Chemical Exchange Effects Research Articles

Ferric nitrate-induced phase transformation of Bi2O3 with ion exchange property for enhanced chloride removal from high-salinity wastewater

The bismuth oxide (Bi2O3)-based method for removing chloride (Cl−) ions has garnered considerable interest because of its superior selectivity and efficiency in removing Cl−. However, the slow release of Bi3+ poses a challenge to the effectiveness of Bi2O3. In this study, a novel chloride removal agent composed mainly of Fe3+-doped Bi5O7NO3 (BF-x) was synthesized through the phase transformation of Bi2O3 by ferric nitrate. By optimizing the Fe/Bi mass ratio to 3 %: 1 (BF-3), a remarkable Cl− removal efficiency of 86.4 % was achieved, representing a substantial improvement compared to the 31.2 % removal efficiency observed for pure Bi2O3. The investigation of the mechanism and formation energy calculation revealed the inherent instability of Bi5O7NO3 in comparison to that of Bi2O3, which facilitated the release of Bi3+ and enabled the direct formation of BiOCl precipitate through ion exchange of NO3– with Cl−. The synergistic effect of chemical precipitation and ion exchange contributes to the excellent Cl− removal performance of BF-3. The final Cl− removal product, comprised of BiOCl and FeOOH, exhibited significant photocatalytic efficacy in the decomposition of organic contaminants. This study proposes a novel strategy for designing and preparing high-efficiency Cl− removal agents.

Essential tools of linear algebra for calculating nuclear spin dynamics of chemically exchanging systems

In this work, we describe essential tools of linear algebra necessary for calculating the effect of chemical exchange on spin dynamics and polarization transfer in various nuclear magnetic resonance (NMR) experiments. We show how to construct matrix representations of Hamiltonian, relaxation, and chemical exchange superoperators in both Hilbert and Liouville space, as well as demonstrate corresponding codes in Python. Examples of applying the code are given for problems involving chemical exchange between NH3 and NH4+ at zero and high magnetic field and polarization transfer from parahydrogen relevant in SABRE (signal amplification by reversible exchange) at low magnetic field (0-20mT). The presented methodology finds utility for describing the effect of chemical exchange on NMR spectra and can be extended further by taking into account non-linearities in the master equation.

Detecting water-protein chemical exchange in membrane-bound proteins/peptides by solid-state NMR spectroscopy

Water plays an important role in many essential biological processes of membrane proteins in hydrated lipid environments. In general, the 1H polarization transfers between water molecules and site-specific protons in proteins can be classified as coherent (via dipolar spin diffusion) and incoherent (via chemical exchange and nuclear Overhauser effect) transfers. Solid-state NMR is the technique of choice for studying such water-protein interactions in membrane-bound proteins/peptides through the detection of 1H polarization transfers from water to the proteins. These polarization transfer mechanisms often exist simultaneously and are difficult to quantify individually. Here, we review water-protein polarization transfer techniques in solid-state NMR, with a focus on the recent progress for the direct detection of site-specific kinetic water-protein chemical exchange processes on the sub-millisecond time scale in membrane-bound proteins. The measurements of the pure chemical exchange kinetics provide a unique opportunity to understand the role that water plays in the structure-function relationships of membrane-bound species at the water-bilayer interface. In addition, the perspective of chemical exchange saturation transfer (CEST) experiments in membrane-bound proteins/peptides is further discussed.

