Nuclear Quadrupole Resonance spectroscopy in studies of biologically active molecular systems—a review

Effect of Dynamical Motion in ab Initio Calculations of Solid-State Nuclear Magnetic and Nuclear Quadrupole Resonance Spectra

Solid-state nuclear magnetic resonance (SSNMR) and nuclear quadrupole resonance (NQR) spectra provide detailed information about the electronic and atomic structure of solids. Modern ab initio methods such as density functional theory (DFT) can be used to calculate NMR and NQR spectra from first-principles, providing a meaningful avenue to connect theory and experiment. Prediction of SSNMR and NQR spectra from DFT relies on accurate calculation of the electric field gradient (EFG) tensor associated with the potential of electrons at the nuclear centers. While static calculations of EFGs are commonly seen in the literature, the effects of dynamical motion of atoms in molecules and solids have been less explored. In this study, we develop a method to calculate EFGs of solids while taking into account the dynamics of atoms through DFT-based molecular dynamics simulations. The method we develop is general, in the sense that it can be applied to any material at any desired temperature and pressure. Here, we focus on application of the method to NaNO2 and study in detail the EFGs of 14N, 17O, and 23Na. We find in the cases of 14N and 17O that the dynamical motion of the atoms can be used to calculate mean EFGs that are in closer agreement with experiments than those of static calculations. For 23Na, we find a complex behavior of the EFGs when atomic motion is incorporated that is not at all captured in static calculations. In particular, we find a distribution of EFGs that is influenced strongly by the local (changing) bond environment, with a pattern that reflects the coordination structure of 23Na. We expect the methodology developed here to provide a path forward for understanding materials in which static EFG calculations do not align with experiments.

Non-invasive authentication of mail packages using nuclear quadrupole resonance spectroscopy

The international postal network is one of the most widely used methods for correspondence throughout the world. Most postal traffic across the globe consists of legitimate interpersonal, business-consumer, and business-business communications. However, the global postal system is also utilized for criminal activity. In particular, it is often utilized to ship and distribute contraband, including illegal psychoactive drugs such as fentanyl and heroin, to consumers. Existing technological solutions are capable of identifying synthetic opioids and other illegal drugs within packages, but are accompanied by several disadvantages that make them unsuitable for large-scale authentication of international mail traffic. This paper presents a novel method for non-invasive authentication of mail packages that overcomes these challenges. The approach uses nuclear quadrupole resonance (NQR) spectroscopy to detect and quantify the presence of known active pharmaceutical ingredients (APIs) within the package. It has been experimentally demonstrated using a bench top prototype. Test results from a variety of package types demonstrate the effectiveness of the proposed authentication approach.

Structural analysis of dexrazoxane: Exploring tautomeric conformations

Structural analysis of dexrazoxane, as a cardioprotective agent, was done in this work by exploring formations of tautomeric conformations and investigating the corresponding effects. Density functional theory (DFT) calculations were performed to optimize the structures to evaluate their molecular and atomic descriptors. In addition to the original structure of dexrazoxane, eight tautomers were obtained with lower stability than the original compound. Movements of two hydrogen atoms in between nitrogen and oxygen atoms of heterocyclic ring put such significant effects. Moreover, electronic molecular orbital features showed effects of such tautomerism processes on distribution patterns and surfaces, in which evaluating the quadrupole coupling constants helped to show the role of atomic sites for resulting the features. As a consequence, the results indicated that the tautomeric formations could significantly change the features of dexrazoxane reminding the importance of carful medication of this drug for patients.

Density functional theory-based electric field gradient database

The deviation of the electron density around the nuclei from spherical symmetry determines the electric field gradient (EFG), which can be measured by various types of spectroscopy. Nuclear Quadrupole Resonance (NQR) is particularly sensitive to the EFG. The EFGs, and by implication NQR frequencies, vary dramatically across materials. Consequently, searching for NQR spectral lines in previously uninvestigated materials represents a major challenge. Calculated EFGs can significantly aid at the search's inception. To facilitate this task, we have applied high-throughput density functional theory calculations to predict EFGs for 15187 materials in the JARVIS-DFT database. This database, which will include EFG as a standard entry, is continuously increasing. Given the large scope of the database, it is impractical to verify each calculation. However, we assess accuracy by singling out cases for which reliable experimental information is readily available and compare them to the calculations. We further present a statistical analysis of the results. The database and tools associated with our work are made publicly available by JARVIS-DFT ( https://www.ctcms.nist.gov/~knc6/JVASP.html ) and NIST-JARVIS API ( http://jarvis.nist.gov/ ).

