Ultrafast Laplace NMR

Abstract This thesis focuses on the developement of novel ultrafast Laplace NMR (UF LNMR) methods. LNMR covers relaxation and diffusion measurements, which provide detailed information about dynamics of molecules. Ultrafast LNMR is based on spatial encoding of multidimensional data, which has been earlier been exploited in ultrafast NMR spectroscopy. The method makes it possible to collect multidimensional data in a single scan, shortening experiment time by one to four orders of magnitude. Furthermore, single-scan approach enables use of modern hyperpolarization methods, such as dynamic nuclear polarization (DNP), parahydrogen induced polarization (PHIP) and spin-exchange optical pumping (SEOP), to boost the sensitivity of the experiments by orders of magnitude. Therefore, the method provides means to study fast molecular processes in real-time, with high sensitivity. In the first part of the thesis work we introduce a novel single scan, spin echo based diffusion experiment (UF PGSE), and compare it to already established single scan, stimulated echo based method (UF PGSTE). We show that the UF PGSE method removes artefacts, which appear in UF PGSTE data. We represent also a thorough theoretical analysis which justifies the feasibility of the method. The analysis reveals also that a conventional exponential fit results in a small overestimation of diffusion coefficient. The second part comprises two scientific articles dealing with a novel two-dimensional D − T₂correlation experiment. We demonstrate the feasibility of the method in chemical analysis and in the investigation of porous materials. In addition we prove that the single-scan approach really makes it possible to exploit nuclear spin hyperpolarization using three different techniques: PHIP, dissolution DNP and SEOP. We show that, with hyperpolarization, single-scan experiments became feasible even with low sensitivity heteronuclei. In the last part, we introduce an ultrafast exchange experiment. It is based on diffusion contrast and called ultrafast diffusion exchange spectroscopy (UF DEXSY). In traditional DEXSY experiment, data of both indirect and direct dimension are collected point-by-point in repeated experiment, while in UF DEXSY whole data is measured in a single scan. This leads to significant, up to four orders of magnitude, reduction of experiment time. Because UF DEXSY provides opportunity to boost the sensitivity of the experiment by orders of magnitude by hyperpolarization, it offers unprecedented opportunities for efficient and high sensitivity analysis of important molecular exchange processes such as cellular metabolism, catalysis and chemical reactions

Determination of Phenolic Hydroxyl Groups in Technical Lignins by Ionization Difference Ultraviolet Spectrophotometry (∆ε-IDUS method)

The amount of hydroxyl groups, particularly phenolic, is one of the most important parameters in lignins, as it is an indicator of lignin reactivity. Ultraviolet (UV) Spectrophotometry is a simple and inexpensive method for determining phenolic hydroxyls in lignin. Ionization Difference Ultraviolet Spectrophotometry (Δε-method) relies on the analysis of solubilized lignin at neutral and alkaline conditions with a UV spectrophotometer. We added a slope analysis to the ∆ε-method and dubbed the resulting method ∆ε-IDUS (Ionization Difference UV Spectrophotometry). We assessed the reliability of ∆ε-IDUS by studying the well-known Indulin AT lignin. Additionally, ∆ε-IDUS was applied to a previously uncharacterized milox lignin. When compared to 13C-NMR, ∆ε-IDUS underestimated the amount of phenolic hydroxyls for Indulin AT, possibly due to neglecting second phenolic hydroxyls in some lignin units, which resist ionization because of steric hindrance. Nevertheless, the results agreed with previously reported values and confirm that ∆ε-IDUS is useful to screen lignins based on their phenolic hydroxyl group content

Ultrafast NMR diffusion measurements exploiting chirp spin echoes

Abstract Standard diffusion NMR measurements require the repetition of the experiment multiple times with varying gradient strength or diffusion delay. This makes the experiment time-consuming and restricts the use of hyperpolarized substances to boost sensitivity. We propose a novel single-scan diffusion experiment, which is based on spatial encoding of two-dimensional data, employing the spin-echoes created by two successive adiabatic frequency-swept chirp π pulses. The experiment is called ultrafast pulsed-field-gradient spin-echo (UF-PGSE). We present a rigorous derivation of the echo amplitude in the UF-PGSE experiment, justifying the theoretical basis of the method. The theory reveals also that the standard analysis of experimental data leads to a diffusion coefficient value overestimated by a few per cent. Although the overestimation is of the order of experimental error and thus insignificant in many practical applications, we propose that it can be compensated by a bipolar gradient version of the experiment, UF-BP-PGSE, or by corresponding stimulated-echo experiment, UF-BP-pulsed-field-gradient stimulated-echo. The latter also removes the effect of uniform background gradients. The experiments offer significant prospects for monitoring fast processes in real time as well as for increasing the sensitivity of experiments by several orders of magnitude by nuclear spin hyperpolarization. Furthermore, they can be applied as basic blocks in various ultrafast multidimensional Laplace NMR experiments

