Quantifying residue-specific conformational dynamics of a highly reactive 29-mer peptide

Understanding structural transitions within macromolecules remains an important challenge in biochemistry, with important implications for drug development and medicine. Insight into molecular behavior often requires residue-specific dynamics measurement at micromolar concentrations. We studied MP01-Gen4, a library peptide selected to rapidly undergo bioconjugation, by using electron paramagnetic resonance (EPR) to measure conformational dynamics. We mapped the dynamics of MP01-Gen4 with residue-specificity and identified the regions involved in a structural transformation related to the conjugation reaction. Upon reaction, the conformational dynamics of residues near the termini slow significantly more than central residues, indicating that the reaction induces a structural transition far from the reaction site. Arrhenius analysis demonstrates a nearly threefold decrease in the activation energy of conformational diffusion upon reaction (8.0 kBT to 3.4 kBT), which occurs across the entire peptide, independently of residue position. This novel approach to EPR spectral analysis provides insight into the positional extent of disorder and the nature of the energy landscape of a highly reactive, intrinsically disordered library peptide before and after conjugation.

Synthesis of Spin-Labelled Bergamottin: A Potent CYP3A4 Inhibitor with Antiproliferative Activity

Bergamottin (BM, 1), a component of grapefruit juice, acts as an inhibitor of some isoforms of the cytochrome P450 (CYP) enzyme, particularly CYP3A4. Herein, a new bergamottin containing a nitroxide moiety (SL-bergamottin, SL-BM, 10) was synthesized; chemically characterized, evaluated as a potential inhibitor of the CYP2C19, CYP3A4, and CYP2C9 enzymes; and compared to BM and known inhibitors such as ketoconazole (KET) (3A4), warfarin (WAR) (2C9), and ticlopidine (TIC) (2C19). The antitumor activity of the new SL-bergamottin was also investigated. Among the compounds studied, BM showed the strongest inhibition of the CYP2C9 and 2C19 enzymes. SL-BM is a more potent inhibitor of CYP3A4 than the parent compound; this finding was also supported by docking studies, suggesting that the binding positions of BM and SL-BM to the active site of CYP3A4 are very similar, but that SL-BM had a better ∆G_bind value than that of BM. The nitroxide moiety markedly increased the antitumor activity of BM toward HeLa cells and marginally increased its toxicity toward a normal cell line. In conclusion, modification of the geranyl sidechain of BM can result in new CYP3A4 enzyme inhibitors with strong antitumor effects.

Measurement of Angstrom to Nanometer Molecular Distances with 19 F Nuclear Spins by EPR/ENDOR Spectroscopy

Spectroscopic and biophysical methods for structural determination at atomic resolution are fundamental in studies of biological function. Here we introduce an approach to measure molecular distances in bio-macromolecules using 19 F nuclear spins and nitroxide radicals in combination with high-frequency (94 GHz/3.4 T) electron-nuclear double resonance (ENDOR). The small size and large gyromagnetic ratio of the 19 F label enables to access distances up to about 1.5 nm with an accuracy of 0.1-1 Å. The experiment is not limited by the size of the bio-macromolecule. Performance is illustrated on synthesized fluorinated model compounds as well as spin-labelled RNA duplexes. The results demonstrate that our simple but strategic spin-labelling procedure combined with state-of-the-art spectroscopy accesses a distance range crucial to elucidate active sites of nucleic acids or proteins in the solution state.

Synthesis of Nanometer Sized Bis- and Tris-trityl Model Compounds with Different Extent of Spin-Spin Coupling

Tris(2,3,5,6-tetrathiaaryl)methyl radicals, so-called trityl radicals, are emerging as spin labels for distance measurements in biological systems based on Electron Paramagnetic Resonance (EPR). Here, the synthesis and characterization of rigid model systems carrying either two or three trityl moieties is reported. The monofunctionalized trityl radicals are connected to the molecular bridging scaffold via an esterification reaction employing the Mukaiyama reagent 2-chloro-methylpyridinium iodide. The bis- and tris-trityl compounds exhibit different inter-spin distances, strength of electron-electron exchange and dipolar coupling and can give rise to multi-spin effects. They are to serve as benchmark systems in comparing EPR distance measurement methods.

Imaging thiol redox status in murine tumors in vivo with rapid-scan electron paramagnetic resonance

Thiol redox status is an important physiologic parameter that affects the success or failure of cancer treatment. Rapid scan electron paramagnetic resonance (RS EPR) is a novel technique that has shown higher signal-to-noise ratio than conventional continuous-wave EPR in in vitro studies. Here we used RS EPR to acquire rapid three-dimensional images of the thiol redox status of tumors in living mice. This work presents, for the first time, in vivo RS EPR images of the kinetics of the reaction of ²H,¹⁵N-substituted disulfide-linked dinitroxide (PxSSPx) spin probe with intracellular glutathione. The cleavage rate is proportional to the intracellular glutathione concentration. Feasibility was demonstrated in a FSa fibrosarcoma tumor model in C3H mice. Similar to other in vivo and cell model studies, decreasing intracellular glutathione concentration by treating mice with l-buthionine sulfoximine (BSO) markedly altered the kinetic images.

Orthogonal spin labeling using click chemistry for in vitro and in vivo applications

Site-directed spin labeling for EPR- and NMR spectroscopy has mainly been achieved exploiting the specific reactivity of cysteines. For proteins with native cysteines or for in vivo applications, an alternative coupling strategy is required. In these cases click chemistry offers major benefits by providing a fast and highly selective, biocompatible reaction between azide and alkyne groups. Here, we establish click chemistry as a tool to target unnatural amino acids in vitro and in vivo using azide- and alkyne-functionalized spin labels. The approach is compatible with a variety of labels including reduction-sensitive nitroxides. Comparing spin labeling efficiencies from the copper-free with the strongly reducing copper(I)-catalyzed azide-alkyne click reaction, we find that the faster kinetics for the catalyzed reaction outrun reduction of the labile nitroxide spin labels and allow quantitative labeling yields within short reaction times. Inter-spin distance measurements demonstrate that the novel side chain is suitable for paramagnetic NMR- or EPR-based conformational studies of macromolecular complexes.

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.

Toward synthesis of third-generation spin-labeled podophyllotoxin derivatives using isocyanide multicomponent reactions

Spin-labeled podophyllotoxins have elicited widespread interest due to their far superior antitumor activity compared to podophyllotoxin. To extend our prior studies in this research area, we synthesized a new generation of spin-labeled podophyllotoxin analogs via isocyanide multicomponent reactions and evaluated their cytotoxicity against four human cancer cell lines (A-549, DU-145, KB and KBvin). Most of the compounds exhibited potent cytotoxic activity against all four cell lines, notably against the drug resistant KBvin cancer cell line. Among the new analogs, compounds 12e (IC50: 0.60-0.75 μM) and 12h (IC50: 1.12-2.03 μM) showed superior potency to etoposide (IC50: 2.03 to >20 μM), a clinically available anticancer drug. With a concise efficient synthesis and potent cytotoxic profiles, compounds 12e and 12h merit further development as a new generation of epipodophyllotoxin-derived antitumor clinical trial candidates.

