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 C-13 relaxation rate constants in RNA and DNA bases: the spin-lattice relaxation rate constant R(C-z), the spin-spin relaxation rate constant R(C+), and the CSA-dipolar cross-correlated relaxation rate constant Gamma(C,CH)(xy). 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% C-13-labelled 33-mer RNA-theophylline complex. The measured R(C+)/Gamma(C,CH)(xy) ratios indicate that C-13 CSA tensors do not vary significantly for the same type of carbon (C-2, C-6, C-8), 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 C-13 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.