Electron spinal resonance analysis of the nitroxide spin label 2,2,6,6-tetramethylpipidone-N-oxyl (Tempone) in single crystals of the reduced Tempone matrix

An analytical treatment of electron spectral saturation for dynamic nuclear polarization NMR of rotating solids

Saturation of electron magnetization by microwave irradiation under magic-angle spinning (MAS) is studied theoretically. The saturation is essential for dynamic nuclear polarization (DNP) enhancement of nuclear magnetic resonance signals. For a spin with a large g-anisotropy and a long T₁ relative to the rotor period, the sample rotation distributes saturation to the whole powder sample spectrum. Analytical expressions for the saturation and frequency profiles are obtained. For a pair of coupled electrons such as those in bis-nitroxides, which are commonly used for MAS DNP, an e_l-e_r model (where e_l and e_r stand for electrons on the left and the right, respectively, in their spectral positions) is introduced to simplify the analysis of a coupled two-spin system under MAS. For such a system, strong electron couplings exchange magnetization during dipolar/J rotor events when the two electron frequencies cross each other. The exchange is equivalent to a swap of the e_l and e_r electrons. This allows for the treatment of a coupled spin pair as two independent spins such that an analytical solution can be obtained for the steady-state magnetization and the difference between the two electrons. The theoretical study with its analytical result provides a simple physical picture of electron saturation under MAS and of how radical properties and experimental parameters affect cross-effect DNP. The effects of depolarization and the extension to more coupled electron spins are also discussed using this approach.

Access to New Cytotoxic Triterpene and Steroidal Acid-TEMPO Conjugates by Ugi Multicomponent-Reactions

Multicomponent reactions, especially the Ugi-four component reaction (U-4CR), provide powerful protocols to efficiently access compounds having potent biological and pharmacological effects. Thus, a diverse library of betulinic acid (BA), fusidic acid (FA), cholic acid (CA) conjugates with TEMPO (nitroxide) have been prepared using this approach, which also makes them applicable in electron paramagnetic resonance (EPR) spectroscopy. Moreover, convertible amide modified spin-labelled fusidic acid derivatives were selected for post-Ugi modification utilizing a wide range of reaction conditions which kept the paramagnetic center intact. The nitroxide labelled betulinic acid analogue 6 possesses cytotoxic effects towards two investigated cell lines: prostate cancer PC3 (IC₅₀ 7.4 ± 0.7 μM) and colon cancer HT29 (IC₅₀ 9.0 ± 0.4 μM). Notably, spin-labelled fusidic acid derivative 8 acts strongly against these two cancer cell lines (PC3: IC₅₀ 6.0 ± 1.1 μM; HT29: IC₅₀ 7.4 ± 0.6 μM). Additionally, another fusidic acid analogue 9 was also found to be active towards HT29 with IC₅₀ 7.0 ± 0.3 μM (CV). Studies on the mode of action revealed that compound 8 increased the level of caspase-3 significantly which clearly indicates induction of apoptosis by activation of the caspase pathway. Furthermore, the exclusive mitochondria targeting of compound 18 was successfully achieved, since mitochondria are the major source of ROS generation.

Concentration Effects in the Interaction of Monoclonal Antibodies (mAbs) with their Immediate Environment Characterized by EPR Spectroscopy

Monoclonal antibodies (mAbs) are often needed and applied in high concentration solutions, >100 mg/mL. Due to close intermolecular distances between mAbs at high concentrations (~10–20 nm at 200 mg/mL), intermolecular interactions between mAbs and mAbs and solvent/co-solute molecules become non-negligible. Here, EPR spectroscopy is used to study the high-concentration solutions of mAbs and their effect on co-solvated small molecules, using EPR “spin probing” assay in aqueous and buffered solutions. Such, information regarding the surrounding environments of mAbs at high concentrations were obtained and comparisons between EPR-obtained micro-viscosities (rotational correlation times) and macroscopic viscosities measured by rheology were possible. In comparison with highly viscous systems like glycerol-water mixtures, it was found that up to concentrations of 50 mg/mL, the mAb-spin probe systems have similar trends in their macro- (rheology) and micro-viscosities (EPR), whereas at very high concentrations they deviate strongly. The charged spin probes sense an almost unchanged aqueous solution even at very high concentrations, which in turn indicates the existence of large solvent regions that despite their proximity to large mAbs essentially offer pure water reservoirs for co-solvated charged molecules. In contrast, in buffered solutions, amphiphilic spin probes like TEMPO interact with the mAb network, due to slight charge screening. The application of EPR spectroscopy in the present work has enabled us to observe and discriminate between electrostatic and hydrophobic kinds of interactions and depict the potential underlying mechanisms of network formation at high concentrations of mAbs. These findings could be of importance as well for the development of liquid-liquid phase separations often observed in highly concentrated protein solutions.