The Predictive Power of Different Projector-Augmented Wave Potentials for Nuclear Quadrupole Resonance

The projector-augmented wave (PAW) method is used to calculate electric field gradients (EFG) for various PAW potentials. A variety of crystals containing reactive nonmetal, simple metal, and transition elements, are evaluated in order to determine the predictive ability of the PAW method for the determination of nuclear quadrupole resonance frequencies in previously unstudied materials and their polymorphs. All results were compared to experimental results and, where possible, to previous density functional theory (DFT) calculations. The EFG at the 14N site of NaNO2 is calculated by DFT for the first time. The reactive nonmetal elements were not very sensitive to the variation in PAW potentials, and calculations were quite close to experimental values. For the other elements, the various PAW potentials led to a clear spread in EFG values, with no one universal potential emerging. Within the spread, there was agreement with other ab initio models.

35 Cl NQR frequency and spin lattice relaxation study in 3,4-dichloronitrobenzene as a function of temperature and pressure

³⁵ Cl NQR frequency and spin lattice relaxation time in 3,4-dichloronitrobenzene have been measured as a function of temperature and pressure. Two NQR signals were observed in the temperature range 77 to 300 K and pressure up to 5.1 kbar at 300 K. The contributions to the relaxation from the torsional motion of the molecule and reorientational motion of the nitro group have been analyzed on the basis of the Woessner and Gutowsky model. The temperature dependence of the average torsional lifetimes of the molecules, transition probabilities, and the activation energy for the reorientation of the nitro group was estimated. The pressure dependence of the NQR frequency in 3,4-Dichloronitrobenzene shows a nonlinear increase in NQR frequency with increase in pressure, indicating increased contribution from the static effects at higher pressures. A thermodynamic analysis of the data was carried out to determine the constant-volume temperature coefficients of the NQR frequency. The spin-lattice relaxation was found to be weakly dependent on pressure.

Repetitive experiments of one or two-pulse sequences in NQR of spins I=3/2: Liouville space, steady-state, Ernst angle and optimum signal

In NMR, the repetition of pulse sequences with a recycle time that does not allow the spin system to completely relax back to equilibrium is a well known and often used method to increase the signal to noise ratio at given total measuring time. For isolated spins I=1/2, the steady-state of a train of strictly identical pulse sequences separated by free evolution periods of same duration is described by the well known Ernst-Anderson model, and the optimum pulse angle is given by the Ernst angle. We showed recently that equivalent formula, but with super-operators in the Liouville space, can be obtained for general spins I. In this article, this formalism is generalized to pure NQR of spins I=3/2, and applied to calculate the signal resulting from single and solid-echo sequences, in the limit when the recycle time T>5T_2q, where T_2q is the transverse (coherence) quadrupolar relaxation time. In particular, we show that powder samples have a behaviour that is very close to NMR of spins I=1/2. For instance, the generalized Ernst angle β_M that maximizes the signal amplitude for a single pulse train is well described by the simple formula cos(1.52β_M)≈exp(-T/T_1q), whatever the quadrupolar asymmetry parameter η, T_1q being the longitudinal (population) quadrupolar relaxation time. Moreover, a simplified NMR-like formula that describes the overall behaviour of nutation curves is proposed, and it is shown that the signal to noise ratio (SNR) at given experimental time is exactly the same as in NMR of spins I=1/2 as a function of recycle time, when properly normalized. Some theoretical predictions for the single pulse and solid-echo sequence were compared to experiments, and validated, by performing ³⁵Cl pure NQR experiment on chloranil (C₆Cl₄O₂ tetrachloro-1,4-benzoquinone) powder.

Authentication of Medicines Using Nuclear Quadrupole Resonance Spectroscopy

The production and sale of counterfeit and substandard pharmaceutical products, such as essential medicines, is an important global public health problem. We describe a chemometric passport-based approach to improve the security of the pharmaceutical supply chain. Our method is based on applying nuclear quadrupole resonance (NQR) spectroscopy to authenticate the contents of medicine packets. NQR is a non-invasive, non-destructive, and quantitative radio frequency (RF) spectroscopic technique. It is sensitive to subtle features of the solid-state chemical environment and thus generates unique chemical fingerprints that are intrinsically difficult to replicate. We describe several advanced NQR techniques, including two-dimensional measurements, polarization enhancement, and spin density imaging, that further improve the security of our authentication approach. We also present experimental results that confirm the specificity and sensitivity of NQR and its ability to detect counterfeit medicines.

