Magnetic field effect on recombination of radicals diffusing on a two-dimensional plane.

Magnetic Field Effects (MFEs) on the recombination of radicals, which diffuse on an infinite plane, are studied theoretically. The case of spin-selective diffusion-controlled recombination of Radical Pairs (RPs) starting from a random spin state is considered assuming uniform initial distribution of the radicals. In this situation, reaction kinetics is described by a time-dependent rate coefficient K(t), which tends to zero at long times. Strong MFEs on K(t) are predicted that originate from the Δg and hyperfine driven singlet-triplet mixing in the RP. The effects of spin relaxation on the magnetic field are studied, as well as the influence of the dipole-dipole interaction between the electron spins of the RP. In the two-dimensional case, this interaction is not averaged out by diffusion and it strongly affects the MFE. The results of this work are of importance for interpreting MFEs on lipid peroxidation, a magnetosensitive process occurring on two-dimensional surfaces of cell membranes.

Challenges of the Human Proteome Project: 10-Year Experience of the Russian Consortium.

This manuscript collects all the efforts of the Russian Consortium, bottlenecks revealed in the course of the C-HPP realization, and ways of their overcoming. One of the main bottlenecks in the C-HPP is the insufficient sensitivity of proteomic technologies, hampering the detection of low- and ultralow-copy number proteins forming the "dark part" of the human proteome. In the frame of MP-Challenge, to increase proteome coverage we suggest an experimental workflow based on a combination of shotgun technology and selected reaction monitoring with two-dimensional alkaline fractionation. Further, to detect proteins that cannot be identified by such technologies, nanotechnologies such as combined atomic force microscopy with molecular fishing and/or nanowire detection may be useful. These technologies provide a powerful tool for single molecule analysis, by analogy with nanopore sequencing during genome analysis. To systematically analyze the functional features of some proteins (CP50 Challenge), we created a mathematical model that predicts the number of proteins differing in amino acid sequence: proteoforms. According to our data, we should expect about 100 000 different proteoforms in the liver tissue and a little more in the HepG2 cell line. The variety of proteins forming the whole human proteome significantly exceeds these results due to post-translational modifications (PTMs). As PTMs determine the functional specificity of the protein, we propose using a combination of gene-centric transcriptome-proteomic analysis with preliminary fractionation by two-dimensional electrophoresis to identify chemically modified proteoforms. Despite the complexity of the proposed solutions, such integrative approaches could be fruitful for MP50 and CP50 Challenges in the framework of the C-HPP.

Evaluation of sample preparation protocols for quantitative NMR-based metabolomics.

INTRODUCTION: Quantification of metabolites in biological fluids and tissues by NMR spectroscopy is challenged by the presence of abundant macromolecules and lipoproteins in samples, which give broad signals in the NMR spectra. To improve the quality of NMR spectra the different protocols for protein and lipid removal from the sample are used. OBJECTIVES: This work is aimed at the evaluation of the effectiveness of various methods of purification of blood serum from proteins and lipids for 1H NMR metabolomic profiling. METHODS: The advantages and limitations of different methods of the sample preparation for NMR-based quantitative metabolomics have been compared, including ultrafiltration, methanol and ethanol extractions with and without additional lipid removal, and methanol-chloroform extraction. RESULTS: The concentrations of 30 abundant metabolites extracted from human blood serum have been measured. It is found that ultrafiltration provides the best lipid removal, but causes significant and inhomogeneous metabolite losses. Ethanol and methanol extractions demonstrate similar performance with the minimal metabolite losses, and are ideal for fluids and tissues with low lipid content. The additional purification of alcohol extracts from lipids allows for the significant improving of NMR spectra, but causes additional metabolite losses. CONCLUSIONS: The methanol-chloroform extraction seems to be an optimal method for tissues with the high lipid content, providing a satisfactory lipid removal and low metabolite losses. The ultrafiltration leads to large losses of metabolites (up to 60%) and for this reason is not suitable for quantitative analysis.

Constant-adiabaticity radiofrequency pulses for generating long-lived singlet spin states in NMR.