Heteronuclear transfers from labile protons in biomolecular NMR: Cross polarization, revisited

INEPT- and HMQC-based pulse sequences are widely used to transfer polarization between heteronuclei, particularly in biomolecular spectroscopy: they are easy to setup and involve low power deposition. Still, these short-pulse polarization transfers schemes are challenged by fast solvent chemical exchange. An alternative to improve these heteronuclear transfers is J-driven cross polarization (J-CP), which transfers polarization by spin-locking the coupled spins under Hartmann-Hahn conditions. J-CP provides certain immunity against chemical exchange and other T2-like relaxation effects, a behavior that is here examined in depth by both Liouville-space numerical and analytical derivations describing the transfer efficiency. While superior to INEPT-based transfers, fast exchange may also slow down these J-CP transfers, hurting their efficiency. This study therefore explores the potential of repeated projective operations to improve 1H→15N and 1H→15N→13C J-CP transfers in the presence of fast solvent chemical exchanges. It is found that while repeating J-CP provides little 1H→15N transfer advantages over a prolonged CP, multiple contacts that keep both the water and the labile protons effectively spin-locked can improve 1H→15N→13C transfers in the presence of chemical exchange. The ensuing Looped, Concatenated Cross Polarization (L-CCP) compensates for single J-CP losses by relying on the 13C’s longer lifetimes, leading to a kind of “algorithmic cooling” that can provide high polarization for the 15N as well as carbonyl and alpha 13Cs. This can facilitate certain experiments, as demonstrated with triple resonance experiments on intrinsically disordered proteins involving labile, chemically exchanging protons.

Pulse sequences for measuring exchange rates between proton species: From unlocalised NMR spectroscopy to chemical exchange saturation transfer imaging

Within the field of NMR spectroscopy, the study of chemical exchange processes through saturation transfer techniques has a long history. In the context of MRI, chemical exchange techniques have been adapted to increase the sensitivity of imaging to small fractions of exchangeable protons, including the labile protons of amines, amides and hydroxyls. The MR contrast is generated by frequency-selective irradiation of the labile protons, which results in a reduction of the water signal associated with transfer of the labile protons’ saturated magnetization to the protons of the surrounding free water. The signal intensity depends on the rate of chemical exchange and the concentration of labile protons as well as on the properties of the irradiation field. This methodology is referred to as CEST (chemical exchange saturation transfer) imaging. Applications of CEST include imaging of molecules with short transverse relaxation times and mapping of physiological parameters such as pH, temperature, buffer concentration and chemical composition due to the dependency of this chemical exchange effect on all these parameters. This article aims to describe these effects both theoretically and experimentally. In depth analysis and mathematical modelling are provided for all pulse sequences designed to date to measure the chemical exchange rate. Importantly, it has become clear that the background signal from semi-solid protons and the presence of the Nuclear Overhauser Effect (NOE), either through direct dipole-dipole mechanisms or through exchange-relayed signals, complicates the analysis of CEST effects. Therefore, advanced methods to suppress these confounding factors have been developed, and these are also reviewed. Finally, the experimental work conducted both in vitro and in vivo is discussed and the progress of CEST imaging towards clinical practice is presented.

Structure or Exchange? On the Feasibility of Chemical Exchange Detection with Balanced Steady-State Free Precession in Tissue - An In Vitro Study.

Balanced steady-state free precession imaging has recently been suggested for chemical exchange detection (bSSFPX). The objective of this work is to investigate the contributions of microstructural, chemical shift and chemical exchange effects to the asymmetry of the bSSFP profile at field strengths of 3 T and 9.4 T. To this end, in vitro bSSFPX experiments are performed for a range of repetition times and flip angles in glucose water solutions with different MnCl2 concentrations and tissue homogenates obtained from the brainstem of pig brains. The experimental results are compared to multi-pool Bloch-McConnell simulations. Additionally, the influence of white matter tract geometry is analyzed ex vivo in pig brain hemispheres measured at two different angles with respect to B0 . The detectable bSSFP profile asymmetry in glucose solutions with tissue-like relaxation times and white matter homogenates was consistent with Bloch-McConnell simulations but relatively small. In intact white matter tracts, the asymmetry was dominated by structural effects with a strong dependency on tract orientation relative to B0 . In tracts perpendicular to B0 , the asymmetry was ≈ 3-4 times higher than in the homogenates, thus barely affected by chemical exchange effects. In conclusion, chemical exchange-related bSSFP profile asymmetries are detectable in tissue homogenates, however, the observed asymmetry level is generally low and prone to confounding structural effects rendering in vivo chemical exchange detection with bSSFP challenging in the brain.

