Artificial Metalloproteins for Binding and Stabilization of a Semiquinone Radical

The interaction of a number of first-row transition-metal ions with a 2,2′-bipyridyl alanine (bpyA) unit incorporated into the lactococcal multidrug resistance regulator (LmrR) scaffold is reported. The composition of the active site is shown to influence binding affinities. In the case of Fe(II), we demonstrate the need of additional ligating residues, in particular those containing carboxylate groups, in the vicinity of the binding site. Moreover, stabilization of di-tert-butylsemiquinone radical (DTB-SQ) in water was achieved by binding to the designed metalloproteins, which resulted in the radical being shielded from the aqueous environment. This allowed the first characterization of the radical semiquinone in water by resonance Raman spectroscopy.