A method is implemented to perform "fast" adiabatic variation of the spin Hamiltonian by imposing the constant adiabaticity condition. The method is applied to improve the performance of singlet-state Nuclear Magnetic Resonance (NMR) experiments, specifically, for efficient generation and readout of the singlet spin order in coupled spin pairs by applying adiabatically ramped RF-fields. Test experiments have been performed on a specially designed molecule having two strongly coupled 13C spins and on selectively isotopically labelled glycerol having two pairs of coupled protons. Optimized RF-ramps show improved performance in comparison, for example, to linear ramps. We expect that the methods described here are useful not only for singlet-state NMR experiments but also for other experiments in magnetic resonance, which utilize adiabatic variation of the spin Hamiltonian.

Metabolomic composition of normal aged and cataractous human lenses.

Quantitative metabolomic profiles of normal and cataractous human lenses were obtained with the combined use of high-frequency nuclear magnetic resonance (NMR) and high-performance liquid chromatography with high-resolution mass-spectrometric detection (LC-MS) methods. The concentration of more than fifty metabolites in the lens cortex and nucleus has been determined. For the majority of metabolites, their concentrations in the lens cortex and nucleus are similar, which confirms low metabolic activity in the lens core. The difference between the metabolite levels in the cortex and nucleus of the normal lens is observed for antioxidants and UV filters, which demonstrates the activity of redox processes in the lens. A huge difference is found between the metabolomic compositions of normal and age-matched cataractous lenses: the concentrations of almost all metabolites in the normal lens are higher than in the cataractous one. The most pronounced difference is observed for compounds playing a key role in the lens cell protection and metabolic activity, including antioxidants, UV filters, and osmolytes. The results obtained imply that the development of the age-related cataracts might originate from the metabolic dysfunction of the lens epithelial cells.

FTIR study of thermally induced magnetostructural transitions in breathing crystals.

"Breathing crystals" based on copper(II) hexafluoroacetylacetonates and pyrazolyl-substituted nitronyl nitroxides comprise the exchange-coupled clusters within the polymeric chains. Owing to an interplay of exchange interaction between copper(II) and nitroxide spins and Jahn-Teller nature of copper(II) complex, the breathing crystals demonstrate thermally and light-induced magnetostructural transitions in many aspects similar to the classical spin crossover. Herewith, we report the first application of variable temperature (VT) far/mid Fourier transform infrared (FTIR) spectroscopy and mid FTIR microscopy to breathing crystals. This VT-FTIR study was aimed toward clarification of the transitions mechanism previously debated on the basis of superconducting quantum interference device, X-ray diffraction, and electron paramagnetic resonance data. VT-FTIR showed the onset of new vibrational bands during phase transitions occurring at the expense of several existing ones, whose intensity was significantly reduced. The most pronounced spectral changes were assigned to corresponding vibrational modes using quantum chemical calculations. A clear-cut correlation was found between temperature-dependent effective magnetic moment of studied compounds and the observed VT-FTIR spectra. Importantly, VT-FTIR confirmed coexistence of two types of copper(II)-nitroxide clusters during gradual magnetostructural transition. Such clusters correspond to weakly coupled and strongly coupled spin states, whose relative contribution depends on temperature. The pronounced difference in the VT-FTIR spectra of two states in breathing crystals is a fingerprint of magnetostructural transition, and understanding of these characteristics achieved by us will be useful for future studies of breathing crystals as well as their diamagnetic analogues.

Photoswitching of a thermally unswitchable molecular magnet Cu(hfac)2L(i-Pr) evidenced by steady-state and time-resolved electron paramagnetic resonance.

Most photoswitchable molecular magnets exhibit thermally induced switching, as is typical of spin crossover (SCO), valence tautomerism and SCO-like phenomena. We report a rare case of a copper-nitroxide based molecular magnet Cu(hfac)2L(i-Pr) that does not exhibit quantitative SCO-like behavior in the temperature range of its chemical stability (2-350 K); however, it can be switched to a metastable thermally inaccessible spin state via visible/near-IR light at cryogenic temperatures. By means of photogeneration, unique information on this otherwise unobservable spin state has been obtained using steady-state Q-band (34 GHz) and time-resolved W-band (94 GHz) electron paramagnetic resonance (EPR) spectroscopy. In particular, we have found that the electronic structure and relaxation properties of the photoinduced state in Cu(hfac)2L(i-Pr) are very similar to those in its sister compound Cu(hfac)2L(n-Pr) that is thermally switchable and has been exhaustively characterized by many analytical methods, previously. The first observation of photoswitchable behavior in a thermally unswitchable copper-nitroxide based molecular magnet Cu(hfac)2L(i-Pr) paves the way for photoswitching applications of this and similar compounds in the remarkably broad temperature range of 2-350 K.