Chemical-shift encoding-based water-fat separation with multifrequency fat spectrum modeling in spin-lock MRI.

Chemical exchange saturation transfer is used commonly to generate MRI contrast based on the chemical exchange effect. The spin-lock techniques can also be used to probe the chemical exchange and other molecular motion processes in tissues. The presence of fat can cause errors in spin-lock MRI. Signals from fat are typically suppressed based on spectral selectivity or T1 nulling approaches in spin-lock imaging. However, these methods cannot be used to suppress fat signals from multiple fat peaks. To address this problem, we report chemical-shift encoding-based water-fat separation approaches with multifrequency fat spectrum modeling. Both the conventional spin-lock and the adiabatic continuous-wave constant-amplitude spin lock (ACCSL) with multi-echo acquisitions are investigated for chemical-shift encoding-based water-fat separation in spin-lock imaging. A comparison is made of reconstructions based on 3 models: a single-peak fat spectrum model, a standard precalibrated proton density 6-peak fat spectrum model, and the self-calibrated relaxation-dependent 3-peak fat spectrum model. Comparisons were performed using Bloch simulations, phantom, and in vivo experiments at 3 T. Conventional spin-lock acquisitions cannot be used for reliable water-fat separation with a multipeak fat spectrum model. Water-fat separation based on ACCSL acquisitions achieves superior performance compared with the use of conventional spin-lock acquisitions. The best result is achieved from ACCSL acquisition with self-calibrated relaxation-dependent multipeak fat spectrum modeling. The ACCSL acquisition can be used for chemical-shift encoding-based water-fat separation with multipeak fat spectrum modeling. This approach has the potential to improve quantitative analysis using spin-lock MRI for assessing the biochemical properties of tissues.

A pure shift and spin echo based approach for high-resolution diffusion-ordered NMR spectroscopy.

Diffusion-ordered NMR spectroscopy (DOSY) can be used for separating mixture components according to their individual diffusion behaviors, thus offering a powerful tool for the analysis of compound mixtures. However, conventional DOSY experiments generally encounter the problem of limited resolution in the spectral domain, particularly for applications to complex mixtures that contains crowed resonances in 1D NMR. In addition, chemical exchange effects, bringing about spurious component signals, pose another limitation for interpreting DOSY measurements. Here, a general DOSY method is proposed based on pure shift extraction and spin echo evolution to obtain high-resolution 2D DOSY spectra, along with the suppression on effects of chemical exchange and J coupling. Both theoretical analyses and experimental results suggest that the proposed method is useful for high-resolution DOSY measurements on complex mixtures that contains crowded or even overlapped NMR resonances and exchanging spin systems.

Probing chemical exchange using quantitative spin-lock R1ρ asymmetry imaging with adiabatic RF pulses.

CEST is commonly used to probe the effects of chemical exchange. Although R1ρ asymmetry quantification has also been described as a promising option for detecting the effects of chemical exchanges, the existing acquisition approaches are highly susceptible to B1 RF and B0 field inhomogeneities. To address this problem, we report a new R1ρ asymmetry imaging approach, AC-iTIP, which is based on the previously reported techniques of irradiation with toggling inversion preparation (iTIP) and adiabatic continuous wave constant amplitude spin-lock RF pulses (ACCSL). We also derived the optimal spin-lock RF pulse B1 amplitude that yielded the greatest R1ρ asymmetry. Bloch-McConnell simulations were used to verify the analytical formula derived for the optimal spin-lock RF pulse B1 amplitude. The performance of the AC-iTIP approach was compared to that of the iTIP approach based on hard RF pulses and the R1ρ -spectrum acquired using adiabatic RF pulses with the conventional fitting method. Comparisons were performed using Bloch-McConnell simulations, phantom, and in vivo experiments at 3.0T. The analytical prediction of the optimal B1 was validated. Compared to the other 2 approaches, the AC-iTIP approach was more robust under the influences of B1 RF and B0 field inhomogeneities. A linear relationship was observed between the measured R1ρ asymmetry and the metabolite concentration. The AC-iTIP approach could probe the chemical exchange effect more robustly than the existing R1ρ asymmetry acquisition approaches. Therefore, AC-iTIP is a promising technique for metabolite imaging based on the chemical exchange effect.