Structure and dynamics of an imidazoline nitroxide side chain with strongly hindered internal motion in proteins

A disulfide-linked imidazoline nitroxide side chain (V1) has a similar and highly constrained internal motion at diverse topological sites in a protein, unlike that for the disulfide-linked pyrroline nitroxide side chain (R1) widely used in site directed spin labeling EPR. Crystal structures of V1 at two positions in a helix of T4 Lysozyme and quantum mechanical calculations suggest the source of the constraints as intra-side chain interactions of the disulfide sulfur atoms with both the protein backbone and the 3-nitrogen in the imidazoline ring. These interactions apparently limit the conformation of the side chain to one of only three possible rotamers, two of which are observed in the crystal structure. An inter-spin distance measurement in frozen solution using double electron-electron resonance (DEER) gives a value essentially identical to that determined from the crystal structure of the protein containing two copies of V1, indicating that lattice forces do not dictate the rotamers observed. Collectively, the results suggest the possibility of predetermining a unique rotamer of V1 in helical structures. In general, the reduced rotameric space of V1 compared to R1 should simplify interpretation of inter-spin distance information in terms of protein structure, while the highly constrained internal motion is expected to extend the dynamic range for characterizing large amplitude nanosecond backbone fluctuations.

Pulsed ESR dipolar spectroscopy for distance measurements in immobilized spin labeled proteins in liquid solution

Pulsed electron spin resonance (ESR) dipolar spectroscopy (PDS) in combination with site-directed spin labeling is unique in providing nanometer-range distances and distributions in biological systems. To date, most of the pulsed ESR techniques require frozen solutions at cryogenic temperatures to reduce the rapid electron spin relaxation rate and to prevent averaging of electron-electron dipolar interaction due to the rapid molecular tumbling. To enable measurements in liquid solution, we are exploring a triarylmethyl (TAM)-based spin label with a relatively long relaxation time where the protein is immobilized by attachment to a solid support. In this preliminary study, TAM radicals were attached via disulfide linkages to substituted cysteine residues at positions 65 and 80 or 65 and 76 in T4 lysozyme immobilized on Sepharose. Interspin distances determined using double quantum coherence (DQC) in solution are close to those expected from models, and the narrow distance distribution in each case indicates that the TAM-based spin label is relatively localized.

Neuroprotective effects of tempol acyl esters against retinal ganglion cell death in a rat partial optic nerve crush model

PURPOSE

The aim of this study is to search for more effective derivatives of the superoxide dismutase mimetic tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl). Although tempol is neuroprotective in a rat partial optic nerve crush (PONC) model, relatively high doses are required to exert this effect.

METHODS

Tempol acyl esters with different-length fatty acids (tempol-C4, tempol-C8, tempol-C12 and tempol-C16) were synthesized and the following properties were evaluated: water-octanol partition coefficient, liposome-liposome energy transfer, and electron paramagnetic resonance (EPR). Brown Norway rats underwent PONC and received tempol or acyl esters intraperitoneally once daily for 7 consecutive days. We then compared the effects of tempol and its four esters on retinal ganglion cell (RGC) damage using a retrograde labelling method.

RESULTS

The water-octanol partition coefficient increased with increasing length of attached acyl chain. However, the energy of the liposome-liposome transfer seemed to be optimal for tempol-C8 and tempol-C12. The EPR signal was very similar for all tested compounds, suggesting similar efficiency of superoxide scavenging. Partial optic nerve crush in vehicle-treated animals reduced RGC numbers by approx. 59% when compared with sham-operated eyes. Tempol did not affect RGC loss at a dose of 1 mg/kg. In contrast, at molar doses equivalent to 1 mg/kg of tempol, tempol-C8 showed a significant neuroprotective effect, whereas tempol-C4, tempol-C12 and tempol-C16 did not act neuroprotectively.

CONCLUSION

Manipulating the hydrophobicity of tempol seems to be a promising tool for developing more potent neuroprotectants in the PONC degeneration model. However, the resulting compounds need further pharmacological evaluation.

A rigid disulfide-linked nitroxide side chain simplifies the quantitative analysis of PRE data

The measurement of (1)H transverse paramagnetic relaxation enhancement (PRE) has been used in biomolecular systems to determine long-range distance restraints and to visualize sparsely-populated transient states. The intrinsic flexibility of most nitroxide and metal-chelating paramagnetic spin-labels, however, complicates the quantitative interpretation of PREs due to delocalization of the paramagnetic center. Here, we present a novel, disulfide-linked nitroxide spin label, R1p, as an alternative to these flexible labels for PRE studies. When introduced at solvent-exposed α-helical positions in two model proteins, calmodulin (CaM) and T4 lysozyme (T4L), EPR measurements show that the R1p side chain exhibits dramatically reduced internal motion compared to the commonly used R1 spin label (generated by reacting cysteine with the spin labeling compound often referred to as MTSL). Further, only a single nitroxide position is necessary to account for the PREs arising from CaM S17R1p, while an ensemble comprising multiple conformations is necessary for those observed for CaM S17R1. Together, these observations suggest that the nitroxide adopts a single, fixed position when R1p is placed at solvent-exposed α-helical positions, greatly simplifying the interpretation of PRE data by removing the need to account for the intrinsic flexibility of the spin label.

Pulsed dipolar spectroscopy distance measurements in biomacromolecules labeled with Gd(III) markers

This work demonstrates the feasibility of using Gd(III) tags for long-range Double Electron Electron Resonance (DEER) distance measurements in biomacromolecules. Double-stranded 14- base pair Gd(III)-DNA conjugates were synthesized and investigated at K(a) band. For the longest Gd(III) tag the average distance and average deviation between Gd(III) ions determined from the DEER time domains was about 59±12Å. This result demonstrates that DEER measurements with Gd(III) tags can be routinely carried out for distances of at least 60Å, and analysis indicates that distance measurements up to 100Å are possible. Compared with commonly used nitroxide labels, Gd(III)-based labels will be most beneficial for the detection of distance variations in large biomacromolecules, with an emphasis on large scale changes in shape or distance. Tracking the folding/unfolding and domain interactions of proteins and the conformational changes in DNA are examples of such applications.

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

The number of dicarbonyl-o-semiquinonato (o-iminosemiquinonato) rhodium complexes was isolated and characterized. Some reactions of these compounds with tertiary phosphines (substitution, addition) were studied by EPR in solution using o-semiquinones (o-iminosemiquinones) as spin labels.

