A new class of contrast agents for MRI based on proton chemical exchange dependent saturation transfer (CEST)

It has been previously shown that intrinsic metabolites can be imaged based on their water proton exchange rates using saturation transfer techniques. The goal of this study was to identify an appropriate chemical exchange site that could be developed for use as an exogenous chemical exchange dependent saturation transfer (CEST) contrast agent under physiological conditions. These agents would function by reducing the water proton signal through a chemical exchange site on the agent via saturation transfer. The ideal chemical exchange site would have a large chemical shift from water. This permits a high exchange rate without approaching the fast exchange limit at physiological pH (6.5-7.6) and temperature (37 degrees C), as well as minimizing problems associated with magnetic field susceptibility. Numerous candidate chemicals (amino acids, sugars, nucleotides, heterocyclic ring chemicals) were evaluated in this preliminary study. Of these, barbituric acid and 5, 6-dihydrouracil were more fully characterized with regard to pH, temperature, and concentration CEST effects. The best chemical exchange site found was the 5.33-ppm indole ring -NH site of 5-hydroxytryptophan. These data demonstrate that a CEST-based exogenous contrast agent for MRI is feasible.     

Acid-base high temperature proton exchange membranes prepared from phosphonic acid functionalized siloxane

One kind of acid-base high temperature proton exchange membranes has been prepared from amino trimethylene phosphonic acid (ATMP), epoxycyclohexyethyltrimethoxysilane (EHTMS), and 3-aminopropyltriethoxysilane (APTES) by sol-gel process. The structural characteristics of these membranes with different amount of APTES were investigated by FT-IR, XRD, and SEM. These membranes showed excellent dimensional stability in water with the contribution of flexible ionic network structure and were thermally stable up to about 200 °C. In addition, the proton conductivity of the membranes increased with increasing temperature over the range of 20 to 140 °C, up to a maximum of 2.63 × 10^?2 S cm^?1 at 140 °C under anhydrous condition. The high proton conductivity was attributed to the formation of hydrogen bond network through the synergistic effect of N and P. The activation energy value of membranes became lower from 0.46 to 0.30 eV because of the acid-base pairs. The variable-temperature FT-IR further proved the formation of hydrogen bond network in the membrane.     

Hydrogen's isotopic exchange reaction in the C‐methyl sides in the medicinal agent xymedon: NMR spectroscopy and ab initio calculations

The kinetics of intramolecular and intermolecular exchange processes in xymedon (1‐(2‐hydroxyethyl)‐4,6‐dimethyl‐1,2‐dihydropyrimidin‐2‐one, a regeneratory, wound‐healing drug) and its analogue were investigated in the solution. Hydrogen's mobility was detected in the C‐methyl sides of these compounds. This mobility was monitored via NMR in the hydrogen/deuterium exchange reaction in water. Two models were proposed as explanations for this hydrogen‐deuterium exchange. According to the main model, the key intermediates of these reactions are low‐energy tautomers of xymedon in which the N³ is protonated following which one proton leaves either 6‐Me or 4‐Me and thus its hybridization is changed. This hydrogen‐to‐deuterium exchange reaction is much faster under acidic conditions although it also occurs in alkaline conditions. Methylation via MeOTs or MeI leads to products with a quaternized ring N³ atom in which a hydrogen‐to‐deuterium exchange reaction also takes place, although the rates of the 6‐Me and 4‐Me hydrogens exchange are reversed. According to density functional theory calculations, the presence of methyl groups at the C⁴/C⁶ positions and of the C═O fragment is crucial to remarkably lower the energies of these “rare” tautomers. The exact position of the C═O in heterocycle is also very important in the tautomers' relative stability.     

