N. Chandrakumar

Chemical‐Shift‐Resolved 19F NMR Spectroscopy between 13.5 and 135 MHz: Overhauser–DNP‐Enhanced Diagonal Suppressed Correlation Spectroscopy

Overhauser-DNP-enhanced homonuclear 2D (19)F correlation spectroscopy with diagonal suppression is presented for small molecules in the solution state at moderate fields. Multi-frequency, multi-radical studies demonstrate that these relatively low-field experiments may be operated with sensitivity rivalling that of standard 200-1000 MHz NMR spectroscopy. Structural information is accessible without a sensitivity penalty, and diagonal suppressed 2D NMR correlations emerge despite the general lack of multiplet resolution in the 1D ODNP spectra. This powerful general approach avoids the rather stiff excitation, detection, and other special requirements of high-field (19)F NMR spectroscopy.

Developmental patterning and segregation of alkaloids in areca nut (seed of Areca catechu) revealed by magnetic resonance and mass spectrometry imaging

Areca nut (seed of Areca catechu) is consumed by people from different parts of Asia, including India. The four major alkaloids present in areca nut are arecoline, arecaidine, guvacoline and guvacine. Upon cutting, the nut reveals two kinds of regions; white and brown. In our present study, we have monitored the formation of these two regions within the nut during maturation, using the non-invasive techniques of magnetic resonance imaging (MRI) and volume localized magnetic resonance spectroscopy (MRS). Electrospray ionization mass spectrometry (ESI MS) and desorption electrospray ionization mass spectrometry (DESI MS) imaging have been used to study the associated change in the alkaloid contents of these two regions during the growth of the nut. Our study reveals that white and brown regions start forming within the nut when the liquid within starts solidifying. At the final stage of maturity, arecoline, arecaidine and guvacoline get segregated in the brown region whereas guvacine gets to the white region of the nut. The transport of molecules with maturity and corresponding pattern formation are expected to be associated with a multitude of physiochemical changes.

Communication: Ultrafast homonuclear correlation spectroscopy with diagonal suppressiona)

A novel ultrafast 2D NMR experiment is introduced for homonuclear correlation spectroscopy in solution state, with diagonal peak suppression in each scan of a two scan procedure. This experiment permits clear visualization of cross peaks between spins whose chemical shifts are very close, which could otherwise be masked by diagonal peaks. The present report describes the principles of its design and illustrates actual performance.

119/117/115Sn low-abundance single-transition correlation spectroscopy (LASSY): Sensitivity-enhanced homonuclear correlation experiments for Sn NMR

We report the implementation of our novel rare‐spin homonuclear correlation experiment, namely, Low‐Abundance Single‐transition correlation SpectroscopY (LASSY), for 119/117/115Sn NMR at natural abundance. Our pulse sequence results in diagonal suppressed COSY‐style display and outperforms the optimal homonuclear correlation experiment for rare spins, which involves double quantum evolution (INADEQUATE CR). The new experiment maximizes efficiency both in respect of pulse transformations as well as relaxation effects, and gives rise to a simplified two‐dimensional (2D) spectrum with considerably reduced crowding, exhibiting only one transition in each cross peak, instead of four. Performance optimization of LASSY is carried out in light of the relatively ‘large’ line widths typical of Sn NMR in solution state. The superior performance of the sequence is demonstrated on dimeric tetraorganodistannoxane samples. Copyright

Slow Molecular Motions in Ionic Liquids Probed by Cross-Relaxation of Nuclear Spins During Overhauser Dynamic Nuclear Polarization

Solution-state Overhauser dynamic nuclear polarization (ODNP) at moderate fields, performed by saturating the electron spin resonance (ESR) of a free radical added to the sample of interest, is well known to lead to significant NMR signal enhancements in the steady state, owing to electron-nuclear cross-relaxation. Here it is shown that under conditions which limit radical access to the molecules of interest, the time course of establishment of ODNP can provide a unique window into internuclear cross-relaxation, and reflects relatively slow molecular motions. This behavior, modeled mathematically by a three-spin version of the Solomon equations (one unpaired electron and two nuclear spins), is demonstrated experimentally on the 19 F/1 H system in ionic liquids. Bulky radicals in these viscous environments turn out to be just the right setting to exploit these effects. Compared to standard nuclear Overhauser effect (NOE) work, the present experiment offers significant improvement in dynamic range and sensitivity, retains usable chemical shift information, and reports on molecular motions in the sub-megahertz (MHz) to tens of MHz range-motions which are not accessed at high fields.

1H Indirect Detected 13C Low-Abundance Single-Transition Correlation Spectroscopy (HICLASS)—13C Homonuclear Correlation at Natural Abundance

A novel proton-detected (13)C homonuclear correlation experiment is reported at natural abundance, viz., (1)H Indirect detected (13)C Low-Abundance Single-transition correlation Spectroscopy (HICLASS). HICLASS is based on the evolution of (13)C single-quantum single transitions, followed by their mixing, and (1)H detection subsequent to heteronuclear transfer. Reduced relaxation losses during the evolution time and partial selectivity in the (1)H multiplet structure result in enhanced sensitivity of HICLASS. The superior performance of HICLASS is demonstrated for (1)H-detected (13)C correlation work.