Development of a DNA-binding TEMPO derivative for evaluation of nuclear oxidative stress and its application in living cells

Oxidative stress in nuclei is known to induce either oxidative modification of DNA bases or single/double-strand breaks, which may lead to carcinogenesis. To evaluate the redox status in nuclei in living cells, we designed a novel nucleus-localizing redox spin probe, F-DisT, which contains a fluorescein fluorophore linked to a DNA minor-groove-binding moiety. Nuclear distribution of the probe was easily confirmed by colocalization with a nuclear stain, Hoechst 33342, in confocal microscopy. Measurement of oxidative stress with F-DisT in a murine macrophage cell line exposed to endotoxin (lipopolysaccharide) showed a remarkable increase in the ESR signal decay rate. This increase was significantly inhibited by N(ω)-nitro-l-arginine (nitric oxide synthase inhibitor) and diphenyleneiodonium chloride (NADPH oxidase inhibitor). These results indicate that nitric oxide and superoxide contribute to oxidative stress in nuclei. Similar studies in membrane or mitochondria using respective organelle-specific spin probes indicated that the redox microenvironments in these organelles are markedly different from that in nuclei. Thus, subcellular redox microenvironments show marked variability in endotoxin-stimulated living cells.

Trityl-based EPR probe with enhanced sensitivity to oxygen

An asymmetric derivative of the triarylmethyl radical, TAM-H, containing one aldehyde and two carboxyl groups, was synthesized. The electron paramagnetic resonance (EPR) spectrum of TAM-H is characterized by a doublet of narrow lines with a linewidth of 105 mG under anoxic conditions and hyperfine interaction constant of 245 mG. The partial overlap of the components of the doublet results in enhanced sensitivity of the spectral amplitudes ratio to oxygen compared with oxygen-induced linewidth broadening of a single line. Application of the TAM-H probe allows for EPR measurements in an extended range of oxygen pressures from atmospheric to 1 mm Hg, whereas the EPR spectrum linewidth of the popular TAM-based oxygen sensor Oxo63 is practically insensitive to oxygen partial pressures below 20 mm Hg. Enhanced sensitivity of the TAM-H probe relative to Oxo63 was demonstrated in the detection of oxygen consumption by Met-1 cancer cells. The TAM-H probe allowed prolonged measurements of oxygen depletion during the hypoxia stage and down to true anoxia (<or=1.5 mm Hg).

Synthesis and ESR studies of a novel cyclic nitrone spin trap attached to a phosphonium group-a suitable trap for mitochondria-generated ROS?

In this study we report the synthesis and biological application of a novel cyclic nitrone spin trap containing a phosphonium cation. This new spin trap ([4-(2-methyl-1-oxy-3, 4-dihydro-2H-pyrrole-2-carbonyloxy)-butyl]-triphenyl-phosphonium bromide, MitoBMPOBr) is a derivative of the cyclic nitrone, 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO). MitoBMPOBr forms radical adducts upon trapping of superoxide and hydroxyl radicals that exhibit highly distinct and characteristic EPR spectra. The stability of these adducts is comparable to those of BMPO. Because of the presence of a positively-charged phosphonium moiety, MitoBMPOBr may be suitable for trapping reactive oxygen species (ROS) in the mitochondria.

Synthesis and application of a novel sunscreen-antioxidant

Background information on the inefficacy of sunscreens to provide free radical protection in skin, despite their usefulness in preventing sunburn/erythema, prompted us to synthesize a compound which would display in the same molecule both UV-absorbing and antioxidant capacities. For this purpose, the UVB absorber, 2-ethylhexyl-4-methoxycinnamate (OMC) was combined with the piperidine nitroxide TEMPOL, which has antioxidant properties. The spectral properties of the new nitroxide-based sunscreen (MC-NO) as well as its efficacy to prevent photo-oxidative damage to lipids induced by UVA, natural sunlight and 4-tert-butyl-4-methoxydibenzoylmethane (BMDBM), a photo-unstable sunscreen which generates free radicals upon UV radiation, was studied. The results obtained demonstrate that MC-NO: (a) absorbs in the UVB region even after UVA irradiation; (b) acts as free radical scavenger as demonstrated by EPR experiments; (c) strongly reduces both UVA-, sunlight- and BMDBM-induced lipid peroxidation in liposomes, measured as reduced TBARS levels; and (d) has comparable antioxidant activity to that of commonly used vitamin E and BHT in skin care formulations. These results suggest that the use of the novel sunscreen-antioxidant or of other nitroxide-based sunscreens in formulations aimed at reducing photoinduced skin damage may be envisaged.

2-Substituted-3H-indol-3-one-1-oxides: Preparation and Radical Trapping Properties

A series of 2-alkyl and 2-aryl substituted-3H-indol-3-one-1-oxides was prepared and evaluated for its radical trapping properties. Spin trapping and electron paramagnetic resonance experiments demonstrate the ability of these indolone-1-oxides to trap hetero- and carbon-centered radicals. The most stable spin adducts (lifetime of several hours) are obtained with 2-alkyl substituted nitrones, the 2-ethyl-5,6-dioxolo-3H-indolone-1-oxide, 5e and the 2-secbutyl-3H-indolone-1-oxide, 5f. These two nitrones are also sensitive to redox reactions in solution. Therefore this indolone-1-oxide series lacking a beta-hydrogen atom gives rise to highly stable adducts with free radicals.

Synthesis and properties of umbelliferone-nitroxide radical hybrid compounds as fluorescence and spin-label probes

Two hybrid compounds 6 and 7, linked via an ester-bond between the 7-hydroxyl residue of an umbelliferone 1 and a carboxylic acid residue of two nitroxide radicals 3 and 4, and one hybrid compound 8, linked via an ester-bond between a 3-carboxylic acid residue of umbelliferone 2 and a hydroxyl residue of nitroxide radical 5, were synthesized in good yields, and their fluorescence and ESR spectra before and after the addition of L-ascorbic acid sodium salt in PBS (pH 7.0) were measured. The ESR intensities of 6 and 7 were proportionally reduced after the addition of ascorbic acid sodium salt, and their fluorescence intensities were increased maximally by eight- and nine-fold, respectively. However, the fluorescence intensity of 8 was essentially unchanged.