A study of dipolar interactions and dynamic processes of water molecules in tendon by 1H and 2H homonuclear and heteronuclear multiple-quantum-filtered NMR spectroscopy

The effect of proton exchange on the measurement of 1H-1H, 1H-2H, and 2H-2H residual dipolar interactions in water molecules in bovine Achilles tendons was investigated using double-quantum-filtered (DQF) NMR and new pulse sequences based on heteronuclear and homonuclear multiple-quantum filtering (MQF). Derivation of theoretical expressions for these techniques allowed evaluation of the 1H-1H and 1H-2H residual dipolar interactions and the proton exchange rate at a temperature of 24 degrees C and above, where no dipolar splitting is evident. The values obtained for these parameters at 24 degrees C were 300 and 50 Hz and 3000 s-1, respectively. The results for the residual dipolar interactions were verified by repeating the above measurements at a temperature of 1.5 degrees C, where the spectra of the H2O molecules were well resolved, so that the 1H-1H dipolar interaction could be determined directly from the observed splitting. Analysis of the MQF experiments at 1.5 degrees C, where the proton exchange was in the intermediate regime for the 1H-2H dipolar interaction, confirmed the result obtained at 24 degrees C for this interaction. A strong dependence of the intensities of the MQF signals on the proton exchange rate, in the intermediate and the fast exchange regimes, was observed and theoretically interpreted. This leads to the conclusion that the MQF techniques are mostly useful for tissues where the residual dipolar interaction is not significantly smaller than the proton exchange rate. Dependence of the relaxation times and signal intensities of the MQF experiments on the orientation of the tendon with respect to the magnetic field was observed and analyzed. One of the results of the theoretical analysis is that, in the fast exchange regime, the signal decay rates in the MQF experiments as well as in the spin echo or CPMG pulse sequences (T2) depend on the orientation as the square of the second-rank Legendre polynomial.     

Residue-specific NH exchange rates studied by NMR diffusion experiments

We present a novel approach to the investigation of rapid (>2s(-1)) NH exchange rates in proteins, based on residue-specific diffusion measurements. (1)H, (15)N-DOSY-HSQC spectra are recorded in order to observe resolved amide proton signals for most residues of the protein. Human ubiquitin was used to demonstrate the proposed method. Exchange rates are derived directly from the decay data of the diffusion experiment by applying a model deduced from the assumption of a two-site exchange with water and the "pure" diffusion coefficients of water and protein. The "pure" diffusion coefficient of the protein is determined in an experiment with selective excitation of the amide protons in order to suppress the influence of magnetization transfer from water to amide protons on the decay data. For rapidly exchanging residues a comparison of our results with the exchange rates obtained in a MEXICO experiment showed good agreement. Molecular dynamics (MD) and quantum mechanical calculations were performed to find molecular parameters correlating with the exchangeability of the NH protons. The RMS fluctuations of the amide protons, obtained from the MD simulations, together with the NH coupling constants provide a bilinear model which shows a good correlation with the experimental NH exchange rates.     

A classical reactive potential for molecular clusters of sulphuric acid and water

We present a two-state empirical valence bond (EVB) potential describing interactions between sulphuric acid and water molecules and designed to model proton transfer between them within a classical dynamical framework. The potential has been developed in order to study the properties of molecular clusters of these species, which are thought to be relevant to atmospheric aerosol nucleation. The particle swarm optimisation method has been used to fit the parameters of the EVB model to density functional theory (DFT) calculations. Features of the parametrised model and DFT data are compared and found to be in satisfactory agreement. In particular, it is found that a single sulphuric acid molecule will donate a proton when clustered with four water molecules at 300 K and that this threshold is temperature dependent.     

Temperature dependence of proton relaxation times in vitro

Accurate measurement of tissue relaxation characteristics is dependent on many factors, including field strength and temperature. The purpose of this study was to evaluate the relationship between sample temperature, viscosity and proton spin-lattice relaxation time (T1) and spin-spin relaxation time (T2). A review of two basic models of relaxation the simple molecular motion model and the fast exchange two state model is given with reference to their thermal dependencies. The temperature dependence for both T1 and T2 was studied on a 0.15 Tesla whole body magnetic resonance imager. Thirteen samples comprising both simple and complex materials were investigated by using a standard spin-echo (SE) technique and a modified Carr-Purcell-Meiboom-Gill (CPMG) multi-echo sequence. A simple linear relationship between T1 and temperature was observed for all samples over the range of 20 degrees C to 50 degrees C. There is an inverse relationship between viscosity and T1 and T2. A quantity called the temperature dependence coefficient (TDC) is introduced and defined as the percent rate of change of the proton relaxation time referenced to a specific temperature. The large TDC found for T1 values, e.g. 2.37%/degrees C for CuSO4 solutions and 3.59%/degrees C for light vegetable oils at 22 degrees C, indicates that a temperature correction should be made when comparing in-vivo and in-vitro T1 times. The T2 temperature dependence is relatively small.     

