ABSTRACT
Metal ions are known to play an essential role in living systems, both in growth and in metabolism. In fact the role of iron as an essential metal has been known since the eighteenth century, whereas the importance of other elements, such as cobalt, copper, manganese, and zinc, has only been known for more than half a century. It has also been known for a long time that an excess of these elements can be very dangerous, and, as a matter of fact, a narrow concentration window exists for most of the so-called trace elements. Even though many elements of the periodic table are nowadays known and accepted to be essential or beneficial for life on earth, the molecular role of these elements is only beginning to be understood [1-4].
