Abstract
We demonstrate the efficacy of the REDOR-type sequences in determining dipolar coupling strength in a paramagnetic environment. Utilizing paramagnetic effects of enhanced relaxation rates and rapid electronic fluctuations in Cu(II)-(DL-Ala)2.H2O, the dipolar coupling for the methyl C–H that is 4.20 Å (methyl carbon) away from the Cu2+ ion, was estimated to be 8.8 ± 0.6 kHz. This coupling is scaled by a factor of ~0.3 in comparison to the rigid limit value of ~32 kHz, in line with partial averaging of the dipolar interaction by rotational motion of the methyl group. Limited variation in the scaling factor of the dipolar coupling strength at different temperatures is observed. The C–H internuclear distance derived from the size of the dipolar coupling is similar to that observed in the crystal structure. The errors in the dipolar coupling strength observed in the REDOR-type experiments are similar to those reported for diamagnetic systems. Increase in resolution due to the Fermi contact shifts, coupled with MAS frequencies of 30–35 kHz allowed to estimate the hyperfine coupling strengths for protons and carbons from the temperature dependence of the chemical shift and obtain a high resolution 1H–1H spin diffusion spectrum. This study shows the utility of REDOR-type sequences in obtaining reliable structural and dynamical information from a paramagnetic complex. We believe that this can help in studying the active site of paramagnetic metalloproteins at high resolution.
Highlights
Paramagnetic solid-state NMR has undergone a revolution in the past decade due to the development of high spinning frequency probes and tailored pulse sequences
We find that internuclear dipolar couplings can be determined with high accuracy that is comparable to what has been achieved for diamagnetic systems
While there was no anomaly in the implementation of shifted-REDOR for Cu(II)-(DL-Ala)2.H2O, in shifted-REDOR (Fig. 3c), the oscillatory behavior observed in the REDOR (Fig. 3a) is diminished which can be due to the presence of additional protons (NH2 and Hα). This is consistent with the observations of Schanda et al (2011), who showed that the presence of additional protons near the observed nuclei reduces the oscillatory nature of the shifted-REDOR for ubiquitin crystals [5,6]
Summary
Paramagnetic solid-state NMR has undergone a revolution in the past decade due to the development of high spinning frequency probes and tailored pulse sequences. The J-coupling between the copper ions determines the fluctuation frequencies for averaging dipolar couplings, which are traceless without isotropic component and is intrinsic to a one-dimensional chain. This results in the motional narrowing from spin waves running along the chain [15]. This opens up a window of opportunity, since essential advantages of paramagnetism, such as rapid acquisition and increased spectral dispersion can be combined with the power of dipolar correlation spectroscopy for resolving static or dynamic structure quantitatively. The Cu(II)-(DL-Ala).H2O form 1-dimensional chains in which the Cu2þ
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