Metabolomics of the rat lens: a combined LC-MS and NMR study.

This work is the first comprehensive report on the quantitative metabolomic composition of the rat lens. Quantitative metabolomic profiles of lenses were acquired with the combined use of high-frequency nuclear magnetic resonance (NMR) and high-performance liquid chromatography with high-resolution mass-spectrometric detection (LC-MS) methods. More than forty low molecular weight compounds found in the lens have been reliably identified and quantified. The most abundant metabolites in the 3-month-old Wistar rat lens are taurine, hypotaurine, lactate, phosphocholine and reduced glutathione. The analysis of age-related changes in the lens metabolomic composition shows a gradual decrease of the content of most metabolites. This decrease is the most pronounced between 1 and 3 months, which probably corresponds to the completion of the lens maturation in one-month-old rats and to the high rate of the young lens growth. The enhanced levels of tryptophan, tyrosine, carnitine, glycerophosphate, GSH and GSSG were found in lenses of senescence-accelerated OXYS rats; for some metabolites, this effect may probably be attributed to the compensatory response to oxidative stress.

Nanoscale solvent organization in metal-organic framework ZIF-8 probed by EPR of flexible ß-phosphorylated nitroxides.

Metal-organic frameworks (MOFs) draw increasing attention as nanoenvironments for chemical reactions, especially in the field of catalysis. Knowing the specifics of MOF cavities is decisive in many of these cases; yet, obtaining them in situ remains very challenging. We report the first direct assessment of the apparent polarity and solvent organization inside MOF cavities using a dedicated structurally flexible spin probe. A stable ß-phosphorylated nitroxide radical was incorporated into the cavities of a prospective MOF ZIF-8 in trace amounts. The spectroscopic properties of this probe depend on local polarity, structuredness, stiffness and cohesive pressure and can be precisely monitored by Electron Paramagnetic Resonance (EPR) spectroscopy. Using this approach, we have demonstrated experimentally that the cavities of bare ZIF-8 are sensed by guest molecules as highly non-polar inside. When various alcohols fill the cavities, remarkable self-organization of solvent molecules is observed leading to a higher apparent polarity in MOFs compared to the corresponding bulk alcohols. Accounting for such nanoorganization phenomena can be crucial for optimization of chemical reactions in MOFs, and the proposed methodology provides unique routes to study MOF cavities inside in situ, thus aiding in their various applications.

Cataract-specific posttranslational modifications and changes in the composition of urea-soluble protein fraction from the rat lens.

PURPOSE: To determine age-related changes in the composition of the urea-soluble (US) protein fraction from lenses of senescence-accelerated OXYS (cataract model) and Wistar (control) rats and to establish posttranslational modifications (PTMs) occurring under enhanced oxidative stress in OXYS lenses. METHODS: The identity and the relative abundance of crystallins in the US fractions were determined using two-dimensional gel electrophoresis (2-DE) and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MS). The identities and the positions of PTMs were established using MS/MS measurements. RESULTS: Two-dimensional gel electrophoresis maps of US protein fractions were obtained for lenses of 3-, 12-, and 62-week-old Wistar and OXYS rats, and the relative abundance of different isoforms of α-, ß-, and γ-crystallins was determined. ß-Crystallins were the major contributor of the US fraction in 3-week-old lenses (above 50%), γ-crystallins in 12-week-old lenses (50-60%), and in 62-week-old lenses, the contributions from all three crystallin families leveled out. The major interstrain difference was the elevated level of α-crystallins in the US fraction from 12-week-old OXYS lenses. Spots with increased relative abundance in OXYS maps were attributed to the cataract-specific spots of interest. The crystallins from these spots were subjected to MS/MS analysis, and the positions of acetylation, oxidation, deamidation, and phosphorylation were established. CONCLUSIONS: The increased relative abundance of α-crystallins in the US fraction from 12-week-old OXYS lenses points to the fast insolubilization of α-crystallins under oxidative stress. Most of the PTMs attributed to the cataract-specific modifications also correspond to α-crystallins. These PTMs include oxidation of methionine residues, deamidation of asparagine and glutamine residues, and phosphorylation of serine and threonine residues.

