Distance Measurements in Multiply Labeled Crystalline Cytidines by Dipolar Recoupling Solid State NMR

Abstract

To systematically explore the effects of spin system size and geometry on the precision and accuracy of two-dimensional solid state NMR distance measurements, we have applied two homonuclear dipolar recoupling experiments, 2D DRAWS and 2D RFDR, to five polycrystalline samples of uniformly or selectively 13 Clabeled cytidine. Distance information has been obtained from the intensities and time behavior of crosspeaks observed in the resulting two-dimensional spectra. The experimental cross-peak buildup curves obtained from these crystalline cytidine samples have been analyzed by comparison with simulations. In uniformly 13 C-labeled cytidine, indirect coherence transfer mechanisms lead to low-precision distance measurements not unlike those measured in solution state NOESY experiments. In the selectively labeled cytidines, the distance measurements are considerably more precise, allowing the possibility of very accurate structure determinations from selectively or randomly labeled spin systems. Of the two techniques, the 2D DRAWS method allows identification of indirect coherence transfer mechanisms that hinder accurate distance measurement.

Precision 1H–1H distance measurement via 13C NMR signals: utilization of 1H–1H double‐quantum dipolar interactions recoupled under magic angle spinning conditions

“…The precision in the measured H˛-Hˇdistance, 1.6% for d˛ˇD 248 pm in our experiment, is better than that previously reported on the internuclear distance measurements in solids: a 1 H-1 H distance determined by the DQ sideband method 8 , 260 š 15 pm (precision: 5.8%), and 13 C-13 C distances determined by DRAWS DQ-dipolar correlation experiment, 28 303 š 20 pm (6.6%) and 246 š 15 pm (6.1%). Recently, similar experiments to ours were reported, 29,30 in which the 1 H-1 H proximity was qualitatively discussed.…”

Sensitivity Enhancement in Two‐Dimensional Solid‐State NMR Spectroscopy by Transverse Mixing

“…The experiments most similar to those described herein are those of Kiihne et al, in which 2D spectra of 13 C-labeled compounds were obtained with evolution of transverse spin polarization under the dipolar recoupling with a windowless sequence (DRAWS) pulse sequence in the mixing period. [10,11] Because the effective Hamiltonian for dipole±dipole couplings under DRAWS contains both zero-quantum and double-quantum terms, Equation (5) C correlation spectroscopy in which heteronuclear polarization transfer occurs under an isotropic mixing sequence, that is, a sequence for which the effective heteronuclear dipole±dipole coupling has scalar symmetry. [12,13] In the experiments of Oliver and Titman, a p/2 pulse at the end of t 1 converts one transverse polarization component to longitudinal polarization.…”

Spinning-Frequency-Dependent Narrowband RF-Driven Dipolar Recoupling

“…The broadband nature of the recoupling permits observation of exchange between several spin pairs simultaneously. This is advantageous for correlation spectroscopy of multiple spins (6,28), but not for accurate distance measurements (29). The bandwidth of frequencies over which recoupling takes place can be narrowed by extending the number of rotor periods N R between consecutive pulses.…”

“…Its magnitude has been discussed extensively in the past (3,20,25,29), and its orientation dependence has been formalized (26). In most cases a single isotropic T 2 ZQ relaxation time is used to reflect all mechanisms that diminish m x (⍀ r , t) and m y (⍀ r , t) for all ⍀ r (13,23).…”