Determination of molecular structure by carbon-13 autocorrelation N.M.R

Analysis and elimination of artifacts in indirect covariance NMR spectra via unsymmetrical processing

Indirect covariance NMR offers an alternative method of extracting spin-spin connectivity information via the conversion of an indirect-detection heteronuclear shift-correlation data matrix to a homonuclear data matrix. Using an IDR (inverted direct response)-HSQC-TOCSY spectrum as a starting point for the indirect covariance processing, a spectrum that can be described as a carbon-carbon COSY experiment is obtained. These data are analogous to the autocorrelated 13C-13C double quantum INADEQUATE experiment except that the indirect covariance NMR spectrum establishes carbon-carbon connectivities only between contiguous protonated carbons. Cyclopentafuranone and the complex polynuclear heteroaromatic naphtho[2',1':5,6]-naphtho[2',1':4,5]thieno[2,3-c]quinoline are used as model compounds. The former is a straightforward example because of its well-resolved proton spectrum, while the latter, which has considerable resonance overlap in its congested proton spectrum, gives rise to two types of artifact responses that must be considered when using the indirect covariance NMR method.

Practical aspects of 2D NMR for assigning the non-exchangeable protons in DNA-RNA fragments

Using the branched trimer A2'-5'A3'-5'A as an example, different 2D NMR experiments such as homonuclear correlations via single-quantum coherence, double-quantum filtration via double-quantum coherence, isotropic mixing, relayed connectivities, cross relaxation and 31P/1H correlations are presented together with the corresponding spectra. The discussion pointed out the advantages and the difficulties of 14 pulse sequences, used for assigning non-exchangeable protons in DNA or RNA fragments. The emphasis has been put on the methodological aspects of 2D NMR as a simple technique, allowing an easier assimilation of the new coming 2D NMR pulse sequences.