Ligand-directed acid-sensitive amidophosphate 5-trifluoromethyl-2'-deoxyuridine conjugate as a potential theranostic agent.

Herein, we report a novel strategy to engineer an acid-sensitive anticancer theranostic agent using a vector-drug ensemble. The ensemble was synthesized by directly conjugating the linoleic acid (LA)-modified branched polyethyleneimine with a chemotherapeutic drug trifluorothymidine. Linoleic acid residues were grafted onto 25 kDa polyethyleneimine (PEI) by treating PEI with linoleic acid chloroanhydride. 5-Trifluoromethyl-2'-deoxyuridine (trifluorothymidine, TFT) was introduced into LA-PEI conjugate by phosphorylating the conjugate with amidophosphate of trifluorothymidine 5'-monophosphate (pTFT), which had been activated by its conversion into the N,N-dimethylaminopyridine derivative. The extent of mononucleotide analog incorporation in the polymer was regulated by the ratio of pTFT to the polymer during the synthesis. Samples containing 20-70 TFT residues per PEI molecule were obtained. The cytotoxicity of PEI-LA-pTFT conjugates decreased with increasing nucleotide content, as examined using the MTT method. Due to the presence of fluorine atoms, TFT-based conjugates could be detected directly in the animals by (19)F magnetic resonance imaging. In addition, the presence of the amidophosphate group in PEI-LA-pTFT conjugates allowed their detection by in vivo(31)P NMR spectroscopy. Indeed, the (31)P NMR signal of a phosphoramide (δ ~ 12 ppm) was observed in the mouse muscle tissue treated with PEI-LA-pTFT conjugate along with the signals from endogenous phosphorus-containing compounds. At the same time, the use of PEI-LA-pTFT conjugate for chemotherapeutic drug delivery is limited due to the low release of pTFT from the carrier. To enhance the release of the drug from the conjugate in the endosomes, PEI-LA polymer was coupled with urocanic acid (UA), which bears imidazole ring and thus can form an acid-labile P-N bond with pTFT. The PEI-LA-UA-pTFT conjugate containing 30 residues of UA and 40 residues of pTFT was tested against the murine Krebs-II ascites carcinoma, grown as an ascetic tumor. The intraperitoneal injection of the conjugates resulted in prolongation of the animals' life and to the complete disappearance of the tumor after three injections.

Light-induced magnetostructural anomalies in a polymer chain complex of Cu(hfac)2 with tert-butylpyrazolylnitroxides.

We report the study of light-induced magnetostructural anomalies in a polymer chain complex of Cu(hfac)2 (hfac = hexafluoroacetylacetonate) with an unusual acyclic tert-butylpyrazolylnitroxide radical (Ltert(Me)) using EPR. This complex ([Cu(hfac)2Ltert(Me)]n) belongs to the family of thermo- and photoswitchable molecular magnets "breathing crystals". Compared to previously studied breathing crystals with nitronyl nitroxides, [Cu(hfac)2Ltert(Me)]n shows much weaker absorption bands in the visible spectral region and therefore is superior for optical manipulation of the spin states. Illumination with light (λ ≈ 540 nm) at cryogenic temperatures leads to formation of a metastable weakly coupled spin state, which relaxes to the ground strongly coupled spin state on a time scale of hours. These phenomena are in many aspects similar to the light-induced excited spin state trapping (LIESST) well-known for spin-crossover compounds. Remarkably, the photoinduced spin state in [Cu(hfac)2Ltert(Me)]n is metastable at temperatures up to TLIESST ≈ 60 K, which is a significant improvement compared to that of previously studied breathing crystals with nitronyl nitroxides (TLIESST ≈ 20 K). We describe LIESST-like behavior observed in [Cu(hfac)2Ltert(Me)]n and discuss possible reasons for the increased stability of the photoinduced spin state.

Influence of intense THz radiation on spin state of photoswitchable compound Cu(hfac)2L(Pr).

