Oxygen, hyperfine coupling constant

DET calculations on the hyperfine coupling constants of ethyl imidazole as a model for histidine support experimental results that the preferred histidine radical is formed by OH addition at the C5 position [00JPC(A)9144]. The reaction mechanism of compound I formation in heme peroxidases has been investigated at the B3-LYP level [99JA10178]. The reaction starts with a proton transfer from the peroxide to the distal histidine and a subsequent proton back donation from the histidine to the second oxygen of the peroxide (Scheme 8). 

The ozonide ion has widely spaced energy levels, and to first order the g values are not influenced by the host lattice or the surface. Thus, the absolute values of the g values are useful in the identification of the ion. These g values, along with the hyperfine coupling constants, are given in Table I. The three sets of hyperfine constants indicate that the oxygen atoms are not equivalent, at least when the ozonide ion is formed according to reaction 2. The geometry of the ion on MgO is believed to be... 

Hyperfine coupling constant, 22 267, 269 Hyperfine interaction, ESR data for, 22 274 Hyperfine parameters for O, 32 128-130 Hyperfine splitting, 31 81 Hyperfine structure, trimer species, 31 98-99 Hyperfine tensor, 22 267, 273-279, 336, 340 constants, 32 20-21 dioxygen species, 32 18-25 equivalent oxygen nuclei, 32 18-21 ionic oxides, 32 40... 

Early ESR studies demonstrated that the hyperfine coupling constant (ac 13) for 13C(car-bonyl)-substituted fluorenone radical anion is counterion-dependent. For the free ion, ac 13 = 2.75 Gauss. In contrast, when the counterion is Li+, ac 13 = 6.2 Gauss23. Consider Scheme 4 For the free ion, canonical structure 1 and 2 are contributors to the resonance hybrid. For the >C=0 / Li+ ion pair, association of Li+ with oxygen increases the relative contribution of canonical structure 1 to the resonance hybrid, resulting in greater spin density at carbon. The fact that spin (and charge density) varies as a function of counterion (and presumably solvent) will certainly affect the reactivity of the radical ion. However, very few quantitative studies exist which directly address this point. 

Attempts were made to explain trends in the magnitudes of hyperfine coupling constants in terms of hyperconjugation, localization on oxygen, and so forth (Symons, 1959). Also, results for radicals in solids were being related to their more stable, and generally more complicated, counterparts in solution. It was realized that the equivalence of /3-protons, in radicals such as MeaCOH, must be the result of rapid... 

Copper-proteins are wide-spread in both animals and plants and have been related to many metabolic processes, as oxygen transport, electron transfer and hydroxylation Copper-containing sites are usually classified in three different types " the type 1, or blue center is characterized by a combination of properties that has not yet been reproduced in model complexes (an intense absorption band at 600 nm, a very small copper hyperfine coupling constant A and a high positive redox potential for the Cu(II)/ Cu(I) couple) the type 2, or non-blue center has properties comparable to those of low molecular weight cupric complexes the type 3 consists of an antiferromagnetically coupled copper(II) pair. 

---