An EPR Study on Intramolecular Spin Exchange and Rotational Mobility of Nitroxides Linked by a Longchain

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An EPR Study on Intramolecular Spin Exchange,Rotational Mobility of Nitroxides Linked by a Long-chain

The effects of pressure, solvent on the intramolecular spin exchange of biradicals having two nitroxide fragments linked by a long flexible chain were studied by means of highpressure EPR technique. It was found that the intramolecular exchange interaction between nitroxides of biradical took place through the direct contact between them. By analyzing the observed EPR spectra, we have estimated the ratio(Τ_in/Τ_out) value of the average lifetime of the radical fragments inside a cage(Τ_in) to that outside the cage(Τ_out). The Τ_in/Τ_out values decreased with decreasing temperature, increasing pressure. The results suggest that the nearly cyclic conformation in a cage is favorable in solution. Fur ther, the rotational correlation time of individual nitroxide was estimated from the anisotropic EPR signal,, the information on the segmental motion of the nitroxide group in biradical was obtained.     

Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents.     

Thermodynamics of some long-chain biradicals studied by EPR spectroscopy

Four biradicals of differing chain lengths have been prepared by spin-labelling the sulphhydryl groups of dithiothreitol. Temperature-dependent EPR spectra of these biradicals indicate that the dynamics of biradicals is associated with both fast and slow exchanges of conformations. The three-conformational model incorporating cage configuration is found to be more appropriate. Certain thermodynamic parameters of conformationalchanges have been calculated from the experimental EPR parameters, obtained by the use of simulation procedures developed recently.     

Synthesis of trityl radical-conjugated disulfide biradicals for measurement of thiol concentration

Measurement of thiol concentrations is of great importance for characterizing their critical role in normal metabolism and disease. Low-frequency electron paramagnetic resonance (EPR) spectroscopy and imaging, coupled with the use of exogenous paramagnetic probes, have been indispensable techniques for the in vivo measurement of various physiological parameters owing to the specificity, noninvasiveness and good depth of magnetic field penetration in animal tissues. However, in vivo detection of thiol levels by EPR spectroscopy and imaging is limited due to the need for improved probes. We report the first synthesis of trityl radical-conjugated disulfide biradicals (TSSN and TSST) as paramagnetic thiol probes. The use of trityl radicals in the construction of these biradicals greatly facilitates thiol measurement by EPR spectroscopy since trityls have extraordinary stability in living tissues with a single narrow EPR line that enables high sensitivity and resolution for in vivo EPR spectroscopy and imaging. Both biradicals exhibit broad characteristic EPR spectra at room temperature because of their intramolecular spin-spin interaction. Reaction of these biradicals with thiol compounds such as glutathione (GSH) and cysteine results in the formation of trityl monoradicals which exhibit high spectral sensitivity to oxygen. The moderately slow reaction between the biradicals and GSH (k(2) ～ 0.3 M(-1) s(-1) for TSSN and 0.2 M(-1) s(-1) for TSST) allows for in vivo measurement of GSH concentration without altering the redox environment in biological systems. The GSH concentration in rat liver was determined to be 3.49 ± 0.14 mM by TSSN and 3.67 ± 0.24 mM by TSST, consistent with the value (3.71 ± 0.09 mM) determined by the Ellman's reagent. Thus, these trityl-based thiol probes exhibit unique properties enabling measurement of thiols in biological systems and should be of great value for monitoring redox metabolism.     

Nitroxide biradicals from substituted bis(hydroxy-ureas)

The EPR spectra of several nitroxide biradicals derived from substituted bis(hydroxy-ureas) have been determined. The main differences in these spectra arose dur to intramolecular interaction of the nitroxide groups. Interaction was only observed for the case where the nitroxide group was bonded to an aliphatic group.     

Thermally induced spin transitions in nitroxide-copper(II)-nitroxide spin triads studied by EPR

Thermally induced spin transitions in a family of heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides are studied using electron paramagnetic resonance (EPR) spectroscopy. The structural rearrangements at low temperatures induce spin transitions in exchange-coupled spin triads of nitroxide-copper(II)-nitroxide. The values of exchange interactions in spin triads of studied systems are typically on the order of tens to hundreds of inverse centimeters. The large magnitude of exchange interaction determines the specific and very informative peculiarities in EPR spectra due to the predominant population of the ground state of a spin triad and spin exchange processes. The variety of these manifestations depending on structure and magnetic properties of spin triads are described. EPR is demonstrated as an efficient tool for the characterization of spin transitions and for obtaining information on the temperature-dependent sign and value of the exchange interaction in strongly coupled spin triads.     

