Lanthanide binding site translation linearly along a coiled coil has a large impact on stability, coordination chemistry, and MRI relaxivity.

Bioinorganic chemists aspire to achieve the same exquisite and highly controlled inorganic chemistry featured in biology. An exciting mimetic approach involves the use of miniature artificial protein scaffolds designed de novo (often based on the coiled coil (CC) scaffold), for reproducing native metal ion sites and their function. Recently, there is increased interest, instead, in the design of xeno-metal sites within CC assemblies. This involves incorporating either non-biological metal ions, cofactors or non-proteinogenic amino acid ligands for metal ion coordination, whilst retaining a minimal CC protein scaffold. Using this approach, one should be able to create functional designs with unique and unusual properties, which combine the advantages of both biology and 'traditional' non-biological inorganic chemistry. It is the recent progress with respect to the design of xeno-metallo CCs which will be discussed in this Focus Review.

The hydration state of designed metal binding sites in coiled coils can be tuned by terminal second sphere residues.

Herein we report unprecedented location-dependent, size-selective binding to designed lanthanide (Ln 3+ ) sites within miniature protein coiled coil scaffolds. Not only do these engineered sites display unusual Ln 3+ selectivity for moderately large Ln 3+ ions (Nd to Tb), for the first time we demonstrate that selectivity can be location-dependent and can be programmed into the sequence. A 1 nm linear translation of the binding site towards the N-terminus can convert a selective site into a highly promiscuous one. An X-ray crystal structure, the first of a lanthanide binding site within a coiled coil to be reported, coupled with CD studies, reveal the existence of an optimal radius that likely stems from the structural constraints of the coiled coil scaffold. To the best of our knowledge this is the first report of location-dependent metal selectivity within a coiled coil scaffold, as well as the first report of locationdependent Ln 3+ selectivity within a protein.