Metallothioneins (MTs) are small cysteine-rich proteins which exhibit high affinities for various metal ions and play roles in storage of essential metals and detoxification of toxic metals. Studies on the redox properties of MTs have been quite limited. Recently, we focused on the a-domain of MT (MT alpha) as a protein matrix and incorporated a tetranuclear metal cluster as a reductant. UV-visible. CD and MS data indicate the formation of the stable tetranuclear metal-cysteine cluster in the MTa matrix with Fe-4(II)-MT alpha and Co-4(II)-MT alpha species existing in water. Furthermore, the Fe-4(II)-MT alpha species was found to promote the reduction of met-myoglobin and azobenzene derivatives under mild conditions. Particularly, the stoichiometric reduction of methyl red with Fe-4(II)-MT alpha (1:1) was found to proceed with a conversion of 98% over a period of 6 h at 25 degrees C. This indicates that all of the four Fe(II) cores contribute to the reduction. In this paper, we describe the preparation and reactivity of the tetranuclear iron cluster in the protein matrix. (C) 2011 Elsevier Inc. All rights reserved.