Polarity dependence of EPR parameters for TOAC and MTSSL spin labels: correlation with DOXYL spin labels for membrane studies

TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) is a nitroxyl amino acid that can be incorporated in the backbone of peptides. DOXYL (4,4-dimethyl-oxazolidine-1-oxyl) is a nitroxyl ring that can be attached rigidly at specific C-atom positions in the acyl chains of phospholipids. Spin-labelled phosphatidylcholines of the DOXYL type have been used previously to establish the transmembrane polarity profile in biological lipid bilayers [D. Marsh, Polarity and permeation profiles in lipid membranes, Proc. Natl. Acad. Sci. USA 87 (2001) 7777-7782]. Here, we determine the polarity dependence of the isotropic (14)N-hyperfine couplings, a(o)(N), and g-values, g(o), in a wide range of protic and aprotic media, for a TOAC-containing dipeptide (Fmoc-TOAC-Aib-OMe) and for a DOXYL-containing fatty acid (12-DOXYL-stearic acid). The correlation between datasets for TOAC and DOXYL nitroxides in the various solvents is used to establish the polarity profile for isotropic hyperfine couplings of TOAC in a transmembrane peptide. This calibration can be used to determine the location of TOAC at selected residue positions in a transmembrane or surface-active peptide. A similar calibration procedure is also applied to a(o)(N) and g(o) for the pyrroline methanethiosulphonate nitroxide (MTSSL) that is used in site-directed spin-labelling studies of membrane proteins.

Spin-labeled rifamycin: biological activity

3-[(2,2,6,6-Tetramethylpiperidine-4-ylimino)methyl]rifamycin (4) and spin-labeled rifamycin-3-[(2,2,6,6- tetramethyl-1-oxyl-piperidine-4-ylimino)methyl]rifamycin (1) were prepared. The structures of these compounds were determined by IR, UV, MS and 1H NMR of 4. The ESR-spectrum of 1 is a symmetric triplet signal, characteristic of nitroxyl radicals, g = 2.0025. An in vitro comparative study of the cytotoxicity and antitumor activity of 1, 4 and the initial 3-formyl-rifamycin was carried out in concentrations from 0.1 to 0.001 mM on cells before and after oxidative stress (preliminary irradiation 7Gy) on MH3924A-hepatoma rat cells, 293 transformed human fibroblasts, NBK transformed human fibroblasts and HT 1080 human fibrosarcoma. The compounds showed a cytostatic effect to 85%, with 1 being less toxic in the hepatoma cell line. In human melanoma cell lines 1 showed a higher toxicity than 4. All the derivatives (1 and 4) have in vitro antibacterial activity comparable with that of rifampicin.

Design of New Derivatives of Nitrone DEPMPO Functionalized at C-4 for Further Specific Applications in Superoxide Radical Detection

A general synthetic route to prepare derivatives of the DEPMPO nitrone (5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide) functionalized at C-4 was established via the synthesis of 4-HMDEPMPO nitrone (5-diethoxyphosphoryl-4-hydroxymethyl-5-methyl-1-pyrroline-N-oxide) that was obtained from reduction of the nitro compound 1. (4R*,5S*)-4-HMDEPMPO was successfully separated from its minor diastereoisomer and could be used to generate various substituted analogues. Among them, 4-NHSDEPMPO, 5-diethoxyphosphoryl-4-succinimidyloxycarbonyloxymethyl-5-methyl-1-pyrroline-N-oxide, constitutes a NH2-reactive precursor for further conjugation to relevant moieties such as targeting groups, labels, or drugs. From 4-NHSDEPMPO, a biotinylated nitrone was synthesized offering new perspectives for targeted delivery applications. A short study of the trapping behaviors of the (4R*,5S*)-isomer of these 4-HMDEPMPO analogues proved that they are as good as DEPMPO for detecting superoxide. For each isomer, only one diastereoisomer adduct was obtained, resulting from the addition of superoxide on the less hindered face of the nitrone, that is, trans to the phosphoryl group and the C-4 substituent. From spectra simulation and experiments in various solvents, we proved that ESR patterns of each adduct corresponded to the superimposed signals of two sets of conformers in a sufficiently slow chemical exchange to induce a widening and a dissymmetry of some of the signal lines. This phenomenon was drastically reduced when compared with that observed for DEPMPO superoxide and attributed to a similar chemical exchange, and it did not hamper spectrum assignment. Determination of the decay rate of the superoxide adduct of (4R*,5S*)-4-HMDEPMPO proved that it has a 25% longer half-life time than the superoxide adduct of DEPMPO.

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 of TOAC spin-labeled proteins and reconstitution in lipid membranes

A procedure is described for the synthetic incorporation into membrane proteins of the non-natural amino acid TOAC (2,2,6,6-tetramethyl-piperidine-1-oxyl-4-amino-4-carboxylic acid), which is coupled rigidly to the alpha-carbon, providing direct detection of peptide backbone dynamics by electron paramagnetic resonance (EPR). Also included is a protocol for the functional reconstitution of the spin-labeled protein in lipid vesicles. This protocol can be completed in 17 d.

Development of molecular probes for second-site screening and design of protein tyrosine phosphatase inhibitors

We report on the design, synthesis, and evaluation of a series of furanyl-salicyl-nitroxide derivatives as effective chemical probes for second-site screening against phosphotyrosine phosphatases (PTPs) using NMR-based techniques. The compounds have been tested against a panel of PTPs to assess their ability to inhibit a broad spectrum of these phosphatases. The utility of the derived compounds is illustrated with the phosphatase YopH, a bacterial toxin from Yersinia pestis. Novel chemical fragments were identified during an NMR-based screen for compounds that are capable of binding on the surface of YopH in regions adjacent the catalytic site in the presence of the spin-labeled compounds. Our data demonstrate the value of the derived chemical probes for NMR-based second-site screening in PTPs.

Novel membrane-localizing TEMPO derivatives for measurement of cellular oxidative stress at the cell membrane

Oxidative stress affecting lipid membranes is considered to be closely related to cardiovascular disease and brain ischemia. In this study, we designed and synthesized membrane-localizing TEMPO derivatives and demonstrated that one of these synthesized probes, compound 1, localized and detected oxidative stress in the cell membrane in an endotoxic model of a mouse macrophage-like cell line. Compound 1 is therefore a potentially useful probe for evaluating oxidative stress at the cell membrane.

Attaching a spin to a protein -- site-directed spin labeling in structural biology

Site-directed spin labeling and electron paramagnetic resonance spectroscopy have recently experienced an outburst of multiple applications in protein science. Numerous interesting strategies have been introduced for determining the structure of proteins and its conformational changes at the level of the backbone fold. Moreover, considerable technical development in the field makes the technique an attractive approach for the study of structure and dynamics of membrane proteins and large biological complexes at physiological conditions. This review focuses on a brief description of site-directed spin labeling-derived techniques in the context of their recent applications.

Molecular design of novel spin-labeled nucleosides directly attached base moiety

N-tert-Butylaminoxyl nucleosides (1 and 2) were synthesized starting from 6-chloropurine derivative (3). These compounds are expected as a new tool to elucidate structure of DNA. Their properties and pH-dependent change of EPR spectra were studied.

