A probe to monitor performance of ¹⁵N longitudinal relaxation experiments for proteins in solution

The magnitude of the ¹⁵N longitudinal relaxation rate typically decreases as magnetic field strength increases in globular proteins in solution. Thus, it is important to test the performance of ¹⁵N longitudinal relaxation experiments at high field strength. Herein, a tool to investigate systematic errors in ¹⁵N longitudinal relaxation rate, R₁, is introduced. The tool, a difference in R₁ values between the two components of the ¹H-coupled ¹⁵N magnetizations, R₁(1)-R₁(2), conveniently detects inefficiencies in cancellation of cross correlation between ¹H-¹⁵N dipolar coupling and ¹⁵N chemical shift anisotropy. Experiments, in varying conditions, and simulations of a two-spin system indicate that insufficient cancellation of the cross correlation is due to (1) ¹H pulse imperfection and (2) ¹H off-resonance effect, and (3) is further amplified by residual ¹⁵N transverse magnetization that is caused by the ¹⁵N off-resonance effect. Results also show that this problem can be easily and practically remedied by discarding the initial decay points when recording ¹⁵N longitudinal relaxation in proteins.