The optical absorption spectra of native and N-3(-)-reacted Cu,Zn superoxide dismutase (SOD) has been studied in the temperature range 300-10 K. The broad d-d bands observed in the room temperature spectrum, centered at 14 700 cm(-1) (native enzyme) and at 15 550 cm(-1) (N-3(-)-reacted enzyme), are clearly split at low temperature into two bands each, centered at 12 835 and 14 844 cm(-1) and at 14 418 and 16 300 cm(-1), respectively. The thermal behavior of the 23 720 cm(-1) band present in the spectrum of the native enzyme indicates that this band belongs to the His61-->Cu(IT) ligand to metal charge transfer transition. Analysis of the zeroth, first, and second moments of the various bands as a function of temperature allowed us to obtain useful information on the stereodynamic properties of the metal site in SOD. In particular for the native protein, it was possible to infer a variation in the metal ligand relative position that occurs as the temperature is lowered and that likely involves all of the ligands except His61. On the other hand, the site is stabilized upon N-3(-) binding, and in this case a variation in the metal ligand position is observed only at the level of the bound anion. The possible relation of these properties to the catalytic mechanism of the enzyme is discussed
