Narayanan D. Kurur

Bimodal Floquet description of heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance

A theoretical treatment of heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance is presented here based on bimodal Floquet theory. The conditions necessary for good heteronuclear decoupling are derived. An analysis of a few of the decoupling schemes implemented until date is presented with regard to satisfying such decoupling conditions and efficiency of decoupling. Resonance conditions for efficient heteronuclear dipolar decoupling are derived with and without the homonuclear (1)H-(1)H dipolar couplings and their influence on heteronuclear dipolar decoupling is pointed out. The analysis points to the superior efficiency of the newly introduced swept two-pulse phase-modulation (SW(f)-TPPM) sequence. It is shown that the experimental robustness of SW(f)-TPPM as compared to the original TPPM sequence results from an adiabatic sweeping of the modulation frequencies. Based on this finding alternative strategies are compared here. The theoretical findings are corroborated by both numerical simulations and representative experiments.

Swept-frequency two-pulse phase modulation (SWf-TPPM) sequences with linear sweep profile for heteronuclear decoupling in solid-state NMR

Recently, a pulse scheme for heteronuclear spin decoupling in solid‐state NMR, called swept‐frequency two‐pulse phase modulation (SWf‐TPPM), was introduced which outperforms the standard TPPM and small phase incremental alteration (SPINAL) schemes. It has also been shown that the frequency‐sweep profile can be varied to achieve optimal efficiency for crystalline and liquid‐crystalline samples, respectively. Here we present a detailed comparison of the proton decoupling performance for SWf‐TPPM sequences with tangential sweep profiles (SWftan‐TPPM) and linear sweep profiles (SWflin‐TPPM). Using the 13CH2 resonance of crystalline tyrosine as a model system, it is shown that linear profiles have a decoupling performance which is at least as good and in some instances slightly better than that obtained from tangential sweep profiles. While tangential sweep profiles require a tangent cut‐off angle as an additional parameter, the lack of that parameter makes linear sweep profiles easier to implement and optimise. Copyright

An analysis of phase-modulated heteronuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance.

The design of variants of the swept-frequency two-pulse phase modulation sequence for heteronuclear dipolar decoupling in solid-state NMR is reported, their performance evaluated, and compared with other established sequences like TPPM and SPINAL. Simulations performed to probe the role of the homonuclear (1)H-(1)H bath show that the robustness of the decoupling schemes improves with the size of the bath. In addition, these simulations reveal that the homonuclear (1)H-(1)H bath also leads to broad baselines at high MAS rates. Results from a study of the SPINAL decoupling scheme indicate that optimisation of the starting phase and phase increment improves its performance and efficiency at high MAS rates. Additionally, experiments performed on a liquid crystal display the role of the initial phase in SPINAL-64 and sequences in the SW(f)-TPPM family.

Heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance under ultra-high magic-angle spinning

We compare in this communication several heteronuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance experiments under a magic-angle spinning frequency of 60 kHz. The decoupling radiofrequency field amplitudes considered are 190 and 10 kHz. No substantial difference was found among the sequences considered here in performance barring the difference in the optimisation protocol of the various schemes, an aspect that favours the use of swept-frequency two pulse phase modulation (SW(f)-TPPM).

On the choice of heteronuclear dipolar decoupling scheme in solid-state NMR.

We present here a comparison of different heteronuclear dipolar decoupling sequences at the moderate magic-angle spinning (MAS) frequency (ν(r)) of 30 kHz. The radio-frequency (RF) amplitude (ν₁) ranges from the low power (ν₁2ν(r)) and includes the rotary resonance conditions (ν₁=nν(r)) where n=1, 2. For decoupling at the rotary resonance condition, we recently introduced a modification of TPPM, namely high-phase TPPM, whose properties will be discussed here. Finally, based on earlier published and current experimental results we suggest the optimal sequence for heteronuclear dipolar decoupling at any RF amplitude and MAS frequencies up to 35 kHz.

Efficient heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance at rotary resonance conditions.