Using bidirectional chemical exchange for improved hyperpolarized [13 C]bicarbonate pH imaging.

Rapid chemical exchange can affect SNR and pH measurement accuracy for hyperpolarized pH imaging with [13 C]bicarbonate. The purpose of this work was to investigate chemical exchange effects on hyperpolarized imaging sequences to identify optimal sequence parameters for high SNR and pH accuracy. Simulations were performed under varying rates of bicarbonate-CO2 chemical exchange to analyze exchange effects on pH quantification accuracy and SNR under different sampling schemes. Four pulse sequences, including 1 new technique, a multiple-excitation 2D EPI (multi-EPI) sequence, were compared in phantoms using hyperpolarized [13 C]bicarbonate, varying parameters such as tip angles, repetition time, order of metabolite excitation, and refocusing pulse design. In vivo hyperpolarized bicarbonate-CO2 exchange measurements were made in transgenic murine prostate tumors to select in vivo imaging parameters. Modeling of bicarbonate-CO2 exchange identified a multiple-excitation scheme for increasing CO2 SNR by up to a factor of 2.7. When implemented in phantom imaging experiments, these sampling schemes were confirmed to yield high pH accuracy and SNR gains. Based on measured bicarbonate-CO2 exchange in vivo, a 47% CO2 SNR gain is predicted. The novel multi-EPI pulse sequence can boost CO2 imaging signal in hyperpolarized 13 C bicarbonate imaging while introducing minimal pH bias, helping to surmount a major hurdle in hyperpolarized pH imaging.

Separation of boron isotopes by chemical exchange in liquid-liquid systems

The currently used technologies for the production of boron isotopes for nuclear power and nuclear medicine are based on the application of toxic boron halides. The report is devoted to alternative processes for enrichment of 10B and 11B by chemical exchange in extraction systems using boric acid, that is, without of boron halides. The results of studying phase and isotope equilibrium in two liquid-liquid systems are considered: an aqueous solution of boric acid – an organic phase in the form of tributyl phosphate or a solution of trioctylamine in o-xylene. The obtained values of the separation factor for boron isotopes characterize the predominantly phase isotope effect in the first system, and in the second system the effect of chemical isotope exchange. Accordingly, the values of the single stage separation factor are 1.0067 ± 0.0035 and 1.025 ± 0.005 and are close to similar values for rectification of boron trifluoride and chemical exchange in the anisole process of boron isotope separation. The practical use of the studied extraction systems may allow the separation of boron isotopes in the horizontal cascades of centrifugal extractors in the absence of an environmental hazard.

Efficient Removal of Lead Ions from Water by a Low-Cost Alginate-Melamine Hybrid Sorbent

A low-cost alginate-melamine hybrid sorbent (named as Alg-Mel) was designed and synthesized for the removal of Pb2+ from water. The as-prepared Alg-Mel sorbent exhibited high affinity and selectivity to Pb2+. The selectivity coefficients of the Alg-Mel for Pb2+/Cd2+, Pb2+/Cu2+, Pb2+/Cr3+ and Pb2+/Co2+ were all over 7. It is found that the hybrid sorbent could uptake 95.4% of Pb2+ from Pb2+-containing solutions (0.48 mM), and the maximum adsorption capacity for Pb2+ reaches 1.39 mmol/g (287.7 mg/g), which is much higher than that of most reported lead ion-sorbents. Furthermore, the Alg-Mel can be regenerated by a simple acid-washing process and used repeatedly. The results of adsorption mechanism analysis reveal that the adsorption of Pb2+ by Alg-Mel is mainly ascribed to the chemical coordination and ion exchange effects.

