Abstract
The systematic difference between T2 values obtained from CPMG and T1p 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 degree 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.
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