Electron paramagnetic resonance and ultraviolet/visible study of compounds I and II in the horseradish peroxidase-H(2)O(2)-silk fiber reaction system.

The enzymatic oxidation of silk with H(2)O(2) in the presence of horseradish peroxidase (HRP) has been investigated. Two intermediate complexes have been observed during this reaction. Both can be attributed to Fe(4+) ions axially bonded to an oxygen atom and to a porphyrin radical (P). In the most unstable of them, indicated as compound II, the chemical bond between [Fe(IV)=O](2+) and P was weaker than in the other, indicated as compound I. The former compound disappeared within 1 h of the reaction, at difference with the latter, traces of which were observed even after 3 weeks with dried samples. However, the chemical bond between [Fe(IV)=O](2+) and P in compound I weakened during the sample ageing. All these phenomena have been enlightened by electron paramagnetic resonance (EPR) and spectrophotometric ultraviolet/visible (UV/Vis) measurements.