Quantum chemistry-based NMR spin Hamiltonian parameters of GABA for quantitation in magnetic resonance spectroscopy

Theoretical and experimental NMR studies on muscimol from fly agaric mushroom (Amanita muscaria)

In this article we report results of combined theoretical and experimental NMR studies on muscimol, the bioactive alkaloid from fly agaric mushroom (Amanita muscaria). The assignment of (1)H and (13)C NMR spectra of muscimol in DMSO-d6 was supported by additional two-dimensional heteronuclear correlated spectra (2D NMR) and gauge independent atomic orbital (GIAO) NMR calculations using density functional theory (DFT). The effect of solvent in theoretical calculations was included via polarized continuum model (PCM) and the hybrid three-parameter B3LYP density functional in combination with 6-311++G(3df,2pd) basis set enabled calculation of reliable structures of non-ionized (neutral) molecule and its NH and zwitterionic forms in the gas phase, chloroform, DMSO and water. GIAO NMR calculations, using equilibrium and rovibrationally averaged geometry, at B3LYP/6-31G* and B3LYP/aug-cc-pVTZ-J levels of theory provided muscimol nuclear magnetic shieldings. The theoretical proton and carbon chemical shifts were critically compared with experimental NMR spectra measured in DMSO. Our results provide useful information on its structure in solution. We believe that such data could improve the understanding of basic features of muscimol at atomistic level and provide another tool in studies related to GABA analogs.

SENSI: signal enhancement by spectral integration for the analysis of metabolic mixtures

The method provided here can overcome the low S/N problem in ¹³C NMR by the integration of plural spectra to increase the resolution based on non-bucketing analysis without measurements.

J-difference editing of gamma-aminobutyric acid (GABA): simulated and experimental multiplet patterns

PURPOSE

To investigate factors that influence the multiplet pattern observed in J-difference editing of gamma-aminobutyric acid (GABA).

METHODS

Density matrix simulations were applied to investigate the shape of the 3 ppm GABA multiplet as a function of the editing sequence's slice-selective refocusing pulse properties, in particular bandwidth, transition width, and flip angle. For comparison to the calculations, experimental measurements were also made at 3 T on a 10 mM GABA solution using the MEGA-PRESS sequence at various refocusing pulse flip angles.

RESULTS

Good agreement was found between experiments and simulations. The edited multiplet consists of two outer lines of slightly unequal intensity due to strong coupling, and a smaller central line, the result of the unequal J-couplings between the C4 and C3 protons. The size of the center peak increases with increasing slice-selective refocusing pulse transition width, and deviation of the flip angle from 180°.

CONCLUSION

The 3 ppm GABA multiplet pattern observed in the MEGA-PRESS experiment depends quite strongly on the properties of the slice-selective refocusing pulses used. Under some circumstance, the central peak can be quite large; this does not necessarily indicate inefficient editing, or a subtraction artifact, but should be recognized as a property of the pulse sequence itself.

Vibrational circular dichroism of Africanane and lippifoliane sesquiterpenes from Lippia integrifolia

The agreement between theoretical and experimental vibrational circular dichroism curves of (4R,9R,10R)-(+)-african-1(5)-ene-2,6-dione (1) and (4R,9S,10R)-lippifoli-1(6)-en-5-one (2), isolated from the widely used plant Lippia integrifolia, allowed the determination of the conformation and absolute configuration of 1 and confirmed both structural features for 2. Molecular modeling of 1 and 2 was carried out by means of a systematic and a Monte Carlo search protocol followed by geometry optimization employing density functional theory calculations with the B3LYP/6-31G(d), B3LYP/DGDZVP, and/or B3PW91/DGDZVP2 functionals/basis sets. Validation of the minimum energy conformations for both tricyclic substances was achieved by comparison of the experimental and theoretical vicinal (1)H-(1)H NMR coupling constants obtained by DFT-GIAO calculations.