Detection of natural abundance 1H-13C correlations of cholesterol in its membrane environment using a gradient enhanced HSQC experiment under high resolution magic angle spinning

The quality and signal to noise ratio of a J-based HETCOR performed on a standard MAS probe have been compared with a gradient enhanced HSQC performed on a HR-MAS probe at 500 MHz. The sample selected was cholesterol, inserted at 30 mol% in acyl chain deuterated phospholipids (DMPC-d54), at a temperature where the bilayer is in a liquid crystalline phase (310 K). It is representative of any rigid molecule undergoing fast axial diffusion in a bilayer as the main movement. After optimization of the spinning rate and carbon decoupling conditions, it is shown that the ge-HSQC/MAS approach is far superior to the more conventional J-HETCOR/MAS in terms of signal to noise ratio, and that it allows the detection of all the natural abundance cross peaks of cholesterol in a membrane environment. Clear differences between the 1H and 13C chemical shifts of cholesterol in a membrane and in chloroform solution were thus revealed.

Spin trapping of superoxide, alkyl- and lipid-derived radicals with derivatives of the spin trap EPPN

The N-t-butyl-alpha-phenylnitrone derivative N-2-(2-ethoxycarbonyl-propyl)-alpha-phenylnitrone (EPPN) has recently been reported to form a superoxide spin adduct (t(1/2)=5.25 min at pH 7.0), which is considerably more stable than the respective N-t-butyl-alpha-phenylnitrone or 5,5-dimethylpyrroline N-oxide adducts (t(1/2) approximately 10 and 45s, respectively). In continuation of our previous studies on structure optimization of 5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide derivatives, a series of six different EPPN derivatives was synthesized and characterized by 1H NMR, 13C NMR and IR spectroscopy. The ethoxy group of EPPN was replaced by a propoxy, iso-propoxy, n-butoxy, sec-butoxy, and tert-butoxy moiety, as well as the phenyl by a pyridyl ring. Electron spin resonance spectra and stabilities of the superoxide adducts of the propoxy derivatives were found to be similar to those of the respective EPPN adduct, whereas the electron spin resonance spectra of the superoxide adducts of N-2-(2-ethoxycarbonyl-propyl)-alpha-(4-pyridyl) nitrone and the butoxy derivatives were accompanied by decomposition products. In contrast to the 5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide series, no significant improvement of the superoxide adduct stability could be obtained when the ethoxy group was replaced by other substituents. Carbon centered radical adducts derived from methanol, ethanol, formic acid and linoleic acid hydroperoxide were more stable than those of 5,5-dimethylpyrroline N-oxide, whereas among the alkoxyl radicals only the methoxyl radical adduct could be detected.

Synthesis and antioxidant efficiency of a new amphiphilic spin-trap derived from PBN and lipoic acid

The synthesis of a new amphiphilic antioxidant called PBNLP and derived from both alpha-phenyl-N-tert-butyl nitrone (PBN) and lipoic acid was described. Grafting a lactobionamide moiety onto the aromatic group of the PBN provided the water solubility of this compound. In vitro preliminary biological evaluations of its antioxidant capacity were performed using the KRL biological test based on free radical-induced hemolysis. The PBNLP induces a protection of erythrocytes against exogenous free radicals higher than that measured with lipoic acid or PBN alone or with lipoic acid or PBN derivatives in admixtures.

Synthesis and properties of a new class of nucleosides with spin source

Novel spin-labeled ribonucleosides 1-5 were synthesized to investigate stability and behavior of N-tert-butyl aminoxyl radical on nucleobase. Site selective lithiation of tri-O-protected ribonucleosides followed by the reaction with 2-methyl-2-nitrosopropane (MNP) resulted in introduction of N-tert-butylhydroxylamino group into various positions of purine or pyrimidine nucleus. Oxidation of the obtained hydroxylamines with Ag2O led to formation of 1-5. EPR study showed that the unpaired electron of the aminoxyl radical was delocalized into the nucleobase and hyperfine structures were dependent on the position of the radical.