The family of magnetoactive compounds Cu(hfac)(2)L(R) exhibits thermo- and photoswitching phenomena promising for various applications. Photoswitching of the Cu(hfac)(2)L(Pr) compound can be observed at temperatures below 20 K and is accompanied by transition to metastable structural state. Reverse conversion to stable structure could not be induced by light of near-IR-vis-UV regions up to date. The far-IR spectra of metastable and stable structural states are different and show characteristic absorption lines in the range of 170-240 cm(-1). These frequencies are accessible by NovoFEL - high-power THz free-electron laser user facility in Novosibirsk. We investigate selective influence of THz radiation on relaxation processes from metastable to stable structural state, which can be monitored by electron paramagnetic resonance (EPR). For this purpose, the experimental station based on X-band EPR spectrometer has been constructed by the THz beamline of NovoFEL and equipped with multimodal THz waveguide allowing to fed radiation directly into the EPR resonator. It has been found that irradiation of studied compound with high-power THz light causes significant but nondestructive increase of its temperature. Apart from this effect, no resonant influence of THz irradiation on relaxation processes has been observed. The experimental results have been rationalized taking into account vibrational relaxation times of the studied compound. Further experiments based on pulse heating by THz radiation have been proposed.

W-band time-resolved electron paramagnetic resonance study of light-induced spin dynamics in copper-nitroxide-based switchable molecular magnets.

Molecular magnets Cu(hfac)(2)L(R) represent a new type of photoswitchable materials based on exchange-coupled clusters of copper(II) with stable nitroxide radicals. It was found recently that the photoinduced spin state of these compounds is metastable on the time scale of hours at cryogenic temperatures, similar to the light-induced excited spin state trapping phenomenon well-known for many spin-crossover compounds. Our previous studies have shown that electron paramagnetic resonance (EPR) in continuous wave (CW) mode allows for studying the light-induced spin state conversion and relaxation in the Cu(hfac)(2)L(R) family. However, light-induced spin dynamics in these compounds has not been studied on the sub-second time scale so far. In this work we report the first time-resolved (TR) EPR study of light-induced spin state switching and relaxation in Cu(hfac)(2)L(R) with nanosecond temporal resolution. To enhance spectral resolution we used high-frequency TR EPR at W-band (94 GHz). We first discuss the peculiarities of applying TR EPR to the solid-phase compounds Cu(hfac)(2)L(R) at low (liquid helium) temperatures and approaches developed for photoswitching/relaxation studies. Then we analyze the kinetics of the excited spin state at T = 5-21 K. It has been found that the photoinduced spin state is formed at time delays shorter than 100 ns. It has also been found that the observed relaxation of the excited state is exponential on the nanosecond time scale, with the decay rate depending linearly on temperature. We propose and discuss possible mechanisms of these processes and correlate them with previously obtained CW EPR data.

Self-decelerating relaxation of the light-induced spin states in molecular magnets Cu(hfac)2L(R) studied by electron paramagnetic resonance.

Molecular magnets Cu(hfac)(2)L(R) (hfac = hexafluoroacetylacetonate) called "breathing crystals" exhibit thermally and light-induced magnetic anomalies very similar to iron(II) spin-crossover compounds. They are physically different systems, because the spin-state switching occurs in exchange-coupled nitroxide-copper(II)-nitroxide clusters, in contrast to classical spin crossover in d(4)-d(7) transition ions. Despite this difference, numerous similarities in physical behavior of these two types of compounds have been observed, including light-induced excited spin-state trapping (LIESST) phenomenon recently found in the Cu(hfac)(2)L(R) family. Similar to iron(II) spin-crossover compounds, the excited spin state in breathing crystals relaxes to the ground state on the time scale of hours at cryogenic temperatures. In this work, we investigate this slow relaxation in a series of breathing crystals using electron paramagnetic resonance (EPR). Three selected compounds represent the cases of relatively strong or weak cooperativity and different temperature of thermal spin transition. They all were studied in a neat magnetically concentrated form; however, sigmoidal self-accelerating relaxation was not observed. On the contrary, the relaxation shows pronounced self-decelerating character for all studied compounds. Relaxation curves and their temperature dependence could be fitted assuming a tunneling process and broad distribution of effective activation energies in these 1D materials. A number of additional experimental and theoretical arguments support the distribution-based model. Because self-decelerating relaxation behavior was also found in 1D polymeric iron(II) spin-crossover compounds previously, we compared general relaxation trends and mechanisms in these two types of systems. Both similarities and differences of copper-nitroxide-based breathing crystals as compared to iron(II) spin-crossover compounds make future research of light-induced phenomena in these new types of spin-crossover-like systems topical in the field of molecule-based magnetic switches.

