TEMPO probe

Interestingly, K-TEMPO probes in monoionic block copolymers of type polystyrene-polyisoprene (notation PS-PI-S, where S stands for the spin probe) exhibited homogeneous dynamics, ie, a relatively narrow, monomodal distribution of rotational correlation times by contrast, the same probes in zwitterionic block copolymers of type Q-PS-PI-S (where Q stands for a quaternary ammonium group) exhibited heterogeneous dynamics, ie, a bimodal distribution of rotational correlation times (87). By comparison of results for telechelic homopolsrmeric... 

Next we need to define the form of the time evolution operator (Liouvillian) for the density matrix described by the SLE. The molecule being partitioned in two fragments, as described above, we have (i) two local frames respectively fixed on the pahnitate chain (CF) and on the tempo probe (PB) these are chosen with their respective z axes directed along the rotating bond, for convenience (ii) the molecular frame (MF), fixed on the pahnitate chain this is the frame which diagonalizes the... 

FIGURE 11.6 A The dependence of effective time correlation of TEMPO probe on time of PHB mat exposition in ozone atmosphere. 1 - the initial fiber mat 2 - the same mat after cold-rolling treatment. B The ratio of fast and slow components in ESR probe spectra as function of time oxidation. 

Coordination causes electron-spin density redistribution in the N-O fragment the contribution of resonance structure II increase. The redistribution of spin density results in changes in the parallel component of the nitrogen hyperfine tensor. TEMPO and anthraquinone (AQ) have been used in this way to probe the Lewis acidity of alumina and Li and Mg doped alumina matrices.176 The differences in the Lewis acidic strength towards TEMPO and anthraquinone are discussed. An interesting study has appeared aimed to study the guest-host interaction between poly(amidoamine) dendrimers labelled with nitroxides and several porous solids including alumina.177... 

The 2,2,6,6-tetramethylpiperidinoxyl radical (TEMPO) was first prepared in 1960 by Lebedev and Kazarnovskii by oxidation of its piperidine precursor.18 The steric hindrance of the NO bond in TEMPO makes it a highly stable radical species, resistant to air and moisture. Paramagnetic TEMPO radicals can be employed as powerful spin probes for elucidating the structure and dynamics of both synthetic and biopolymers (e.g., proteins and DNA) by ESR spectroscopy.19 Unlike solid-phase 1H-NMR where magic angle spinning is required in order to reduce the anisotropic effects in the solid-phase environment, solid-phase ESR spectroscopy can be conducted without specialized equipment. Thus, we conducted comparative ESR studies of various polymers with persistent radical labels, and we also determined rotational correlation times as a function of... 

Figure 7.2.12 shows the principal set-up for this experiment. The column, containing the immobilized free radicals consists of an adapted PEEK tube, which fits into the flow probe below the detection cell. Figure 7.2.13 depicts a spectrum of d-n-butylphthalate recorded under the influence of a ( free-radical ) column filled with 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO), immobilized with an aminopropyl spacer on silica. The spectrum is taken from an on-line separation of a two-compound mixture. The line width is of the same order as... 

Experimental evidence for the presence of radical intermediates is provided by the identification of expected products from radical rearrangements, by the use of appropriate radical probes and by direct detection by electron spin resonance (ESR). Other mechanistic evidence includes inhibition by radical traps, such as di-t-butylnitroxide (DTBN), TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy), galvinoxyl and oxygen, and by radical anion scavengers such as p-dinitrobenzene (p-DNB). 

Hydrophilic spin probe 4-Oxo- TEMPO (Sigma) of 0.1 mM concentration was introduced in water dispersions of ShC nanoparticles of different concentrations (0.1, 1.0 and 10 mg/ml). Paramagnetic spin probe like TEMPOL effectively dissolves in hydration water [7,8] owing to capability of polar and paramagnetic NO group of probe to form hydrogen bonds with water molecules. 

Apart from those of RNO discussed previously, others include those of nitroxide radicals such as Bu2NO and 2,2,6,6-tetramethylpiperidinyl-l-oxy (tempo) (9-XXXIII) used as probes in biological systems. The bonding can be if-O or T -tyO where the ligand is bound as R2NO . 

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...

More recently, the new photochromic spin probe 74 was synthesized.67 As the paramagnetic unit is provided by a 4-(2,2,6,6,-tetramethylpiperidinyl-l-oxyl) (TEMPO) moiety, it is not unexpected that the spectral parameters of this radical, i.e., aN= 15.5 G and g = 2.0055, did not differ from those of TEMPO itself. 

Simatos et al. (1981) measured the mobility of a spin-label probe, TEMPO, a stable free radical commonly used for electron paramagnetic resonance (EPR) spectroscopy. She found that the probe showed no mobility below a critical that correlated to Wq. A critical a also existed at which the probe demonstrated a partitioning into a dissolved and a solid-like state. This critical a could represent the moisture content correlating to Tg, though this concept had not been introduced in foods at that time. The partitioning of a... 

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. 

The mechanistic details of these laccase/mediator catalyzed aerobic oxidations are still a matter of conjecture (51-54). However, experiments with a probe alcohol point towards one-electron oxidation of the mediator by the oxidized (cupric) form of the laccase followed by reaction of the oxidized mediator with the substrate, either via electron transfer (ET), e.g., with ABTS, or via hydrogen atom transfer (HAT), e.g., with N-hydroxy compounds which form N-oxy radicals (55). TEMPO and its derivatives form a unique case one-electron oxidation of TEMPO affords the oxoammonium cation which oxidizes the alcohol via a heterolytic pathway (Fig. 6), giving the carbonyl product and the hydroxylamine. The Tl copper center in fungal laccases has a redox potential of ca. 0.8 V vs. NHE. Consequently, fungal laccases can easily oxidize TEMPO to the corresponding oxoammonium cation, since the oxidation potential of the latter, which was first measured by Golubev and co-workers (55,57), is 0.75 V. This was confirmed by EPR measurements, which showed that laccase is reduced in the presence of TEMPO One equivalent of laccase could oxidize at least three equivalents of TEMPO within a few minutes under anaerobic conditions (58). 

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]. 

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, the EPR spectroscopic characterization of thermoresponsive polymeric systems is presented. The polymeric systems are water-swollen at lower temperatures and upon temperature increase the incorporated water is driven out and the system undergoes a reversible phase separation. Simple CW EPR spectroscopy (see above), carried out on a low-cost, easy-to-use benchtop spectrometer, is used here to reveal and characterize inhomogeneities on a scale of several nanometers during the thermal collapse. Further, neither any physical model of analysis nor chemical synthesis to introduce radicals had to be utilized. Adding amphiphilic TEMPO spin probes as guest molecules to the polymeric systems leads to self-assembly of these tracer molecules in hydrophilic and hydrophobic regions of the systems. These probes in different environments can be discerned and one... 

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