HEMOPROTEIN MODELS BASED ON A COVALENT HELIX-HEME-HELIX SANDWICH .2. STRUCTURAL CHARACTERIZATION OF CO-III MIMOCHROME-I-DELTA AND DELTA-ISOMERS

Fe-III mimochrome I is the prototype of a new class of hemoprotein mod els characterized by a covalent helix-heme-helix sandwich. It contains deuterohemin bound through two propionyl groups to two identical N- a nd 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 structur e of Fe-III mimochrome I and its correlation with spectral properties, we have characterized the fully diamagnetic parent compound Co-III mi mochrome I by UV/visible, CD, and NMR spectroscopy, coupled with confo rmational energy calculations. Co-III mimochrome I is a highly water-s oluble compound present in solution as two isomers, which slowly inter convert only at very low pH values. These isomers were isolated and se parately characterized. Their UV/visible spectral properties are very similar, while their CD spectral properties differ markedly in both th e far UV and Soret regions. The isomers were identified by (HNMR)-H-1 spectroscopy as diastereomers of the Delta and Lambda type. This is th e first example of an accurate three-dimensional structure determinati on in solution of a hemoprotein mimetic that allows a straightforward correlation between structure and spectral properties.