Hydrophilic Reduction-Resistant Spin Labels of Pyrrolidine and Pyrroline Series from 3,4-Bis-hydroxymethyl-2,2,5,5-tetraethylpyrrolidine-1-oxyl

Highly resistant to reduction nitroxides open new opportunities for structural studies of biological macromolecules in their native environment inside living cells and for functional imaging of pH and thiols, enzymatic activity and redox status in living animals. 3,4-Disubstituted nitroxides of 2,2,5,5-tetraethylpyrrolidine and pyrroline series with a functional group for binding to biomolecules and a polar moiety for higher solubility in water and for more rigid attachment via additional coordination to polar sites were designed and synthesized. The EPR spectra, lipophilicities, kinetics of the reduction in ascorbate-containing systems and the decay rates in liver homogenates were measured. The EPR spectra of all 3,4-disubstituted pyrrolidine nitroxides showed additional large splitting on methylene hydrogens of the ethyl groups, while the spectra of similar pyrroline nitroxides were represented with a simple triplet with narrow lines and hyperfine structure of the nitrogen manifolds resolved in oxygen-free conditions. Both pyrrolidine and pyrroline nitroxides demonstrated low rates of reduction with ascorbate, pyrrolidines being a bit more stable than similar pyrrolines. The decay of positively charged nitroxides in the rat liver homogenate was faster than that of neutral and negatively charged radicals, with lipophilicity, rate of reduction with ascorbate and the ring type playing minor role. The EPR spectra of N,N-dimethyl-3,4-bis-(aminomethyl)-2,2,5,5-tetraethylpyrrolidine-1-oxyl showed dependence on pH with pKa = 3, ΔaN = 0.055 mT and ΔaH = 0.075 mT.

2,5-Di-tert-butyl-2,5-diethylpyrrolidine-1-oxyls: Where Is a Reasonable Limit of Sterical Loading for Higher Resistance to Reduction?

The pyrrolidine nitroxides with four bulky alkyl substituents adjacent to the N-O∙ group demonstrate very high resistance to reduction with biogenic antioxidants and enzymatic systems. This makes them valuable molecular tools for studying the structure and functions of biomolecules directly in a living cell and for functional EPR and NMR tomography in vivo. The first example of highly strained pyrrolidine nitroxides with both ethyl and tert-butyl groups at each of the α-carbon atoms of the nitroxide moiety with cis-configuration of the tert-butyl groups was prepared using a three-component domino reaction of tert-leucine and 2,2-dimethylpentan-3-one with dimethyl fumarate with subsequent conversion of the resulting strained pyrrolidine into 1-pyrroline-1-oxide and addition of EtLi. The nitroxide has demonstrated unexpectedly fast reduction with ascorbate, the rate constant k2 = (2.0 ± 0.1) × 10-3 M-1s-1. This effect was explained by destabilization of the planar nitroxide moiety due to repulsion with the two neighboring tert-butyl groups cis to each other.

Origin of Long-Range Hyperfine Couplings in the EPR Spectra of 2,2,5,5-Tetraethylpyrrolidine-1-oxyls

Cyclic nitroxides with several bulky alkyl substituents adjacent to the nitroxide group are known to demonstrate a much higher stability to bioreduction than their tetramethyl analogues. Among these so-called "sterically shielded" nitroxides, the pyrrolidine derivatives are the most stable. The EPR spectra of some sterically shielded pyrrolidine-1-oxyls were reported to show one or two large additional doublet splittings with a hyperfine coupling (hfc) constant (ca. 0.2-0.4 mT). To determine the origin of these hfc, a series of 2-R-2,5,5-triethyl-3,4-bis(hydroxymethyl)-pyrrolidine-1-oxyls with methylene groups stereospecifically enriched with deuterium were prepared, and their CW EPR spectra were studied. In addition, these nitroxides were investigated using quantum chemical calculations on the UB3LYP/def2-TZVP level and NBO analysis. The apparent constants were assigned to hfc with γ-hydrogen in the side chain, with the contribution of the NBO orbital βπ*(N-O) to the natural localized molecular orbital βσ(C-H) playing the major role. This interaction is efficient if the ethyl substituent is in the pseudoaxial position of the ring and the CH2-CH3 bond is codirected with (parallel to) N-O. The apparent constant aH increases with the Boltzmann population of this conformation.

Synthesis and Properties of (1R(S),5R(S),7R(S),8R(S))-1,8-Bis(hydroxymethyl)-6-azadispiro[4.1.4.2]tridecane-6-oxyl: Reduction-Resistant Spin Labels with High Spin Relaxation Times

Site-directed spin labeling followed by investigation using Electron Paramagnetic Resonance spectroscopy is a rapidly expanding powerful biophysical technique to study structure, local dynamics and functions of biomolecules using pulsed EPR techniques and nitroxides are the most widely used spin labels. Modern trends of this method include measurements directly inside a living cell, as well as measurements without deep freezing (below 70 K), which provide information that is more consistent with the behavior of the molecules under study in natural conditions. Such studies require nitroxides, which are resistant to the action of biogenic reductants and have high spin relaxation (dephasing) times, T_m. (1R(S),5R(S),7R(S),8R(S))-1,8-bis(hydroxymethyl)-6-azadispiro[4.1.4.2]tridecane-6-oxyl is a unique nitroxide that combines these features. We have developed a convenient method for the synthesis of this radical and studied the ways of its functionalization. Promising spin labels have been obtained, the parameters of their spin relaxation T₁ and T_m have been measured, and the kinetics of reduction with ascorbate have been studied.

Electron Paramagnetic Resonance Implemented with Multiple Harmonic Detections Successfully Maps Extracellular pH In Vivo

Extracellular acidification indicates a metabolic shift in cancer cells and is, along with tissue hypoxia, a hallmark of tumor malignancy. Thus, non-invasive mapping of extracellular pH (pHe) is essential for researchers to understand the tumor microenvironment and to monitor tumor response to metabolism-targeting drugs. While electron paramagnetic resonance (EPR) has been successfully used to map pHe in mouse xenograft models, this method is not sensitive enough to map pHe with a moderate amount of exogenous pH-sensitive probes. Here, we show that a modified EPR system achieves twofold higher sensitivity by using the multiple harmonic detection (MHD) method and improves the robustness of pHe mapping in mouse xenograft models. Our results demonstrate that treatment of a mouse xenograft model of human-derived pancreatic ductal adenocarcinoma cells with the carbonic anhydrase IX (CAIX) inhibitor U-104 delays tumor growth with a concurrent tendency toward further extracellular acidification. We anticipate that EPR-based pHe mapping can be expanded to monitor the response of other metabolism-targeting drugs. Furthermore, pHe monitoring can also be used for the development of improved metabolism-targeting cancer treatments.

