Hc reacted with nitrite forms a half met form in which nitrite is bound to Cu(II). Three imidazoles are coordinated to Cu(II), based on a 3-pulse ESEEM study, two weaker coupled, the third stronger. Relative coupling differences for Hc are species specific. The g and ACu as well as the hyperfine coupling constants suggest a distorted tetragonal 3 N 1 O structure for Cu(II), with the O arising from nitrite.