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Q-band continuos wave (QW) and pulsed electron-nuclear double resonance spectroscopy (ENDOR) were used to study the chloroperoxidase heme in situ following the inactivation with allylbenzene. Samples prepared in natural isotopic abundance, with N-labeled enzyme, and with allylbenzene labeled with 2H or 13C in specific vinylic positions were used. QW and ENDOR together with molecular modeling calculations determined the in situ structure of the alkene-modified heme.

Continuous-wave and 1H electron-nuclear double resonance spectroscopy identified the aqua ligand nature of the axial ligand of native low-spin chloroperoxidase (PO). The ligand has two major conformations. The identical nature of heme iron coordination sphere associated with CPO and cytochrome P-450(sub a) is proved using this scheme. The heme ligation structures have significant differences. Quadrupole coupling and 14N hyperfine coupling for these two enzyme are different due to out-of-planarity and/or heme ruffling.