Dynamic nuclear polarization mechanisms

On the other hand, in accord with the free radical mechanism peroxynitrite is dissociated into free radicals, which are supposed to be genuine reactive species. Although free radical mechanism was proposed as early as in 1970 [111], for some time it was not considered to be a reliable one because a great confusion ensued during the next two decades because of misinterpretations of inconclusive experiments, sometimes stimulated by improper thermodynamic estimations [85]. The latest experimental data supported its reliability [107-109]. Among them, the formation of dityrosine in the reaction with tyrosine and 15N chemically induced dynamic nuclear polarization (CIDNP) in the NMR spectra of the products of peroxynitrite reactions are probably the most convincing evidences (see below). 

This mechanism is still quite controversial However, CIDNP (chemically induced dynamic nuclear polarization) experiments on a magnesium-dependent DNA-I polymerase (which hydrolyzes the P —O—P j linkage in a NTP in order to insert a new nucleoside in a growing DNA chain) being fed MgGTP revealed that GMP radical anions may indeed be involved . 

The second approach is the use of the dynamic nuclear polarization (DNP) detection principle. Dorn and co-workers have pioneered the application of this technique [9,10], Whereas the NOE enhancement of 13C nuclei in the conventional 13C H recording is dependent upon the 7h/7c ratio (NOE = Th/ Tc = 2 1), the DNP enhancement relates to the ye/yuc ratio (2640 1). In an electron-nucleus spinsystem, the electron-electron transitions are saturated by microwave irradiation and magnetization transfer from electron to nucleus (Overhauser effect) occurs via a scalar and/or dipolar mechanism. The DNP enhancement, A, is described by the following equation ... 

L13. Lehnig, M., Radical mechanisms of the decomposition of peroxynitrite and the peroxynitrite-CO(2) adduct and of reactions with 1-tyrosine and related compounds as studied by (15)N chemically induced dynamic nuclear polarization. Arch. Biochem. Biophys. 368, 303-318... 

Such anomalous NMR spectra as observed in the above reactions have been called Chemically Induced Dynamic Nuclear Polarization (CIDNP) . CINDP should be due to nonequilibrium nuclear spin state population in reaction products. At first, the mechanism of CIDNP was tried to be explained by the electron-nuclear cross relaxation in free radicals in a similar way to the Overhauser effect [4b, 5b]. In 1969, however, the group of Closs and Trifunac [6] and that of Kaptain and Oosterhoff [7] showed independently that all published CIDNP spectra were successfully explained by the radical pair mechanism. CIDEP could also be explained by the radical pair mechanism as CIDNP. In this and next chapters, we will see how CIDNP and CIDEP can be explained by the radical pair mechanism, respectively. 

Stoicheiometric hydroformylation of alkenes with [MnH(CO)s] has recently been reported. Treatment of 1,2-diphenyl 3,3-dimethylcyclo-propene in hexane under CO with [MnH(CO)s] at 55 °C gave a mixture of cis-(S7%) and trans-(l37o) aldehydes together with some alkane. The proposed mechanism is shown in Scheme 3. A chemically induced dynamic nuclear polarization (CIDNP) effect was observed in the H n.m.r. spectrum during the reaction and this was ascribed to the initial formation of an alkyl radical (10). The final step, reaction of a metal hydride with a metal acyl to give free aldehyde, is similar to that proposed in Co-catalysed hydro-... 

When discussing the general aspects of FTNMR, we have to remember that all principal statements about Fourier methods have been introduced for a strictly linear system (mechanical oscillator) in Chapter 1. In Chapter 2, on the other hand, we have seen that the nuclear spin system is not strictly linear (with Kramer-Kronig-relations between absorption mode and dispersion mode signal >). Moreover, the spin system has to be treated quantummechanically, e.g. by a density matrix formalism. Thus, the question arises what are the conditions under which the Fourier transform of the FID is actually equivalent to the result of a low-field slow-passage experiment Generally, these conditions are obeyed for systems which are at thermal equilibrium just before the initial pulse but are mostly violated for systems in a non-equilibrium state (Oberhauser effect, chemically induced dynamic nuclear polarization, double resonance experiments etc.). 

Arakawa T, Timasheff SN (1984) Mechanism of protein salting in and salting out by divalent cation salts balance between hydration and salt binding. Biochemistry 23 5912-5923 Armstrong BD, Choi J, Lopez C, Wesener DA, HubbeU W, Cavagnero S, Han S (2011) Site-specific hydration dynamics in the nonpolar core of a molten globule by dynamic nuclear polarization of water. J Am Chem Soc 133 5987-5995... 

The most striking evidence for a possible radical pair mechanism in the case of vinylarenes is the observation of chemically induced dynamic nuclear polarization (CIDNP). CIDNP was observed in the product of the stoichiometric hydrogenation of 1,1-diphenylethene (156) and in both carbonylation and saturation products of styrene (164). Scheme 11 shows the proposed mechanism of the reaction of styrene with HCo(CO)4. 

Chemical Induced Dynamic Nuclear Polarization is a useful source of information about chemical mechanisms involving free radicals and it has been largely exploited in and spectroscopies. Due to... 

A search for an ionic Cope rearrangement has included (608). A concerted mechanism can only interchange (608) and (609), whereas (610) may intrude if ions (radicals) are involved. In fact, (610) is the product, and the absence of chemically induced dynamic nuclear polarization (CIDNP) effects together with trapping of the intermediate carbonium ion with borohydride as anethole support an ionic mechanism. ... 

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