Off-resonance effects in 15N T2 CPMG measurements

The systematic difference between T₂ values obtained from CPMG and T₁ experiments was observed for backbone ¹⁵N nuclei of bacterial ribonuclease barnase. Theoretical consideration suggests that the observed difference is caused by off-resonance effects of 180° pulses of the CPMG pulse train. Namely, at off-resonance conditions T₁-dependent secondary echo coherence pathways considerably contribute to the signal decay in the CPMG experiment and result in systematic (up to 10%) offset-dependent overestimation of ¹⁵N T₂ measured by the CPMG technique. Under certain circumstances off-resonance effects result in dependence of ¹⁵N T₂ on CPMG frequency, which might be erroneously interpreted as conformational exchange on the millisecond time-scale. A procedure for numerical correction of ¹⁵N T₂ (CPMG) data is proposed.