Nitrogen-sulfur-containing ligands

The high affinity of zinc for nitrogeneous and sulfur-containing ligands seems chiefly responsible for the occurrence of zinc in a wide variety of biological compounds, such as proteins, amino acids, nucleic acids,... 

In contrast to oxygen and nitrogen as donor atoms, sulfur has low lying, unoccupied 3 -orbitals available. MO calculations reported in the literature, however, indicate that this influence might be overestimated. It has to be assumed that the high polarizability of the electrons on the sulfur atoms is responsible for the variety of structures and the reactivity of metal complexes with sulfur-containing ligands. 

Estimated A/h values (MHz) for H, 13C and l4N nuclei of nitrogen- and sulfur-containing ligands of nickel(II)... 

Iron-containing proteins are those in which iron atoms are bound with sulfur-containing ligands. The number of labile sulfur atoms is usually equal to that of iron atoms which varies between 1 and 18. The molecular mass of these proteins is in the range of 6000 to 750,000 units. Such proteins effect electron transfer in photosynthesis, nitrogen fixation, and respiration in mitochondria. They are capable of transferring electrons under a potential close to that of a reversible hydrogen electrode. Their oxidation-reduction potential is between +0.35 and -0.49 V. 

In more acidic solutions, protonated forms of the zinc—cysteine complex are present also (Shindo and Brown, 1965). It is evident that sulfur-containing ligands give a more complicated pattern of complexes than the simple patterns produced by oxygen- and nitrogen-containing ligands, which are more common and to which the next section will be devoted. 

The two subunits of CODH/ACS have been dissociated to offer a clearer picture of the ACS active site 135). The holoenzyme contains 2 Ni, 12 Fe, and 14 S 120) that are organized into 3 discrete clusters, whereas the isolated a subunit contains only 1 Ni and 4 Fe and has spectroscopic properties similar to those of Cluster A in the native enzyme 186). Based on EXAFS spectroscopy of the a. subunit, the Ni site in Cluster A has been proposed to be coordinated to 2 sulfur ligands at 2.19 A and 2 nitrogen or oxygen ligands at 1.89 A in a distorted square plane 186). 

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