Synthesis of Sterically Shielded Nitroxides Using the Reaction of Nitrones with Alkynylmagnesium Bromides

Sterically shielded nitroxides, which demonstrate high resistance to bioreduction, are the spin labels of choice for structural studies inside living cells using pulsed EPR and functional MRI and EPRI in vivo. To prepare new sterically shielded nitroxides, a reaction of cyclic nitrones, including various 1-pyrroline-1-oxides, 2,5-dihydroimidazole-3-oxide and 4H-imidazole-3-oxide with alkynylmagnesium bromide wereused. The reaction gave corresponding nitroxides with an alkynyl group adjacent to the N-O moiety. The hydrogenation of resulting 2-ethynyl-substituted nitroxides with subsequent re-oxidation of the N-OH group produced the corresponding sterically shielded tetraalkylnitroxides of pyrrolidine, imidazolidine and 2,5-dihydroimidazole series. EPR studies revealed large additional couplings up to 4 G in the spectra of pyrrolidine and imidazolidine nitroxides with substituents in 3- and/or 4-positions of the ring.

Coupling of phagocytic NADPH oxidase activity and mitochondrial superoxide production

Superoxide radical plays an important role in redox cell signaling and physiological processes; however, overproduction of superoxide or insufficient activity of antioxidants leads to oxidative stress and contributes to the development of pathological conditions such as endothelial dysfunction and hypertension. Meanwhile, the studies of superoxide in biological systems represent unique challenges associated with short lifetime of superoxide, insufficient reactivity of the superoxide probes, and lack of site-specific detection of superoxide. In this work we have developed 15N-and deuterium-enriched spin probe 15N-CAT1H for high sensitivity and site-specific detection of extracellular superoxide. We have tested simultaneous tracking of extracellular superoxide by 15N-CAT1H and intramitochondrial superoxide by conventional 14N-containing spin probe mitoTEMPO-H in immune cells isolated from spleen, splenocytes, under basal conditions or stimulated with inflammatory cytokines IL-17A and TNFα, NADPH oxidase activator PMA, or treated with inhibitors of mitochondrial complex I rotenone or complex III antimycin A. 15N-CAT1H provides two-fold increase in sensitivity and improves detection since EPR spectrum of 15N-CAT1 nitroxide does not overlap with biological radicals. Furthermore, concurrent use of cell impermeable 15N-CAT1H and mitochondria-targeted 14N-mitoTEMPO-H allows simultaneous detection of extracellular and mitochondrial superoxide. Analysis of IL-17A- and TNFα-induced superoxide showed parallel increase in 15N-CAT1 and 14N-mitoTEMPO signals suggesting coupling between phagocytic NADPH oxidase and mitochondria. The interplay between mitochondrial superoxide production and activity of phagocytic NADPH oxidase was further investigated in splenocytes isolated from Sham and angiotensin II infused C57Bl/6J and Nox2KO mice. Angiotensin II infusion in wild-type mice increased the extracellular basal splenocyte superoxide which was further enhanced by complex III inhibitor antimycin A, mitochondrial uncoupling agent CCCP and NADPH oxidase activator PMA. Nox2 depletion attenuated angiotensin II mediated stimulation and inhibited both extracellular and mitochondrial PMA-induced superoxide production. These data indicate that splenocytes isolated from hypertensive angiotensin II-infused mice are "primed" for enhanced superoxide production from both phagocytic NADPH oxidase and mitochondria. Our data demonstrate that novel 15N-CAT1H provides high sensitivity superoxide measurements and combination with mitoTEMPO-H allows independent and simultaneous detection of extracellular and mitochondrial superoxide. We suggest that this new approach can be used to study the site-specific superoxide production and analysis of important sources of oxidative stress in cardiovascular conditions.

Stability of ZIF-8 Nanoparticles in Most Common Cell Culture Media

Zeolite imidazolate framework-8 (ZIF-8) is a promising platform for drug delivery, and information regarding the stability of ZIF-8 nanoparticles in cell culture media is essential for proper interpretation of in vitro experimental results. In this work, we report a quantitative investigation of the ZIF-8 nanoparticle's stability in most common cell culture media. To this purpose, ZIF-8 nanoparticles containing sterically shielded nitroxide probes with high resistance to reduction were synthesized and studied using electron paramagnetic resonance (EPR). The degradation of ZIF-8 in cell media was monitored by tracking the cargo leakage. It was shown that nanoparticles degrade at least partially in all studied media, although the degree of cargo leakage varies widely. We found a strong correlation between the amount of escaped cargo and total concentration of amino acids in the environment. We also established the role of individual amino acids in ZIF-8 degradation. Finally, 2-methylimidazole preliminary dissolved in cell culture media partially inhibits the degradation of ZIF-8 nanoparticles. The guidelines for choosing the proper cell culture medium for the in vitro study of ZIF-8 nanoparticles have been formulated.

3,4-Unsubstituted 2-tert-Butyl-pyrrolidine-1-oxyls with Hydrophilic Functional Groups in the Side Chains

Pyrrolidine nitroxides with four bulky alkyl substituents adjacent to N–O group are known for their high resistance to bioreduction. The 3,4-unsubstituted 2-tert-butyl-2-ethylpyrrolidine-1-oxyls were prepared from the corresponding 2-tert-butyl-1-pyrroline-1-oxides via either the addition of ethinylmagnesium bromide with subsequent hydrogenation or via treatment with ethyllithium. The new nitroxides showed excellent stability to reduction with ascorbate with no evidence for additional large hyperfine couplings in the EPR spectra.

The effects of nitroxide structure upon 1H Overhauser dynamic nuclear polarization efficacy at ultralow-field

The efficacy in ¹H Overhauser dynamic nuclear polarization in liquids at ultralow magnetic field (ULF, B₀ = 92 ± 0.8 µT) and polarization field (B_p = 1-10 mT) was studied for a broad variety of 26 different spin probes. Among others, piperidine, pyrrolidine, and pyrroline radicals specifically synthesized for this study, along with some well-established commercially available nitroxides, were investigated. Isotope-substituted variants, some sterically shielded reduction-resistant nitroxides, and some biradicals were included in the measurements. The maximal achievable enhancement, E_max, and the radio frequency power, P_1/2, needed for reaching E_max/2 were measured. Physico-chemical features such as molecular weight, spectral linewidth, heterocyclic structure, different types of substituents, deuteration, and ¹⁵N-labeling as well as the difference between monoradicals and biradicals were investigated. For the unmodified nitroxide radicals, the E_max values correlate with the molecular weight. The P_1/2 values correlate with the spectral linewidth and are additionally influenced by the type of substituents neighboring the nitroxide group. The nitroxide biradicals with high intramolecular spin-spin coupling show low performance. Nitroxides enriched with ¹⁵N and/or ²H afford significantly higher |E_max| and require lower power to do so, compared to their unmodified counterparts containing at natural abundance predominantly ¹⁴N and ¹H. The results allow for a correlation of chemical features with physical hyperpolarization-related properties and indicate that small nitroxides with narrow spectral lines have clear advantages for the use in Overhauser dynamic nuclear polarization experiments. Perdeuteration and ¹⁵N-labeling can be used to additionally boost the spin probe performance.