Simultaneous pH and Temperature Measurements Using Pyranine as a Molecular Probe

Steep variations in concentration and temperature frequently occur in small fluid compartments such as those found in cells or microfluidic devices. A quantitative characterization of concentration and temperature gradients is therefore required before these systems can be fully understood. Although different spatially resolved fluorescence methods have been developed to measure either the temperature or the concentration of ions such as proton or calcium, often concentration measurements depend on temperature and vice versa. Here, we describe a method allowing simultaneous measurement of pH and temperature. This method is based on the detection of the blinking of the fluorescent pH indicator pyranine, a process due to its alternating between a basic form and an acidic form. Fluorescence correlation spectroscopy allows measuring both the protonation and deprotonation rates of pyranine, and each pair of rates can be uniquely related to a pair of pH and temperature values. We show, however, that the relationship between rates, pH and temperature, is very sensitive to the presence of other acid-base molecules in solution. We also show that it is influenced by the overall ionic strength of the solution, in a manner that depends on buffer composition.     

Measuring fast hydrogen exchange rates by NMR spectroscopy

We introduce a method to measure hydrogen exchange rates based on the observation of the coherence of a neighboring spin S such as (15)N that has a scalar coupling J(IS) to the exchanging proton I. The decay of S(x) coherence under a Carr-Purcell-Meiboom-Gill (CPMG) multiple echo train is recorded in the presence and absence of proton decoupling. This method allows one to extract proton exchange rates up to 10(5)s(-1). We could extend the pH range for the study of the indole proton in tryptophan, allowing the determination of the exchange constants of the cationic, zwitterionic, and anionic forms of tryptophan.     

The effect of perfusion on the temperature distribution during thermotherapy: Study on perfused porcine kidneys

The effect of perfusion on the temperature distribution during radio-frequency hyperthermia and laser-induced thermotherapy was investigated with the perfused porcine kidney model. The phase shift-based proton resonance frequency shift method was used to map the temperature distribution. In experiments with modulated perfusion rates it was demonstrated that perfusion dissipates a significant amount of the absorbed energy and, therefore, the resulting heat distribution is strongly dependent on the perfusion rate. The measured time course of the temperature distribution was used to estimate the thermal conductivity, local perfusivity and heat absorption rate of the tissue. These parameters were in a good agreement with literature data. This approach can also be extended to measure heat absorption and heat transfer parameters in vivo, which can significantly improve the accuracy of thermotherapy session planning.     

NMR proton relaxation and chemical exchange in the system H16 2O/H17 2O-[2H6]dimethylsulphoxide

NMR proton relaxation rates of normal and ¹⁷O enriched water in a mixture of 68 mol% water and 32 mol% [²H₆]dimethylsulphoxide were measured for temperatures between 298 K and 183 K. In the range between 240 K and 204 K the limit of fast proton-proton exchange between H¹⁶ ₂O and H¹⁷ ₂O is not obeyed, and relaxation curves deviate from mono-exponential behaviour. By fitting the relaxation curves to a model of NMR two-phase relaxation the proton-proton exchange rate within the aqueous component could be obtained. With decreasing temperature, proton-proton exchange slows down and a residence time of about 125 ms at 215 K is found, but it becomes faster again for still lower temperatures. From the phase-averaged relaxation rates of water in the ¹⁷O enriched mixtures, the ¹⁷O induced proton relaxation rate was derived as a function of temperature. This yields the rotational correlation times of the water molecule in the mixture and the dipolar spin-lattice coupling parameter. The latter is considerably lower than the one predicted from the geometry of water.     

