Hemoprotein Models Based on Covalent Helix-Heme-Helix sandwich: 2. Structural Characterization of Co(III)-Mimochrome I delta and lambda Isomers

FeIII 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 α-helical nonapeptides, each of which bears a His residue (a potential axial lig-and of the iron ion) in the central position. In order to understand better the three-dimensional structure of FeIII mimochrome I and its correlation with spectral properties, we have characterized the fully diamagnetic parent compound CoIII mimochrome I by UV/visible, CD, and NMR spectroscopy, coupled with conformational energy calculations. CoIII 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 1H NMR spectroscopy as diastereomers of the δ and δ 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.