Effect of ¹³C-¹⁴N Dipolar Interactions on Spin-Lattice Relaxation Times and Intensities of Nonprotonated Carbon Resonances

Spin-lattice relaxation times (T₁) and integrated intensities of the resonances in proton-decoupled natural-abundance ¹³C Fourier transform NMR spectra of adenosine 5′-monophosphate and guanosine S'-monophosphate (in H₂O and D₂O, at 40-44 °C, at 15.18 MHz, in 20-mm sample tubes) are compared with calculated values that take into account ¹³C-¹H and 13C-14N dipolar relaxation. In each case, T₁ values of methine carbons of the base were used to obtain a rotational correlation time, which was then used, together with interatomic distances from crystallographic data, to compute T₁ values of nonprotonated carbons. Nonprotonated carbons which are directly bonded to nitrogens and which have no hydrogens two bonds removed yield theoretical T₁ values strongly affected by ¹³C-¹⁴N dipolar interactions. For carbons in this category, calculated T₁ values which include ¹³C-¹⁴N dipolar interactions are in much better agreement with experimental values than calculated values which consider only ¹³C-¹H interactions. Integrated intensities were calculated by considering variations in the nuclear Overhauser enhancement that result from differences in relative contributions to 1/T₁ from ¹³C-¹H and ¹³C-¹⁴N dipolar relaxation. The calculated intensities are in excellent agreement with the experimental ones.