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It was attempted to examine whether or not isotope labeling may possibly affect an oligonuclear metal-thiolate cluster. Cu-metallothioneins are known to contain strongly distorted Cu-thiolate clusters and seemed appropriate for this study. Thus, yeast 13C-and 15N-Cu-metallothioneins were isolated from Saccharomyces cerevisiae cells grown in a minimal synthetic medium and some physicochemical parameters were compared with those of the unlabeled Cu-thionein. Surprisingly, the 13C- and 15N- labeled Cu7-thioneins are distinctly different in their characteristic spectroscopic properties. The electronic absorption was blue-shifted while both luminescence emission and chiroptic features display a distinct red shift with markedly diminished intensities, respectively. Contrary to common knowledge that isotope labeling does not affect the molecular architecture of a protein the present results support such a phenomenon. Attributable to the fortunate happenstance that there is a strongly distorted structural situation in the oligonuclear Cu-thiolate cluster this isotope effect came to light. [ABSTRACT FROM AUTHOR]