Dynamic nuclear polarization properties of nitroxyl radicals used in Overhauser-enhanced MRI for simultaneous molecular imaging

DNP parameters relevant to Overhauser-enhanced magnetic resonance imaging (OMRI) are reported for a few nitroxyl radicals and their corresponding (15)N and (2)H enriched analogues, used in simultaneous imaging by OMRI. DNP enhancement was measured at 14.529 mT, using a custom-built scanner operating in a field-cycled mode, for different concentrations, ESR irradiation times and RF power levels. DNP enhancements increased with agent concentration up to 2.5 mM and decreased above 3 mM, in tune with ESR line broadening measured at X-band as a function of the agent concentration. The proton spin-lattice relaxation times (T(1)) measured at very low Zeeman field (14.529 mT) and the longitudinal relaxivity parameters were estimated. The relaxivity parameters were in good agreement with those independently computed from the linear region of the concentration dependent enhancement. The leakage factor showed an asymptotic increase with increasing agent concentration. The coupling parameters of (14)N- and (15)N-labeled carbamoyl-PROXYL showed the interaction between the electron and nuclear spins to be mainly dipolar in origin. Upon (2)H labeling, about 70% and 40% increases in enhancement for (15)N- and (14)N-labeled nitroxyl agents were observed, respectively. It is envisaged that the results reported here may enable better understanding of the factors determining DNP enhancement to design suitable 'beacons' for simultaneous molecular imaging by OMRI.

Synthesis and spin-trapping behavior of 5-ChEPMPO, a cholesteryl ester analogue of the spin trap DEPMPO

[structure: see text] 5-(Cholesteryloxyethoxyphosphoryl)-5-methylpyrroline N-oxide (5-ChEPMPO), a DEPMPO analogue bearing a cholesterol group on the phosphorus atom, has been prepared and used to trap peroxyl-, alkoxyl-, thiyl-, and carbon-centered radicals in organic solvent. The important steric hindrance in 5-ChEPMPO does not affect the properties of 5-ChEPMPO in comparison to DEPMPO for the spin trapping of an enantiopure linoleic acid hydroperoxide. The 5-ChEPMPO-OOL spin adduct was observed by ESR and confirmed by ESI-MS/MS experiments. The relaxation terms of the 5-ChEPMPO-lipid peroxyl spin adduct were compared with those of other peroxyl spin adducts, and it was shown that the cholesteryl group has only a weak influence on the exchange rate between adduct conformers.

Cytotoxicity of novel derivatives of the spin trap EMPO

Free radicals are involved in different regulatory and pathological processes. The formation of superoxide in living cells or whole organisms is of major interest. ESR spin trapping allows identification of the radicals if proper spin traps are available. Our study investigates the toxicity of novel derivatives of the spin trap EMPO to cultured human lung carcinoma cells (A549), breast carcinoma cells (SKBR3), colon carcinoma cells (SW480) as well as to human fibroblasts (F2000). A dose-dependent decrease of the cell number was observed for all spin traps. At 100mM BuMPO, t-BuMPO and s-BuMPO caused pronounced cell loss (>90%) and increased LDH-release, while DEPMPO, EMPO, PrMPO and i-PrMPO caused only moderate cell loss (<60%) without any effect on the LDH-release after 24h. At 10mM and 50mM the latter agents even decreased LDH-release. 10mM and 50mM of i-PrMPO as well as 10mM PrMPO increased intracellular GSH content acting like antioxidants, whereas 50mM s-BuMPO and PrMPO decreased GSH content by 67% and 38%, respectively. Staining for apoptotic nuclei did not reveal any differences between controls and treated cultures indicating necrotic cell death possibly due to membrane toxicity. The following toxicity ranking was obtained: t-BuMPO>BuMPO>s-BuMPO>PrMPO>i-PrMPO approximately DEPMPO approximately EMPO. The least toxic compounds were DEPMPO (LD(50)=143 mM for SW480, 117 mM for A549 or 277 mM for F2000) and i-PrMPO (LD(50)=114 mM for SKBR3), the most toxic one was t-BuMPO (LD(50)=5-6mM for all cell types). In conclusion, up to 50mM i-PrMPO (t(1/2)=18.8 min) and up to 10 mM s-BuMPO (t(1/2)=26.3 min) can be recommended for further investigation of superoxide in biological systems.

Synthesis and Biological Evaluation of Spin-Labeled Alkylphospholipid Analogs

Alkylphospholipid analogues of perifosine and miltefosine bearing a nitroxide moiety at different positions on an alkyl chain were synthesized as electron paramagnetic resonance (EPR) probes. Their amphiphilic properties were characterized by determining their critical micelle concentration (cmc) and hemolytic activity on erythrocytes both in free and liposomal form. Spin-labeled analogues as membrane components of large unilamellar liposomes containing cholesterol and dicetyl phosphate or in free solution were evaluated using the MTT assay to determine growth inhibition on MT1, MT3, and MCF7 breast cancer cell lines. 4a (IC50 = 56.4 microM) was found to be significantly more active than the perifosine against the MCF-7 cell line. Its high cmc (194.03 microM) and low hemolytic activity shows that its cytotoxic activity might be more specific; therefore, 4a can be an important molecular tool for further EPR investigations.

Development of spin-labeled probes for adenosine receptors

Functionalized xanthine derivatives bearing a nitroxide moiety at the 3- or 8-position were synthesized as electron paramagnetic resonance (EPR) probes. The 8-cyclopentyl-1-propylxanthine derivative 4, spin-labeled at N3 by substitution with a nitroxide-bearing dihydropyrrole moiety, was a potent and selective A(1) adenosine receptor antagonist (K(i) for A(1) 5.5 nM, 1600-fold selectivity vs A(2A), >200-fold vs A(2B), and 310-fold vs A(3) adenosine receptors). 8-(1-Oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-3-yl)-1,3-dipropylxanthine 10 (K(i) for A(1) 8.2 nM) was similarly potent and selective, while 8-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-1,3-dipropylxanthine 11 (K(i) for A(1) 160 nM) exhibited significantly lower affinity for A(1) adenosine receptors. 8-[4-(((1-Oxyl-2,2,6,6-tetramethylpiperidin-4-yl)amino)-2-oxoethoxy)phenyl]-1-propylxanthine14, a 3-unsubstituted xanthine derivative, was found to be a potent A(2B) adenosine receptor antagonist (K(i) for A(2B) 48 nM) but also exhibited high affinity for A(1) receptors (K(i) for A(1) 15.7 nM). An X-ray structure of compound 10 was obtained, confirming the proposed structure. The novel spin-labeled A(1)-selective or A(1)/A(2B)-nonselective adenosine receptor antagonists may become useful probes for biophysicochemical investigations of adenosine receptors in their membrane environment.