Conformation of bis-nitroxide polarizing agents by multi-frequency EPR spectroscopy

The chemical structure of polarizing agents critically determines the efficiency of dynamic nuclear polarization (DNP). For cross-effect DNP, biradicals are the polarizing agents of choice and the interaction and relative orientation of the two unpaired electrons should be optimal. Both parameters are affected by the molecular structure of the biradical in the frozen glassy matrix that is typically used for DNP/MAS NMR and likely differs from the structure observed with X-ray crystallography. We have determined the conformations of six bis-nitroxide polarizing agents, including the highly efficient AMUPol, in their DNP matrix with EPR spectroscopy at 9.7 GHz, 140 GHz, and 275 GHz. The multi-frequency approach in combination with an advanced fitting routine allows us to reliably extract the interaction and relative orientation of the nitroxide moieties. We compare the structures of six bis-nitroxides to their DNP performance at 500 MHz/330 GHz.

Electronic and Geometric Structures of Paramagnetic Diazadiene Complexes of Lithium and Sodium

The electronic and molecular structures of the lithium and sodium complexes of 1,4-bis(2,6-diisopropylphenyl)-2,3-dimethyl-1,4-diazabutadiene (Me2DADDipp) were fully characterized by using a multi-frequency electron paramagnetic resonance (EPR) spectroscopy approach and crystallography, together with density functional theory (DFT) calculations. EPR measurements, using T1 relaxation-time-filtered pulse EPR spectroscopy, revealed the diagonal elements of the A and g tensors for the metal and ligand sites. It was found that the central metals in the lithium complexes had sizable contributions to the SOMO, whereas this contribution was less strongly observed for the sodium complex. Such strong contributions were attributed to structural specifications (e.g. geometrical data and atomic size) rather than electronic effects.

Self-reporting and refoldable profluorescent single-chain nanoparticles

We pioneer the formation of self-reporting and refoldable profluorescent single-chain nanoparticles (SCNPs) via the light-induced reaction (λ_max = 320 nm) of nitroxide radicals with a photo-active crosslinker.

We pioneer the formation of self-reporting and refoldable profluorescent single-chain nanoparticles (SCNPs) via the light-induced reaction (λ_max = 320 nm) of nitroxide radicals with a photo-active crosslinker. Whereas the tethered nitroxide moiety in these polymers fully quenches the luminescence (i.e. fluorescence) of the aromatic backbone, nitroxide trapping of a transient C-radical leads to the corresponding closed shell alkoxyamine thereby restoring luminescence of the folded SCNP. Hence, the polymer in the folded state is capable of emitting light, while in the non-folded state the luminescence is silenced. Under oxidative conditions the initially folded SCNPs unfold, resulting in luminescence switch-off and the reestablishment of the initial precursor polymer. Critically, we show that the luminescence can be repeatedly silenced and reactivated. Importantly, the self-reporting character of the SCNPs was followed by size-exclusion chromatography (SEC), dynamic light scattering (DLS), fluorescence, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR) and diffusion ordered NMR spectroscopy (DOSY).

Verdazyl-ribose: A new radical for solid-state dynamic nuclear polarization at high magnetic field