Formation of linear polymers with pendant vinyl groups via inclusion complex mediated polymerization of divinyl monomers

Ethylene glycol dimethacrylate (EGDMA) and ethylene glycol methacrylate 4-vinyl benzoate (EGMAVB) were shown to form 1:1 inclusion complexes with cyclodextrin and were characterized by instrumental techniques. Computational analysis showed that the bent conformation of the included divinyl monomer was more stable than its linear conformation. Complexation of the divinyl monomer with the first CD molecule offered substantial stabilization than with the second CD molecule. The vinyl group included in the CD cavity did not participate in polymerization. As a result, solvent soluble, linear polymers with pendant vinyl unsaturation per repeat unit were obtained. This was unequivocally established by the polymerization of a complex comprising CD and EGMAVB. The unreacted vinyl group can be polymerized in the subsequent step to yield cross-linked products.     

EPR study of dialkyl nitroxides as probes to investigate the exchange of solutes between the ligand shell of monolayers of protected gold nanoparticles and aqueous solutions

EPR spectroscopy has been used to study the interaction of para-substituted benzyl hydroxyalkyl nitroxides with the monolayer of water-soluble protected gold cluster made by a short alkyl chain and a triethylene glycol monomethyl ether unit. The inclusion of nitroxide probes in the more hydrophobic environment of the monolayer gave rise to a reduction of the value of both nitrogen and beta-proton hyperfine splittings. The spectra also showed selective line broadening attributed to modulation of the spectroscopic parameters as the result of exchange between free and complexed nitroxide. The rate constants were obtained by analyzing the EPR line shape variations as functions of nanoparticle concentration and temperature. This represents, to the best of our knowledge, the first determination of rate constants for the solubilization of organic substrates in a monolayer-protected cluster.     

Spin-labelled mechanically interlocked molecules as models for the interpretation of biradical EPR spectra

Biradical spin probes can provide detailed information about the distances between molecules/regions of molecules because the through-space coupling of radical centres, characterised by J, is strongly distance dependent. However, if the system can adopt multiple configurations, as is common in supramolecular complexes, the shape of the EPR spectrum is influenced not only by J but also the rate of exchange between different states. In practice, it is often hard to separate these variables and as a result, the effect of the latter is sometimes overlooked. To demonstrate this challenge unequivocally we synthesised rotaxane biradicals containing nitronyl nitroxide units at the termini of their axles. The rotaxanes exchange between the available biradical conformations more slowly than the corresponding non-interlocked axles but, despite this, in some cases, the EPR spectra of the axle and rotaxane remain remarkably similar. Detailed analysis allowed us to demonstrate that the similar EPR spectral shapes result from different combinations of J and rates of conformational interconversion, a phenomenon suggested theoretically more than 50 years ago. This work reinforces the idea that thorough analysis must be performed when interpreting the spectra of biradicals employed as spin probes in solution.

Using a rotaxane biradical, we unambiguously demonstrate the need consider both J and rate of conformational interconversion carefully when interpreting the spectra of such systems when they are employed as spin probes in solution.     

Pseudo cyclic ionophores: ‘Binary-effect’ of quinolinyloxy groups at both ends of oligoethylene glycols on the conformational stabilization of their complexes with alkali metal salts

A series of noncyclic neutral ionophores has been synthesized by the reaction of oligoethylene glycol dihalides with 8-quinolinol. Complexation properties for alkali metal picrates were evaluated from solvent extraction and bulk liquid membrane transport experiments. Complexation profiles of the newly synthesized ionophores with a hexyl chain were similar to those of their homologues without the hexyl chain in the extraction experiments. Among them, the pentaethylene glycol derivatives showed the highest extraction efficiency and selectivity towards potassium ion. From the¹H NMR spectra (400 MHz), the change in chemical shifts of the aromatic protons upon the addition of alkali metal thiocyanates suggested the existence of a stabilization effect which is caused by intramolecular stacking conformations between the quinoline rings during complexation. Aryl stacking interactions depend on the size of the cations and on the chain length of the oligoethylene glycol. The relationship between transport ability towards alkali metal cations and lipophilicity of these ionophores is also discussed.     

