Reaction monitoring of hydrogen peroxide decomposition by NMR relaxometry

Earth's field NMR relaxation of pre-polarised water protons for real-time detection of free-radical formation

Real-time imaging of free-radical formation is important in physical chemistry, biochemistry, and radiobiology, especially for the study of radiation dose-rate effects. Herein, we show for the first time that the formation of free radicals during the time course of a chemical reaction can be imaged through NMR relaxation measurements of water protons in the Earth's magnetic field, in an open-coil spectrometer. The relaxation rate constants of water magnetisation are enhanced as reactions leading to the formation of hydroxyl radicals and oxygen proceed on the timescale of tens of minutes. The reaction rate of iodide-catalysed H2O2 decay was followed by Earth-field 1H NMR relaxation in real time. The relaxivities of the reaction product and several other paramagnetic compounds were determined. Spin-trap molecules were then used to capture ˙OH radical species, thus altering the reaction rate in proportion to the formation of new paramagnetic compounds. Thereby, a new experimental method for magnetic resonance imaging of the formation of intermediate and stable radical species in water is proposed.

Oxidation of Drugs during Drug Product Development: Problems and Solutions

Oxidation is the second most common degradation pathway for pharmaceuticals, after hydrolysis. However, in contrast to hydrolysis, oxidation is mechanistically more complex and produces a wider range of degradation products; oxidation is thus harder to control. The propensity of a drug towards oxidation is established during forced degradation studies. However, a more realistic insight into degradation in the solid state can be achieved with accelerated studies of mixtures of drugs and excipients, as the excipients are the most common sources of impurities that have the potential to initiate oxidation of a solid drug product. Based on the results of these studies, critical parameters can be identified and appropriate measures can be taken to avoid the problems that oxidation poses to the quality of a drug product. This article reviews the most common types of oxidation mechanisms, possible sources of reactive oxygen species, and how to minimize the oxidation of a solid drug product based on a well-planned accelerated study.

NMR characterization of H2O2 hydrogen exchange

Quantification of H2O2 concentration in aqueous solutions is of interest in many fields. It usually is based on indirect methods that rely on oxidation reactions that turn on/off fluorescent probes. Such methods can suffer from reaction incompleteness and interfering chemical species. We describe optimization of NMR detection that enables direct quantification of H2O2 down to the nanomolar range. Taking advantage of fast hydrogen exchange (HX) between H2O2 and water permits the use of very short interscan delays, greatly increasing sensitivity. The specific acid-, base- and water-catalyzed HX rates at 2 °C were measured to be 2.1 × 107, 6.1 × 109, and 1.4 × 10-1 M-1s-1, respectively, which result in a minimum HX rate at pH 6.2. Furthermore, the exchange is accelerated by general acid/base catalysis. MES and phosphate buffers catalyze HX strongest in their unprotonated forms. For imidazole, only the unprotonated form catalyzes HX, which contrasts with acetic acid where only the protonated state catalyzes exchange. Inorganic salts such as sodium chloride and azide have negligible effect on HX. We present optimal conditions for accurate measurement of H2O2 concentrations as low as 40 nM in aqueous samples in a few hours.

Diffusion measurements with continuous hydrogenation in PHIP

DOSY is a powerful spectroscopic NMR technique that resolves components in mixtures through the evaluation of different diffusion coefficients. The application of DOSY to dilute mixtures is hampered by the low signal to noise ratios (SNR), leading to long acquisition times. The use of PHIP may resolve this issue as long as reproducible signals are obtained in order to perform 2D experiments. Here we show that the use of hollow membranes and adequate gas flow produce constant polarization for a time-span that enables the acquisition of 2D experiments. A pressure gradient is evidenced by the presence of convection, which is accounted for by using a DPGSE sequence. The influence of J-coupling evolution during the sequence is studied both numerically and experimentally, to determine the optimum echo-time. The applicability of the method for samples with poor SNR is explored by setting the reaction rate to achieve a low intensity of polarized signals.

Magnetic resonance imaging of catalytically relevant processes

The main aim of this article is to provide a state-of-the-art review of the magnetic resonance imaging (MRI) utilization in heterogeneous catalysis. MRI is capable to provide very useful information about both living and nonliving objects in a noninvasive way. The studies of an internal heterogeneous reactor structure by MRI help to understand the mass transport and chemical processes inside the working catalytic reactor that can significantly improve its efficiency. However, one of the serious disadvantages of MRI is low sensitivity, and this obstacle dramatically limits possible MRI application. Fortunately, there are hyperpolarization methods that eliminate this problem. Parahydrogen-induced polarization approach, for instance, can increase the nuclear magnetic resonance signal intensity by four to five orders of magnitude; moreover, the obtained polarization can be stored in long-lived spin states and then transferred into an observable signal in MRI. An in-depth account of the studies on both thermal and hyperpolarized MRI for the investigation of heterogeneous catalytic processes is provided in this review as part of the special issue emphasizing the research performed to date in Russia/USSR.

