Spin probes, nitroxide

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... 

Electron spin resonance (ESR) is a useful technique for investigating the mobility and orientation of exchange cations at the surface of layer silicate clays in various states of hydration. Using Cu2+ and the charged nitroxide spin probe, TEMPAMINE+... 

The exact value of g depends on the magnitude of local magnetic fields induced in the atom by the applied external field, H. Nitroxide spin probes possess an unpaired electron which is largely localized on the 2px orbital of the N atom (16), as is shown in the molecular structure of TEMPAMINE" " below ... 

Figure 4. First-derivative ESR spectra of a nitroxide spin probe with the x-axis (a), y-axis (b), and z-axis (c) aligned parallel to H. The rigid-limit "powder" spectrum (d) is also shown. The dashed line marks the free electron field position (g = 2.0027). Reproduced from Ref. (jl) with permission of D. Reidel Publishing Co.

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... 

In conclusion, our analysis is directly concerned with relatively fast and local solvent motions and the results highlight the importance of careful computational modelling for the interpretation of experimental data on the behaviour of nitroxide spin probes in water and other protic solvents. 

Improta R, Scalmani G, Barone V (2001) Quantum mechanical prediction of the magnetic titration curve of a nitroxide spin probe . Chem Phys Lett 336 349—356... 

The equation 4.2 allows one to calculate the ZpZ product using the appropriate experimentally-measured log (k+/k°) or log (k7k°) value and an r value estimated for the encounter complex. From the value of ZxZp one can ascertain the Zx charge in the vicinity of the region of the paramagnetic complex encountered by the charged nitroxide spin-probe since the Zp charge of the nitroxide spin-probe is known. 

Figure 4.3. Spin relaxation rate of charged molecules as a function of nitroxide spin probe concentration. Neutral, negative, and positive charged probes denoted are respectively denoted as 0, -, and + (Likhtenshtein et al., 1999). Reproduced with permission.

TABLE 4.2. Apparent spin-exchange rate-constants (k+, k, and k°) of the positively charged (VI), negatively charged (V) and neutral (IV) nitroxide spin-probes with hemin, ferricyanide anion, and heme proteins in aqueous solutions and apparent local charges (ZH) in the vicinity of the heme groups3 (Likhtenshtein, 2000). Reproduced with permission. 

Fig. 23. Plot of the rotational correlation time (tr) for nitroxide spin-probes (TEMPO and 4-oxo-TEMPO), in chloroform swollen (Sty)98(DVB)2 versus the logarithm of the corresponding residual number, a of sorbed CHC13 per phenyl group of polymer...

Measurements of the dynamic properties of the surface water, particularly NMR measurements, have shown that the characteristic time of the water motion is slower than the bulk water value by a factor of less than 100. The motion is anisotropic. There is litde or no irrotadonally bound water. Study of a protein labeled covalently with a nitroxide spin probe (Polnaszek and Bryant, 1984a,b) has shown that the diffusion constant of the surface water is about 5-fold below the bulk water value. The NMR results are in agreement with measurements of dielectric relaxation of water in protein powders (Harvey and Hoekstra, 1972). 

Figure 13 shows the absorption spectra for a nitroxide spin probe and compares the case of free diffusion (P2 0) with those obtained for various values of the orienting potential. Such spectra have been calculated for an isotropic diffusion coefficient. Note the drastic change in lineshape at the onset of molecular order. 

In this appendix we shall provide exphdt expressions for the recursion formulas of Eq. (3.13) for the case of a nitroxide spin probe and a copper complex in solution. To this end we need these preliminary definitions ... 

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. 

Introduction of a nitroxide spin probe into a membrane can be accomplished in... 

Fig. 4. (a) Energy levels for the system S = l, I = 1 and allowed transitions, (b) ESR spectrum observed from the nitroxide spin probe TEMPO. 

The molecular motion of redox couples within polymer-coated electrodes has recently been investigated by making use of both nitroxide spin probes and various cationic spin probes [94-97]. Spin probes, such as the nitroxide probe TEMPO (see Sect. 2.1.2) and its derivatives, have well-defined electrochemistry and their ESR spectra in viscous media exhibit effects due to incomplete rotational averaging of the g and hyperfine coupling constant tensors. Analysis of the spectra [98] allows deductions to be made concerning the molecular rotation. Such analysis has been performed for spin probes incorporated into various polymer films. 

The modification of electrodes with PVC membranes has found applicability in ion selective electrode work [99] (so-called "coated wire electrode ). The molecular motion of species within such electrodes has been investigated by Compton and Waller [100]. Using a range of derivatives of the nitroxide spin probe TEMPO, they were able to show how the rotational correlational time was dependent upon the molecular volume of the probe and, by use of variable-temperature apparatus, how this varied with temperature. The effect of various plasticizers upon the molecular motion within the PVC membrane was investigated, rotational correlational times being dependent upon the nature of the plasticizer and the loading level. The effect of loading level upon the correlation time was shown to correlate with data obtained by Compton Maxwell [101] for the response times of K+ ion selective electrodes based upon PVC modified electrodes. 

Fig. 2 Chemical structures of common nitroxide spin probes Fremy s salt (potassium nitrosodi-sulfonate) TEMPO and derivatives (2,2,6,6-tetramethylpiperidine-l-oxyl), DOXYL (4,4-dimethyl-oxazolidine-l-oxyl) PROXYL (2,2,5,5-tetramethylpyrrolidine-l-oxyl) Dehydro-PROXYL (2,2,5,5-tetramethylpyrroline-l-oxyl) 5-DSA (5-DOXYL stearic acid) 16-DSA (16-DOXYL stearic...

In this section, it is shown that EPR spectroscopy of amphiphilic nitroxide spin probes offers an interesting way to study the release of small molecules from PNIPAAM hydrogels. EPR spectroscopy on these reporter molecules shows high selectivity and site-specificity and delivers a large variety of information on local guest-host interaction, the distribution of guest molecules (on the nanometer scale), and accessibility by solvents. A picture of the collapse process at a molecular level can be drawn that is specifically based on variations of the chemical environment and rotational dynamics [32, 59-61]. 

Bush AI (2003) The metallobiology of Alzheimer s disease. Trends Neurosci 26 207—214 Butterfield DA, Martin L, Camey JM, Hensley K (1996) A beta (25-35) peptide displays H202-Uke reactivity towards aqueous Fe2-t, nitroxide spin probes, and synaptosomal membrane proteins. Life Sci 58 217-228... 

Fig. 1 Orientation dependence of nitroxide spin-probe EPR spectra and dynamical effects on line shapes (simulations). Maximum hyperfine splitting A, is observed along the lobes of the singly occupied molecular orbital (j direction). Within the xy plane, spectra at X band frequencies differ only slightly due to g anisotropy. Dynamic spectra are shown for rotational correlation times Tr corresponding to the fast limit (10 ps), slow tumbling (x, = 1/Av=5.25 ns), and the rigid limit (1 ys). (View this art in color at www.dekker.com.)...

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