13C spin relaxation measurements in RNA: Sensitivity and resolution improvement using spin-state selective correlation experiments

A set of new NMR pulse sequences has been designed for the measurement of 13C relaxation rate constants in RNA and DNA bases: the spin-lattice relaxation rate constant R(Cz), the spin-spin relaxation rate constant R(C+), and the CSA-dipolar cross-correlated relaxation rate constant ΓC,CHxy. The use of spin-state selective correlation techniques provides increased sensitivity and spectral resolution. Sensitivity optimised C-C filters are included in the pulse schemes for the suppression of signals originating from undesired carbon isotopomers. The experiments are applied to a 15% 13C-labelled 33-mer RNA-theophylline complex. The measured R(C+)/ΓC,CHxy ratios indicate that 13C CSA tensors do not vary significantly for the same type of carbon (C2, C6, C8), but that they differ from one type to another. In addition, conformational exchange effects in the RNA bases are detected as a change in the relaxation decay of the narrow 13C doublet component when varying the spacing of a CPMG pulse train. This new approach allows the detection of small exchange effects with a higher precision compared to conventional techniques