Volume localized shift selective 13C spectroscopy using pulsed rotating frame transfer sequences with windows (PRAWN)

Some novel techniques for volume localized, chemical shift selective 13C spectroscopy are described in this work. These techniques are based on rotating frame J cross polarization and are reported for both direct and indirect modes of 13C detection. The performance of two selective mixing sequences, viz., pulsed rotating frame transfer sequences with windows (PRAWN) and PRAWN‐π has been studied systematically with different liquid and gel phantoms. Two different front‐end modules are used for volume localization, viz., point resolved spectroscopy (PRESS) and localized distortionless enhancement by polarization transfer (LODEPT). It is shown experimentally that both the selective J cross polarization sequences can operate efficiently with very low radiofrequency duty cycle; further, they have considerable tolerance to Hartmann‐Hahn mismatch. A simple theoretical analysis is also presented to understand J cross‐polarization dynamics at low RF field amplitudes. Finally, the performance of LODEPT‐PRAWN‐π is demonstrated for the selective detection of saturated fat in pigeon egg in indirect detection mode. Magn Reson Med, 2011.

Transferred Overhauser DNP: A Fast, Efficient Approach for Room Temperature 13C ODNP at Moderately Low Fields and Natural Abundance

Overhauser dynamic nuclear polarization (ODNP) is investigated at a moderately low field (1.2 T) for natural abundance 13C NMR of small molecules in solution state at room temperature. It is shown that ODNP transferred from 1H to 13C by NMR coherence transfer is in general significantly more efficient than direct ODNP of 13C. Compared to direct 13C ODNP, we demonstrate over 4-fold higher 13C sensitivity (signal-to-noise ratio, SNR), achieved in one-eighth of the measurement time by transferred ODNP (t-ODNP). Compared to the 13C signal arising from Boltzmann equilibrium in a fixed measurement time, this is equivalent to about 1500-fold enhancement of 13C signal by t-ODNP, as against a direct 13C ODNP signal enhancement of about 45-fold, both at a moderate ESR saturation factor of about 0.25. This owes in part to the short polarization times characteristic of 1H. Typically, t-ODNP reflects the essentially uniform ODNP enhancements of all protons in a molecule. Although the purpose of this work is to establish the superiority of t-ODNP vis-à-vis direct 13C ODNP, a comparison is also made of the SNR in t-ODNP experiments with standard high resolution NMR as well. Finally, the potential of t-ODNP experiments for 2D heteronuclear correlation spectroscopy of small molecules is demonstrated in 2D 1H-13C HETCOR experiments at natural abundance, with decoupling in both dimensions.

Triple quantum filtered spectroscopy of homonuclear three spin-1/2 systems employing isotropic mixing ☆

We report the design and performance evaluation of novel pulse sequences for triple quantum filtered spectroscopy in homonuclear three spin-1/2 systems, employing isotropic mixing (IM) to excite triple quantum coherence (TQC). Our approach involves the generation of combination single quantum coherences (cSQC) from antisymmetric longitudinal or transverse magnetization components employing isotropic mixing (IM). cSQCs are then converted to TQC by a selective 180° pulse on one of the spins. As IM ideally causes magnetization to evolve under the influence of the spin coupling Hamiltonian alone, TQC is generated at a faster rate compared to sequences involving free precession. This is expected to be significant when the spins have large relaxation rates. Our approach is demonstrated experimentally by TQC filtered 1D spectroscopy on a (1)H AX2 system (propargyl bromide in the presence of a paramagnetic additive), as well as a (31)P linear AMX system (ATP in agar gel). The performance of the IM-based sequences for TQC excitation are compared against the standard three pulse sequence (Ernst et al., 1987) and an AX2 spin pattern recognition sequence (Levitt and Ernst, 1983). The latter reaches the unitary bound on TQC preparation efficiency starting from thermal equilibrium in AX2 systems, not considering relaxation. It is shown that in systems where spins relax rapidly, the new IM-based sequences indeed perform significantly better than the above two known TQC excitation sequences, the sensitivity enhancement being especially pronounced in the case of the proton system investigated. An overview of the differences in relaxation behavior is presented for the different approaches. Applications are envisaged to Overhauser DNP experiments and to in vivo NMR.

ADEQUATE CR: 13C connectivity mapping in indirect detection mode with composite refocusing

We report a novel rare spin correlation experiment termed ADEQUATE with composite refocusing (CR), which is the 1H‐detected version of 2D INADEQUATE CR. ADEQUATE CR begins with a polarization transfer from protons to the attached carbon, followed by 13C–13C double‐quantum (DQ) preparation. Unlike the ADEQUATE class of experiments, 13C DQ coherence is converted after evolution to single‐quantum single transitions (SQ‐STs) by CR. 13C SQ‐ST is then transferred back to the coupled protons by a coherence order selective reconversion. The present sequence produces partial transition selectivity in the 1H dimension as does 1H Indirect detected 13C Low‐Abundance Single‐transition correlation Spectroscopy (HICLASS), thereby mitigating the reduction in sensitivity enhancement because of the presence of homonuclear proton couplings. However, unlike HICLASS (which is an experiment that involves SQ‐TS evolution), no homonuclear zero quantum mixing is required on the 13C channel in the present experiment. Experimental results are demonstrated on a variety of samples, establishing the efficiency of the proposed method. Copyright