Inclusion complexes of PBN-type nitrone spin traps and their superoxide spin adducts with cyclodextrin derivatives: parallel determination of the association constants by NMR titrations and 2D-EPR simulations

(1)H NMR and electron paramagnetic resonance (EPR) titrations were used to determine the association constants of the complexes of alpha-phenyl-N-tert-butylnitrone (PBN) analogues and their superoxide spin adducts, respectively, with methylated beta-cyclodextrins. A 1:1 stoichiometry for the nitrones with randomly methylated beta-cyclodextrin and 2,6-di-O-methyl-beta-cyclodextrin and 1:1 and 1:2 stoichiometries for the corresponding cyclodextrin-nitroxide complexes were observed. After the superoxide radical spin trapping reaction, EPR titrations afforded the association constants of the corresponding cyclodextrin-nitroxide complexes. Two-dimensional EPR simulations indicated a bimodal inclusion of the nitroxide free radical spin adducts into the cyclodextrins. For all the nitrone-cyclodextrin and nitroxide-cyclodextrin complexes, the association constants were always higher for the nitroxide complexes than for the nitrone complexes. A cooperative system concerning the complexation of the nitroxide spin adduct with a cyclodextrin was evidenced by EPR titrations. The efficiency of the cyclodextrin inclusion technique to trap superoxide and to resist bioreduction by sodium l-ascorbate was also investigated.     

Evaluation of the β value of the phenylene unit by probing exchange interaction between two nitroxides

Oligophenylene molecular rods with bicyclo[2.2.2]octane having two nitronyl nitroxide radicals were synthesized to investigate the decay constant of p-phenylene. By the measurement and simulation of the ESR spectra of the biradicals with different rod length, it was found that the exchange interaction was decreased with the decay constant β of 0.51 ± 0.01 ?(-1). This result indicates that the spin-spin exchange interaction between neutral radicals has a decay constant similar to the molecular conductance.     

Structures of ionic species and electronic interactions in radical ions; ketyls with divalent cations

EPR and optical spectra of magnesium and calcium ketyls were investigated in MTHF and mixtures of DMF and MTHF at various concentrations. It was shown that these ketyls form aggregates in ethereal solvents and dissociate into biradicals in more polar solutions. Models of ion aggregate to explain the observed EPR and optical spectra are suggested.     

Rapamycin-cyclodextrin complexation: improved solubility and dissolution rate

The objective of this study was to improve poor aqueous solubility and dissolution properties of anticancer drug rapamycin through formation of inclusion complexes with natural and modified cyclodextrins. Of the cyclodextrins tested, ??-cyclodextrin and hydroxypropyl-??-cyclodextrin did not complex with rapamycin. However, complexes of rapamycin with ??-cyclodextrin, methyl-??-cyclodextrin and hydroxypropyl-??-cyclodextrin were prepared and characterized by techniques such as Fourier Transform infrared spectroscopy, differential scanning calorimetry, phase solubility analysis and in vitro dissolution studies. According to the characterization data for the complexes, rapamycin water solubility was highly enhanced by all three ??-cyclodextrins with methyl-??-cyclodextrin complex resulting in particularly higher solubility enhancement. FTIR spectra and DSC thermograms supported the formation of inclusion complexes. The complexes showed highly improved dissolution rate in water. Complexation with cyclodextrin derivatives such as methyl-??-cyclodextrin and hydroxypropyl-??-cyclodextrin can provide promising alternatives for the formulation of rapamycin.     

酸性染料与吐温-80相互作用的研究

通过对铬天青S（CAS）、麝香草酚兰（TB）等与吐温－80（Tween-80)的相互作用的研究,测定了二者之间的络合常数K和结合比N,证明了这二者之间有很强的相互作用力,并通过比较染料在不同溶剂的吸收光谱,推断了表面活性剂与染料相互作用点的位置,证明了极性较大的CAS位于聚氧乙烯链的外围“栅栏”中,而极性较小的TB则位于胶束的烷基“内核”。     

Study on the heteroatom influence in pyridine-based nitronyl nitroxide biradicals with phenylethynyl spacers on the molecular ground state