Na3GaF6– A crystal chemical and solid state NMR spectroscopic study

AbstractNa3GaF6and Na3GaF6:Mn4+samples were obtained from NaNO3and Ga(NO3)3·9H2O in hydrofluoric acid using K2MnF6or NaMnO4as manganese sources. The structure of Na3GaF6was studied by single crystal X-ray diffraction at 90, 293, 440 and 500 K, confirming the monoclinic cryolite type structure, space groupP21/c. The gallium atoms show slightly distorted octahedral coordination by fluorine atoms, similar to the Na1 atoms. Coordination number 8 is observed for Na2. Both sodium sites are clearly distinguished by23Na MAS-NMR spectroscopy. Above 400 K the spectra reveal distinct chemical exchange effects, signifying sodium ion hopping between these two sites. At the same time static19F NMR spectra indicate pronounced motional narrowing effects in this temperature region. The nearly invariant69Ga MAS-NMR spectra suggest that any reorientational motion involving the GaF63−ions (if present) occurs with preservation of the center of mass of these octahedra.

Spin-lock imaging of early tissue pH changes in ischemic rat brain.

We have previously reported that the dispersion of spin-lattice relaxation rates in the rotating frame (R1ρ ) of tissue water protons at high field can be dominated by chemical exchange contributions. Ischemia in brain causes changes in tissue pH, which in turn may affect proton exchange rates. Amide proton transfer (APT, a form of chemical exchange saturation transfer) has been shown to be sensitive to chemical exchange rates and able to detect pH changes non-invasively following ischemic stroke. However, the specificity of APT to pH changes is decreased because of the influence of several other factors that affect magnetization transfer. R1ρ is less influenced by such confounding factors and thus may be more specific for detecting variations in pH. Here, we applied a spin-locking sequence to detect ischemic stroke in animal models. Although R1ρ images acquired with a single spin-locking amplitude (ω1 ) have previously been used to assess stroke, here we use ΔR1ρ , which is the difference in R1ρ values acquired with two different locking fields to emphasize selectively the contribution of chemical exchange effects. Numerical simulations with different exchange rates and measurements of tissue homogenates with different pH were performed to evaluate the specificity of ΔR1ρ to detect tissue acidosis. Spin-lock and APT data were acquired on five rat brains after ischemic strokes induced via middle cerebral artery occlusions. Correlations between these data were analyzed at different time points after the onset of stroke. The results show that ΔR1ρ (but not R1ρ acquired with a single ω1 ) was significantly correlated with APT metrics consistent with ΔR1ρ varying with pH.

Spin-lock imaging of 3-o-methyl-D glucose (3oMG) in brain tumors.

PurposeTo evaluate the ability of spin‐lock imaging to detect the uptake of 3‐o‐methyl‐D‐glucose (3oMG) in normal brain and brain tumors in animals.MethodsMeasurements of the longitudinal relaxation rate in the rotating frame (R1ρ) were made over a range of spin‐lock powers in rat brains bearing 9L tumors. The dispersion of R1ρ values was quantified by ΔR1ρ, the difference of R1ρ values acquired with low and high locking powers. The glucose analogue 3oMG was administered intravenously and the differences of ΔR1ρ values ( ) before and as a function of time after administration were calculated to isolate the contribution of 3oMG to the dispersions, which at high fields primarily reflects chemical exchange effects. In addition, the ratio of image signals from low and high locking fields (the spin‐lock ratio, SLR), which requires fewer acquisitions and varies directly with ΔR1ρ, was computed as an alternative measure of the variation with locking power, and changes in SLR (SLRdiff) after 3oMG were evaluated.ResultsBoth and SLRdiff in tumors increased rapidly after injection, whereas intact brain showed a gradual increase up to 1 h. The and SLRdiff were significantly different between tumors and contralateral normal tissues.ConclusionSpin‐lock methods can be used to detect 3oMG in vivo after injection, and appropriate analyses of MRI signals allow tumors to be distinguished from normal brain.

