Heterochromia in Designed Metallopeptides: Geometry-Selective Binding of Cd in a De Novo Peptide**

One of the most important chemical concepts is defining how one molecule recognizes and controls the properties of another molecule or ion. Nowhere is this issue more significant than in the field of biomolecular recognition. Metalloproteins efficiently control the geometry and coordination number of metal ions as well as the types of ligands bound to them. Incorporation of metals in the correct binding site is essential for proper biological activity of the protein, regardless of whether the function is catalytic, structural, or regulatory. Despite the importance of metalloproteins, the major factors determining metal-ion selectivity and specificity are not yet well understood. Recent studies on metalloregulatory proteins and carbonic anhydrase suggest that coordination number and geometry of the metal ion are key elements. This control becomes even more significant when proteins can bind two metal ions with different coordination environments. This is the case for the enzyme 5’-aminolevulinic acid dehydratase, which contains two Zn centers, one bound in a five-coordinate geometry and the other bound in a pseudotetrahedral geometry. Interestingly, lead toxicity is mainly due to the replacement by a Pb ion of the Zn ion bound to the tetrahedral site. Thus, a deep knowledge of the factors determining this delicate ion recognition is crucial to understanding the principles that govern metalloprotein structure and function. Using the de novo designed TRI peptide family (TRI= Ac-G-(LKALEEK)4-G-NH2), we have been investigating the influence of protein environment on metal-ion specificity and how subtle changes in the amino acid sequence can fully control the coordination number of the metal ion. By exploiting our previous observations that we could influence the metal binding affinity in the TRI coiled coils by placing cysteine in either position a or d of the heptad repeat unit, we designed a TRI derivative with two cysteine binding sites (TRI L9CL19C, Table 1) that shows sequential and selective binding of Cd ions, first to the a site and then to the d site.