Conformational dynamics of the active site loop of S-adenosylmethionine synthetase illuminated by site-directed spin labeling

S-adenosylmethionine synthetase (ATP: L-methionine S-adenosyltransferase, methionine adenosyltransferase, a.k.a. MAT) is one of numerous enzymes that have a flexible polypeptide loop that moves to gate access to the active site in a motion that is closely coupled to catalysis. Crystallographic studies of this tetrameric enzyme have shown that the loop is closed in the absence of bound substrates. However, the loop must open to allow substrate binding and a variety of data indicate that the loop is closed during the catalytic steps. Previous kinetic studies indicate that during turnover loop motion occurs on a time scale of 10(-2)s, ca. 10-fold faster than chemical transformations and turnover. Site-directed spin labeling has been used to introduce nitroxide groups at two positions in the loop to illuminate how the motion of the loop is affected by substrate binding. The two loop mutants constructed, G105C and D107C, retain wild type levels of MAT activity; attachment of a methanethiosulfonate spin label to convert the cysteine to the "R1" residue reduced the k(cat) only for the labeled D107R1 form (7-fold). The K(m) value for methionine increased 2- to 4-fold for the cysteine mutants and 2- to 7-fold for the labeled proteins, whereas the K(m) for ATP was changed by at most 2-fold. EPR spectra for both labeled proteins are nearly identical and show the presence of two major spin label environments with rotational diffusion rates differing by approximately 10-fold; the slower rate is ca. 4-fold faster than the estimated protein rotational rate. The spectra are not altered by addition of substrates or products. At both positions the less mobile conformation constitutes ca. 65% of the total species, indicating an equilibrium that only slightly favors one form, that in which the label is more immobilized. The equilibrium constant that relates the two forms is comparable to the equilibrium constant of 1.5 for a conformational change that was previously deduced from the viscosity dependence of the rate of AdoMet formation. The results suggest that the motion of the loop may be an intrinsic property of the protein and not be strictly ligand modulated.

Solution-state dynamics of sugar-connected spin probes in sucrose solution as studied by multiband (L-, X-, and W-band) electron paramagnetic resonance

A multiband (L-band, 0.7GHz; X-band, 9.4GHz; and W-band, 94GHz) electron paramagnetic resonance (EPR) study was performed for two glycosidated spin probes, 4-(alpha,beta-D-glucopyranosyloxy)-TEMPO (Glc-TEMPO) and 4-(alpha,beta-D-lactopyranosyloxy)-TEMPO (Lac-TEMPO), and one non-glycosylated spin probe, 4-hydroxy-TEMPO (TEMPOL), where TEMPO=2,2,6,6-tetramethyl-1-piperidinyloxyl, to characterize fundamental hydrodynamic properties of sugar-connected spin probes. The linewidths of these spin probes were investigated in various concentrations of sucrose solutions (0-50wt%). The multiband approach has allowed full characterization of the linewidth parameters, providing insights into the molecular shapes of the spin probes in sucrose solution. The analysis based on the fast-motional linewidth theory has yielded anisotropy parameters of rho(x) approximately 2.6 and rho(y) approximately 0.9 for Glc-TEMPO, and rho(x) approximately 4.2 and rho(y) approximately 0.9 for Lac-TEMPO. These values indicate that the glycosidated spin probes have a prolate-type molecular shape elongated along the x-axis (NO(rad) axis) with Lac-TEMPO elongated more remarkably, consistent with their molecular structures. The interaction parameters k (the ratios of the effective hydrodynamic volumes to the real ones) corrected for the difference in molecular shape have been estimated and found to have the relation k(TEMPOL)<k(Glc-TEMPO) approximately k(Lac-TEMPO). This agrees with the expectation that glycosidated spin probes can have stronger hydrogen bonding to water. Glycosidated spin probes are expected to be useful for probing sugar-involving interactions, which commonly occur in biological systems. Thus this study will provide an indispensable basis for such spin-probe studies.

