LUMINESCENCE PROPERTIES OF THE DINUCLEAR COPPER COMPLEX IN THE ACTIVE-SITE OF HEMOCYANINS

The deoxygenated form of hemocyanin, containing a dinuclear Cu(I) acti ve site, emits luminescence in the red with maximum around 1.54 mu m(- 1) (650 nm). The luminescence of deoxyhemocyanin (deoxy-Hc) from arthr opod species is detectable at room temperature, the quantum yield bein g 2.4-2.7 X 10(-3); in contrast, the emission from molluscan proteins can be detected only at liquid nitrogen temperature. The luminescence emission is an inherent property of the bis[Cu(I)-(histidine)(3)] comp lex of the deoxygenated form of the protein to which both Cu(I) ions c ontribute equally to the overall emission. Luminescence is not observe d with the oxygenated and the oxidized forms of hemocyanin, in which t he metal is in the Cu(II) state, and in the metal-depleted or apo-Hc f orm. Based on steady-state and time-resolved measurements and referenc es to Cu(I) model compounds, the luminescence emission is attributed t o a triplet excited state of a Cu(I)-to-N (histidine) charge transfer transition (3)d-pi. Acrylamide quenching experiments indicate that th e metal active site is very shielded from the solvent. This property o f deoxy-Hc enables us to directly follow reactions that modify either the copper oxidation number or the metal-to-protein stoichiometry. (C) 1994 Academic Press, Inc.