Non-water-suppressed 1 H FID-MRSI at 3T and 9.4T.

This study investigates metabolite concentrations using metabolite-cycled 1 H free induction decay (FID) magnetic resonance spectroscopic imaging (MRSI) at ultra-high fields. A non-lipid-suppressed and slice-selective ultra-short echo time (TE) 1 H FID MRSI sequence was combined with a low-specific absorption rate (SAR) asymmetric inversion adiabatic pulse to enable non-water-suppressed metabolite mapping using metabolite-cycling at 9.4T. The results were compared to a water-suppressed FID MRSI sequence, and the same study was performed at 3T for comparison. The scan times for performing single-slice metabolite mapping with a nominal voxel size of 0.4 mL were 14 and 17.5 min on 3T and 9.4T, respectively. The low-SAR asymmetric inversion adiabatic pulse enabled reliable non-water-suppressed metabolite mapping using metabolite cycling at both 3T and 9.4T. The spectra and maps showed good agreement with the water-suppressed FID MRSI ones at both field strengths. A quantitative analysis of metabolite ratios with respect to N-acetyl aspartate (NAA) was performed. The difference in Cre/NAA was statistically significant, ∼0.1 higher for the non-water-suppressed case than for water suppression (from 0.73 to 0.64 at 3T and from 0.69 to 0.59 at 9.4T). The difference is likely because of chemical exchange effects of the water suppression pulses. Small differences in mI/NAA were also statistically significant, however, are they are less reliable because the metabolite peaks are close to the water peak that may be affected by the water suppression pulses or metabolite-cycling inversion pulse. We showed the first implementation of non-water-suppressed metabolite-cycled 1 H FID MRSI at ultra-high fields. An increase in Cre/NAA was seen for the metabolite-cycled case. The same methodology was further applied at 3T and similar results were observed. Magn Reson Med 80:442-451, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

R1ρ dispersion and sodium imaging in human calf muscle

PurposeTo evaluate the magnitude of chemical exchange effects and R1ρ dispersion in muscle and their relationship to tissue sodium levels with aging. MethodsSeven healthy volunteers (aged 24 to 87years, median age 47) underwent MRI to assess tissue sodium levels and water T1ρ values at different spin-locking frequencies in calf muscles. T1ρ values at each locking field were computed based on a three-parameter mono-exponential model to fit signals obtained at different locking times, and R1ρ (=1/T1ρ) rates were compared at different locking fields. In particular, the dispersion of R1ρ (ΔR1ρ=R1ρ(0Hz)−R1ρ(500Hz)) was examined as a function of subject age. Muscle sodium content was calculated by comparing signal intensities between tissues and reference standards within the same image. The variations of ΔR1ρ with age and sodium were analyzed by linear regression. ResultsT1ρ values and sodium content both increased with age. R1ρ dispersion also increased with age and showed a strong linear correlation (correlation coefficient r=0.98, P=0.000578) with sodium content. ConclusionΔR1ρ reports on the contribution of labile protons such as hydroxyls which may be associated with macromolecule accumulation in the extracellular matrix (ECM). An increase of sodium signal suggests an enlarged ECM volume fraction and/or an increase in sodium concentration, which occurs during normal aging. The strong correlation between ΔR1ρ and sodium is likely the consequence of increased ECM and density of total charged sites within the matrix from molecules such as collagens and proteoglycans. The results from this study show the potential use of R1ρ dispersion and sodium imaging in the assessment of pathological changes in muscle such as fibrosis.

Modeling of Polarization Transfer Kinetics in Protein Hydration Using Hyperpolarized Water.

