Spectroscopic studies on Neurospora copper metallothionein

The spectral properties of Neurospora copper metallothionein were investigated and compared with those of the Cu(I)-2-mercaptoethanesulfonic acid complex. In both cases, the absorption spectra are rather similar, showing a characteristic shoulder at approximately 250 nm. However, marked differences were observed in their emissive properties. Thus, only metallothionein emits detectable luminescence in solution, but both the copper protein and the Cu(I) complex are luminescent at 77 K. The circular dichroism spectrum of Neurospora copper metallothionein shows several Cotton extrema attributable to asymmetry in metal coordination. The influence of HgCl2 and p-(chloromercuri)benzoate on the spectral properties of metallothionein was also investigated. The two mercurials exerted a pronounced effect on the electronic absorption, chiroptical, and emissive properties of the protein. Spectroscopic titrations followed by gel filtration experiments indicate that two mercurials can be bound per metallothionein molecule without loss of copper. This binding is responsible for the disappearance of the emissive properties of metallothionein and for the distinct changes in its electronic absorption and circular dichroism spectra. From these data, it is suggested that the Cu(I) ions are coordinated to the cysteinyl residues in the form of a single metal cluster.