| Abstract
| - Two different tryptophan radicals (Wa•and Wb•) with lifetimes of several minutes atroom temperatureare formed during the reconstitution of the diiron center in theEscherichia coli ribonucleotide reductasemutantprotein R2 Y122F. Detailed hyperfine parameters are for the firsttime determined for protein-linked oxidized neutraltryptophan radicals. Wa• isfreeze-trapped and investigated by EPR and ENDOR in protonated andselectivelydeuterated proteins at 20 K. Two hyperfine couplings from theβ-methylene protons, hyperfine tensors of twoα-protons, and the complete nitrogen hyperfine tensor are determined.Based on the absence of a large hyperfinecoupling from the N−H proton, which would be expected for a cationradical, and on comparison of the experimentaldata with theoretical spin densities from density functionalcalculations, Wa• is assigned to an oxidizedneutraltryptophan radical. A small anisotropic hyperfine couplingdetected in selectively deuterated Wa• istentatively assignedto a proton which is hydrogen bonded to the nitrogen ofWa•. A similar spin density distributionas for Wa• isobtained also for the second tryptophan radical,Wb•, observed by EPR at room temperature,which is also assignedto an oxidized neutral radical.
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