Nitrogen-14 nuclear quadrupole resonance spectroscopy: a promising analytical methodology for medicines authentication and counterfeit antimalarial analysis

We report the detection and analysis of a suspected counterfeit sample of the antimalarial medicine Metakelfin through developing nitrogen-14 nuclear quadrupole resonance ((14)N NQR) spectroscopy at a quantitative level. The sensitivity of quadrupolar parameters to the solid-state chemical environment of the molecule enables development of a technique capable of discrimination between the same pharmaceutical preparations made by different manufacturers. The (14)N NQR signal returned by a tablet (or tablets) from a Metakelfin batch suspected to be counterfeit was compared with that acquired from a tablet(s) from a known-to-be-genuine batch from the same named manufacturer. Metakelfin contains two active pharmaceutical ingredients, sulfalene and pyrimethamine, and NQR analysis revealed spectral differences for the sulfalene component indicative of differences in the processing history of the two batches. Furthermore, the NQR analysis provided quantitative information that the suspected counterfeit tablets contained only 43 ± 3%, as much sulfalene as the genuine Metakelfin tablets. Conversely, conventional nondestructive analysis by Fourier transform (FT)-Raman and FT-near infrared (NIR) spectroscopies only achieved differentiation between batches but no ascription. High performance liquid chromatography (HPLC)-UV analysis of the suspect tablets revealed a sulfalene content of 42 ± 2% of the labeled claim. The degree of agreement shows the promise of NQR as a means of the nondestructive identification and content-indicating first-stage analysis of counterfeit pharmaceuticals.

A THEORETICAL INVESTIGATION OF SUBSTITUENT EFFECTS ON STRUCTURE AND NQR PARAMETERS OF PHENACYL PHENYL SULFIDE DERIVATIVES

This paper describes a theoretical investigation of the geometrical parameters and the 17 O and 33 S nuclear quadrupole resonance (NQR), parameters of phenacyl phenyl sulfide derivatives. The calculations were carried out using Gaussian 98 package by applying the Hartree–Fock method (HF), the second-order Møller–Plesset perturbation theory (MP2), and the density functional theory (DFT) and by employing the 3-21G, 6-31G, and 6-311G basis sets in the Townes–Daily approximation. It was shown that all of the methods can be used to predict the NQR parameters; however, the DFT is the best method of all the methods in use. The nuclear quadrupole coupling constants, CQ, and the asymmetry parameter, η, were calculated for all of the nuclei in each molecule, separately. The results indicated that the value of CQ and η for 17 O nucleus increases in the order of H > Br > NH 2 > CH 3 and NH 2 > H > Br > CH 3, respectively, as well as, for the 33 S nucleus the increase in the order of substituents was NH 2 > H > Br CH 3 and CH 3 > H > Br > NH 2, respectively.

The application of frequency swept pulses for the acquisition of nuclear quadrupole resonance spectra

The acquisition of nuclear quadrupole resonance (NQR) spectra with wideband uniform rate and smooth truncation (WURST) pulses is investigated. (75)As and (35)Cl NQR spectra acquired with the WURST echo sequence are compared to those acquired with standard Hahn-echo sequences and echo sequences which employ composite refocusing pulses. The utility of WURST pulses for locating NQR resonances of unknown frequency is investigated by monitoring the integrated intensity and signal to noise of (35)Cl and (75)As NQR spectra acquired with transmitter offsets of several hundreds kilohertz from the resonance frequencies. The WURST echo sequence is demonstrated to possess superior excitation bandwidths in comparison to the pulse sequences which employ conventional monochromatic rectangular pulses. The superior excitation bandwidths of the WURST pulses allows for differences in the characteristic impedance of the receiving and excitation circuits of the spectrometer to be detected. Impedance mismatches have previously been reported by Marion and Desvaux [D.J.Y. Marion, H. Desvaux, J. Magn. Reson. (2008) 193(1) 153-157] and Muller et al. [M. Nausner, J. Schlagnitweit, V. Smrecki, X. Yang, A. Jerschow, N. Muller, J. Magn. Reson. (2009) 198(1) 73-79]. In this regard, WURST pulse sequences may afford an efficient new method for experimentally detecting impedance mismatches between receiving and excitation circuits, allowing for the optimization of solids and solution NMR and NQR spectrometer systems. The use of the Carr-Purcell Meiboom-Gill (CPMG) pulse sequence for signal enhancement of NQR spectra acquired with WURST pulses and conventional pulses is also investigated. Finally, the utility of WURST pulses for the acquisition of wideline NQR spectra is demonstrated by acquiring part of the (63/65)Cu NQR spectrum of CuCN.

Computational modeling of biologically active molecules using NMR spectra

The molecular structure and NMR chemical shift information of a compound can be combined to form powerful models of biological activity. NMR spectral data and structure information can be combined on a structural template analogous to 3D-QSAR methodology or orientation independently in spectral space. Surprisingly, quantitative spectrometric data-activity relationship (QSDAR) models built on structure templates are inferior to multi-dimensional QSDAR models built in spectral space. 3D-QSDAR modeling could be useful for estimating chemical toxicity, risk assessment of environmental contaminants and drug lead-compound identifications.