Optimized phases for the acquisition of J-spectra in coupled spin systems for thermally and PHIP polarized molecules

We demonstrate that the relative phases in the refocusing pulses of multipulse sequences can compensate for pulse errors and off-resonant effects, which are commonly encountered in J-spectroscopy when CPMG is used for acquisition. The use of supercycles has been considered many times in the past, but always from the view point of time-domain NMR, that is, in an effort to lengthen the decay of the magnetization. Here we use simple spin-coupled systems, in which the quantum evolution of the system can be simulated and contrasted to experimental results. In order to explore fine details, we resort to partial J-spectroscopy, that is, to the acquisition of J-spectra of a defined multiplet, which is acquired with a suitable digital filter. We unambiguously show that when finite radiofrequency pulses are considered, the off-resonance effects on nearby multiplets affects the dynamics of the spins within the spectral window under acquisition. Moreover, the most robust phase cycling scheme for our setup consists of a 4-pulse cycle, with phases yyyy‾ or xxxx‾ for an excitation pulse with phase x. We show simulated and experimental results in both thermally polarized and PHIP hyperpolarized systems.

Flow reactors integrated with in-line monitoring using benchtop NMR spectroscopy

The state-of-the-art flow reactors integrated with in-line benchtop NMR are thoroughly discussed with highlights on the strengths and weaknesses of this emerging technology.

Detection and Quantification of Hydrogen Peroxide in Aqueous Solutions Using Chemical Exchange Saturation Transfer

The development of new analytical methods to accurately quantify hydrogen peroxide is of great interest. In the current study, we developed a new magnetic resonance (MR) method for noninvasively quantifying hydrogen peroxide (H2O2) in aqueous solutions based on chemical exchange saturation transfer (CEST), an emerging MRI contrast mechanism. Our method can detect H2O2 by its specific CEST signal at ∼6.2 ppm downfield from water resonance, with more than 1000 times signal amplification compared to the direct NMR detection. To improve the accuracy of quantification, we comprehensively investigated the effects of sample properties on CEST detection, including pH, temperature, and relaxation times. To accelerate the NMR measurement, we implemented an ultrafast Z-spectroscopic (UFZ) CEST method to boost the acquisition speed to 2 s per CEST spectrum. To accurately quantify H2O2 in unknown samples, we also implemented a standard addition method, which eliminated the need for predetermined calibration curves. Our results clearly demonstrate that the presented CEST-based technique is a simple, noninvasive, quick, and accurate method for quantifying H2O2 in aqueous solutions.

Imaging short-lived reactive oxygen species (ROS) with endogenous contrast MRI

PURPOSE

To characterize the relaxation properties of reactive oxygen species (ROS) for the development of endogenous ROS contrast magnetic resonance imaging (MRI).

MATERIALS AND METHODS

ROS-producing phantoms and animal models were imaged at 9.4T MRI to obtain T₁ and T₂ maps. Egg white samples treated with varied concentrations of hydrogen peroxide (H₂ O₂ ) were used to evaluate the effect of produced ROS in T₁ and T₂ for up to 4 hours. pH and temperature changes due to H₂ O₂ treatment in egg white were also monitored. The influences from H₂ O₂ itself and oxygen were evaluated in bovine serum albumin (BSA) solution producing no ROS. In addition, dynamic temporal changes of T₁ in H₂ O₂ -treated egg white samples were used to estimate ROS concentration over time and hence the detection sensitivity of relaxation-based endogenous ROS MRI. The relaxivity of ROS was compared with that of Gd-DTPA as a reference. Finally, the feasibility of in vivo ROS MRI with T₁ mapping acquired using an inversion recovery sequence was demonstrated with a well-established rotenone-treated mouse model (n = 6).