Very stable superoxide radical adducts of 5-ethoxycarbonyl- 3,5-dimethyl-pyrroline N-oxide (3,5-EDPO) and its derivatives

Oxygen radicals are involved in the onset of many diseases. Adequate spin traps are required for identification and localisation of free radical formation in biological systems. Superoxide spin adducts with half-lives up to 20 min at physiological pH have recently been reported to be formed from derivatives of the spin trap 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO). This is a major improvement over DMPO (t(1/2) ca. 45 s), and even DEPMPO (t(1/2) ca. 14 min). In this study, an additional methyl group was introduced into position 3 or 4 of the pyrroline ring which greatly increases the stability of the respective superoxide spin adducts. In addition, the ethoxy group of EMPO was exchanged by either a propoxy- or an iso-propoxy group in order to test the influence of increasing lipophilic properties of the investigated spin traps. The structure of all compounds was confirmed by (1)H and (13)C-NMR with full signal assignment. In comparison with EMPO (t(1/2) ca. 8 min) or DEPMPO (t(1/2) ca. 14 min), the superoxide adducts of all novel spin traps were considerably higher (t(1/2) ca. 12-55 min). In addition, various other spin adducts obtained from oxygen-centered as well as carbon-centered radicals (e.g. derived from methanol or linoleic acid hydroperoxide) were also detected.

Properties of the Silica Layer during the Formation of MCM-41 Studied by EPR of a Silica-Bound Spin Probe

The properties of the silica layer during the formation of the mesoporous material MCM-41 were investigated by electron paramagnetic resonance (EPR) experiments carried out on a specifically designed, organo(trialkoxy)silane spin probe, SL1SiEt. Minute amounts of the spin probe were co-condensed with the silica source, tetraethyl orthosilicate (TEOS), in the synthesis of MCM-41 with cetyltrimethylammonium bromide (CTAB) under basic conditions. The mobility and location of the spin probe were followed in the CTAB micellar solution before the reaction, in the reaction mixture and in the final ordered material. It was found that the EPR spectra of hydrolyzed SL1SiEt throughout the room temperature part of the reaction are characteristic of a fast tumbling species, indicating that the silica is highly fluid prior to drying. After filtering, a slow motion type spectrum was observed, showing that the spin-label experiences considerable motional hindrance. The liquidlike behavior could be restored upon stirring the material in water. When the reaction is performed with a hydrothermal stage, the spectrum of SL1SiEt in the final product is the same as that of the room temperature synthesized material, but the addition of water did not restore the high mobility, due to a higher degree of silica cross-linking. The location of SL1SiEt throughout the formation process was obtained from electron spin-echo envelope modulation (ESEEM) measurements on MCM-41 prepared with CTAB deuterated either at the N-methyl or the alpha position and in a reaction carried out in D2O. Comparing the deuterium modulation depth, k(2H), induced by CTAB-alpha-d2, CTAB-d9, or D2O in CTAB micellar solutions of a number of reference spin probes with those of SL1SiEt revealed that the hydrolyzed SL1SiEt is located near the polar heads of the surfactant in the absence of base and TEOS. This supports the postulation of charge matching at the interface as a driving force for the formation of the mesostructure. Similar experiments carried out on reaction mixtures containing SL1SiEt showed a decrease of k(2H) from CTAB-alpha-d2 and CTAB-d9 compared to the micellar solution, exhibiting practically no time dependence. This indicates that the spin probe is pulled away from the micelle-water interface into the loosely linked, forming silica network. After drying, the modulation depth induced by CTAB-alpha-d2 and CTAB-d9 increases, showing that, once the water is removed, the silica walls contract around the micelles, pushing the silica-linked spin probe into the organic phase within the mesopores.

Synthesis and enzyme-catalyzed hydrolysis of a radical-masked glycosylated spin-label reagent

N1-Acetoxy-2,2,6,6-tetramethylpiperidin-4-yl 2,3,4,6-tetra-O-benzyl-alpha- and -beta-D-glucopyranosides (3-alpha, beta) and N1-acetoxy-2,2,5,5-tetramethylpyrrolin-3-oyl 2,3,4,6-tetra-O-benzyl-alpha- and -beta-D-glucopyranosylamines (9-alpha, beta) were synthesized in good yield by Schmidt's glycosylation method. Their subsequent O-debenzylation was proceeded successfully to give the desired products 1-alpha, and 1-beta in good yield, and 2-alpha in a low yield, without 2-beta by only short-timed hydrogenolysis in the presence of palladium-on-carbon (Pd-C) in a CHCl3-MeOH solvent system that included concentrated HCl. Upon enzyme-catalyzed hydrolysis, only 2-alpha was hydrolyzed by the esterase, while both of 1-alpha and 1-beta were not hydrolyzed by any other enzyme such as lipase. These 2-alpha can likely be used as a new water-soluble radical-masked glycosylated spin-label reagent.

Development of a new EPR spin trap, DOD-8C (N-[4-dodecyloxy-2-(7′-carboxyhept-1′-yloxy)benzylidene]-N-tert-butylamine N-oxide), for the trapping of lipid radicals at a predetermined depth within biological membranes

We report on the development of the first member of a new family of EPR spin-trapping agents designed to trap radicals at a predetermined depth within biological membranes. By analogy to the use of nitroxide spin labels to 'report' on the environment at specific depths within biological membranes, we set out to prepare similar reporter molecules, but with a nitrone in place of the nitroxide function. The prototype compounds were tested in a model system consisting of large unilamellar vesicles exposed to a copper-dependent radical generating system. This entailed the reduction of tert-butylhydroperoxide to the tert-butoxyl radical ((t)BuO(.-)) by a membrane-permeable Cu(I) complex, which was generated in situ by reduction of the Cu(II) complex by ascorbate. To assist in the identification of the radicals detected, preliminary studies were performed in methanolic solution, where the major radical trapped was shown to be (.-)CH(2)OH, resulting from H-atom abstraction from the alcohol by (t)BuO(.-). This conclusion was shown to be in agreement with predictions based on chemical kinetics, which were then used to support the proposal that the primary species trapped in the lipid vesicles were radicals derived from membrane fatty acids. This molecule represents the first of a new generation of spin traps which, through modification, can be used to position the radical-trapping nitrone moiety at chosen depths within biological membranes.