Thermodynamic properties of the kinesin neck-region docking to the catalytic core

Kinesin motors move on microtubules by a mechanism that involves a large, ATP-triggered conformational change in which a mechanical element called the neck linker docks onto the catalytic core, making contacts with the core throughout its length. Here, we investigate the thermodynamic properties of this conformational change using electron paramagnetic resonance (EPR) spectroscopy. We placed spin probes at several locations on the human kinesin neck linker and recorded EPR spectra in the presence of microtubules and either 5'-adenylylimidodiphosphate (AMPPNP) or ADP at temperatures of 4-30 degrees C. The free-energy change (DeltaG) associated with AMPPNP-induced docking of the neck linker onto the catalytic core is favorable but small, about 3 kJ/mol. In contrast, the favorable enthalpy change (DeltaH) and unfavorable entropy change (TDeltaS) are quite large, about 50 kJ/mol. A mutation in the neck linker, V331A/N332A, results in an unfavorable DeltaG for AMPPNP-induced zipping of the neck linker onto the core and causes motility defects. These results suggest that the kinesin neck linker folds onto the core from a more unstructured state, thereby paying a large entropic cost and gaining a large amount of enthalpy.

Purity of different preparations of sodium 3,5-dibromo-4-nitrosobenzenesulphonate and their applicability for EPR spin trapping

During the preparation of sodium 3,5-dibromo-4-nitrosobenzenesulphonate (DBNBS) of high purity for electron paramagnetic resonance (EPR) spin-trapping purposes, it was found that the material synthesised as part of the present study differed significantly from some commercially available samples of DBNBS. A thorough chemical characterisation of the contents of the various samples led to the conclusion that the preparations synthesised in the present study, as well as one of four commercially available samples, contained essentially pure DBNBS and had efficient spin-trapping activity. In contrast, the remaining three commercially available samples contained almost exclusively sodium 3,5-dibromo-4-nitrobenzenesulphonate, i.e. a one-oxygen oxidation product of DBNBS, and had little spin-trapping activity. The two compounds were readily separated by reverse-phase high performance liquid chromatography (HPLC). It was further found that the quality of DBNBS preparations may be determined by NMR spectrometry, IR spectrometry, fast atom bombardment-mass spectrometry (FAB-MS) and EPR spectrometry. In particular, UV-Visible spectroscopy may be used to determine A308/A280, which should be greater than 1.8 for a high purity DBNBS preparation.

Probing triplex formation by EPR spectroscopy using a newly synthesized spin label for oligonucleotides

Spin labels have been extensively used to study the dynamics of oligonucleotides. Spin labels that are more rigidly attached to a base in an oligonucleotide experience much larger changes in their range of motion than those that are loosely tethered. Thus, their electron paramagnetic resonance spectra show larger changes in response to differences in the mobility of the oligonucleotides to which they are attached. An example of this is 5-(2,2,5,5-tetramethyl-3-ethynylpyrrolidine-1-oxyl)-uridine (1). How ever, the synthesis of this modified DNA base is quite involved and, here, we report the synthesis of a new spin-labeled DNA base, 5-(2,2,6,6-tetramethyl-4-ethynylpiperidyl-3-ene-1-oxyl)-uridine (2). This spin label is readily prepared in half the number of steps required for 1, and yet behaves in a spectroscopically analogous manner to 1 in oligonucleotides. Finally, it is shown here that both spin labels 1 and 2 can be used to detect the formation of both double-stranded and triplex DNA.

A Novel Electron Paramagnetic Resonance Approach to Determine the Mechanism of Drug Transport by P-glycoprotein

ATP-driven pumping of a variety of drugs out of cells by the human P-glycoprotein poses a serious problem to medical therapy. High level heterologous expression of human P-glycoprotein, in the yeast Saccharomyces cerevisiae, has facilitated biophysical studies in purified proteoliposome preparations. Membrane permeability of transported drugs and consequent lack of an experimentally defined drug position have made resolution of the transport mechanism difficult by classical techniques. To overcome these obstacles we devised a novel EPR spin-labeled verapamil for use as a transport substrate. Spin-labeled verapamil was an excellent transport substrate with apparent turnover number, K(m) and K(i) values of 5.8 s(-1), 4 microm, and 210 microm, respectively, at pH 7.4 and 37 degrees C. The apparent affinities were approximately 10-fold higher than for unlabeled verapamil. Spin-labeled verapamil stimulated ATPase activity approximately 5-fold, was relatively hydrophilic, and had a very low flip-flop rate, making it an ideal transport substrate. The K(m) for MgATP activation of transport was 0.8 mm. By measuring the mobility of spin-labeled verapamil during transport experiments, we were able to resolve the location of the drug in proteoliposome suspensions. Steady state gradients of spin-labeled verapamil within the range of K(i)/K(m) ratios were observed.

