Measuring transverse relaxation rates provides insight into the dynamics of molecules. For example, measurements of relaxation dispersion allow the study of invisible conformations of proteins. Such applications are currently restricted to sparse spin systems in which homonuclear couplings can be neglected, such as 15NH or selectively labelled 13CHD2 groups in proteins. The accurate measurement of transverse relaxation rates is also essential if quantitative results are to be obtained from multipulse NMR experiments where there are T2 differences among the spins observed.
The common experiments for measuring T2 in coupled spin systems, CPMG  and PROJECT , can suppress the signal modulations caused by homonuclear J couplings. However in both cases the relaxation measured is not specific to a given signal, but contains contributions from all the spins in a coupled system because of the sharing of coherence. Measured T2 values therefore differ from the true T2s. We propose a new approach, Active Spin Refocusing (ASR) T2 measurement, which allows broadband measurement of ‘true’ transverse relaxation rates even in coupled spin systems. It uses a single spin echo, with the refocusing element flanked by variable delays instead of trains of pulses so that the measured signal decay reflects the true loss of transverse magnetisation during free precession. J modulation is suppressed by using an active spin refocusing element of the sort now commonly used in pure shift NMR methods such as that of Zangger and Sterk . This allows true T2 measurements on all signals in a spectrum in a single experiment, albeit at the price of a reduction in sensitivity.
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