Article

A convenient method for the measurements of transverse relaxation rates in homonuclear scalar coupled spin systems.

Laboratoire Chimie Provence, Spectrométries Appliquées à la Chimie Structurale, UMR 6264, Université Aix-Marseille I-CNRS, 13397 Marseille, France.
Chemical Communications (Impact Factor: 6.38). 08/2011; 47(32):9209-11. DOI: 10.1039/c1cc13042k
Source: PubMed

ABSTRACT A new solution-state NMR method is proposed to determine apparent transverse NMR relaxation rates in both weakly and strongly scalar coupled two-spin systems.

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    ABSTRACT: Nuclear magnetic resonance (NMR) was discovered in the first half of the 20th century. Today, neither analytical chemistry without NMR spectroscopy nor medical diagnostics without magnetic resonance imaging (MRI) could be imagined. A magnetic resonance signal decays with a time constant T2, the transverse relaxation time. This parameter contains a great deal of information about structure and dynamics of the molecule, where the observed nuclear spin is located. A method to measure T2 is the so-called spin echo, discovered by Erwin Hahn in 1950. Usually, the spin echo can generate an exponential decay of the signal and T2 can be extracted. But if the indirect spin-spin interaction, the J-coupling, connects nuclear spins of the same isotope, the echo signal is modulated. The present work discusses novelmethods to quench these echo modulations. This enables one, on the one hand, to measure the transverse relaxation in homonuclear spin systems. On the other hand, this decoupling technique can simplify NMR spectra by making splittings due to J-couplings disappear. These two processes can be seen to be equivalent, since the Fourier transform of a oscillating time signal corresponds to a line splitting of the frequency signal. Tomeasure T2’s of protons (the nuclei of the most abundant hydrogen isotope 1H) or molecules isotopically enriched in carbon-13, we use a “train of echo pulses” with a moderate amplitude of the irradiated radio-frequency. To decouple various nuclear spins in a 1H NMR spectrum, we use a selective radio-frequency irradiation. These experiments are performed as Fourier transformspectroscopy, introduced by Ernst and Anderson in 1966. Based on the same pulse sequence, we measure so-called “spin tickling” experiments, described by Freeman and Anderson in 1962. All lines in the spectrum split that share a common energy-level with the irradiated resonance. “Tickling” and decoupling are complementary and belong both to the category of double resonance NMR experiments.
    03/2013, Degree: PhD (Docteur ès Sciences), Supervisor: Geoffrey Bodenhausen
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    ABSTRACT: Homonuclear (1)H-(1)H J-modulation leads to J-multiplets in F1 dimension of 2D (1)H-(13)C HMQC spectra. This hampers unambiguous signal assignment for overcrowded (13)C spectra. Broadband homonuclear decoupling has been achieved in the indirect t1 evolution period by incorporating blocks of perfect echo. This method of perfect echo HMQC demonstrates better resolution and sensitivity than conventional HMQC spectra. The results on Cyclosporine demonstrate that the method is very efficient for refocusing geminal couplings in weakly coupled -(13)CH2 groups. Partial refocusing of vicinal couplings is also observed for -(13)CH and -(13)CH3 groups. Interpretation of the result based on product operator formalism is also given. Comparison of pe-HMQC, HMQC and HSQC reveals that the F1 linewidth of pe-HMQC is much narrower than HMQC and very close to that of HSQC for CH2 groups.
    Journal of Magnetic Resonance 06/2013; 234C:67-74. · 2.30 Impact Factor
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    ABSTRACT: Homogeneous line-widths that arise from transverse relaxation tend to be masked by B0 field inhomogeneity and by multiplets due to homonuclear J-couplings. Besides well-known spin-locking sequences that lead to signals that decay with a rate R1ρ without any modulations, alternative experiments allow one to determine the transverse relaxation rates R2 in systems with scalar-coupled spins. We evaluate three recent strategies by experiment and simulation: (i) moderate-amplitude SITCOM-CPMG sequences (Dittmer and Bodenhausen, 2006 [2]), (ii) multiple-quantum filtered (MQF) sequences (Barrère et al., 2011 [4]) and (iii) PROJECT sequences (Aguilar et al., 2012 [5]). Experiments where the J-evolution is suppressed by spin-locking measure the pure relaxation rate R2(Ix) of an in-phase component. Experiments based on J-refocusing yield a mixture of in-phase rates R2(Ix) and antiphase rates R2(2IySz), where the latter are usually faster than the former. Moderate-amplitude SITCOM-CPMG and PROJECT methods can be applied to systems with many coupled spins, but applications of MQF sequences are limited to two-spin systems since modulations in larger systems can only partly be suppressed.
    Journal of Magnetic Resonance 10/2013; 237C:139-146. · 2.30 Impact Factor

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