Two spin labeled triazenes: relationship between biochemical and biological activities

Biochemical and biological activities of two recently synthesized spin labeled triazenes, containing the nitroxyl free radical moiety at different places of the triazene structure have been studied and compared with those of the antitumor drug Dacarbazine (DTIC). Tissue distribution of the triazenes was investigated in vitro in organ homogenates, tumor (B16 melanoma) and blood of C57BL mice using the electron paramagnetic resonance (EPR) method. The spin labeled triazenes were mainly localized in the tumor and in the brain. Normal leucocites, YAC-1 mNK target Moloney lymphoma cells and B16 melanoma cells were treated with spin labeled triazenes in vitro and the effects on cell viability were compared. Spin labeled 3,3-dimethyl triazene with nitroxyl radical as a substituent in the benzen ring was more cytotoxic to B16 melanoma cells than to YAC-1 Moloney lymphoma cells and normal leucocites in comparison to the spin labeled monomethyl triazene. The spin labeled derivatives were assessed with low toxicity for BDF1 mice hybrids in vivo. These results could be interpreted in terms of a possible correlation between tissue distribution and the selective antimelanoma activity of the spin labeled triazenes.

Acyl-protected hydroxylamines as spin label generators for EPR brain imaging

In a search for novel electron paramagnetic resonance (EPR) brain imaging agents, we have designed and synthesized the acyl-protected hydroxylamines 1-acetoxy-4-methoxycarbonyl-2,2,6,6-tetramethylpiperidine (AMCPe), 1-acetoxy-3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine (AMCPy), and 1-acetoxy-3-(acetoxymethoxy)carbonyl-2,2,5,5-tetramethylpyrrolidine (DACPy), in which both the ring size and the number of ester functions were varied. In all of them, the nitroxide was first reduced and the resultant hydroxylamine was then protected with an acetyl group. These compounds are lipophilic, which is a major prerequisite for blood-brain barrier penetration. Once in the brain, esterases and oxidants quickly convert these derivatives into ionic, water-soluble radicals and thus EPR detectable species that then reside in the central nervous system for periods of time sufficient for detection and imaging. The biological relevancy of these new compounds in mice has been assessed, and their biodistribution patterns have been compared. The five-membered ring derivative AMCPy emerged as a potent EPR brain imaging agent while the other two derivatives, AMCPe and DACPy, were quite ineffective.

Spin adducts of superoxide, alkoxyl, and lipid-derived radicals with EMPO and its derivatives

The compound 5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide (EMPO) is a hydrophilic cyclic nitrone spin trap, which, in contrast to DMPO, forms a relatively stable superoxide adduct (t(1/2)=8.6 min) with an EPR spectrum similar to the respective DMPO adduct. In order to find the optimal degree of lipophilicity of this novel type of spin trap with respect to the detection of radicals formed during lipid peroxidation, the ethoxy group of EMPO was replaced by alkoxy substituents of increasing chain length, leading to the methoxy- (MeMPO), 1-propoxy- (PrMPO), 1-butoxy- (BuMPO), and 1-octyloxy- (OcMPO) derivatives of EMPO. The stability of their superoxide adducts was found to be strongly dependent on the size of the alkoxycarbonyl group. Increasing chain length of the alkoxyl substituent decreased the stability of alkoxyl radical adducts of MeMPO, EMPO, and PrMPO, but increased the stability of OcMPO adducts. The stability of alkoxyl radical adducts of BuMPO, on the other hand, were practically independent of the size of the alkoxyl group. Detection of lipid alkoxyl radicals formed by peroxidizing linoleic acid in a stationary system was therefore only possible with the most lipophilic spin trap, OcMPO. However, with the more hydrophilic spin traps MeMPO, EMPO, PrMPO, and BuMPO optimal EPR signal intensity could be obtained when a slow-flow system was used. Thus, within this series EMPO is the best spin trap for the detection of superoxide; OcMPO, on the other hand, is most suitable for the detection of lipid alkoxyl radicals.

