Spin probes nitroxide radicals

Line Shape Analysis for Nitroxide Spin Probes. Nitroxide radicals as spin probes and labels are useful for the determination of the motional mechanism, rotational correlation time, to, and local polarity. Figure 2a demonstrates that the relative line widths and line heights depend on the rotational correlation time to, which is inversely proportional to the diffusion constant of rotational diffusion. 

Electron Spin Resonance.—Nitroxide radicals of varying structiu-e have been employed in studies of micelle structure and mobility. The basic spectrum is a triplet due to N-electron coupling, which may show hyperfine coupling to 3-C-H in appropriate cases. On micelle formation or incorporation of the probe the spectrum normally broadens because of reduction in the rotational correlation time and shows enhanced broadening and change in positions of the high-field line. Cationic micelles incorporate the probe (10) with an association constant of 3 x 10 and at low surfactant concentrations there is... 

Method involves measuring the change in rotational correlation time for a free radical probe (e.g., nitroxide spin probes) introduced into a sample being studied. The temperature associated with a decrease in the rotational correlation time of a spin probe is assigned as Tg... 

Very shortly after the technique of spin trapping was first proposed as a versatile general method for probing free-radical reactions, it was pointed out by Forrester and Hepburn (1971) that both nitroso-compounds and nitrones are susceptible to nucleophilic attack. The resulting hydroxylamines are particularly sensitive to oxidation (14a,b) and this gives nitroxides Under many circumstances, the possibility of such a reaction is remote, but it should... 

Spectroscopic techniques such as electron spin resonance (ESR) offer the possibility to "probe" the chemical environment of the interlayer regions. With the ESR technique, an appropriate paramagnetic ion or molecule is allowed to penetrate the interlayer, and chemical information is deduced from the ESR spectrum. Transition metal ions, such as Cu2+, and nitroxide radical cations, such as TEMPAMINE (4-amino-2,2,6,6-tetramethylpiperidine N-oxide) have been used as probes in this manner (6-14). Since ESR is a sensitive and non-destructive method, investigations of small quantities of cations on layer silicate clays at various stages... 

Belkin S, Mehlhorn RJ, Hideg K, Hankovsky O, Packer L (1987) Reduction and destruction rates of nitroxide spin probes. Arch Biochem Biophys 256 232-243 Berdnikov VM, Bazhin NM, Fedorov VK, Polyakov OV (1972) Isomerization of the ethoxyl radical to thea-hydroxyethyl radical in aqueous solution. KinetCatal (English translation) 13 986-987 BerkowitzJ, Ellison GB, Gutman D (1994) Three methods to measure RH bond energies. J Phys Chem 98 2744-2765... 

Oxidation of 2,2,4,4-tetraalkyloxazolidines gives stable nitroxide radicals (265) which are used as spin labels for probing biomolecular structures (69ACR17, B-76MI41800). The stearic acid derivative 12-doxylstearic acid (266) is commercially available. 

An additional step in the cascade reaction scheme is the quenching of the sensitizer triplet state with relatively low-concentration radicals (Fig. 1.5) (Papper et al 1999, 2000 Papper and Likhtenshtein, 2001). The entire investigated reaction that is shown in Fig. 1.5 is the sequence of the four kinetic processes and serves as a basis for the spin-triplet-photochrome labeling technique. This technique combines the three types of biophysical probes stilbene photochrome probe, triplet probe and stable nitroxide-radical spin probe, which depresses the sensitiser exited triplet state. 

From the point of view of the biochemist, there are three major classes of samples that might be investigated with ESR spectroscopy (1) samples containing transition or lanthanide elements (2) samples containing free radicals (3) samples that have been extrinsically labeled by the introduction of nitroxide radical spin labels or spin probes. We provide an overview of these three subjects in this chapter. Basic references are given at the beginning of each section. The material described is by no means comprehensive the topics reviewed have become very massive subjects. Rather, we have tried to provide information, often from our own work and experience, that we hope will be of general interest. 

Nitroxide radical spin labels and spin probes... 

Hindered nitroxides are a class of free radicals that are unusually long-lived. The presence of methyl groups y to the radical center decreases the rate of self reaction to such an extent that the radical becomes kinetically stable. Hindered nitroxides can be obtained as crystalline solids or pure liquids, which in the absence of other materials are very stable. These materials have found extensive use as spin labels and spin probes. Indeed, almost all spin-label studies have employed nitroxides spin-probe studies have used either nitroxides or paramagnetic metal ions. 

D15.6 The ESR spectra of a spin probe, such as the di-terr-butyl nitroxide radical, broadens with restricted motion of the probe. This suggests that the width of spectral lines may correlate with the depth to which a probe may enter into a biopolymer crevice. Deep crevices are expected to severely restrict probe motion and broaden the spectral lines. Additionally, the splitting and center of ESR spectra of an oriented sample can provide information about the shape of the biopolymer-probe environment because the probe ESR signal is anisotropic and depends upon the orientation of the probe with the external magnetic held. Oriented biopolymers occur in lipid membranes and in muscle fibers. 

The starting point for every kind of EPR study is the choice of the radical that is best suited to deliver valuable information on the systems that are investigated. In this chapter only three types of radicals are used for this purpose. Nitroxide radicals such as TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl Pig. 2), its derivatives, and its five-ring analogs (Fig. 2) are the by far most widely used spin probes and spin labels and they are the spin probes of choice for the majority of EPR spectroscopic applications in naturally diamagnetic systems [2, 19]. 

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