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.

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.