Untitled

Abstract

An approach to the determination of the 2-(13)C' chemical shift (CS) tensor orientation in pyrimidine bases via heteronuclear MAS NMR spectroscopy is presented. Considering a dipolar coupled spin 1/2 network of the type S1-I-S2 consisting of directly bonded heteronuclear spins, we have carried out numerical simulations to assess the sensitivity of I-S REDOR spinning sidebands to the Euler angles defining the orientation of the I-S1 and I-S2 dipolar vectors in the I spin CS tensor principal axes system. Our investigations clearly demonstrate the potential of I-S REDOR studies in IS1S2 systems for obtaining with high reliability and accuracy the I spin chemical shift tensor orientation in the molecular frame spanned by the two internuclear vectors I-S1 and I-S2. The significant contribution to the observed REDOR sideband intensities from anti-phase operator terms which are present at the start of the data acquisition is illustrated. The procedure for the recording and analysis of the I-S REDOR spectra in IS1S2 systems is presented and the measurement of the 2-(13)C' CS tensor orientation in a polycrystalline sample of [1,3-(15)N2, 2-(13)C] uracil, which is one of the four bases in RNA, is experimentally demonstrated.

Orientational Information from TEDOR Spectral Sidebands

“…The efficacy of REDOR [15,16] for obtaining CS tensor orientation has also been shown recently by Goetz and Schaefer [17]. As part of our ongoing efforts on the characterization of 15 N and 13 C chemical shift tensor orientations in peptides and ribonucleic acids, we have been recently exploring the potential of heteronuclear MAS NMR spectroscopy [18][19][20]. To define in the molecular frame the CS tensor orientation of an NMR active nucleus at a given site of interest, following the work of Hartzell et al [21], we obtain the Euler angles defining the orientation of two dipolar vectors in the CS tensor principal axes system.…”

“…1, assuming ideal experimental conditions and neglecting scalar and homonuclear dipolar interactions, the evolution of the dipolar-coupled spin-1/2 network 15 N1-13 C-15 N2 has been studied via standard density matrix calculations. Making use of the expression for the MAS Hamiltonian for this three-spin network given recently [20], the observed signal is calculated via…”

“…The Euler angles of the dipolar vectors in the rotor and CS tensor frames are related to each other via the Euler angles defining the orientation of the CS tensor principal axes system in the rotor frame [20].…”

“…To define in the molecular frame the CS tensor orientation of an NMR active nucleus at a given site of interest, following the work of Hartzell et al [21], we obtain the Euler angles defining the orientation of two dipolar vectors in the CS tensor principal axes system. Based on such an approach we have shown recently [18,19] that a combination of 15 N- 13 C REDOR [15][16][17] and 15 N-1 H DIPSHIFT [22][23][24] experiments can be conveniently employed, for example, in the characterization of the CS tensor orientation of the amide nitrogens in a peptide backbone and the imino nitrogens of nucleic acid bases. It was also shown that a single 13 C- 15 N REDOR experiment would suffice to obtain the 13 C spin tensor orientation in the molecular frame in systems having a dipolar-coupled spin-1/2 network of the type 15 N1- 13 C-15 N2 [20].…”

“…Hence the measurement of the 2- 13 C CS tensor orientation of pyrimidine bases is of great relevance in structural investigations of RNAs. Employing a 13 C- 15 N REDOR approach we have characterized the 2-13 C CS tensor orientation in a polycrystalline sample of [1 3-15 N 2 2-13 C]uracil via an analysis of the REDOR sideband intensities in terms of the Euler angles defining the orientation of the two 13 C- 15 N dipolar vectors in the 2-13 C CS tensor principal axes system [20]. The results of our investigation are in good agreement with the 13 C tensor orientation reported in the literature on a similar system [12].…”