We introduce here a heteronuclear dipolar decoupling scheme in solid-state nuclear magnetic resonance that performs efficiently at the rotary resonance conditions, where otherwise dipolar couplings are re-introduced. Results are shown proving the efficiency of this scheme at two magnetic fields under magic-angle spinning frequencies of 30 and 20 kHz.

Supercycled SWf-TPPM sequence for heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance

The performance of a supercycled SW(f)-TPPM sequence for heteronuclear dipolar decoupling in solid-state NMR is analyzed here. The decoupling performance of this sequence with respect to experimental parameters, such as, the phase angle, proton offset and MAS frequency is studied. A comparison is made with two other commonly used decoupling schemes in solid-state NMR namely, SPINAL-64 and SW(f)-TPPM, on a sample of U-¹³C-labeled tyrosine. Our results show that supercycled SW(f)-TPPM performs better than the former sequences. Also, numerical spin dynamics studies are presented which support the experimentally observed efficiency in the decoupling.

Heteronuclear dipolar decoupling in liquid‐crystal NMR using supercycled SWf‐TPPM sequences

Heteronuclear dipolar decoupling is an essential requirement for extracting structural information from the 13C NMR spectra of liquid crystals. Efficient schemes for heteronuclear dipolar decoupling in such systems are formulated here by supercycling SWf‐TPPM, a sequence introduced recently for this purpose in rotating solids. These sequences are compared with two other commonly used decoupling schemes in liquid‐crystal NMR, SPINAL‐64 and SWf‐TPPM, by analyzing the intensities of various resonances in the proton decoupled 13C spectrum of the liquid‐crystal 4‐n‐pentyl‐4′‐cyanobiphenyl (5CB). The effectiveness of the decoupling programs with respect to experimental parameters such as RF field strength, decoupler offset frequency and phase angle is also presented. Copyright

Relaxation Editing Using Long-Lived States and Coherences for Analysis of Mixtures

Nuclear magnetic resonance (NMR) is a powerful tool for structural and dynamical studies of molecules. Although widely applicable, the search for novel spectral editing methods that facilitate spectral assignment of peaks in high-resolution NMR is highly desirable. Earlier, the sensitivity of lifetime of spin states (spin-lattice relaxation time, T1) and coherences (spin-spin relaxation time, T2) to the immediate environment was utilized for spectral editing in solution NMR. Long-lived states (LLS) and coherences (LLCs) were recently uncovered to have longer and more domain sensitive lifetime than other type of states and coherences. Herein, this longevity and increased sensitivity of LLS and LLC lifetime is utilized for more enhanced dispersion in relaxation editing in NMR. The generality of the method as a powerful tool in spectral editing is confirmed with molecules containing a mixture of strongly and weakly coupled spin systems and finally with metabolomic mixture. Extension to insensitive nuclei enhanced by polarization transfer (INEPT), correlation spectroscopy (COSY), and heteronuclear single quantum coherence (HSQC) are also demonstrated.

Paramagnetic Relaxation of Long‐Lived Coherences in Solution NMR

Long-lived coherences (LLCs) are known to have lifetimes much longer than transverse magnetization or single quantum coherences (SQCs). The effect of paramagnetic ions on the relaxation of LLCs is not known. This is particularly important, as LLCs have potential applications in various fields like analytical NMR, in vivo NMR and MR imaging methods. We study here the behaviour of LLCs in the presence of paramagnetic relaxation agents. The stepwise increase in the concentration of the metal ion is followed by measuring various relaxation rates. The effect of paramagnetic ions is analysed in terms of the external random fields contribution to the relaxation of two coupled protons in 2,3,6-trichlorobenzaldehyde. The LLCs relax faster than ordinary SQCs in the presence of paramagnetic ions of varying character. This is explained on the basis of an increase in the contribution of the external random field to relaxation due to a paramagnetic relaxation mechanism. Comparison is also made with ordinary Zeeman relaxation rates like R1, R2, R1ρ and also with rate of relaxation of long-lived states RLLS which are known to be less sensitive to paramagnetically induced relaxation. Also, the extent of correlation of random fields at two proton sites is studied and is found to be strongly correlated with each other. The obtained correlation constant is found to be independent of the nature of added paramagnetic impurities.