Water-protein interactions play a central role in protein structure, dynamics, and function. These interactions, traditionally, have been studied using nuclear magnetic resonance (NMR) by measuring chemical exchange and nuclear Overhauser effect (NOE). Polarization transferred from hyperpolarized water can result in substantial transient signal enhancements of protein resonances due to these processes. Here, we use dissolution dynamic nuclear polarization and flow-NMR for measuring the pH dependence of transferred signals to the protein trypsin. A maximum enhancement of 20 is visible in the amide proton region of the spectrum at pH 6.0, and of 47 at pH 7.5. The aliphatic region is enhanced up to 2.3 times at pH 6.0 and up to 2.5 times at pH 7.5. The time dependence of these observed signals can be modeled quantitatively using rate equations incorporating chemical exchange to amide sites and, optionally, intramolecular NOE to aliphatic protons. On the basis of these two- and three-site models, average exchange (kex) and cross-relaxation rates (σ) obtained were kex = 12 s-1, σ = -0.33 s-1 for pH 7.5 and kex = 1.8 s-1, σ = -0.72 s-1 for pH 6.0 at a temperature of 304 K. These values were validated using conventional EXSY and NOESY measurements. In general, a rapid measurement of exchange and cross-relaxation rates may be of interest for the study of structural changes of the protein occurring on the same time scale. Besides protein-water interactions, interactions with cosolvent or solutes can further be investigated using the same methods.

Rapid and simultaneous measurement of phosphorus metabolite pool size ratio and reaction kinetics of enzymes in vivo.

The metabolites phosphocreatine (PCr), adenosine triphosphate (ATP), and in-organic phosphate (Pi) are biochemically coupled. Their pool sizes, assessed by their magnetization ratios, have been extensively studied and reflect bioenergetics status in vivo. However, most such studies have ignored chemical exchange and T1 relaxation effects. In this work, we aimed to extend the T1nom method to simultaneously quantify the reaction rate constants as well as phosphorus metabolite pool size ratios under partially relaxed conditions. Modified Bloch-McConnell equations were used to simulate the effects of chemical exchanges on T1 relaxation times and magnetization ratios among PCr, γ-ATP, and Pi. The T1nom method with iteration approach was used to measure both reaction constants and metabolite pool size ratios. To validate our method, in vivo data from rat brains (N = 8) at 9.4 Tesla were acquired under two conditions, i.e., approximately full relaxation (TR = 9 s) and partial relaxation (TR = 3 s). We compared metabolite pool size ratios and reaction constants before and after correcting the chemical exchange and T1 relaxation effects. There were significant errors in underestimation of PCr/γATP by 12 % (P = 0.03) and overestimation of ATP/Pi ratios by 14 % (P = 0.04) when not considering chemical exchange effects. These errors were minimized using our iteration approach, resulting in no significant differences (PCr/γATP, P = 0.47; ATP/Pi, P = 0.81) in metabolite pool size ratios and reaction constants between the two measurements (i.e., short versus long TR conditions). Our method can facilitate broad biomedical applications of 31 P magnetization saturation transfer spectroscopy, requiring high temporal and/or spatial resolution for assessment of altered bioenergetics. 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:210-221.

Longitudinal relaxation optimized amide 1H-CEST experiments for studying slow chemical exchange processes in fully protonated proteins.

Chemical Exchange Saturation Transfer (CEST) experiments are increasingly used to study slow timescale exchange processes in biomolecules. Although 15N- and 13C-CEST have been the approaches of choice, the development of spin state selective 1H-CEST pulse sequences that separate the effects of chemical and dipolar exchange [T. Yuwen, A. Sekhar and L. E. Kay, Angew Chem Int Ed Engl 2016 doi: 10.1002/anie.201610759 (Yuwen et al. 2017)] significantly increases the utility of 1H-based experiments. Pulse schemes have been described previously for studies of highly deuterated proteins. We present here longitudinal-relaxation optimized amide 1H-CEST experiments for probing chemical exchange in protonated proteins. Applications involving a pair of proteins are presented establishing that accurate 1H chemical shifts of sparsely populated conformers can be obtained from simple analyses of 1H-CEST profiles. A discussion of the inherent differences between 15N-/13C- and 1H-CEST experiments is presented, leading to an optimal strategy for recording 1H-CEST experiments.