The Reactions of 6-(Hydroxymethyl)-2,2-dimethyl-1-azaspiro[4.4]nonanes with Methanesulfonyl Chloride or PPh3-CBr4

Activation of a hydroxyl group towards nucleophilic substitution via reaction with methanesulfonyl chloride or PPh₃-CBr₄ system is a commonly used pathway to various functional derivatives. The reactions of (5R(S),6R(S))-1-X-6-(hydroxymethyl)-2,2-dimethyl- 1-azaspiro[4.4]nonanes 1a–d (X = O·; H; OBn, OBz) with MsCl/NR₃ or PPh₃-CBr₄ were studied. Depending on substituent X, the reaction afforded hexahydro-1H,6H-cyclopenta[c]pyrrolo[1,2-b]isoxazole (2) (for X = O), a mixture of 2 and octahydrocyclopenta[c]azepines (4–6) (for X = OBn, OBz), or perhydro-cyclopenta[2,3]azeto[1,2-a]pyrrol (3) (for X = H) derivatives. Alkylation of the latter with MeI with subsequent Hofmann elimination afforded 2,3,3-trimethyl-1,2,3,4,5,7,8,8a-octahydrocyclopenta[c]azepine with 56% yield.

Uptake of Cell-Penetrating Peptide RL2 by Human Lung Cancer Cells: Monitoring by Electron Paramagnetic Resonance and Confocal Laser Scanning Microscopy

RL2 is a recombinant analogue of a human κ-casein fragment, capable of penetrating cells and inducing apoptosis of cancer cells with no toxicity to normal cells. The exact mechanism of RL2 penetration into cells remains unknown. In this study, we investigated the mechanism of RL2 penetration into human lung cancer A549 cells by a combination of electron paramagnetic resonance (EPR) spectroscopy and confocal laser scanning microscopy. EPR spectra of A549 cells incubated with RL2 (sRL2) spin-labeled by a highly stable 3-carboxy-2,2,5,5-tetraethylpyrrolidine-1-oxyl radical were found to contain three components, with their contributions changing with time. The combined EPR and confocal-microscopy data allowed us to assign these three forms of sRL2 to the spin-labeled protein sticking to the membrane of the cell and endosomes, to the spin-labeled protein in the cell interior, and to spin labeled short peptides formed in the cell because of protein digestion. EPR spectroscopy enabled us to follow the kinetics of transformations between different forms of the spin-labeled protein at a minimal spin concentration (3-16 μM) in the cell. The prospects of applications of spin-labeled cell-penetrating peptides to EPR imaging, DNP, and magnetic resonance imaging are discussed, as is possible research on an intrinsically disordered protein in the cell by pulsed dipolar EPR spectroscopy.

Acidic and Electrosurface Properties of Binary TiO2-SiO2 Xerogels Using EPR of pH-Sensitive Nitroxides

The binary xerogels TiO2-SiO2 are widely used as catalysts and their carriers in organic synthesis. Characterization and adjustment of the electrostatic properties of the surface and the local acidity inside the pores, are necessary for the further development of TiO2-SiO2 xerogels applications. This research investigates acid-base equilibria in the pores, and the surface electrostatic potential (SEP) of binary TiO2-SiO2 xerogels, by the EPR of stable pH-sensitive nitroxide radicals. These radicals are small enough to penetrate directly into the pores, and to be adsorbed onto the surface of the material under study. This makes it possible to obtain valuable information on the acidic and electrosurface properties of the studied system. The highest negative surface electrical charge associated with surface electrical potential (SEP) was equal to -196 ± 6 mV. It was induced by the surface of the sample with a 7% TiO2 content. The local acidity inside the pores of this sample was found to be higher, by approximately 1.49 pH units, as compared to that in the external bulk solution.

The Kinetics of 1,3-Dipolar Cycloaddition of Vinyl Monomers to 2,2,5,5-Tetramethyl-3-imidazoline-3-oxides

In our previous work [Edeleva et al. Chem. Commun. 2019, 55, 190-193], we proposed a versatile approach to the activation of the homolysis of an aldonitrone group-containing alkoxyamine by 1,3-dipolar cycloaddition to a vinyl monomer. Both nitroxide- and alkoxyamine-containing aldonitrones were found to be capable of reacting with the activated alkenes. In the present study, the kinetics of these reactions with 11 different vinyl monomers were investigated using EPR and NMR spectroscopy, and apparent activation energies as well as pre-exponential factors were determined. The influence of monomer structure on the rate of the 1,3-dipolar cycloaddition is discussed. For the vinyl monomers typically used in nitroxide mediated polymerization (styrene, methyl methacrylate) the rate coefficient of cycloaddition to the nitroxide is around k(353 K) ∼4 ⋅ 10-4  L mol-1  s-1 , whereas for n-butyl acrylate and methyl vinyl ketone we observed the fastest cycloaddition reaction with k(353 K)=8 ⋅ 10-3 and 4 ⋅ 10-2  L mol-1  s-1 respectively.

Application of EPR to porphyrin-protein agents for photodynamic therapy

Recently, a new type of spin labels based on photoexcited triplet molecules was proposed for nanometer scale distance measurements by pulsed dipolar electron paramagnetic resonance (PD EPR). However, such molecules are also actively used within biological complexes as photosensitizers for photodynamic therapy (PDT) of cancer. Up to date, the idea of using the photoexcited triplets simultaneously as PDT agents and as spin labels for PD EPR has never been employed. In this work, we demonstrate that PD EPR in conjunction with other methods provides valuable information on the structure and function of PDT candidate complexes, exemplified here with porphyrins bound to human serum albumin (HSA). Two distinct porphyrins with different properties were used: amphiphilic meso-tetrakis(4-hydroxyphenyl)porphyrin (mTHPP) and water soluble cationic meso-tetrakis(N-methyl-4-pyridyl)porphyrin (TMPyP4); HSA was singly nitroxide-labeled to provide a second tag for PD EPR measurements. We found that TMPyP4 locates in a cavity at the center of the four-helix bundle of HSA subdomain IB, close to the interface with solvent, thus being readily accessible to oxygen. As a result, the photolysis of the complex leads to photooxidation of HSA by generated singlet oxygen and causes structural perturbation of the protein. Contrary, in case of mTHPP porphyrin, the binding occurs at the proton-rich pocket of HSA subdomain IIIA, where the access of oxygen to a photosensitizer is hindered. Structural data of PD EPR were supported by other EPR techniques, laser flash photolysis and protein photocleavage studies. Therefore, pulsed EPR on complexes of proteins with photoexcited triplets is a promising approach for gaining structural and functional insights into such PDT agents.