Novel pyridine-based nitronyl nitroxide (NIT) biradicals, 3,5-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (1) and 2,6-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (2), and monoradicals, 4-(5-bromopyridine-3-ylethynyl)-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (3), 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (4), and 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)pyridine (5), were synthesized and investigated by ESR and UV-vis spectroscopy. The solution EPR measurements of the biradicals gave well-resolved, nine-line spectra with exact half line spacing as compared to monoradicals (giso = 2.0067) with isotropic line spacing /aN/= 7.36 G. This indicates strong, intramolecular exchange coupling (J > 7 x 10(-4) cm(-1); J/aN > 1) of the biradicals with in the limit of EPR. The temperature dependence on the Deltams = +/-2 signal intensity of biradicals follow Curie behavior down to 4 K ascertaining the triplet ground state or its near-degeneracy with the singlet state. UV-vis studies of 1-5 show characteristic differences in the extinctions of n-pi transitions around 600 nm. Both biradicals 1 and 2 were crystallized in monoclinic space groups C2/c and P2(1)/a with the intraradical distances 1.54 and 1.47 nm, respectively. Computational studies of the biradicals 1, 2, and 1,3-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]benzene (T) were performed by the AM1/CAS(8,8) method to calculate the singlet-triplet (DeltaEST) energy difference and the spin density distribution. Results show that the position of the pyridyl nitrogen in 1 and 2 in comparison with T does not alter the triplet ground-state spin multiplicities supporting the obtained experimental results.     

Binuclear copper(II) complexes based on acyldihydrazones of imino-, oxo-, and thiodiacetic acids and 2-hydroxyacetophenone

The results of studying the binuclear copper(II) complexes with acyldihydrazones of amino-, oxo-, and thiodiacetic acids and 2-hydroxyacetophenone were described. The temperature variations of the EPR spectra of solutions with a seven-line hyperfine structure indicating an exchange character of interaction between unpaired electrons and the two equivalent nuclei of copper atoms were studied.     

TOTAPOL: a biradical polarizing agent for dynamic nuclear polarization experiments in aqueous media

In a previous publication, we described the use of biradicals, in that case two TEMPO molecules tethered by an ethylene glycol chain of variable length, as polarizing agents for microwave driven dynamic nuclear polarization (DNP) experiments. The use of biradicals in place of monomeric paramagnetic centers such as TEMPO yields enhancements that are a factor of approximately 4 larger (epsilon approximately 175 at 5 T and 90 K) and concurrently the concentration of the polarizing agent is a factor of 4 smaller (10 mM electron spins), reducing the residual electron nuclear dipole broadening. In this paper we describe the synthesis and characterization by EPR and DNP/NMR of an improved polarizing agent 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL). Under the same experimental conditions and using 2.5 mm magic angle rotors, this new biradical yields larger enhancements (epsilon approximately 290) at lower concentrations (6 mM electron spins) and has the additional important property that it is compatible with experiments in aqueous media, including salt solutions commonly used in the study of proteins and nucleic acids.     

Synthesis and spectroscopic studies on copper(II) binuclear complexes of 1-phenylamidino-O-alkylurea (alkyl = n-propyl, n- and iso-butyl) with 1,3-diaminopropane or ethylenediamine

Copper(II) binuclear complexes [Cu(II)(1-phenylamidino-O-n-propylurea)tn]2 (H2O)2(Cl2)2 (1), [Cu(II)(1-phenylamidino-O-n-butylurea)tn]2(H2O)2(Cl2)2(2), [Cu(II)(1-phenylamidino-O-i-butylurea)tn]2(H2O)2(Cl2)2(3), and [Cu(II)(1-phenyamidino-O-i-butylurea)en]2(H2O)2(Cl2)2 (4) have been reported. The binuclear complexes 3 and 4 crystallize in a monoclinic structure with unit cell dimensions a = 15.252(17) A, b = 14.682(10) A, c = 13.606(13) A, and beta = 111.2(1) degrees and a = 15.278(35) A, b = 14.665(21) A, c = 13.603(27) A, and beta = 111.1(1) degrees , respectively. The EPR spectra of all the solid complexes at room temperature consisted of fine-structure transitions (DeltaM(s) = 1) with zero-field splitting (ZFS) of 0.0500 cm(-1) and a half-field signal (DeltaM(s) = 2) at ca. 1600 G, suggesting the formation of binuclear complexes (S = 1). From the observed ZFS, we estimated the average Cu-Cu distance. From the temperature dependence of the EPR signal intensity, we evaluated the isotropic exchange interaction constant J. It appears that the exchange interaction between the two interacting spins of the binuclear complexes is ferromagnetic in nature. The formation of ferromagnetically coupled copper binuclear complexes was further confirmed from the high magnetic-moment values at room temperature. When the EPR spectra were recorded in the temperature range 300-400 K, it was observed that the triplet-state EPR signal completely and irreversibly disappeared at ca. 380 K with the appearance of a new signal attributable to the mononuclear complex (S = 1/2). Thermal studies of these complexes in this temperature range suggested the loss of two water molecules, which might be responsible for binding two mononuclear species. EPR, IR, and thermal studies indicate a long-range ferromagnetic exchange mediated through hydrogen bonding between copper(II) ions (S = 1/2).