Hemoprotein Modes Based on a Covalent Helix-Heme-Helix Sandwich: 2. Structural Characterization of Co(III)-Mimochrome I Delta and Lambda Isomers

Fe-III mimochrome I is the prototype of a new class of hemoprotein models characterized by a covalent helix-heme-helix sandwich. It contains deuterohemin bound through two propionyl groups to two identical N- and C-terminal protected a-helical nonapeptides, each of which bears a His residue (a potential axial ligand of the iron ion) in the central position. In order to understand better the three-dimensional structure of Fe-III mimochrome I and its correlation with spectral properties, we have characterized the fully diamagnetic parent compound Co-III mimochrome I by UV/visible, CD, and NMR spectroscopy, coupled with conformational energy calculations. Co-III mimochrome I is a highly water-soluble compound present in solution as two isomers, which slowly interconvert only at very low pH values. These isomers were isolated and separately characterized. Their UV/visible spectral properties are very similar, while their CD spectral properties differ markedly in both the far UV and Soret regions. The isomers were identified by (HNMR)-H-1 spectroscopy as diastereomers of the Delta and Lambda type. This is the first example of an accurate three-dimensional structure determination in solution of a hemoprotein mimetic that allows a straightforward correlation between structure and spectral properties.