Nuclear Magnetic Resonance Study of Metal-Ion Binding to Adenosine Triphosphate. II. Proton Studies

Abstract

We have studied the high‐resolution proton NMR spectrum of adenosine triphosphate (ATP) bound to the paramagneticmetal ions Mn2+, Ni2+, and Co2+ from 0° to 90°C at different concentrations of metal and ATP. In conjunction with our 31P study, we conclude that in the MnATP and NiATP complexes the metal ion is bound to the adenine ring all the time it is bound to the phosphates. At 298°K the average time the metal ion sticks to the adenine was measured to be (5±1)×10−6 and (1±0.15)×10−3 sec for MnATP and NiATP, respectively. The corresponding times measured for the phosphates were (5±1)×10−6 and (7±2)×10−4 sec. The excellent agreement between these sticking times indicates that the bonds to adenine and phosphates are made and broken simultaneously. The metal ion was located at a position in space between the amino group on C6 and N7 in the adenine ring by triangulation from the three protons H8, H2, and H1′. The triangulation was made from measurements of T 1 under conditions where 1/T 1∝ (μ/r 3)2τ c . The magnetic moment μ and correlation time τ c of the metal ion with respect to the proton were determined independently. Therefore, r, the distances from the H8, H2, and H1′ protons to the metal ion, were determined. In CoATP the distances determined were the same as for the other metal ions. From this and from its similar behavior to CoITP (inosine triphosphate) where chemical evidence favors ring binding, we conclude that in CoATP as well as NiATP and MnATP the metal is bound to the ring.