Protein modification by thiolactone homocysteine chemistry: a multifunctionalized human serum albumin theranostic

As the most abundant protein with a variety of physiological functions, albumin has been used extensively for the delivery of therapeutic molecules. Thiolactone chemistry provides a powerful tool to prepare spin-labeled albumin-based multimodal imaging probes and therapeutic agents. We report the synthesis of a tamoxifen homocysteine thiolactone derivative and its use in thiol-'click' chemistry to prepare multi-functionalized serum albumin. The released sulfhydryl group of the homocysteine functional handle was labeled with a nitroxide reagent to prepare a spin-labeled albumin-tamoxifen conjugate confirmed by MALDI-TOF-MS, EPR spectroscopy, UV-vis and fluorescent emission spectra. This is the basis for a novel multimodal tamoxifen-albumin theranostic with a significant (dose-dependent) inhibitory effect on the proliferation of malignant cells. The response of human glioblastoma multiforme T98G cells and breast cancer MCF-7 cells to tamoxifen and its albumin conjugates was different in tumor cells with different expression level of ERα in our experiments. These results provide further impetus to develop a serum protein for delivery of tamoxifen to cancer cells.

Unexpected one-pot formation of the 1H-6a,8a-epiminotricyclopenta[a,c,e][8]annulene system from cyclopentanone, ammonia and dimethyl fumarate. Synthesis of highly strained polycyclic nitroxide and EPR study

The unexpected formation of a highly strained polycyclic amine was observed in a one-pot synthesis from cyclopentanone, dimethyl fumarate and ammonium acetate. This multistep reaction includes 1,3-dipolar cycloaddition of dimethyl fumarate to the cyclic azomethine ylide formed in situ from cyclopentanone and ammonia. The polycyclic amine product was easily converted into a sterically shielded polycyclic nitroxide. The EPR spectra and spin relaxation behavior of the nitroxide were studied in solution. The spin relaxation seems well suited for the use as a biological spin label and are comparable with those of cyclic nitroxides with two spirocyclic moieties adjacent to the N–O^· group.

Synthesis of 1-azaspiro[4.4]nonan-1-oxyls via intramolecular 1,3-dipolar cycloaddition

Sterically shielded nitroxides of the pyrrolidine series have shown the highest resistance to reduction. Here we report the synthesis of new pyrrolidine nitroxides from 5,5-dialkyl-1-pyrroline N-oxides via the introduction of a pent-4-enyl group to the nitrone carbon followed by an intramolecular 1,3-dipolar cycloaddition reaction and isoxazolidine ring opening. The kinetics of reduction of the new nitroxides with ascorbate were studied and compared to those of previously published (1S,2R,3′S,4′S,5′S,2″R)-dispiro[(2-hydroxymethyl)cyclopentan-1,2′-(3′,4′-di-tert-butoxy)pyrrolidine-5′,1″-(2″-hydroxymethyl)cyclopentane]-1′-oxyl (1).

Simultaneous T2* mapping of 14N- and 15N-labeled dicarboxy-PROXYLs using CW-EPR-based single-point imaging

This article reports a method of simultaneous T₂^* mapping of ¹⁴N- and ¹⁵N-labeled dicarboxy-PROXYLs using 750-MHz continuous-wave electron paramagnetic resonance (CW-EPR) imaging. To separate the spectra of ¹⁴N- and ¹⁵N-labeled dicarboxy-PROXYLs under magnetic field gradients, an optimization problem for spectral projections was formulated with the spatial total variation as a regularization term and solved using a local search based on the gradient descent algorithm. Using the single-point imaging (SPI) method with spectral projections of each radical, simultaneous T₂^* mapping was performed for solution samples. Simultaneous T₂^* mapping enabled visualization of the response of T₂^* values to the level of dissolved oxygen in the solution. Simultaneous T₂^* mapping applied to a mouse tumor model demonstrated the feasibility of the reported method for potential application to in vivo oxygenation imaging.

In Vivo Extracellular pH Mapping of Tumors Using Electron Paramagnetic Resonance

An electron paramagnetic resonance (EPR)-based method for noninvasive three-dimensional extracellular pH mapping was developed using a pH-sensitive nitroxyl radical as an exogenous paramagnetic probe. Fast projection scanning with a constant magnetic field sweep enabled the acquisition of four-dimensional (3D spatial +1D spectral) EPR images within 7.5 min. Three-dimensional maps of pH were reconstructed by processing the pH-dependent spectral information on the images. To demonstrate the proposed method of pH mapping, the progress of extracellular acidosis in tumor-bearing mouse legs was studied. Furthermore, extracellular pH mapping was used to visualize the spatial distribution of acidification in different tumor xenograft mouse models of human-derived pancreatic ductal adenocarcinoma cells. The proposed EPR-based pH mapping method enabled quantitative visualization of regional changes in extracellular pH associated with altered tumor metabolism.

Structural Anomalies in Ionic Liquids near the Glass Transition Revealed by Pulse EPR

Unusual physical and chemical properties of ionic liquids (ILs) open up prospects for various applications. We report the first observation of density/rigidity heterogeneities in a series of ILs near the glass transition temperature ( T_g) by means of pulse electron paramagnetic resonance (EPR). Unprecedented suppression of molecular mobility is evidenced near the glass transition, which is assigned to unusual structural rearrangements of ILs on the nanometer scale. Indeed, pulse and continuous wave EPR clearly indicate the occurrence of heterogeneities near T_g, which exist in a rather broad temperature range of ∼50 K. The two types of local environments are evidenced, being drastically different by their stiffness. The more rigid one suppresses molecular mobility, whereas the softer one instead promotes diffusive molecular rotation. Such properties of ILs near T_g are of general importance; moreover, the observed density/rigidity heterogeneities controlled by temperature might be considered as a new type of tunable reaction nanoenvironment.