A compact optical heterodyne interferometer by optical integration and its application

A compact high-resolution optical heterodyne interferometer combining a two-frequency light module and a minute optical system is described. The light module, which generates two independent frequencies of light, is fabricated by proton exchange method on LiNbO₃ substrate. We report an experiment evaluating measurement accuracy using a micro-displacement measurement system which incorporates this interferometer. Results of the experiment with a standard thickness sample show high thermal stability with maximum measurement error of 1.8 nm at a temperature from 19°C to 33°C. The system was used to measure the hysteresis of a piezoelectric element for displacements of several nm, thereby making it possible to analyze the system quantitatively in practice.     

The effect of Magic Angle Spinning on proton spin–lattice relaxation times in some organic solids

Proton spectra of solids are usually broadened by strong proton homonuclear dipolar interactions. However, substantial line narrowing may be achieved by Magic Angle Spinning (MAS) in systems of low proton density or in systems in which rapid molecular motions occur. In such conditions, T₁(H) measurements are often used to characterise the dynamics of each resolved proton site. We show that T₁(H) values measured for solid organic compounds with high proton abundance, such as adamantane and glycine, may be strongly dependent on the spinning rate employed, so that care is required when values are compared. The effects of molecular motion and proton density on T₁(H) and its dependence on spinning rate were investigated. We found that an increase in molecular motion leads to an increase of T₁(H) at higher spinning rates. The opposite is found for systems with low proton densities which show relatively lower T₁(H), at higher spinning rates. A possible interpretation is suggested in terms of the reduced spin diffusion efficiency at higher spinning rates.     

Polarized parton distribution in the relativistic quark exchange framework

The Spin dependent gluon and sea quark distributions of the proton and the neutron are extracted in the leading order (LO) and the next-to-leading order (NLO) QCD. The relativistic quark exchange model is used to calculate the related valence quark spin dependent structure function. The inverse Mellin transform technique is performed to evaluate the polarized x-dependent distributions of the gluon and the sea quark from the various moments of the valence quarks. It is shown that the calculated spin structure functions (SSF) of the proton and the neutron are in good agreement with the available data, such as E143, SMC, E142, E154 and Hermes experiments. A comparison is also made with the other theoretical models. Finally it is shown that the above calculated parton distributions improve the SSF of the proton and the neutron. Received: 4 January 1999 / Revised version: 12 April 1999     

Mechanism of protonic conductivity in an NH4HSeO4 crystal

The chemical exchange of deuterons in a partly deuterated ammonium hydrogen selenate crystal is investigated by deuteron magnetic resonance (²H NMR) spectroscopy over a wide range of temperatures. The changes observed in the line shape of the NMR spectra at temperatures above 350 K are characteristic of chemical exchange processes. The exchange processes are thoroughly examined by two-dimensional ²H NMR spectroscopy. It is established that, over the entire temperature range, only deuterons of hydrogen bonds are involved in the exchange and the rates of exchange between deuterons of all types are nearly identical. No deuteron exchange between the ND₄ groups and hydrogen bonds is found. A new model of proton transport in ammonium hydrogen selenate is proposed on the basis of the experimental data. This model makes it possible, within a unified context, to explain all the available experimental data, including macroscopic measurements of the electrical conductivity.     

o-Semiquinonato and o-iminosemiquinonato rhodium complexes. EPR study of the reactions in coordination sphere of rhodium

Chemical exchange saturation transfer (CEST) processes in aqueous systems are quantified by evaluation of z-spectra, which are obtained by acquisition of the water proton signal after selective RF presaturation at different frequencies. When saturation experiments are performed in vivo, three effects are contributing: CEST, direct water saturation (spillover), and magnetization transfer (MT) mediated by protons bound to macromolecules and bulk water molecules. To analyze the combined saturation a new analytical model is introduced which is based on the weak-saturation-pulse (WSP) approximation. The model combines three single WSP approaches to a general model function. Simulations demonstrated the benefits and constraints of the model, in particular the capability of the model to reproduce the ideal proton transfer rate (PTR) and the conventional MT rate for moderate spillover effects (up to 50% direct saturation at CEST-resonant irradiation). The method offers access to PTR from z-spectra data without further knowledge of the system, but requires precise measurements with dense saturation frequency sampling of z-spectra. PTR is related to physical parameters such as concentration, transfer rates and thereby pH or temperature of tissue, using either exogenous contrast agents (PARACEST, DIACEST) or endogenous agents such as amide protons and -OH protons of small metabolites.     

