Self-assembled Transition Metal Coordination Frameworks of ...

Chapter 6 _ _ __ 7
the latter have larger size. The coordination number of Zn(II) complexes with
semicarbazones and thiosemicarbazones (lower homologues) ranges from four to
seven while that of Cd(I1) complexes ranges from four to eight [24].
Zinc is an essential element, necessary for sustaining all forms of life. It is
estimated that 3000 of the hundreds of thousands of proteins in the human body
contain zinc prosthetic groups. In addition, there are over a dozen types of cells in the
human body that secrete zinc ions, and the roles of these secreted zinc signals in
medicine and health are now being actively studied [25]. Despite its critical
importance in maintaining such basic functions as proliferation and cell growth, little
is known about how zinc is acquired, stored, and utilized by the cell. Recent
pioneering studies have started to uncover the mysteries of subcellular distribution and
to some extend on the proteins that maintain it. Many zinc complexes serve as models
of biologically active zinc systems [25]. In biological chemistry, zinc serves as an
electrophilic catalyst; that is, it stabilizes negative charges encountered during an
enzyme-catalyzed reaction. Thus, protein-zinc recognition and discrimination requires
proper chemical composition and proper stereochemistry of the metal-ligand
environment. The fact that zinc is not subject to any ligand field stabilization effects
and the change from an octahedral to a tetrahedral ligand field is not energetically
disfavored for Zn(II) also have to be considered in biological studies. Zinc(H) ion has
been found to be of catalytic importance in enzymatic reactions [26]. The
enhancement of antitumor activity of some thiosemicarbazones in the presence of
zinc(II) ions has been reported [27]. Some zinc complexes of thiosemicarbazones
have been shown to be active as anti-tumor agents, are as cytotoxic as cisplatin and
are also effective against cisplatin resistant cell lines [28]. Recently, the in vitro
antiproliferative activity of Zn(II) thiosemicarbazones against the cells of MCF-7 and
T24 human cancer cell lines have been reported [29]. Although, zinc is an important
element in biological systems, cadmium, similar to zinc chemically in many ways,
apparently does not substitute or "stand in" for it. The toxicity of cadmium is
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