Off-resonance effects in N-15 T-2 CPMG measurements

The systematic difference between T-2 values obtained from CPMG and T(1)rho experiments was observed for backbone N-15 nuclei of bacterial ribonucleas e barnase. Theoretical consideration suggests that the observed difference is caused by off-resonance effects of 180 degrees pulses of the CPMG pulse train. Namely, at off-resonance conditions T-1-dependent secondary echo coh erence pathways considerably contribute to the signal decay in the CPMG exp eriment and result in systematic (up to 10%) offset-dependent overestimatio n of N-15 T-2 measured by the CPMG technique. Under certain circumstances o ff-resonance effects result in dependence of N-15 T-2 on CPMG frequency, wh ich might be erroneously interpreted as conformational exchange on the mill isecond time-scale. A procedure for numerical correction of (1)5N T-2 (CPMG ) data is proposed.