Cellular accumulation and antioxidant activity of acetoxymethoxycarbonyl pyrrolidine nitroxides

Nitroxides are widely used in biology as antioxidants, spin labels, functional spin probes for pH, oxygen and thiol levels, and tissue redox status imaging using electron paramagnetic resonance (EPR); however, biological applications of nitroxides is hindered by fast bioreduction to EPR-silent hydroxylamines and rapid clearance. In this work, we have studied pyrrolidine nitroxides with acetoxymethoxycarbonyl groups which can undergo hydrolysis by cellular esterases to hydrophilic carboxylate derivatives resistant to bioreduction. Nitroxides containing acetoxymethoxycarbonyl groups were rapidly absorbed by cells from the media, 3,4-bis-(acetoxymethoxycarbonyl)-proxyl (DCP-AM2) and 3-(2-(bis(2-(acetoxymethoxy)-2-oxoethyl)amino)acetamido)-proxyl (DCAP-AM2) showing the strongest EPR signal of the cellular fraction. Remarkably, the EPR parameters of 3,4-dicarboxy-proxyl (DCP) and its mono- and di-acetoxymethyl esters are different, and consequent intracellular hydrolysis of acetoxymethoxycarbonyl groups in DCP-AM2 can be followed by EPR. To elucidate intracellular location of the resultant DCP, the mitochondrial fraction has been isolated. EPR measurements showed that mitochondria were the main place where DCP was finally accumulated. TEMPO derivatives showed expectedly much faster decay of EPR signal in the cellular fraction, compared to pyrrolidine nitroxides. It was found that supplementation of endothelial cells with 50 nM of DCP-AM2 completely normalised the mitochondrial superoxide level. Moreover, administration of DCP-AM2 to mice (1.4 mg/kg/day) resulted in substantial nitroxide accumulation in the tissues and significantly reduced hypertension. We found that hydroxylamine derivatives of dicarboxyproxyl nitroxide DCP-AM-H can be used for the detection of superoxide in vivo in angiotensin II model of hypertension. Infusion of DCP-AM-H in mice leads to accumulation of persistent EPR signal of nitroxide in the blood and vascular tissue in angiotensin II-infused wild-type but not in SOD2 overexpressing mice. Our data demonstrate that acetoxymethoxycarbonyl group containing nitroxides accumulate in mitochondria and demonstrate site-specific antioxidant activity.

Feasibility of in vivo three-dimensional T 2* mapping using dicarboxy-PROXYL and CW-EPR-based single-point imaging

OBJECTIVES

The aim of this study was to demonstrate the feasibility of in vivo three-dimensional (3D) relaxation time T ₂^* mapping of a dicarboxy-PROXYL radical using continuous-wave electron paramagnetic resonance (CW-EPR) imaging.

MATERIALS AND METHODS

Isotopically substituted dicarboxy-PROXYL radicals, 3,4-dicarboxy-2,2,5,5-tetra(²H₃)methylpyrrolidin-(3,4-²H₂)-(1-¹⁵N)-1-oxyl (²H,¹⁵N-DCP) and 3,4-dicarboxy-2,2,5,5-tetra(²H₃)methylpyrrolidin-(3,4-²H₂)-1-oxyl (²H-DCP), were used in the study. A clonogenic cell survival assay was performed with the ²H-DCP radical using squamous cell carcinoma (SCC VII) cells. The time course of EPR signal intensities of intravenously injected ²H,¹⁵N-DCP and ²H-DCP radicals were determined in tumor-bearing hind legs of mice (C3H/HeJ, male, n = 5). CW-EPR-based single-point imaging (SPI) was performed for 3D T ₂^* mapping.

RESULTS

²H-DCP radical did not exhibit cytotoxicity at concentrations below 10 mM. The in vivo half-life of ²H,¹⁵N-DCP in tumor tissues was 24.7 ± 2.9 min (mean ± standard deviation [SD], n = 5). The in vivo time course of the EPR signal intensity of the ²H,¹⁵N-DCP radical showed a plateau of 10.2 ± 1.2 min (mean ± SD) where the EPR signal intensity remained at more than 90% of the maximum intensity. During the plateau, in vivo 3D T ₂^* maps with ²H,¹⁵N-DCP were obtained from tumor-bearing hind legs, with a total acquisition time of 7.5 min.

CONCLUSION

EPR signals of ²H,¹⁵N-DCP persisted long enough after bolus intravenous injection to conduct in vivo 3D T ₂^* mapping with CW-EPR-based SPI.

Interfacial Electrostatic Properties of Hydrated Mesoporous and Nanostructured Alumina Powders by Spin Labeling EPR

Acid-base equilibria and interfacial electrostatic properties of hydrated mesoporous and nanostructured alumina powders are determining factors for the use of these materials in heterogeneous catalysis and as a sorption media for filtration and chromatographic applications including life sciences. Here spin probe electron paramagnetic resonance spectroscopy of pH-sensitive nitroxides was employed to evaluate the surface charge and interfacial acid-base equilibria at the pore surface of mesoporous powders of α-Al₂O₃, γ-Al₂O₃, Al₂O₃ × nH₂O, and basic γ-Al₂O₃ and nanostructured Al₂O₃ in the form of pristine materials and modified with aluminum-tri-sec-butoxide, hydroxyaluminum glycerate, and several phospholipids. A new pH-sensitive nitroxide probe, 4-dimethylamino-5,5-dimethyl-2-(4-(chloromethyl)phenyl)-2-ethyl-2,5-dihydro-1H-imidazol-1-oxyl hydrochloride semihydrate (nitroxide R1), has been synthesized and characterized. It was found that conditions of preparation of alumina powders exert strikingly large effects on the apparent pK _a of nitroxides measured from electron paramagnetic resonance titration curves. Specifically, while the electron paramagnetic resonance titrations curves for the nitroxide R1 in mesoporous powders prepared from basic γ-Al₂O₃ and Al₂O₃ × nH₂O were shifted by ΔpK _a≈ +0.6 and up to ≈ +1.2 pH units respectively, the shift for γ-Al₂O₃ was found to be much higher: ΔpK _a = +3.5. Assuming approximately the same ∆pH = 0.5-1.0 arising from a difference in the hydrogen ion activity between the bulk solution phase and that in a confined pore volume, the samples were ranked in the following order of descending magnitude of the effective surface electrostatic potential Ψ: mesoporous γ-Al₂O₃ > Al₂O₃ × nH₂O > basic γ-Al₂O₃ > α-Al₂O₃. Conditions of the Al₂O₃ synthesis as well as the surface modification procedures were found to have profound effects on the interfacial electrostatic properties of hydrated samples that are likely related to the nature and concentration of the active sites on the alumina surfaces.