RESULTS

pH and temperature changes in treated egg white samples were insignificant (<0.1 unit and <1°C, respectively). T₁ relaxation time in the H₂ O₂ -treated egg white was reduced significantly (P < 0.05), while there was only small reduction in T₂ (<10%). In the H₂ O₂ -treated BSA solution that produce no ROS, there was a small change in T₁ due to H₂ O₂ itself (±1%), although a significant T₂ -shortening effect was observed (>10%, P < 0.05). Also, there was a small reduction in T₁ (13 ± 1%) and T₂ (1 ± 2%) from molecular oxygen. The detection sensitivity of ROS MRI was estimated around 10 pM. The T₁ relaxivity of ROS was found to be much higher than that of Gd-DTPA (3.4 × 10⁷ vs. 0.9 s^-1 ·mM^-1 ). Finally, significantly reduced T₁ was observed in rotenone-treated mouse brain (5.1 ± 2.5%, P < 0.05).

CONCLUSION

We demonstrated in the study that endogenous ROS MRI based on the paramagnetic effect has sensitivity for in vitro and in vivo applications.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2018;47:222-229.

A novel NMR method for the determination and monitoring of evolution of hydrogen peroxide in aqueous solutions

A novel NMR method that allowed the rapid and direct quantitative analysis of hydrogen peroxide in protic solvents was developed. The method was based on the highly deshielded (1)H NMR signal of the H2O2 protons (δ ∼ 11.15 ppm at 298 K) in H2O and the combined use of cryoprotective (antifreeze) mixtures of H2O-DMSO-d6, low temperatures (∼260 K), and pH effects in order to achieve minimum proton exchange rate and, thus, sharp (1)H line widths. Extremely broad resonances with line widths above 550 Hz at room temperature in H2O were observed in a wide range of pH values, which were reduced below 2 Hz with the use of the above method which resulted in a detection limit of 20.0 μmol L(-1) (in tube) even when using very short total experimental time of 10 min. The method was applied in aqueous extract of Greek oregano and in aqueous instant coffee. Line widths below 10 Hz for oregano samples and 17 Hz for instant coffee samples were obtained which resulted (i) in the unequivocal assignment of H2O2 with spiking experiments precluding any confusion with interferences from intrinsic phenolics in the extracts and (ii) in the quantitative investigation of the evolution of H2O2 in real time with parameters easily accessible experimentally.

Novel 1H low field nuclear magnetic resonance applications for the field of biodiesel

BACKGROUND

Biodiesel production has increased dramatically over the last decade, raising the need for new rapid and non-destructive analytical tools and technologies. 1H Low Field Nuclear Magnetic Resonance (LF-NMR) applications, which offer great potential to the field of biodiesel, have been developed by the Phyto Lipid Biotechnology Lab research team in the last few years.

RESULTS

Supervised and un-supervised chemometric tools are suggested for screening new alternative biodiesel feedstocks according to oil content and viscosity. The tools allowed assignment into viscosity groups of biodiesel-petrodiesel samples whose viscosity is unknown, and uncovered biodiesel samples that have residues of unreacted acylglycerol and/or methanol, and poorly separated and cleaned glycerol and water. In the case of composite materials, relaxation time distribution, and cross-correlation methods were successfully applied to differentiate components. Continuous distributed methods were also applied to calculate the yield of the transesterification reaction, and thus monitor the progress of the common and in-situ transesterification reactions, offering a tool for optimization of reaction parameters.

CONCLUSIONS

Comprehensive applied tools are detailed for the characterization of new alternative biodiesel resources in their whole conformation, monitoring of the biodiesel transesterification reaction, and quality evaluation of the final product, using a non-invasive and non-destructive technology that is new to the biodiesel research area. A new integrated computational-experimental approach for analysis of 1H LF-NMR relaxometry data is also presented, suggesting improved solution stability and peak resolution.

Monitoring mass transport in heterogeneously catalyzed reactions by field-gradient NMR for assessing reaction efficiency in a single pellet

An important aspect in assessing the performance of a catalytically active reactor is the accessibility of the reactive sites inside the individual pellets, and the mass transfer of reactants and products to and from these sites. Optimal design often requires a suitable combination of micro- and macropores in order to facilitate mass transport inside the pellet. In an exothermic reaction, fluid exchange between the pellet and the surrounding medium is enhanced by convection, and often by the occurrence of gas bubbles. Determining mass flow in the vicinity of a pellet thus represents a parameter for quantifying the reaction efficiency and its dependence on time or external reaction conditions. Field gradient Nuclear Magnetic Resonance (NMR) methods are suggested as a tool for providing parameters sensitive to this mass flow in a contact-free and non-invasive way. For the example of bubble-forming hydrogen peroxide decomposition in an alumina pellet, the dependence of the mean-squared displacement of fluid molecules on spatial direction, observation time and reaction time is presented, and multi-pulse techniques are employed in order to separate molecular displacements from coherent and incoherent motion on the timescale of the experiment. The reaction progress is followed until the complete decomposition of H2O2.