Reprint of: Localization of Cl-35 Nuclei in Biological Solids using Rotational-Echo Double-Resonance Experiments.

Chloride ions play important roles in many chemical and biological processes. This paper investigates the possibility of localizing 35Cl nuclei using solid-state NMR. It demonstrates that distances shorter than 3.8Å, between 13C atoms and 35Cl atoms in 10% uniformly labeled 13C L-tyrosine·HCl and natural abundance Glycine·HCl can be measured using rotational-echo (adiabatic passage) double-resonance (RE(AP)DOR). Furthermore the effect of quadrupolar interaction on the REDOR/REAPDOR experiment is quantified. The dephasing curve is plotted in a three dimensional chart as a function of the dephasing time and of the strength of quadrupolar interaction felt by each orientation. During spinning each orientation feels a quadrupolar interaction that varies in time, and therefore at each moment in time we reorder the crystallite orientations as a function of their contribution to the dephasing curve. In this way the effect of quadrupolar interaction on the dipolar dephasing curve can be fitted with a polynomial function. The numerical investigation performed allows us to generate REDOR/REAPDOR curves which are then used to simulate the experimental data.

Localization of Cl-35 nuclei in biological solids using rotational-echo double-resonance experiments.

Chloride ions play important roles in many chemical and biological processes. This paper investigates the possibility of localizing 35Cl nuclei using solid-state NMR. It demonstrates that distances shorter than 3.8Å, between 13C atoms and 35Cl atoms in 10% uniformly labeled 13C L-tyrosine·HCl and natural abundance Glycine·HCl can be measured using rotational-echo (adiabatic passage) double-resonance (RE(AP)DOR). Furthermore the effect of quadrupolar interaction on the REDOR/REAPDOR experiment is quantified. The dephasing curve is plotted in a three dimensional chart as a function of the dephasing time and of the strength of quadrupolar interaction felt by each orientation. During spinning each orientation feels a quadrupolar interaction that varies in time, and therefore at each moment in time we reorder the crystallite orientations as a function of their contribution to the dephasing curve. In this way the effect of quadrupolar interaction on the dipolar dephasing curve can be fitted with a polynomial function. The numerical investigation performed allows us to generate REDOR/REAPDOR curves which are then used to simulate the experimental data.

Population and coherence transfer induced by double frequency sweeps in half-integer quadrupolar spin systems.

We have recently shown that the sensitivity of single- and multiple-quantum NMR experiments of half-integer (N/2) quadrupolar nuclei can be increased significantly by introducing so-called double frequency sweeps (DFS) in various pulse schemes. These sweeps consist of two sidebands generated by an amplitude modulation of the RF carrier. Using a time-dependent amplitude modulation the sidebands can be swept through a certain frequency range. Inspired by the work of Vega and Naor (J. Chem. Phys. 75, 75 (1981)), this is used to manipulate +/-(m - 1) <--> +/-m (3/2 < or = m < or = N/2) satellite transitions in half-integer spin systems simultaneously. For (23)Na (I = 3/2) and (27)Al (I = 5/2) spins in single crystals it proved possible to transfer the populations of the outer +/-m spin levels to the inner +/-1/2 spin levels. A detailed analysis shows that the efficiency of this process is a function of the adiabaticity with which the various spin transitions are passed during the sweep. In powders these sweep parameters have to be optimized to satisfy the appropriate conditions for a maximum of spins in the powder distribution. The effects of sweep rate, sweep range, and RF field strength are investigated both numerically and experimentally. Using a DFS as a preparation period leads to significantly enhanced central transition powder spectra under both static and MAS conditions, compared to single pulse excitation. DFSs prove to be very efficient tools not only for population transfer, but also for coherence transfer. This can be exploited for the multiple- to single-quantum transfer in MQMAS experiments. It is demonstrated, theoretically and experimentally, that DFSs are capable of transferring both quintuple-quantum and triple-quantum coherence into single-quantum coherence in I = 5/2 spin systems. This leads to a significant enhancement in signal-to-noise ratio and strongly reduces the RF power requirement compared to pulsed MQMAS experiments, thus extending their applicability. This is demonstrated by (27)Al 3QMAS experiments on 9Al(2)O(3). 2B(2)O(3) and the mineral andalusite. In the latter compound, Al experiences a quadrupolar-coupling constant of 15.3 MHz in one of the sites. Finally a 5QMAS spectrum on 9Al(2)O(3). 2B(2)O(3) demonstrates the sensitivity enhancement of this experiment using a double frequency sweep.