Water-soluble aminoxyls (nitroxides): 2-methyl-2-[(N-(4-tert-butylphenyl)oxyl]propanesulfonate and (1-oxyl-2,5,5-trimethylpyrrolidin-2-yl)methanesulfonate

Two charged aminoxyls, ammonium (1-oxyl-2,5,5-trimethylpyrrolidin-2-yl)methanesulfonate and ammonium 2-methyl-2-[(N-(4-tert-butylphenyl)oxyl]propanesulfonate, obtained by methods easily adaptable to the preparation of various other aminoxyls are totally soluble in water but are partially associated in other solvents.

Structure of spin-labeled methylmethanethiolsulfonate in solution and bound to TEM-1 beta-lactamase determined by electron nuclear double resonance spectroscopy

Site-directed spin-labeling of proteins whereby the spin-label methyl 3-(2,2,5,5-tetramethyl-1-oxypyrrolinyl)methanethiolsulfonate (SLMTS) is reacted with the -SH groups of cysteinyl residues incorporated into a protein by mutagenesis has been successfully applied to investigate secondary structure and conformational transitions of proteins. In these studies, it is expected that the spin-label moiety adopts different conformations dependent on its local environment. To determine the conformation of SLMTS in solution reacted with L-cysteine (SLMTCys) and bound in the active site of the Glu240Cys mutant of TEM-1 beta-lactamase, we have synthesized SLMTS both of natural abundance isotope composition and in site-specifically deuterated forms for electron nuclear double resonance (ENDOR) studies. ENDOR-determined electron-proton distances from the unpaired electron of the nitroxyl group of the spin-label to the methylene and methyl protons of SLMTS showed three conformations of the oxypyrrolinyl ring with respect to rotation around the S-S bond dependent on the solvent dielectric constant. For SLMTCys, two conformations of the molecule were compatible with the ENDOR-determined electron-nucleus distances to the side-chain methylene protons and to H(alpha) and H(beta1,2) of cysteine. To determine SLMTS conformation reacted with the Glu240Cys mutant of TEM-1 beta-lactamase, enzyme was overexpressed in both ordinary and perdeuterated minimal medium. Resonance features of H(alpha) and H(beta1,2) of the Cys240 residue of the mutant and of the side-chain methylene protons within the spin-label moiety yielded electron-proton distances that sterically accommodated the two conformations of free SLMTCys in solution.

Novel glycosylation of the nitroxyl radicals with peracetylated glycosyl fluorides using a combination of BF(3) x OEt(2) and an amine base as promoters

Glycosylation of the nitroxyl radicals, 4-acetoxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-acetoxy-TEMPO) and 3-carbamoyl-2,2,5,5-tetramethylpyrollin-1-oxyl (3-carbamoyl-PROXYL) with peracetylglycosyl fluoride as the glycosyl donor, in the presence of boron trifluoride diethyl etherate (BF(3) x OEt(2)) and an amine base afforded the corresponding hydroxylamine-O-glycosides in 25-100% yields.

