Structural properties of metal-free apometallothioneins

► Electrospray ionization mass spectrometry study of metal-free metallothionein. ► Zn7-MT and apo-MT at pH 7 exhibit the same charge state distribution. ► Benzoquinone cysteine modifications do not significantly alter the protein volume. ► Molecular modeling calculations provided the volume of each of the proteins. ► The structure of apo-βα-MT is more organized than previously considered.