How Metal Cations can Modulate the Hydrogen Bonding of Azide and Thiocyanate Anions

Luba Tchertanov

Institut de Chimie des Substances Naturelles, UPR 2301, CNRS, 91198 Gif-sur-Yvette, FRANCE Fax: 33-1-69-07-72-47, E-mail: Luba.Tchertanov@icsn.cnrs-gif.fr

Keywords: anion binding, azide, thiocyanate, metal coordination, hydrogen bonding, structure, the Brookhaven Protein Databank, the Cambridge Structural Database.

In the chemistry of life processes, anion binding plays a central role since it concerns essential aspects such as the activity of enzymes, the transport of hormones, protein synthesis and DNA regulation. There is, therefore, intensive effort being devoted to the problem of anion complexation and recognition. The examples of metalloproteins processing small inorganic anions (e.g. superoxide dismutase, carbonic anhydrase etc.) clearly show that in each case, anion recognition is guided by attraction to the metal ion and/or hydrogen-bonding interactions with the donor-groups functionality.

For several years, we have been involved in the study of complexation and molecular recognition of certain anionic groups which vary in shape, geometries and electronic properties.

The anions referred to in this study are thiocyanate SCN- and azide NNN-, potent inhibitors of many enzymes. Both exogenous ligands are small, triatomic, linear monoanions with bidentate function and offer different modes of binding by metals and/or by H-bond donor groups. The following structural aspects will be discussed :
- the hydrogen-bond acceptor function of the anions;
- the analysis of the coordination features of anions with the metals;
- study of the influence of metal complexation on the H-donor accepting properties of the anions.

Lastly, a comparison of the modes of binding of both anionic substrates will be made and the role of structural variability of the anion binding site in human carbonic anhydrase and myoglobin will be considered.

The results provide structural information of the role of anion binding in proteins and, to our knowledge, afford the first study of the binding behaviour of thiocyanate and azide. Our work is based on experimental information obtained from the Brookhaven Protein Databank (PDB), which contains crystal structures of biological macromolecules and the Cambridge Structural Database (CSD), which contains organic and organometallic crystal structures.

Related literature:

L. Chertanova & C. Pascard, ACTA CRYST., Section B. Structural Science (1996) B52, pp. 677-684;
L. Tchertanov & C. Pascard, ACTA CRYST., Section B. Structural Science (1996) B52, pp. 685-690;
L. Tchertanov & C. Pascard, ACTA CRYST., Section B. Structural Science (1997) B53, pp. 904-915.