Influence of Ecological Factors on the Metabolomic Composition of Fish Lenses.

Multiple stressors related to changes in environmental conditions (such as water temperature, salinity, and natural and anthropogenic pollution) may cause biological responses of aquatic organisms that lead to significant variations in the biochemical reactions in their tissues and thereby change the concentrations of metabolites. We used a quantitative NMR-based metabolomic analysis of the fish lens for the evaluation of the influence of environmental factors on metabolic processes in aquatic animals. For this purpose, three species of freshwater fish-Perca fluviatilis, Rutilus rutilus lacustris, and Gymnocephalus cernua-were caught at approximately the same time at three locations in Siberia (Russia) that differed in levels of dissolved oxygen (LDO) and water purity, and the concentrations of 57 major metabolites in the fish lenses were determined. We found that the metabolomic profiles of the fish lenses strongly depended on the location. The obtained data demonstrated that two typical stressors for aquatic animals-a reduced LDO and anthropogenic water pollution-caused a largely similar metabolic response in the fish lenses that led to an increase in the concentrations of several amino acids and a decrease in sarcosine and phosphoethanolamine. At the same time, the composition of the major lens osmolytes depended mostly on the oxygen level, while variations in AMP (decrease) and NAD (increase) corresponded to the water pollution. We suggest that the eye lens is a very convenient tissue for studying the impact of ecological factors on the metabolic state of aquatic animals, fish in particular.

Age-related changes in the water-soluble lens protein composition of Wistar and accelerated-senescence OXYS rats.

PURPOSE: To determine the age-related and the cataract-specific changes in the crystallin composition in lenses of accelerated-senescence OXYS (cataract model) and Wistar (control) rats. METHODS: The water soluble (WS) and insoluble (WIS) fractions of the lens proteins were separated; the identity and relative abundance of each crystallin in WS fraction were determined with the use of two-dimensional electrophoresis (2-DE) and Matrix-Assisted Laser Desorption Ionization-Time Of Flight (MALDI-TOF) mass spectrometry. All statistical calculations were performed using the software package Statistica 6.0 by factor dispersion analysis (ANOVA/MANOVA) and Newman-Keuls post-hoc test for comparison of group mean values. RESULTS: The WIS protein content increased significantly in the aged animal lenses; the WIS/WS ratio increases in approximately 8 times to the age of 62 weeks. The interstrain difference was insignificant in this experiment. 2-DE maps of the young rat lenses (3 weeks) showed single spots for each lens protein while in older lenses (12 and 62 weeks) each crystallin was presented by several spots. The abundance of γA-γF-crystallins in WS fraction significantly decreases with age. A significant increase in the percentage abundance was also found for α-crystallins and ßB2-crystallin from 3 to 12 weeks. The major differences between Wistar and OXYS lenses are the faster decay of the content of γA-γF-crystallins in OXYS lenses, and the significant decrease of unmodified αA-crystallin abundance in old OXYS lenses. CONCLUSIONS: The presented results demonstrate that the increase of the water-insoluble (WIS) protein fraction is rather age-specific than cataract-specific phenomenon. The major age-related changes in WS protein composition are the fast insolubilization of γ-crystallins, and the increase of αB- and ßB2-crystallin abundance. The main interstrain differences, which could be attributed to the cataract-specific processes, are the faster decay of the content of γ-crystallins and the significant decrease of unmodified αA-crystallin abundance in the OXYS lenses.

Synthesis and characterization of fluorinated homocysteine derivatives as potential molecular probes for ¹9F magnetic resonance spectroscopy and imaging.

A N-trifluoroacetyl-protected amino acid containing a thioester function, 2,2,2-trifluoro-N-(2-oxo-tetrahydrothiophen-3-yl)acetamide (TFA-tHcy), has been synthesized and characterized. It was then used to prepare a fluorine-labeled N-homocysteinylated protein, (19)F-Hcy-εN-Lys-albumin, that was characterized by SDS-PAGE, MALDI-TOF-MS, UV-vis and (19)F NMR spectroscopy. On average, four N-trifluoroacetylhomocysteine residues were covalently conjugated to human serum albumin through the N-substituted homocysteine thiolactone. The in situ homocysteinylation of human plasma proteins with TFA-tHcy has also been performed and has led to the formation of N-homocysteinylated proteins, with albumin modification accounting for ca. 75% of all fluorine-labeled human plasma proteins. The synthesized fluorinated molecular probes can be potentially used as informative molecular probes for in vivo (19)F magnetic resonance spectroscopy and imaging.