Room-temperature electron spin relaxation of nitroxides immobilized in trehalose: Effect of substituents adjacent to NO-group

Trehalose has been recently promoted as efficient immobilizer of biomolecules for room-temperature EPR studies, including distance measurements between attached nitroxide spin labels. Generally, the structure of nitroxide influences the electron spin relaxation times, being crucial parameters for room-temperature pulse EPR measurements. Therefore, in this work we investigated a series of nitroxides with different substituents adjacent to NO-moiety including spirocyclohexane, spirocyclopentane, tetraethyl and tetramethyl groups. Electron spin relaxation times (T1, Tm) of these radicals immobilized in trehalose were measured at room temperature at X- and Q-bands (9/34GHz). In addition, a comparison was made with the corresponding relaxation times in nitroxide-labeled DNA immobilized in trehalose. In all cases phase memory times Tm were close to 700ns and did not essentially depend on structure of substituents. Comparison of temperature dependences of Tm at T=80-300K shows that the benefit of spirocyclohexane substituents well-known at medium temperatures (∼100-180K) becomes negligible at 300K. Therefore, unless there are specific interactions between spin labels and biomolecules, the room-temperature value of Tm in trehalose is weakly dependent on the structure of substituents adjacent to NO-moiety of nitroxide. The issues of specific interactions and stability of nitroxide labels in biological media might be more important for room temperature pulsed dipolar EPR than differences in intrinsic spin relaxation of radicals.

Effect of Sterical Shielding on the Redox Properties of Imidazoline and Imidazolidine Nitroxides

The oxidant properties of the series of 2,2,5,5-tetraalkyl imidazoline and imidazolidine nitroxides were investigated. An increase in the number of bulky alkyl substituents leads to a decrease in the rate of reduction with ascorbate, which makes the electrochemical reduction potential more negative and shifts the equilibrium in the mixture of nitroxide and reference hydroxylamine (3-carboxy-1-hydroxy-2,2,5,5-tetramethylpyrrolidine-1-oxyl-1-(15)N) toward the starting compounds. The effect of structural factors on these reactions was analyzed by means of multiple regression using the Fujita steric constant Es and the inductive Hammett constant σI. Satisfactory statistical outputs were obtained in all of the biparameter correlations, denoting that the oxidant properties of the nitroxides are determined by steric and electronic effects of the substituents. The data imply that bulky substituents can stabilize nitroxide and/or destabilize hydroxylamine.

Functional electron paramagnetic resonance imaging of ischemic rat heart: Monitoring of tissue oxygenation and pH

PURPOSE

Electron paramagnetic resonance (EPR) imaging in the spectral-spatial domain with application of soluble paramagnetic probes provides an opportunity for spatially resolved functional measurements of living objects. The purpose of this study was to develop EPR methods for visualization of oxygenation and acidosis of ischemic myocardium.

METHODS

EPR oxygen measurements were performed using isotopically substituted (2) H,(15) N-dicarboxyproxyl. The radical has an EPR line width of 320 mG and oxygen-induced line broadening of 0.53 mG/mm Hg, providing oxygen sensitivity down to 5 μM. pH measurements were performed using previously developed pH-sensitive imidazoline nitroxide. The radical has an EPR spectrum with pH-dependable hyperfine splitting, pK = 6.6, providing pH sensitivity of approximately 0.05 U in the physiological range.

RESULTS

EPR imaging of isolated and perfused rat hearts was performed in the two-dimensional + spectral domain. The spatial resolution of the measurements was about 1.4 mm. Marked tissue hypoxia was observed in the ischemic area of the heart after occlusion of the left anterior descending coronary artery. Tissue oxygenation was partly restored upon reperfusion. EPR mapping of myocardial pH indicated acidosis of the ischemic area down to pH 6.7-6.8.

CONCLUSION

This study demonstrates the capability of low-field EPR and the nitroxide spin probes for mapping of myocardial oxygenation and pH. The developed approaches might be used for noninvasive investigation of microenvironment on living objects. Magn Reson Med 76:350-358, 2016. © 2015 Wiley Periodicals, Inc.

A versatile approach for site-directed spin labeling and structural EPR studies of RNAs

Site-directed spin labeling (SDSL) is widely applied for structural studies of biopolymers by electron paramagnetic resonance (EPR). However, SDSL of long RNA sequences still remains a challenging task. Here, we propose a novel SDSL approach potentially suitable for long natural RNAs, which is based on the attachment of a linker containing an aliphatic amino group to the target nucleotide residue followed by selective coupling of a spin label to this amino group. Such a linker can be attached to the desired RNA residue via a sequence-specific reaction with the derivatives of oligodeoxyribonucleotides. To verify this approach, we applied it to model RNA duplex with known structure and expected distance between corresponding residues. A new 2,5-bis(spirocyclohexane)-substituted spin label with advanced stability and relaxation properties has been used, and the distance distribution measured using Q-band (34 GHz) pulsed double electron-electron resonance corresponds well to the expected one. We have additionally validated the obtained results by studying a similar RNA duplex, where the linker with the aliphatic amino group was introduced via solid-phase synthesis. Although this novel SDSL approach does not provide an advantage in precision of molecular distance measurements, we believe that its applicability to long RNAs is a crucial benefit for future structural studies using pulse EPR.

In vivo proton-electron double-resonance imaging of extracellular tumor pH using an advanced nitroxide probe

A variable radio frequency proton-electron double-resonance imaging (VRF PEDRI) approach for pH mapping of aqueous samples has been recently developed (Efimova et al. J. Magn. Reson. 2011, 209, 227-232). A pH map is extracted from two PEDRI acquisitions performed at electron paramagnetic resonance (EPR) frequencies of protonated and unprotonated forms of a pH-sensitive probe. To translate VRF PEDRI to an in vivo setting, an advanced pH probe was synthesized. Probe deuteration resulted in a narrow spectral line of 1.2 G compared to a nondeuterated analogue line width of 2.1 G allowing for an increase of Overhauser enhancements and reduction in rf power deposition. Binding of the probe to the cell-impermeable tripeptide, glutathione (GSH), allows for targeting to extracellular tissue space for monitoring extracellular tumor acidosis, a prognostic factor in tumor pathophysiology. The probe demonstrated pH sensitivity in the 5.8-7.8 range, optimum for measurement of acidic extracellular tumor pH (pH(e)). In vivo VRF PEDRI was performed on Met-1 tumor-bearing mice. Compared to normal mammary glands with a neutral mean pH(e) (7.1 ± 0.1), we observed broader pH distribution with acidic mean pH(e) (6.8 ± 0.1) in tumor tissue. In summary, VRF PEDRI in combination with a newly developed pH probe provides an analytical approach for spatially resolved noninvasive pHe monitoring, in vivo.

Does scavenging of mitochondrial superoxide attenuate cancer prosurvival signaling pathways?

