An NMR study of 1H, 31P, and 23Na relaxation and molecular dynamics in the polycrystalline sodium salt of adenosine monophosphate

Abstract

Proton spin-lattice relaxation times in the laboratory frame, T 1 (H), have been measured as a function of frequency and temperature (295 K > T > 90 K). The spin-lattice relaxation times in the rotating frame have been measured at two different rotating fields, while the proton second moment has been obtained from the lineshapes. In addition, T 1( 31 P) and T 1 ( 23 Na) have been measured as functions of temperature at 81 and 50 MHz, respectively. The results demonstrate clearly that the water content of the compound influences the results to a large extent. It seems that water molecules at some of the lattice sites can be removed from the structure by evacuation, while others are more tightly bound to the AMP molecule. The more loosely bound water molecules are very mobile and dominate the relaxation results in the high-temperature region via the spin-rotation and dipolar mechanisms. The more tightly bound water molecules rotate about their twofold axes and this motion, characterized by a distribution of correlation times, influences the T 1 (H) results at lower temperatures. The 23Na spin-lattice relaxation rate is dominated by the quadrupolar interactions, which also provide a dominating relaxation mechanism for the proton spins in the rotating frame. T 1 (P) is strongly dependent on the water content of the sample.