gd3+ is only influenced by the ATP concentration. At 310 K, tau(M) is 58.4+/-1.1 mu s for [ATP] = 10 mM. and 101.5 +/-1 mu s for [ATP] = 100 mM. In all ATP-Mn complexe s, longitudinal paramagnetic relaxation is dominated by a dipolar mech anism whereas transverse relaxation proceeds mainly from scalar intera ction. Consequently at 4.7 T and 310 K, linewidths of P-31 peaks are c onsiderably broadened at each ATP concentration. The type of ATP-Mn co mplex is dependent on the ATP concentration. At a high ATP concentrati on (50 to 100 mM) and [ATP]/[Mn2+] much greater than 1.2:1 or larger c omplexes exist that are characterized by (i) a relatively slow exchang e between free and bound phosphates at temperatures lower than 295 K, (ii) long tau(R), and (iii) shorter metal-phosphorus distances for bet a and gamma phosphates than for the alpha phosphate. At a low concentr ation of ATP (5 mM), tau(R) is smaller and the Mn-P distance r(alpha) is 0.31 nm whereas tau(beta) and tau(gamma) are 0.30 nm. A 1:1 complex seems thus to be favored. One and two water molecules would be partic ipating in the innersphere proton relaxation mechanism in the 2:1 and the 1:1 complexes respectively. (C) 1998 Elsevier Science S.A. All rig hts reserved.