Generation and interrogation of a pure nuclear spin state by parahydrogen-enhanced NMR spectroscopy: a defined initial state for quantum computation

We describe a number of studies used to establish that parahydrogen can be used to prepare a two-spin system in a pure state, which is suitable for implementing NMR quantum computation. States are generated by pulsed and continuous-wave (CW) UV laser initiation of a chemical reaction between Ru(CO)(3)(L(2)) [where L(2) = dppe = 1,2-bis(diphenylphosphino)ethane or L(2) = dpae = 1,2-bis(diphenylarsino)ethane] with pure parahydrogen (generated at 18 K). This process forms Ru(CO)(2)(dppe)(H)(2) and Ru(CO)(2)(dpae)(H)(2) on a sub-microsecond time-scale. With the pulsed laser, the spin state of the hydride nuclei in Ru(CO)(2)(dppe)(H)(2) has a purity of 89.8 +/- 2.6% (from 12 measurements). To achieve comparable results by cooling would require a temperature of 6.6 mK, which is unmanageable in the liquid state, or an impractical magnetic field of 0.44 MT at room temperature. In the case of CW initiation, reduced state purities are observed due to natural signal relaxation even when a spin-lock is used to prevent dephasing. When Ru(CO)(3)(dpae) and pulsed laser excitation are utilized, the corresponding dihydride product spin state purity was determined as 106 +/- 4% of the theoretical maximum. In other words, the state prepared using Ru(CO)(3)(dpae) as the precursor is indistinguishable from a pure state.

Synthesis of 1,5-diisopropyl substituted 6-oxoverdazyls

1,5-Diisopropyl-6-oxo-verdazyl free radicals were synthesized via the condensation of BOC protected isopropyl hydrazine with phosgene, deprotection with aqueous HCl, condensation with aldehydes to form tetrazanes and finally oxidation to give the free radicals. The introduction of isopropyl groups results in free radicals that show greater solubility in a variety of solvents and are more stable than their methyl substituted counterparts. ESR shows reduced hyperfine coupling to the isopropyl methine hydrogens consistent with this hydrogen being in the plane of the verdazyl ring.

Spin adduct formation from lipophilic EMPO-derived spin traps with various oxygen- and carbon-centered radicals

Free radicals are involved in the onset of many diseases, therefore the availability of adequate spin traps is crucial to the identification and localization of free radical formation in biological systems. In recent studies several hydrophilic compounds of 2-ethoxycarbonyl-2-methyl-pyrroline-N-oxide (EMPO) have been found to form rather stable superoxide spin adducts with half-lives up to twenty minutes at physiological pH. This is a major improvement over DMPO (t1/2=ca. 45 s), and even over DEPMPO (t1/2=ca. 14 min), the best commercially available spin trap for the unambiguous detection of superoxide radicals. In order to allow the detection of superoxide and also other radicals in lipid environment a series of more lipophilic derivatives of EMPO was synthesized and their structure unambiguously characterized by 1H and 13C NMR spectroscopy. In this way, six different compounds with a n-butyl group in position 5 and either an ethoxy- (EBPO), propoxy- (PBPO), iso-propoxy- (iPBPO), butoxy- (BBPO), sec-butoxy- (sBBPO) or tert-butoxycarbonyl group (tBBPO) in position 5 of the pyrroline ring were obtained and fully analytically characterized (NMR, IR). The stability of the superoxide adducts of all investigated spin traps were comparable with EMPO (t1/2=ca. 8 min), except for the two compounds bearing an additional methyl group in position 3 or 4 of the pyrroline ring, 5-butyl-5-ethoxycarbonyl-3-methyl-pyrroline-N-oxide (BEMPO-3) and 5-butyl-5-ethoxycarbonyl-4-methyl-pyrroline-N-oxide (BEMPO-4), of which the superoxide adducts were stable for more than 30 min. Spin adducts of other carbon- and oxygen-centered radicals were also investigated.

Nitroxides with two pK values—useful spin probes for pH monitoring within a broad range

A series of 4-dialkylamino-2,5-dihydroimidazole nitroxides with pyridine-4-yl, 4-dimethylaminophenyl or 4-hydroxyphenyl groups in position 2 of the imidazole ring were prepared using the reaction of RMgBr with corresponding 5-dialkylamino-4,4-dimethyl-4H-imidazole 3-oxides. The EPR spectra of the nitroxides were shown to be pH-sensitive due to consecutive protonation of the amidino moiety and the basic group(s) at position 2 of the imidazole ring. The 5,5-dimethyl-4-(dimethylamino)-2-ethyl-2-pyridine-4-yl-2,5-dihydro-1H-imidazol-1-oxyl showed a monotonic increase in the isotropic nitrogen hyperfine (hfi) coupling constant alpha(N) of 1 .4 G over a pH range from 2 to 6.5. Such a broad range of pH-sensitivity could be useful for many biophysical and biomedical applications, including pH-monitoring in the stomach.

Mechanism of oxygen-induced EPR line broadening in lithium phthalocyanine microcrystals

EPR oximetry has been recognized as an important tool for determining oxygen concentration in biological tissues, in vivo. The method relies on the use of oxygen-sensitive paramagnetic probes whose linewidth varies predictably, mostly linear, with oxygen concentration. Lithium phthalocyanine (LiPc) radical has emerged as the probe of choice due to its superior EPR sensitivity, oxygen response, and biocompatibility. However, there are certain limitations in the preparation of this material in a pure and usable form. In our efforts to improve the synthesis of this material for reliable use in oximetry applications, we developed microcrystalline particulates that showed several advantages over other probes. Despite its advantages, the probe shows linear response to pO2 only in the range of 0-70 mmHg, beyond which a saturation behavior is observed. The goal of this study was to understand the mechanism of the interaction of oxygen with LiPc in order to interpret the experimentally observed linewidths. We propose a dual-spin model in which the freely diffusing spins of LiPc are converted to fixed spins by adsorption of molecular oxygen. The proposed mechanism was verified from the effect of oxygenation/deoxygenation processes on the linewidth of LiPc. In summary, we demonstrated that adsorption of oxygen molecules on LiPc contributes to a nonlinear line-broadening effect. This understanding is important for the future design of new EPR oximetry probes.

Lateral ordering of lipid chains in cholesterol-containing membranes: high-field spin-label EPR

High-field (i.e., 94 GHz) electron paramagnetic resonance is used to characterize the nonaxial ordering of spin-labeled lipid chains in membranes containing cholesterol. Employing high magnetic fields (and microwave frequencies) allows investigation of both the lateral and transverse ordering of the phospholipid chains by cholesterol, from the x-y and z-elements, respectively, of the spin-label g-tensor. Transverse ordering is described by the conventional order parameter, <P2(cosbeta)>, where beta is the instantaneous inclination of the chain axis to the membrane normal; and lateral ordering is described by the order parameter <cos2(phi - phi)>, where phi is the azimuthal angle about the chain axis and phi is the mean azimuthal orientation about which angular fluctuations take place. To obtain high positional resolution, phosphatidylcholines spin labeled at all odd and even positions from n = 4 to n = 14 in the sn-2 chain (1-acyl-2-[n-(4,4'-dimethyloxazolidine-N-oxyl)]stearoyl-sn-glycero-3-phosphocholine) are used at probe amounts in membranes of dimyristoyl phosphatidylcholine containing either high (40 mol %) or low (5 mol %) concentrations of cholesterol. At high-cholesterol content, lateral ordering of the spin-labeled lipid chains is detected over a wide range of temperature throughout the liquid-ordered phase. The transverse profile of lateral phi-ordering with position, n, of chain labeling follows the profile of the rigid steroid nucleus of cholesterol. It becomes progressively averaged toward the terminal methyl group of the sn-2 chain, in the region of the flexible hydrocarbon chain of cholesterol. At low-cholesterol content, lateral chain ordering is prominent at low temperature, but diminishes at progressively higher chain positions with increasing temperature. The nonaxial lipid ordering may be related to the formation of in-plane lipid domains in membranes containing cholesterol and saturated lipids.

