Introduction: Transition Metals and Sulfur.

The number of transition metal ions which are essential to life - also often called trace elements - increased steadily over the years. In parallel, the list of biological functions in which transition metals are involved, has grown, and is still growing tremendously. Significant progress has been made in understanding the chemistry operating at the biological sites where metal ions have been discovered. Early on, based on the application of physical, chemical, and biological techniques, it became likely that numerous of these metal centers carry sulfur ligands in their coordination sphere, such as sulfide (S2-), cysteine (RS-), or methionine (RSCH3). Notably, the structure and the reactivity of the metal active sites turned out to be quite different from anything previously observed in simple coordination complexes. Consequently, the prediction of active-site structures, based on known properties of transition metal ion complexes, turned out to be difficult and incorrect in many cases. Yet, biomimetic inorganic chemistry, via synthesis and detailed structural and electronic characterization of synthetic analogues, became an important factor and helped to understand the properties of the metal active sites. Striking advances came from molecular biology techniques and protein crystallography, as documented by the publication of the first high-resolution structures of iron-sulfur proteins and the blue copper protein plastocyanin approximately five decades ago. In this volume of METAL IONS IN LIFE SCIENCES the focus will be on some of the most intriguing, in our view, transition metal-sulfur sites discovered in living organisms. These include the type 1 Cu mononuclear center, the purple mixed-valent [Cu1.5+-(Cys)2-Cu1.5+] CuA, the tetranuclear copper-sulfide catalytic center of nitrous oxide reductase, the heme-thiolate site in cytochrome P450, the iron-sulfur proteins with bound inorganic (S2-) and organic (Cys-) sulfur, the pterin dithiolene cofactor (Moco) coordinated to either molybdenum or tungsten, the [8Fe-7S] P-cluster and the [Mo-7Fe-9S-C]-homocitrate catalytic site of nitrogenase, the siroheme-[4Fe-4S] center involved in the reduction of sulfite (SO32-) to hydrogen sulfide (H2S), the NiFeS sites of hydrogenases and CO dehydrogenase, and the zinc finger domains. We apologize to all researchers and their associates who have made tremendous contributions to our current knowledge of the steadily increasing transition metal sulfur sites in proteins and enzymes but are not mentioned here. These omissions are by no means intentional but merely the consequence of time and space. We are fully aware of the excellent books and authoritative reviews on various aspects of the subject, however, it is our motivation to cover in one single volume this exciting domain of bioinorganic chemistry.