Solid-state dynamic nuclear polarization (DNP) using the cross-effect relies on radical pairs whose electron spin resonance (ESR) frequencies differ by the nuclear magnetic resonance (NMR) frequency. We measure the DNP provided by a new water-soluble verdazyl radical, verdazyl-ribose, under both magic-angle spinning (MAS) and static sample conditions at 9.4 T, and compare it to a nitroxide radical, 4-hydroxy-TEMPO. We find that verdazyl-ribose is an effective radical for cross-effect DNP, with the best relative results for a non-spinning sample. Under non-spinning conditions, verdazyl-ribose provides roughly 2× larger 13C cross-polarized (CP) NMR signal than the nitroxide, with similar polarization buildup times, at both 29 K and 76 K. With MAS at 7 kHz and 1.5 W microwave power, the verdazyl-ribose does not provide as much DNP as the nitroxide, with the verdazyl providing less NMR signal and a longer polarization buildup time. When the microwave power is decreased to 30 mW with 5 kHz MAS, the two types of radical are comparable, with the verdazyl-doped sample having a larger NMR signal which compensates for its longer polarization buildup time. We also present electron spin relaxation measurements at Q-band (1.2 T) and ESR lineshapes at 1.2 and 9.4 T. Most notably, the verdazyl radical has a longer T1e than the nitroxide (9.9 ms and 1.3 ms, respectively, at 50 K and 1.2 T). The verdazyl electron spin lineshape is significantly affected by the hyperfine coupling to four 14N nuclei, even at 9.4 T. We also describe 3000-spin calculations to illustrate the DNP potential of possible radical pairs: verdazyl-verdazyl, verdazyl-nitroxide, or nitroxide-nitroxide pairs. These calculations suggest that the verdazyl radical at 9.4 T has a narrower linewidth than optimal for cross-effect DNP using verdazyl-verdazyl pairs. Because of the hyperfine coupling contribution to the electron spin linewidth, this implies that DNP using the verdazyl radical would improve at lower magnetic field. Another conclusion from the calculations is that a verdazyl-nitroxide bi-radical would be expected to be slightly better for cross-effect DNP than the nitroxide-nitroxide bi-radicals commonly used now, assuming the same spin-spin coupling constants.

Spin-labelled diketopiperazines and peptide-peptoid chimera by Ugi-multi-component-reactions

For the first time, spin-labelled coumpounds have been obtained by isonitrile-based multi component reactions (IMCRs). The typical IMCR Ugi-protocols offer a simple experimental setup allowing structural variety by which labelled diketopiperazines (DKPs) and peptide-peptoid chimera have been synthesized. The reaction keeps the paramagnetic spin label intact and offers a simple and versatile route to a large variety of new and chemically diverse spin labels.

Optimization of bicelle lipid composition and temperature for EPR spectroscopy of aligned membranes

We have optimized the magnetic alignment of phospholipid bilayered micelles (bicelles) for EPR spectroscopy, by varying lipid composition and temperature. Bicelles have been extensively used in NMR spectroscopy for several decades, in order to obtain aligned samples in a near-native membrane environment and take advantage of the intrinsic sensitivity of magnetic resonance to molecular orientation. Recently, bicelles have also seen increasing use in EPR, which offers superior sensitivity and orientational resolution. However, the low magnetic field strength (less than 1 T) of most conventional EPR spectrometers results in homogeneously oriented bicelles only at a temperature well above physiological. To optimize bicelle composition for magnetic alignment at reduced temperature, we prepared bicelles containing varying ratios of saturated (DMPC) and unsaturated (POPC) phospholipids, using EPR spectra of a spin-labeled fatty acid to assess alignment as a function of lipid composition and temperature. Spectral analysis showed that bicelles containing an equimolar mixture of DMPC and POPC homogeneously align at 298 K, 20 K lower than conventional DMPC-only bicelles. It is now possible to perform EPR studies of membrane protein structure and dynamics in well-aligned bicelles at physiological temperatures and below.

High-Field EPR Spectroscopic Characterization of Spin Probes in Aqueous Ionic Liquid Mixtures

Binary mixtures of the hydrophilic room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim+][BF4 -]) and water have been probed using high-field/high-frequency electron paramagnetic resonance (EPR) spectroscopy. By adding nitroxide radicals as paramagnetic tracers, the solution structure could be assessed from typical solutes' points of view. Three probe molecules, differing in charge and hydrogen bonding affinity, have been employed: the dianionic Fremy's salt, amphiphilic TEMPO, and hydrophilic TEMPOL. High-field pulse EPR spectroscopy has been used to determine hyperfine interaction tensors and g-matrices under cryogenic conditions. Polarity-proticity plots reveal the interaction of the nitroxide moiety with apolar (TEMPO) and ionic (FS) nano-domains, depending on the probe structure. Only small variations are observed on changing the water content suggesting that the probes are well shielded from aqueous subdomains. The results indicate that addition of water in fact stabilizes IL-rich mesostructures by embedding them in pools of water.

Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: the importance of level crossings

We present theoretical calculations of dynamic nuclear polarization (DNP) due to the cross effect in nuclear magnetic resonance under magic-angle spinning (MAS). Using a three-spin model (two electrons and one nucleus), cross effect DNP with MAS for electron spins with a large g-anisotropy can be seen as a series of spin transitions at avoided crossings of the energy levels, with varying degrees of adiabaticity. If the electron spin-lattice relaxation time T(1e) is large relative to the MAS rotation period, the cross effect can happen as two separate events: (i) partial saturation of one electron spin by the applied microwaves as one electron spin resonance (ESR) frequency crosses the microwave frequency and (ii) flip of all three spins, when the difference of the two ESR frequencies crosses the nuclear frequency, which transfers polarization to the nuclear spin if the two electron spins have different polarizations. In addition, adiabatic level crossings at which the two ESR frequencies become equal serve to maintain non-uniform saturation across the ESR line. We present analytical results based on the Landau-Zener theory of adiabatic transitions, as well as numerical quantum mechanical calculations for the evolution of the time-dependent three-spin system. These calculations provide insight into the dependence of cross effect DNP on various experimental parameters, including MAS frequency, microwave field strength, spin relaxation rates, hyperfine and electron-electron dipole coupling strengths, and the nature of the biradical dopants.

Polarizing agents and mechanisms for high-field dynamic nuclear polarization of frozen dielectric solids

This article provides an overview of polarizing mechanisms involved in high-frequency dynamic nuclear polarization (DNP) of frozen biological samples at temperatures maintained using liquid nitrogen, compatible with contemporary magic-angle spinning (MAS) nuclear magnetic resonance (NMR). Typical DNP experiments require unpaired electrons that are usually exogenous in samples via paramagnetic doping with polarizing agents. Thus, the resulting nuclear polarization mechanism depends on the electron and nuclear spin interactions induced by the paramagnetic species. The Overhauser Effect (OE) DNP, which relies on time-dependent spin-spin interactions, is excluded from our discussion due the lack of conducting electrons in frozen aqueous solutions containing biological entities. DNP of particular interest to us relies primarily on time-independent, spin-spin interactions for significant electron-nucleus polarization transfer through mechanisms such as the Solid Effect (SE), the Cross Effect (CE) or Thermal Mixing (TM), involving one, two or multiple electron spins, respectively. Derived from monomeric radicals initially used in high-field DNP experiments, bi- or multiple-radical polarizing agents facilitate CE/TM to generate significant NMR signal enhancements in dielectric solids at low temperatures (<100 K). For example, large DNP enhancements (∼300 times at 5 T) from a biologically compatible biradical, 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL), have enabled high-resolution MAS NMR in sample systems existing in submicron domains or embedded in larger biomolecular complexes. The scope of this review is focused on recently developed DNP polarizing agents for high-field applications and leads up to future developments per the CE DNP mechanism. Because DNP experiments are feasible with a solid-state microwave source when performed at <20K, nuclear polarization using lower microwave power (<100 mW) is possible by forcing a high proportion of biradicals to fulfill the frequency matching condition of CE (two EPR frequencies separated by the NMR frequency) using the strategies involving hetero-radical moieties and/or molecular alignment. In addition, the combination of an excited triplet and a stable radical might provide alternative DNP mechanisms without the microwave requirement.

Multifrequency electron spin resonance study of the dynamics of spin labeled T4 lysozyme