It has been previously suggested that overexpression of mitochondrial superoxide dismutase (SOD) attenuates cancer development; however, the exact mechanism remains unclear. In this work, we have studied the direct effect of the mitochondria-targeted superoxide scavenger, (2-(2,2,6,6-tetramethylpiperidin-1-oxyl-4-ylamino)-2-oxoethyl)triphenylphosphonium chloride (mitoTEMPO), on B16-F0 mouse melanoma cells and tumor growth in a nude mouse model of human melanoma. We show that scavenging of mitochondrial superoxide inhibited cell growth, reduced viability, and induced apoptosis in melanoma cells, but did not affect nonmalignant skin fibroblasts. Diminished mitochondrial superoxide inhibited redox-dependent Akt, restored activity of mitochondrial pyruvate dehydrogenase, and reduced HIF1-α and lactate dehydrogenase expression in cancer cells. Suppression of glycolysis in mitoTEMPO-treated melanoma cells resulted in a significant drop of cellular adenosine-5'-triphosphate and induced cell death. In vivo mitoTEMPO treatment effectively suppressed growth of established tumor in the mouse model of human melanoma. Therefore, our data lead to the hypothesis that scavenging of mitochondrial superoxide selectively inhibits redox-sensitive survival and metabolic pathways, resulting in cancer cell death. In contrast to existing anticancer therapies, inhibition of mitochondrial superoxide may represent a novel specific anticancer treatment with reduced cytotoxic side effects.

Accelerated rejection of the second transplants of immunogenic tumor in mice under inhibition of indoleamine 2,3-dioxygenase activity by ethyl pyruvate

AIM

A recently discovered enzyme, indoleamine 2,3-dioxygenase (IDO), is expressed in placenta, dendritic cells and also in many kinds of tumors and in tumor-infiltrating macrophages. By catabolizing tryptophan, IDO causes local depletion of this essential amino acid and excess of kinurenin, and suppresses in situ proliferation and functioning of T lymphocytes. Thus, immune resistance of tumors can be overcome by inhibiting IDO activity.

MATERIALS AND METHODS

C3HA mice immunized with non-syngeneic H-29 tumor were used to study the effect of the IDO inhibitor ethyl pyruvate, under systemic or local (at site of tumor cells localization) administration, on the occurrence and rate of rejection of the second transplants of this tumor.

RESULTS

Both systemic and local administration of ethyl pyruvate increases the incidence of and substantially accelerates tumor regression as compared with control.

CONCLUSION

IDO inhibitors impairing immune resistance of tumors may appear useful in leveraging the efficacy of antitumor therapy.

Electron paramagnetic resonance monitoring of ischemia-induced myocardial oxygen depletion and acidosis in isolated rat hearts using soluble paramagnetic probes

A new low-field electron paramagnetic resonance approach for noninvasive measurements of myocardial oxygen tension and tissue acidity was developed. The approach was applied to monitor myocardial pO(2) and pH in a model of global no-flow ischemia (30 min) and reperfusion in isolated perfused rat hearts. The myocardial oxygen measurements were performed using deuterated Finland trityl radical probe. A rapid decrease in myocardial pO(2) from 160 mmHg to about 2 ± 1 mmHg was observed within the first minute of ischemia followed by incomplete restoration of pO(2) to 50 mmHg during 30 min of reperfusion. The lower oxygen concentration after ischemia was attributed to the 50% reduction in coronary flow after ischemia as a consequence of myocardial ischemia and reperfusion damage. Myocardial pH measurements using a specially designed imidazoline pH-sensitive nitroxide showed severe myocardial acidification to pH 6.25 during 30 min of ischemia. Preconditioning of the hearts with two 5-min periods of ischemia significantly reduced the acidification of myocardial tissue during sustained ischemia. Noninvasive electron paramagnetic resonance monitoring of myocardial oxygenation and pH may provide important insights into the mechanisms of ischemia and reperfusion injury and a background for development of new therapeutic approaches.

In vivo monitoring of pH, redox status, and glutathione using L-band EPR for assessment of therapeutic effectiveness in solid tumors

Approach for in vivo real-time assessment of tumor tissue extracellular pH (pH(e)), redox, and intracellular glutathione based on L-band EPR spectroscopy using dual function pH and redox nitroxide probe and disulfide nitroxide biradical, is described. These parameters were monitored in PyMT mice bearing breast cancer tumors during treatment with granulocyte macrophage colony-stimulating factor. It was observed that tumor pH(e) is about 0.4 pH units lower than that in normal mammary gland tissue. Treatment with granulocyte macrophage colony-stimulating factor decreased the value of pH(e) by 0.3 units compared with PBS control treatment. Tumor tissue reducing capacity and intracellular glutathione were elevated compared with normal mammary gland tissue. Granulocyte macrophage colony-stimulating factor treatment resulted in a decrease of the tumor tissue reducing capacity and intracellular glutathione content. In addition to spectroscopic studies, pH(e) mapping was performed using recently proposed variable frequency proton-electron double-resonance imaging. The pH mapping superimposed with MRI image supports probe localization in mammary gland/tumor tissue, shows high heterogeneity of tumor tissue pH(e) and a difference of about 0.4 pH units between average pH(e) values in tumor and normal mammary gland. In summary, the developed multifunctional approach allows for in vivo, noninvasive pH(e), extracellular redox, and intracellular glutathione content monitoring during investigation of various therapeutic strategies for solid tumors.

EPR detection of cellular and mitochondrial superoxide using cyclic hydroxylamines

Superoxide (O₂ⁱ⁻) has been implicated in the pathogenesis of many human diseases, but detection of the O(2)(•-) radicals in biological systems is limited due to inefficiency of O₂ⁱ⁻ spin trapping and lack of site-specific information. This work studied production of extracellular, intracellular and mitochondrial O₂ⁱ⁻ in neutrophils, cultured endothelial cells and isolated mitochondria using a new set of cationic, anionic and neutral hydroxylamine spin probes with various lipophilicity and cell permeability. Cyclic hydroxylamines rapidly react with O₂ⁱ⁻, producing stable nitroxides and allowing site-specific cO₂ⁱ⁻ detection in intracellular, extracellular and mitochondrial compartments. Negatively charged 1-hydroxy-4-phosphono-oxy-2,2,6,6-tetramethylpiperidine (PP-H) and positively charged 1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl-trimethylammonium (CAT1-H) detected only extramitochondrial O₂ⁱ⁻. Inhibition of EPR signal by SOD2 over-expression showed that mitochondria targeted mitoTEMPO-H detected intramitochondrial O₂ⁱ⁻ both in isolated mitochondria and intact cells. Both 1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CP-H) and 1-hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine (CM-H) detected an increase in cytoplasm O₂ⁱ⁻ stimulated by PMA, but only CM-H and mitoTEMPO-H showed an increase in rotenone-induced mitochondrial O₂ⁱ⁻. These data show that a new set of hydroxylamine spin probes provide unique information about site-specific production of the O₂ⁱ⁻ radical in extracellular or intracellular compartments, cytoplasm or mitochondria.