Relationship between the thermally induced reorientations of aromatic solvate molecules in Cu(hfac)2-nitroxide breathing crystals and the character of the magnetic anomaly.

A new group of "breathing" crystals has been synthesized. These are aromatic solvates of the copper(II) hexafluoroacetylacetonate complex with spin-labeled pyrazole Cu(hfac)(2)L·0.5Solv, where L is 2-(1-butyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and Solv is benzene, toluene, ethylbenzene, propylbenzene, butylbenzene, styrene, o-xylene, m-xylene, p-xylene, 1,4-bis(trifluoromethyl)benzene, 1-methyl-4-ethylbenzene, 1-methyl-4-vinylbenzene, 1,4-diethylbenzene, 1,2,3-trimethylbenzene, or 1,2,4-trimethylbenzene. The main feature of Cu(hfac)(2)L·0.5Solv single crystals is their remarkable mechanical stability and ability to undergo thermally induced structural rearrangements accompanied by spin-crossover-like phenomena. The structures of Cu(hfac)(2)L·0.5Solv solvates are similar and based on mutually parallel {Cu(hfac)(2)L}(∞) heterospin chains with a "head-to-head" motif. The localization of voids with guest molecules being the same in all crystals, the temperature dependence of the effective magnetic moment (µ(eff)) for Cu(hfac)(2)L·0.5Solv is determined by the structure of the guest molecules, along which the polymer chains are "gliding" when the temperature changes. When the temperature decreased from 300 to 100-50 K, µ(eff) decreased, abruptly or gradually, from 2.7-2.4 to ~1.8 ß for the majority of Cu(hfac)(2)L·0.5Solv except the solvates with benzene, toluene, and 1,4-bis(trifluoromethyl)benzene. When Cu(hfac)(2)L·0.5C(6)H(6) and Cu(hfac)(2)L·0.5CH(3)-C(6)H(5) were cooled to 50 K, µ(eff) decreased to ~2.1-2.2 ß. When Cu(hfac)(2)L·0.5(1,4-(CF(3))(2)-C(6)H(4)) was cooled to 50 K, µ(eff) initially decreased from ~2.7 to 1.9 ß and then abruptly increased to ~2.4 ß. A single-crystal X-ray diffraction analysis of each solvate within a temperature range wider than the range of magnetic anomaly temperatures revealed a complex interrelated dynamics of the aromatic solvent guest molecules and heterospin chains. The dynamics largely depended on the orientation of the solvent guest molecules relative to the polymer chains. An analysis of the thermally induced phase transformations revealed a relationship between the structural rearrangement of Cu(hfac)(2)L·0.5Solv and the form of the magnetic anomaly on the µ(eff)(T) curve and between the structural rearrangement of the solvate and the temperature of the magnetic effect.

Temperature-dependent exchange interaction in molecular magnets Cu(hfac)2L(R) studied by EPR: methodology and interpretations.

Exchange-coupled spin triads nitroxide-copper(II)-nitroxide are the key building blocks of molecular magnets Cu(hfac)(2)L(R). These compounds exhibit thermally induced structural rearrangements and spin transitions, where the exchange interaction between spins of copper(II) ion and nitroxide radicals changes typically by 1 order of magnitude. We have shown previously that electron paramagnetic resonance (EPR) spectroscopy is sensitive to the observed magnetic anomalies and provides information on both inter- and intracluster exchange interactions. The value of intracluster exchange interaction is temperature-dependent (J(T)), that can be accessed by monitoring the effective g-factor of the spin triad as a function of temperature (g(eff)(T)). This paper describes approaches for studying the g(eff)(T) and J(T) dependences and establishes correlations between them. The experimentally obtained g(eff)(T) dependences are interpreted using three different models for the mechanism of structural rearrangements on the molecular level leading to different meanings of the J(T) function. The contributions from these mechanisms and their manifestations in X-ray, magnetic susceptibility and EPR data are discussed.