Accelerating restricted diffusion NMR studies with time-resolved and ultrafast methods

Abstract Restricted diffusion of fluids in porous materials can be studied by pulsed field gradient nuclear magnetic resonance (NMR) non-invasively and without tracers. If the experiment is repeated many times with varying diffusion delays, detailed information about pore sizes and tortuosity can be recorded. However, the measurements are very time-consuming because numerous repetitions are needed for gradient ramping and varying diffusion delays. In this paper, we demonstrate two different strategies for acceleration of the restricted diffusion NMR measurements: time-resolved diffusion NMR and ultrafast Laplace NMR. The former is based on time-resolved non-uniform sampling, while the latter relies on spatial encoding of two-dimensional data. Both techniques allow similar 1–2 order of magnitude acceleration of acquisition, but they have different strengths and weaknesses, which we discuss in detail. The feasibility of the methods was proven by investigating restricted diffusion of water inside tracheid cells of thermally modified pine wood

Ultrafast Laplace NMR with hyperpolarized xenon gas

Abstract Laplace NMR, consisting of diffusion and relaxation experiments, provides detailed information about dynamics of fluids in porous materials. Recently, we showed that two-dimensional Laplace NMR experiments can be carried out with a single scan based on spatial encoding. The method shortens the experiment time by one to three orders of magnitude, and therefore it is called ultrafast Laplace NMR. Furthermore, the single-scan approach facilitates significantly the use of nuclear spin hyperpolarization for boosting the sensitivity of the experiment, because a laborious hyperpolarization procedure does not need to be repeated. Here, we push the limits of the ultrafast Laplace NMR method by applying it, for the first time, in the investigation of a gas phase substance, namely hyperpolarized xenon gas. We show that, regardless of the fast diffusion of gas, layer-like spatial encoding is feasible, and an ultrafast diffusion — T2 relaxation correlation experiment reveals significantly different signals of free gas and gas adsorbed in a mesoporous controlled pore glass (CPG). The observed diffusion coefficients are many orders of magnitude larger than those detected earlier from liquid phase substances, emphasizing the extended application range of the method. The challenges in the methodology, caused by the fast diffusion, are also discussed

Ultrafast transverse relaxation exchange NMR spectroscopy

Abstract Molecular exchange between different physical or chemical environments occurs due to either diffusion or chemical transformation. Nuclear magnetic resonance (NMR) spectroscopy provides a means of understanding the molecular exchange in a noninvasive way and without tracers. Here, we introduce a novel two dimensional, single-scan ultrafast Laplace NMR (UF LNMR) method to monitor molecular exchange using transverse relaxation as a contrast. The UF T₂–T₂ relaxation exchange spectroscopy (REXSY) method shortens the experiment time by one to two orders of magnitude compared to its conventional counterpart. Contrary to the conventional EXSY, the exchanging sites are distinguished based on T₂ relaxation times instead of chemical shifts, making the method especially useful for systems including physical exchange of molecules. Therefore, the UF REXSY method offers an efficient means for quantification of exchange processes in various fields such as cellular metabolism and ion transport in electrolytes. As a proof of principle, we studied a halogen-free orthoborate based ionic liquid system and followed molecular exchange between molecular aggregates and free molecules. The results are in good agreement with the conventional exchange studies. Due to the single-scan nature, the method potentially significantly facilitates the use of modern hyperpolarization techniques to boost the sensitivity by several orders of magnitude

High-purity lignin fractions and nanospheres rich in phenolic hydroxyl and carboxyl groups isolated with alkaline deep eutectic solvent from wheat straw

Abstract A combined pretreatment based on alkaline deep eutectic solvent (DES) of K₂CO₃ and glycerol and sequential acid fractionation was developed to extract reactive lignin from wheat straw biomass. This process exhibited excellent purification performance in lignin isolation, and the lignin fractionated at low pH displayed high reactivity, having hydroxyl and carboxyl groups up to 9.60 and 2.52 mmol/g, respectively. Silica was selectively separated and removed during the precipitation stage, avoiding the “silica interference”. Moreover, DES-lignin nanospheres created by self-assembly using lignin fractions obtained by acid precipitation possessed a high zeta potential, large particle size and high content of hydrophilic groups. Overall, the findings related to the dissociation mechanism and fractionation of reactive lignin during alkaline DES pretreatment and the acid sequence precipitation are crucial for facilitating lignin valorization in high-added value products