Proton exchange in stable nitroxyl radicals of the imidazoline and imidazolidine series

The influence of pH on the ESR spectra of six nitroxyl radicals was studied. Hyperfine splitting constants, a_N, and g factors differ in protonated (RH⁺) and nonprotonated (R) forms of the radicals. Proton exchange rates between R and RH⁺ are either fast, moderate, or slow on the ESR time scale, depending on the pK_a of the radical. For moderate exchange rates, the temperature dependence of the proton exchange rate was studied. Quantum chemical methods were used to calculate the magnetic parameters of ESR spectra in R and RH⁺ forms for four of the radicals studied. All the radicals studied may be used as “pH probes.” One of the radicals was covalently bound to protein to give a macromolecular probe, which is recommended for pH studies in biological systems.     

Chemical exchange in KHSeO4 and NH4HSeO4 studied by two-dimensional NMR

The microscopic mechanism of proton transport in partially deuterated potassium hydrogen selenate (KHSe) and in partially deuterated ammonium hydrogen selenate (AHSe) were studied by means of one-dimensional Fourier transform²H nuclear magnetic resonance (NMR), two-dimensional²H NMR and dielectric measurements over a wide temperature range. In both systems, KHSe and AHSe, the slow chemical exchange processes of deuterons between different hydrogen bridges occur. It was established that the rates of exchange between deuteron sites, which are involved in infinite chains of hydrogen bonds, are approximately the same for both crystals. The rates of exchange between these positions and the deuterons in the dimer groups of KHSe are approximately hundred times more slowly. On the basis of our findings, we discuss the models of the microscopic mechanism of hydrogen transport for both substances.     

Proton NMR relaxation of adsorbed water in gelatin and collagen

The dependence of the water self-diffusion coefficients as well as of the proton spin-lattice and spin-spin relaxation rates on the concentration have been studied in the gelatin-water system and in hydrated native collagen. The bound and free water fractions and the corresponding spin-spin and spin-lattice relaxation rates have been determined within the multi-phase water proton exchange model. Various theoretical models for the water proton cross-relaxation to the biopolymer have been studied and the results compared with the observed Larmor frequency dependence of the water proton spin-lattice relaxation rate.     

A Study of Dipolar Interactions and Dynamic Processes of Water Molecules in Tendon byH andH Homonuclear and Heteronuclear Multiple-Quantum-Filtered NMR Spectroscopy

The effect of proton exchange on the measurement of¹H–¹H,¹H–²H, and²H–²H residual dipolar interactions in water molecules in bovine Achilles tendons was investigated using double-quantum-filtered (DQF) NMR and new pulse sequences based on heteronuclear and homonuclear multiple-quantum filtering (MQF). Derivation of theoretical expressions for these techniques allowed evaluation of the¹H–¹H and¹H–²H residual dipolar interactions and the proton exchange rate at a temperature of 24°C and above, where no dipolar splitting is evident. The values obtained for these parameters at 24°C were 300 and 50 Hz and 3000 s⁻¹, respectively. The results for the residual dipolar interactions were verified by repeating the above measurements at a temperature of 1.5°C, where the spectra of the H₂O molecules were well resolved, so that the¹H–¹H dipolar interaction could be determined directly from the observed splitting. Analysis of the MQF experiments at 1.5°C, where the proton exchange was in the intermediate regime for the¹H–²H dipolar interaction, confirmed the result obtained at 24°C for this interaction. A strong dependence of the intensities of the MQF signals on the proton exchange rate, in the intermediate and the fast exchange regimes, was observed and theoretically interpreted. This leads to the conclusion that the MQF techniques are mostly useful for tissues where the residual dipolar interaction is not significantly smaller than the proton exchange rate. Dependence of the relaxation times and signal intensities of the MQF experiments on the orientation of the tendon with respect to the magnetic field was observed and analyzed. One of the results of the theoretical analysis is that, in the fast exchange regime, the signal decay rates in the MQF experiments as well as in the spin echo or CPMG pulse sequences (T₂) depend on the orientation as the square of the second-rank Legendre polynomial.