Spin-labeled compound

The u.v.-visible spectrum of the 4-hydroxy-2,2,6,6-tetramethyl-piperidine-N-oxyl-methyl-cobinamide is very similar to methyl-cobin-amide itself and as a result this technique cannot be used to rigorously identify the spin labeled derivative. The spin labeled compound does show a spectral change with pH between pH 7.0 and pH 2.0 which methyl-cobinamide does not exhibit. Despite the similarities between methyl-cobinamide and nitroxylmethylcobinamide, the circular dichroism spectrum of the two derivatives are quite different. Fig. 23 shows the marked difference in C. D. spectra of 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl, methylcobinamide, and a methylcobinamide solution containing an equimolar amount of uncoordinated nitroxide. 

As with the nitroxalkylcobalamins (119) and cobinamides, the co-binamides in which nitroxide is coordinated show electron spin resonance spectra very similar to the spectrum of free nitroxide. The high field line is not broadened as much as in the spectrum of a nitroxalkyl-cobinamide. No hyperfine splitting from methyl protons in the 2 or 6 positions can be observed for the bound nitroxide. However, treatment of the coordinate spin labeled compounds with cyanide releases the nitroxide. When this happens, the proton hyperfine can be observed (Fig. 25). Thus treatment with cyanide simply displaces the nitroxide and a spectrum for free nitroxide is observed. 

The e.s.r. spectra of y-irradiated diethyl phosphate and its salts have been studied111 and the structures of the phosphonates(89)112 and some nucleoside phosphates113 have been studied from the e.s.r. spectra of the nitroxide spin-labelled compounds. An e.s.r. study of electron transfer in dinucleoside phosphate anions indicates preferen-... 

Ahlin P. et al., The effect of lipophilicity of spin-labeled compounds on their distribution in solid lipid nanoparticle dispersions studied by electron paramagnetic resonance, J. Pham. Set, 92, 58, 2003. 

The triphase hydrolysis of 1-bromoadamantane with catalysts 50 (n = 1-16) and 51 was studied kinetically 170). The enthalpies of activation (AH ) for the catalyzed reactions were 6-12 kcal/mol lower than for the uncatalyzed reaction. (The free energies of activation were 1-2 kcal/mol lower). This considerable variation in AH was attributed to a much different microenvironment in catalysts 50 and 51 from that in the absence of the catalyst. Thus Regen 170> pictures both the organic phase and the aqueous phase in the polymer matrix, as if homogeneous. Studies using spin-labeled compounds also indicated that catalyst 50 affects the polarity and the motional freedom of the microenvironment171). 

This nitroxide is more soluble in liquid regions of bilayers than it is in solid regions. As bilayers are warmed in the EPR spectrometer, the solubility of this spin-labeled compound in the lipid can be followed (see figure). The lower of the three spectra approximates that of the spin label in water alone, while the others are composite spectra for which part of the spin label has dissolved in the phospholipid bilayers. 

Since frequencies for EPR spectroscopy are -100 times higher than those for NMR spectroscopy, correlation times (Chapter 3) must be less than 10-9 s if sharp spectra are to be obtained. Sharp bands may sometimes be obtained for solutions, but samples are often frozen to eliminate molecular motion spectra are taken at very low temperatures. For spin labels in lipid bilayers, both the bandwidth and shape are sensitively dependent upon molecular motion, which may be either random or restricted. Computer simulations are often used to match observed band shapes under varying conditions with those predicted by theories of motional broadening of lines. Among the many spin-labeled compounds that have been incorporated into lipid bilayers are the following ... 

Improved syntheses have been described for a variety of spin-labelled compounds, including the aziridine derivatives (27), phosphoramidates (28), esters (29), and ester-amidates, the compound (29a) being the reagent of choice. 

Figure 2, ESR spectra of the spin-labeled compound (4). i, 2,2,6,6-tetramethyl-4-aminopiperidine-1 -oxyl in the solid state ii, in toluene Hi, 4 in aqueous solution,...

In general the labeled compound is an analog of a compound found naturally in the membrane system, and thus it is assumed that the motional behavior of the labeled species is very similar to that of its analog. It is surprising, therefore, that the surface properties of the spin labeled compounds have not been studied as extensively as those of the unlabeled... 

Cellulose, PVA, and their derivatives can be used as polymers which have hydroxy groups on the surface. As azo component the commercially available 4,4 -azobis-4-cyanovaloyl chloride (AIVC) was employed. Whether or not the azo group is really introduced into the polymer can be verified, for instance, by further reaction with a spin-labeled compound like 2,2,6,6-tetramethyl-4-aminopiperidine-l-oxyl (TMPO). This reaction proceeds as follows ... 

The mobility of spin labels grafted on neurotoxin II and the degree of their exposure towards solvent environment containing nonspecific paramagnetic probes K3Fe(CN) (42) or Ni(Ac)2,were examined both for AChR bonded AchR and non-bonded states (Table IV). Isotropic correlation times for spin labeled compounds (x j g) within the 50 ps - 1 ns interval were determined from the EPR spectra as described in(54). For slower rates the x g values were evaluated by comparison with calculated EPR spectra(55), assuming isotropic rotation. 

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