Turning protein into room temperature molecular magnet*

Metallothionein-2 (MT-2) is a cysteine-rich protein that binds seven divalent transition metal ions avidly via its metal-thiol linkages. A magnetic MT-2 containing two Mn and five Cd (Mn,Cd-MT-2) has been synthesized by protein refolding process. No trace of Fe was detected by ICP mass spectroscopy. The uniform size distribution, tested by dynamic light scattering, indicated that each Mn,Cd-MT-2 molecule is a single molecular magnet. Its coercive field of ferromagnetic signals changed slightly from 50 to 300K, but dropped rapidly when the temperature rose from 330 to 395 K. The blocking temperature T$_{B}$ is around 410K, in powder form. These results indicated that the un-paired electron of both Mn$^{2+}$ might be aligned by electron hoping of the bridging sulfurs in the $\beta $-metal binding cluster of MT-2 and when the protein deformed at 410K the ferromagnetic signals disappear correspondingly. This engineered molecule exhibits both molecular magnetization and bio-compatibility. These features make Mn,Cd-MT-2, a good candidate for biological applications and sensing sources of new nano-devices.