An extensive set of electron spin resonance spectra was obtained over a wide range of frequencies (9, 95, 170, and 240 GHz) and temperatures (2 to 32 degrees C) to explore the dynamic modes of nitroxide-labeled T4 lysozyme in solution. A commonly used nitroxide side chain (R1), or a methylated analogue with hindered internal motion (R2), was substituted for the native side chain at solvent-exposed helical sites, 72 or 131. The spectra at all four frequencies were simultaneously fit with the slowly relaxing local structure (SRLS) model. Good fits were achieved at all the temperatures. Two principle dynamic modes are included in the SRLS model, the global tumbling of the protein and the internal motion consisting of backbone fluctuations and side chain isomerizations. Three distinct spectral components were required for R1 and two for R2 to account for the spectra at all temperatures. One is a highly ordered and slow motional component, which is observed in the spectra of both R1 and R2; it may correspond to conformers stabilized by interaction with the protein surface. The fraction of this component decreases with increasing temperature and is more populated in the R2 spectra, possibly arising from stronger interaction of the nitroxide ring with the protein surface due to the additional methyl group. The other two components of R1 and the second component of R2 are characterized by fast anisotropic diffusion and relatively low ordering, most likely corresponding to conformers having little or no interactions with nearby residues. Ficoll of different concentrations was added to increase the solution viscosity, thereby slowing down the global tumbling of the protein. A significant effect of Ficoll on the internal motion of an immobilized component was apparent in R2 but not in R1. The ability of such multifrequency studies to separate the effects of faster internal modes of motion from slower overall motions is clearly demonstrated, and its utility in future studies is considered.

Dinuclear copper(II) complexes containing 6-(benzylamino)purines as bridging ligands: synthesis, characterization, and in vitro and in vivo antioxidant activities

A series of dinuclear copper(II) complexes involving 6-(benzylamino)purine derivatives, (HL(n)), as bridging ligands were synthesized, characterized and tested for both their in vitro and in vivo antioxidant activities. Based on results of elemental analyses, temperature dependence of magnetic susceptibility measurements, UV-vis, FTIR, EPR, NMR and MALDI-TOF mass spectroscopy, conductivity measurements and thermal analyses, the complexes with general compositions of [Cu(2)(mu-HL(n))(4)Cl(2)]Cl(2).2H(2)O (1-4) and [Cu(2)(mu-HL(n))(2)(mu-Cl)(2)Cl(2)] (5-7) were prepared {where n=1-4; HL(1)=6-[(2-methoxybenzyl)amino]purine, HL(2)=6-[(4-methoxybenzyl)amino]purine, HL(3)=6-[(2,3-dimethoxybenzyl)amino]purine and HL(4)=6-[(3,4-dimethoxybenzyl)amino]purine}. In the case of complexes 2, 3, 5 and 7, the antioxidant activities were studied by both in vitro {superoxide dismutase-mimic (SOD-mimic) activity} and in vivo {cytoprotective effect against the alloxan-induced diabetes (antidiabetic activity)} methods. The obtained IC(50) value of the SOD-mimic activity for the complex 5 (IC(50)=0.253 microM) was shown to be even better than that of the native bovine Cu,Zn-SOD enzyme (IC(50)=0.480 microM), used as a standard. As for the antidiabetic activity, the pretreatment of mice with complexes 3 and 7 led to the complete elimination of cytotoxic attack of alloxan and its free radical metabolites, used as a diabetogenic agent. The cytoprotective effect of these compounds was proved by the preservation of the initial blood glucose levels of the pretreated animals, as against the untreated control group.

High-frequency dynamic nuclear polarization using biradicals: a multifrequency EPR lineshape analysis

To date, the cross effect (CE) and thermal mixing (TM) mechanisms have consistently provided the largest enhancements in dynamic nuclear polarization (DNP) experiments performed at high magnetic fields. Both involve a three-spin electron-electron-nucleus process whose efficiency depends primarily on two electron-electron interactions--the interelectron distance R and the correct electron paramagnetic resonance (EPR) frequency separation that matches the nuclear Larmor frequency, /omega(e2)-omega(e1)/ = omega(n). Biradicals, for example, two 2,2,6,6-tetramethyl-piperidine-1-oxyls (TEMPOs) tethered with a molecular linker, can in principle constrain both the distance and relative g-tensor orientation between two unpaired electrons, allowing these two spectral parameters to be optimized for the CE and TM. To verify this hypothesis, we synthesized a series of biradicals--bis-TEMPO tethered by n ethylene glycol units (a.k.a. BTnE)--that show an increasing DNP enhancement with a decreasing tether length. Specifically at 90 K and 5 T, the enhancement grew from approximately 40 observed with 10 mM monomeric TEMPO, where the average R approximately 56 A corresponding to electron-electron dipolar coupling constant omega(d)2 pi = 0.3 MHz, to approximately 175 with 5 mM BT2E (10 mM electrons) which has R approximately 13 A with omega(d)2 pi = 24 MHz. In addition, we compared these DNP enhancements with those from three biradicals having shorter and more rigid tethers-bis-TEMPO tethered by oxalyl amide, bis-TEMPO tethered by the urea structure, and 1-(TEMPO-4-oxyl)-3-(TEMPO-4-amino)-propan-2-ol (TOTAPOL) TOTAPOL is of particular interest since it is soluble in aqueous media and compatible with DNP experiments on biological systems such as membrane and amyloid proteins. The interelectron distances and relative g-tensor orientations of all of these biradicals were characterized with an analysis of their 9 and 140 GHz continuous-wave EPR lineshapes. The results show that the largest DNP enhancements are observed with BT2E and TOTAPOL that have shorter tethers and the two TEMPO moieties are oriented so as to satisfy the matching condition for the CE.

