A unique low frequency Raman band associated with metal binding to metallothionein

We find that the low frequency Raman spectrum of Zn(II) metallothionein has a single prominent band at 138 cm-1 which is absent from the Raman spectrum of the metal-free protein. This feature is also found for Cd(II) binding to both of the independent metallothionein domains and the metallothionein from Neurospora crassa. TcO(III) coordination to metallothionein results in a similar Raman band which is also found for the complex (Ph4As)[ReO(SCH2CH2S)2]. By comparing these results to literature data for metal-thiolate complexes, this feature is identified as a bending vibration which appears to be characteristic of metal ion coordination by the metallothionein cysteines. Two likely assignments are a symmetric metal-centered mode (delta S-M-S) or a bending mode of the metal-coordinated cysteine thiolates (delta M-S-C).