Abstract
1. Acidic solutions (pH 1–5) of copper(II)-adenosine triphosphate yield ultrasonic attenuation spectra consisting of three maxima. From the concentration and pH dependence of these peaks, and by comparison with the spectra of low pH ATP, CuCl2 and Cu3(PO4)2 solutions, we have assigned the spectrum as follows. The overall reaction which fits the ultrasonic attenuation data best is
The shortest relaxation time (approx. 2 · 10−9 s) is ascribed to penetration of the inner copper(II) hydration shell by the ligand. The next slower process (approx. 1 · 10−8 s) is then due to chelate ring closure of the species formed in the faster step. The lowest frequency peak (< 2 MHz) has been taken to be either a characteristic of aqueous copper(II) ion, as already observed in other systems (e.g. CuSO4), or to be due to formation of another (minority) complex species such as CuH2ATP. In either case, the presence of this peak does not interfere with the kinetic analysis of the faster processes.
2. On the assumption that formation of copper(II) complexes follows the established pattern of other divalent transition metal ions, we obtain at 25 ·C and variable ionic strength (0.1–0.8 M), 8.8 · 108 s−1 for the water elimination rate constant, and 6.3 · 107 s−1 for the sterically controlled ring closure. The ion-pair formation constant consistent with the data is 16 M−1. The absolute value of the standard volume change, |ΔVc| has been estimated to be 42 ml · mole−1 for the above overall reaction.
3.Although the experimental error associated with the determination of the relaxation maxima is on the order of 10–20%, the values of the rate constants, ion-pair formation constant and standard volume changes are reliable to only a factor of approximately two, due to uncertainties in solution composition and approximations made in solving the rate equations.