Comparing continuous wave progressive saturation EPR and time domain saturation recovery EPR over the entire motional range of nitroxide spin labels

The measurement of spin-lattice relaxation rates from spin labels, such as nitroxides, in the presence and absence of spin relaxants provides information that is useful for determining biomolecular properties such as nucleic acid dynamics and the interaction of proteins with membranes. We compare X-band continuous wave (CW) and pulsed or time domain (TD) EPR methods for obtaining spin-lattice relaxation rates of spin labels across the entire range of rotational motion to which relaxation rates are sensitive. Model nitroxides and spin-labeled biological species are used to illustrate the potential complications that arise in extracting relaxation data under conditions typical to biological experiments. The effect of super hyperfine (SHF) structure is investigated for both CW and TD spectra. First and second harmonic absorption and dispersion CW spectra of the nitroxide spin label, TEMPOL, are all fit simultaneously to a model of SHF structure over a range of microwave amplitudes. The CW spectra are novel because all harmonics and microwave phases were acquired simultaneously using our homebuilt CW/TD spectrometer. The effect of the SHF structure on the pulsed free induction decay (FID) and pulsed saturation recovery spectrum is shown for both protonated and deuterated TEMPOL. We present novel pulsed saturation recovery measurements on biological molecules, including spin-lattice relaxation rates of spin-labeled proteins and spin-labeled double-stranded DNA. The impact of structure and dynamics on relaxation rates are discussed in the context of each of these examples. Collisional relaxation rates with oxygen and transition metal paramagnetic relaxants are extracted using both continuous wave and time domain methods. The extent of the errors inherent in the CW method and the advantages of pulsed methods for unambiguously measuring collisional relaxation rates are discussed. Spin-lattice relaxation rates, determined by both CW and pulsed methods, are used to determine the electrostatic potential on the surface of a protein.

[A comparative study on the resolution of second derivative and Fourier self-deconvolution]

Infrared spectra of 2-alkyl-7,7,8,8-tetracyanoquinodimethane (C12H25 TCNQ, C15H31 TCNQ, C18H37 TCNQ) were measured with the resolution of 1 and 4 cm(-1). In order to identify the peak number correctly in the CH2 stretching region, second derivative and Fourier self-deconvolution were applied to the infrared spectra, respectively. The overlapping bands in the CH2 stretching region could be identified when the infrared spectra, which were measured with the resolution of 4 cm(-1), were dealt with by Fourier self-deconvolution. However, the bands overlapped in the CH2 symmetric stretching region could not be observed when these infrared spectra were dealt with by second derivative. The above results reveal that Fourier self-deconvolution method is more powerful than second derivative in identifying bands that are involved in an overlapping band feature.

Membrane binding, structure, and localization of cecropin-mellitin hybrid peptides: a site-directed spin-labeling study

The interaction of antimicrobial peptides with membranes is a key factor in determining their biological activity. In this study we have synthesized a series of minimized cecropin-mellitin hybrid peptides each containing a single cysteine residue, modified the cysteine with the sulfhydryl-specific methanethiosulfonate spin-label, and used electron paramagnetic resonance spectroscopy to measure membrane-binding affinities and determine the orientation and localization of peptides bound to membranes that mimic the bacterial cytoplasmic membrane. All of the peptides were unstructured in aqueous solution but underwent a significant conformational change upon membrane binding that diminished the rotational mobility of the attached spin-label. Apparent partition coefficients were similar for five of the six constructs examined, indicating that location of the spin-label had little effect on peptide binding as long as the attachment site was in the relatively hydrophobic C-terminal domain. Depth measurements based on accessibility of the spin-labeled sites to oxygen and nickel ethylenediaminediacetate indicated that at high lipid/peptide ratios these peptides form a single alpha-helix, with the helical axis aligned parallel to the bilayer surface and immersed approximately 5 A below the membrane-aqueous interface. Such a localization would provide exposure of charged/polar residues on the hydrophilic face of the amphipathic helix to the aqueous phase, and allow the nonpolar residues along the opposite face of the helix to remain immersed in the hydrophobic phase of the bilayer. These results are discussed with respect to the mechanism of membrane disruption by antimicrobial peptides.

Preparation of Cellouronic Acids and Partially Acetylated Cellouronic Acids by TEMPO/NaClO Oxidation of Water-Soluble Cellulose Acetate

Water-soluble cellulose acetates with a degree of substitution (DS) of 0.5, prepared by partial deacetylation of cellulose acetate of DS=2.5, were oxidized with catalytic amount of 2,2,6,6,-tetramethyl-1-piperidinyloxy radical (TEMPO), sodium hypochlorite, and sodium bromide to provide useful cellouronic acids. The oxidation was conducted at a constant pH of 10 and at 2 degrees C to avoid the occurrence of side products. Whereas only the primary hydroxyl groups of cellulose acetate were oxidized, a variable degree of oxidation (DO) resulted in a range of 0.33 to 1.0, depending on the concentration in sodium hypochlorite. Thus, polyglucuronic acid as well as partially acetylated cellouronic acid, having a range of DO were obtained.

NMR assignment of protein side chains using residue-correlated labeling and NOE spectra

A new approach for the isotopic labeling of proteins is proposed that aims to facilitate side chain resonance assignments. Residue-correlated (RC) labeling is achieved by the expression of a protein on a medium containing a mixture of labeled, e.g., [U-13C,15N]amino acids, and NMR silent, [U-2H]amino acids. De novo synthesis of amino acids was suppressed by feedback inhibition by the amino acids in the growth medium and by the addition of beta-chloro-L-alanine, a transaminase inhibitor. Incorporation of these amino acids into synthesized proteins results in a relative diminution of inter-residue NOE interactions and a relative enhancement of intra-residue NOEs. Comparison of the resulting NOE spectra with those obtained from a uniformly labeled sample allows identification of intra-residue NOE peaks. Thus, this approach provides direct information for sidechain assignments in the NOE spectra, which are subsequently used for structural analysis. We have demonstrated the feasibility of this strategy for the 143 amino acid nuclease inhibitor NuiA, both at 35 degrees C, corresponding to a rotational correlation time of 9.5 ns, and at 5 degrees C, corresponding to a rotational correlation time of 22 ns.