Involvement of Propionate Side Chains of the Heme in Circular Dichroism of Myoglobin: Experimental and Theoretical Analyses

Incorporation of the heme into globin induces a prominent circular dichroism (CD) band in the Soret region. The appearance of heme optical activity is widely believed to arise from the interaction between the heme and aromatic residues of the globin. However, hemoglobin (Hb) containing the reversed heme exhibits a CD spectrum obviously different from that of native Hb, indicating that the interactions of heme side chains with globin contribute to the appearance of heme optical activity. We examined this possibility by comparing CD spectra of native myoglobin (Mb) and those of Mb reconstituted with synthetic hemes lacking vinyl and/or propionate. Replacement of 2,4-vinyl groups with methyl induced moderate changes. In contrast, replacement of 6,7-propionate groups with carboxylate resulted in complete disappearance of the positive Soret CD band. To get theoretical basis for the contributions of 6,7-side chains on the band, we investigated the CD spectra at a time-dependent density functional theory level. In the antiparallel conformation of the 6,7-side chains, the rotational strengths were calculated to be positive, on the other hand in the parallel conformation to be negative. We also found that the weak Soret CD band in 2,4-dimethyl-6,7-dicarboxyheme can be explained by canceling between different carboxyl conformers.