Spin-labeled pH-sensitive phospholipids for interfacial pKa determination: synthesis and characterization in aqueous and micellar solutions

The synthesis and characterization of spin-labeled phospholipids (SLP)--derivatives of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (PTE)--with pH-reporting nitroxides that are covalently attached to the lipid's polar headgroup are being reported. Two lipids were synthesized by reactions of PTE with thiol-specific, pH-sensitive methanethiosulfonate spin labels methanethiosulfonic acid S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-ylmethyl) ester (IMTSL) and S-4-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-1-oxyl-2,5-dihydro-1H-imidazol-2-yl)benzyl methanethiosulfonate (IKMTSL). The pKa values of the IMTSL-PTE lipid measured by EPR titration in aqueous buffer/isopropyl alcohol solutions of various compositions were found to be essentially the same (pKa approximately 2.35), indicating that in mixed aqueous/organic solvents, the amphiphilic lipid molecules could be shielded from changing bulk conditions by a local shell of solvent molecules. To overcome this problem, the spin-labeled lipids were modeled by synthesizing IMTSL- and IKMTSL-2-mercaptoethanol adducts. These model compounds yielded the intrinsic pKa0's for IMTSL-PTE and IKMTSL-PTE in aqueous buffers as 3.33 +/- 0.03 and 5.98 +/- 0.03, respectively. A series of EPR titrations of IMTSL-PTE in mixed water/isopropyl alcohol solution allowed for calibrating the polarity-induced pKa shifts, deltapKapol, vs bulk solvent dielectric permittivity. These calibration data allowed for estimating the local dielectric constant, epsilon(eff), experienced by the reporter nitroxide of the IMTSL-PTE lipid incorporated into the nonionic Triton X-100 micelles as 60 +/- 5 and 57 +/- 5 at 23 and 48 degrees C, respectively. For micelles formed from an anionic surfactant sodium dodecyl sulfate (SDS) the electrostatic-induced pKa shift, deltapKael = 2.06 +/- 0.04 units of pH, was obtained by subtracting the polarity-induced contribution. This shift yields psi = -121 mV electric potential of the SDS micelle surface.

Design of liposome-based pH sensitive nanoSPIN probes: nano-sized particles with incorporated nitroxides

Liposome-based nanoSized Particles with Incorporated Nitroxides, or nanoSPINs, were designed for EPR applications as pH probes in biological systems. Phospholipid membrane of the liposomes with incorporated gramicidin A showed selective permeability to a small analyte, H(+), while protecting entrapped sensing nitroxide from biological reductants. An application of the pH-sensitive nanoSPIN in an ischemia model in rat heart homogenate allows for monitoring ischemia-induced acidosis while protecting encapsulated nitroxide against bioreduction.

Spin-probes designed for measuring the intrathylakoid pH in chloroplasts

Nitroxide radicals are widely used as molecular probes in different fields of chemistry and biology. In this work, we describe pH-sensitive imidazoline- and imidazolidine-based nitroxides with pK values in the range 4.7-7.6 (2,2,3,4,5,5-hexamethylperhydroimidazol-1-oxyl, 4-amino-2,2,5,5-tetramethyl-2,5-dihydro-1H-imidazol-1-oxyl, 4-dimethylamino-2,2-diethyl-5,5-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl, and 2,2-diethyl-5,5-dimethyl-4-pyrrolidyline-1-yl-2,5-dihydro-1H-imidazol-1-oxyl), which allow the pH-monitoring inside chloroplasts. We have demonstrated that EPR spectra of these spin-probes localized in the thylakoid lumen markedly change with the light-induced acidification of the thylakoid lumen in chloroplasts. Comparing EPR spectrum parameters of intrathylakoid spin-probes with relevant calibrating curves, we could estimate steady-state values of lumen pHin established during illumination of chloroplasts with continuous light. For isolated bean (Vicia faba) chloroplasts suspended in a medium with pHout=7.8, we found that pHin approximately 5.4-5.7 in the state of photosynthetic control, and pHin approximately 5.7-6.0 under photophosphorylation conditions. Thus, ATP synthesis occurs at a moderate acidification of the thylakoid lumen, corresponding to transthylakoid pH difference DeltapH approximately 1.8-2.1. These values of DeltapH are consistent with a point of view that under steady-state conditions the proton gradient DeltapH is the main contributor to the proton motive force driving the operation of ATP synthesis, provided that stoichiometric ratio H+/ATP is n> or =4-4.7.

Reversible reduction of nitroxides to hydroxylamines: roles for ascorbate and glutathione

Biological applications of stable nitroxyl radicals, NR, include their use as contrast agents for magnetic resonance imaging, spin labels, superoxide dismutase mimics, and antioxidants. The rapid reduction of NR in biological samples into hydroxylamines (HA) significantly limits their application. In turn, reoxidation of HA back to the NR has been used for detection of reactive oxygen species (ROS). In this work comparative studies of the reduction of pyrrolidine, imidazoline, and imidazolidine NR by ascorbate were performed taking advantage of recently synthesized tetraethyl-substituted NR with much higher stability toward reduction both in vitro and in vivo. Surprisingly, these NR kept 10-50% of initial intensity of electron paramagnetic resonance signal for about 1 h in the presence of 100-fold excess of ascorbate. To explain these data, reoxidation of the corresponding HA by ascorbate radical and dehydroascorbic acid back to the NR was proposed. This hypothesis was supported by direct measurement of the NR appearance from the HA on ascorbate radical generation by ascorbate oxidase, or in the presence of the dehydroascorbic acid. The reversible reaction between NR and ascorbate was observed for the various types of NR, and the rate constants for direct and reverse reactions were determined. The equilibrium constants for one-electron reduction of the tetraethyl-substituted NR by ascorbate were found to be in the range from 2.65x10(-6) to 10(-5) which is significantly lower than corresponding values for the tetramethyl-substituted NR (more or about 10(-4)). This explains the establishment of an EPR-detectable quasi-equilibrium level of tetraethyl-substituted NR in the presence of an excess of ascorbate. The redox reactions of the NR-HA couple in ascorbate-containing media were found to be significantly affected by glutathione (GSH). This effect was attributed to the reduction of ascorbate radicals by GSH, and the rate constant of this reaction was found to be equal to 10 M-1 s-1. In summary, the data provide new insight into the redox chemistry of NR and HA, and significantly affect interpretation and strategy of their use as redox- and ROS-sensitive probes, or as antioxidants.