Fmoc-POAC: [(9-fluorenylmethyloxycarbonyl)-2,2,5,5-tetramethylpyrrolidine-N-oxyl-3-amino-4-carboxylic acid]: a novel protected spin labeled beta-amino acid for peptide and protein chemistry

The stable free radical 2,2,6,6-tetramethylpiperidine-N-oxyl-4-amino-4-carboxylic acid (TOAC) is the only spin labeled amino acid that has been used to date to successfully label peptide sequences for structural studies. However, severe difficulty in coupling the subsequent amino acid has been the most serious shortcoming of this paramagnetic marker. This problem stems from the low nucleophilicity of TOAC's amine group towards the acylation reaction during peptide chain elongation. The present report introduces the alternative beta-amino acid 2,2,5,5-tetramethylpyrrolidine-N-oxyl-3-amino-4-carboxylic acid (POAC), potentially useful in peptide and protein chemistry. Investigations aimed at addressing the stereochemistry of this cyclic molecule through X-ray diffraction measurements of crystalline and bulk samples revealed that it consists only of the trans conformer. The 9-fluorenylmethyloxycarbonyl group (Fmoc) was chosen for temporary protection of the POAC amine function, allowing insertion of the probe at any position in a peptide sequence. The vasoactive octapeptide angiotensin II (All, DRVYIHPF) was synthesized by replacing Pro7 with POAC. The reaction of Fmoc-POAC with the peptidyl-resin occurred smoothly, and the coupling of the subsequent amino acid showed a much faster reaction when compared with TOAC. POAC7-AII was obtained in good yield, demonstrating that, in addition to TOAC, POAC is a convenient amino acid for the synthesis of spin labeled peptide analogues. The present findings open the possibility of a wide range of chemical and biological applications for this novel beta-amino acid derivative, including structural investigations involving its differentiated bend-inducing characteristics.

Molecular Design of a New Class of Spin-Labeled Ribonucleosides with N-tert-Butylaminoxyl Radicals

We designed a new type of spin-labeled nucleosides with an N-tert-butylaminoxyl radical which is introduced to the nucleobase directly. Purine and pyrimidine ribonucleosides containing the aminoxyl radical such as 1a-d, 2, 3, and 4 were synthesized to investigate the stability and behavior of the N-tert-butylaminoxyl radical on a nucleobase. Lithiation of tri-O-silylated 6-chloropurine ribonucleoside (5) followed by reaction with 2-methyl-2-nitrosopropane (MNP) gave the key compound 6a, which was further converted to 6b-d. Oxidation of the obtained 6a-d and their triols (7a-d) with Ag(2)O led to formation of the corresponding stable spin-labeled nucleosides (8a-d and 1a-d), which were confirmed by EPR spectroscopy. Similarly, the precursors of spin-labeled pyrimidines (13, 20, and 23) were synthesized by site-selective lithiation of tri-O-protected pyrimidine derivatives (9, 18, and 21) followed by the reaction with MNP and deprotection. An EPR study showed that the aminoxyl radicals (2, 3, and 4) were stable and that their hyperfine structures were dependent on the position of the radical. Electron densities of pyrimidine also affected hyperfine structures.

Molecular motion of spin labeled side chains in alpha-helices: analysis by variation of side chain structure

Two single cysteine substitution mutants at helix surface sites in T4 lysozyme (D72C and V131C) have been modified with a series of nitroxide methanethiosulfonate reagents to investigate the structural and dynamical origins of their electron paramagnetic resonance spectra. The novel reagents include 4-substituted derivatives of either the pyrroline or pyrrolidine series of nitroxides. The spectral line shapes were analyzed as a function of side chain structure and temperature using a simulation method with a single order parameter and diffusion rates about three orthogonal axes as parameters. Taken together, the results provide strong support for an anisotropic motional model of the side chain, which was previously proposed from qualitative features of the spectra and crystal structures of spin labeled T4 lysozyme. Site-specific differences in apparent order parameter are interpreted in terms of backbone dynamics modes with characteristic correlation times in the nanosecond or faster time scale. The saturated 4-substituted pyrrolidine nitroxides are shown to be a suitable template for novel "functionalized" side chains designed to mimic salient features of the native side chains they replace.

A new facile method for spin-labeling of oligonucleotides

A new facile method for spin-labeling suitable for DNA and RNA oligonucleotides is presented. The nitroxide 3-ethenyl-2,2,5,5-tetramethyl-pyrrolin-1-yloxy was directly introduced during automated solid-phase synthesis by a Pd(0) cross coupling reaction. The main advantages of this procedure are the small amount of spin-label needed for the derivatisation of the oligonucleotide and the high coupling efficiency on the solid phase.