Dynamics and aggregation of the peptide ion channel alamethicin. Measurements using spin-labeled peptides

Two spin-labeled derivatives of the ion conductive peptide alamethicin were synthesized and used to examine its binding and state of aggregation. One derivative was spin labeled at the C-terminus and the other, a leucine analogue, was spin labeled at the N-terminus. In methanol, both the C and N terminal labeled peptides were monomeric. In aqueous solution, the C-terminal derivative was monomeric at low concentrations, but aggregated at higher concentrations with a critical concentration of 23 microM. In the membrane, the C-terminal label was localized to the membrane-aqueous interface using 13C-NMR, and could assume more than one orientation. The membrane binding of the C-terminal derivative was examined using EPR, and it exhibited a cooperativity seen previously for native alamethicin. However, this cooperativity was not the result of an aggregation of the peptide in the membrane. When the spectra of either the C or N-terminal labeled peptide were examined over a wide range of membrane lipid to peptide ratios, no evidence for aggregation could be found and the peptides remained monomeric under all conditions examined. Because electrical measurements on this peptide provide strong evidence for an ion-conductive aggregate, the ion-conductive form of alamethicin likely represents a minor fraction of the total membrane bound peptide.

ESR correlation times of 2,2,6,6-tetramethyl piperidone-N-oxyl (Tempone) in solutions of hemoglobin A and hemoglobin S

Correlation times for the tumbling motion of the spin probe 2,2,6,6,-tetramethyl piperidone-N-oxyl (Tempone) were obtained in the presence of different concentrations of oxyhemoglobin A, oxyhemoglobin S, and deoxyhemoglobin S and compared to the viscosity of non-gelling hemoglobin solutions. Reorientational motion (or tumbling) of Tempone in gelled solutions of deoxyhemoglobin S is as great as that in non-gelled hemoglobins of the same total concentration. It is concluded that the gel does not exclusively partition Tempone into an aqueous phase of lower solute concentration after gel formation. The gel at room temperature is a highly mobile and dynamic structure on the microscopic level.

The viscosity of mammalian nerve axoplasm measured by electron spin resonance

1. The microviscosity of the axoplasm of can sciatic nerve was determined by an in vitro electron spin resonance (e.s.r.) method using the spin label tempone. To identify the spin label signal as one arising only from within the axoplasm, Ni2+ was used as a line broadening agent. In one series of experiments in nerves with sheath intact the Ni2+ ion was shown to eliminate the tempone signal arising from the surface water, and in another series of experiments, with the sheath slit, to eliminate the signal from the extracellular space as well. 2. A microviscosity of less than 5 centipoise (cP), i.e. 5x that of water, was determined for the axoplasm. Changes in the viscosity of the nerve axoplasm as a function of temperature over a range of 38 degrees down to 2 degrees C were seen to follow closely the viscosity change found for a water solution. 3. The microviscosity of nerve axoplasm and its change with temperature were related to axoplasmic transport of material in nerve fibres. The results were used to exclude a large increase in viscosity at low temperatures as the cause for the cold-block of fast axoplasmic transport.

An electron spin resonance study of synaptosome opiate receptors. The preparation and use of a spin labeled morphine

Morphine spin labeled on the phenolic hydroxy group has been prepared using commercially available reagents and characterized by thin layer chromatography, mass spectroscopy, and electron spin resonance spectroscopy. It has been shown that morphine modified in this way retains some opiate activity, does not pass through the blood-brain barrier, and specifically binds to isolate rat brain synaptosomes. Spin labeled morphine has been shown to be an effective biophysical probe complementing radioactive tracer techniques in the study of the narcotic receptor site.