Proton and phosphorus‐31 magnetic relaxation studies on the interaction of polyriboadenylic acid with Mn 2+

Proton and phosphorus‐31 nuclear spin–lattice relaxation times T 1 and spin–spin relaxation times T 2 have been measured on the single‐stranded polyriboadenylic acid [poly(A)]–Mn 2+ system in a neutral D 2 O solution in the temperature range 10°–90°C at 100 and 40.5 MHz, respectively, with the Fourier transform nmr method. Minimum values of T 1 have been found for all these nuclei, which have enabled the exact estimation of apparent distances from Mn 2+ to H 2 , H 8 , H 1′ , and the phosphorus nucleus to be 4.7, 4.1, 5.2, and 3.0 Å, respectively. The electron spin of Mn 2+ penetrates into the phosphorus nucleus, giving 31 P hyperfine coupling of more than 10 6 Hz. Evidence of penetration of the electron spin into H 8 and H 2 is also obtained, suggesting direct coordination of nitrogen atoms of the adenine ring to the Mn 2+ Ion. Combined with the result from proton relaxation enhancement of water, it is concluded that every Mn 2+ ion added is bound directly to two phosphate groups with a Mn 2+ –phosphorus distance of 3.3 Å, while a part of the Mn 2+ ions are simultaneouly bound to the adenine ring. It is estimated that 39 ± 13% and 13 ± 5% of Mn 2+ are coordinated by N 7 and N 3 (or N 1 ), respectively. The motional freedom of poly(A) in the environment of the Mn 2+ binding site has been found to be quenched to the extent that the rotational motion becomes several times slower than that of the corresponding Mn 2+ –free poly(A). The activation energies for the molecular motion are, however, practically unchanged from those for Mn 2+ –free poly(A), and are found to be 8.3, 8.5, 6.1, and 8.7 kcal/mol for H 8 , H 2 , H 1′ , and phosphorus, respectively. T 2 of phosphorus is determined by the dissociation rate ( k −1 ) of Mn 2+ from the phosphate group for the whole temperature range studied with activation enthalpy of 6.5 kcal/mol. The dissociation rates of Mn 2+ from the adenine ring are also estimated from proton T 2 values below 50°C.