Spin trapping of lipid radicals with DEPMPO-derived spin traps: detection of superoxide, alkyl and alkoxyl radicals in aqueous and lipid phase

The spin trap 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO) forms a superoxide adduct with a half-life of almost 15 min. DEPMPO is very hydrophilic and its use for the detection of radicals in the lipid phase (lipid-derived radicals and superoxide generated in the lipid phase) is therefore limited due to its very low concentration in the lipid phase. For the detection of lipid-derived radicals, three derivatives of DEPMPO with increasing degree of lipid solubility have been investigated: 5-(di-n-propoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DPPMPO), 5-(di-n-butoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DBPMPO), and 5-(bis-(2-ethylhexyloxy)phosphoryl)-5-methyl-1-pyrroline N-oxide (DEHPMPO). As compared with the spin trap DMPO, the half-lives of the respective superoxide adducts were clearly higher in aqueous solutions of the spin traps, which facilitates qualitative ESR measurements. The stability of the superoxide spin adducts formed with the various lipophilic spin traps in aqueous buffer were similar to those observed with DEPMPO (half-life: 7-11 min.). In model experiments using Fe(3+)-catalyzed nucleophilic addition of methanol or tert-butanol to the respective spin trap the respective alkoxyl radical adducts were formed in aqueous solution as transient species in the presence of high concentrations of the alcohol. Upon dilution with water the alkoxyl group was substituted by water, giving the respective hydroxyl adduct of the spin trap. Care must therefore be taken when Fenton-type reactions are used for the generation of radicals such as the use of Fe(2+) complexes with phosphate or DTPA or inactivation of iron by addition of "Desferal" (Novarti's Pharma GmbH, Vienna, Austria) after a short incubation time. Addition of Fe(2+) under anaerobic conditions to an aqueous suspension of linoleic acid hydroperoxide and the spin trap resulted in the detection of three different species: a carbon-centered radical adduct, an acyl radical adduct, and the hydroxyl adduct. In the presence of oxygen a different species was observed with DEPMPO, DPPMPO, and DBPMPO, which was only slightly suppressed upon the addition of SOD, possibly the respective spin adduct of either the alkylperoxyl radical or, in analogy to DMPO, a secondary alkoxyl radical.

Oligonucleotide labeling: synthesis of a new spin-labeled 2'-deoxyguanosine analogue

In order to achieve an EPR sensitive probe for DNA, 3-carboxy-Proxyl free radical was linked to O-6 of dG through a five-atoms-tether. The modified base was incorporated into a 30-mer ODN, then annealed to its complementary DNA strand. Hydrodynamic parameters show only a slight destabilization with respect to the equivalent unlabeled hybrid. EPR could monitor the hybrid formation showing a progressive enlargement of the upfield signal in passing from the labeled ss- to the ds-30-mer.

Detection of superoxide anion using an isotopically labeled nitrone spin trap: potential biological applications

We describe the synthesis and biological applications of a novel nitrogen-15-labeled nitrone spin trap, 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide ([(15)N]EMPO) for detecting superoxide anion. Superoxide anion generated in xanthine/xanthine oxidase (100 nM min(-1)) and NADPH/calcium-calmodulin/nitric oxide synthase systems was readily detected using EMPO, a nitrone analog of 5,5'-dimethyl-1-pyrroline N-oxide (DMPO). Unlike DMPO-superoxide adduct (DMPO-OOH), the superoxide adduct of EMPO (EMPO-OOH) does not spontaneously decay to the corresponding hydroxyl adduct, making spectral interpretation less confounding. Although the superoxide adduct of 5-(diethoxyphosphoryl)-5-methyl-pyrroline N-oxide is more persistent than EMPO-OOH, the electron spin resonance spectra of [(14)N]EMPO-OOH and [(15)N]EMPO-OOH are less complex and easier to interpret. Potential uses of [(15)N]EMPO in elucidating the mechanism of superoxide formation from nitric oxide synthases, and in ischemia/reperfusion injury are discussed.