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Z. Naturforsch. 69a, 313 – 325 (2014)
doi:10.5560/ZNA.2014-0004
The Role of Heme Chirality in the Circular Dichroism of Heme Proteins
Robert W. Woody1 and Gennaro Pescitelli2
1 Department of Biochemistry and Molecular Biology, Colorado State University, Fort Collins, CO 80523, USA
2 Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, I-56126 Pisa, Italy
Received December 15, 2013 / revised December 27, 2013 / published online July 15, 2014
Reprint requests to: R. W. W.; Fax: 1-970 491 0494, E-mail: robert.woody@colostate.edu
The rotational strength (R) of the Soret transition in sperm-whale myoglobin (SW Mb), the hemoglobin from Chironomus thummi thummi (CTT Hb), and human hemoglobin (hHb) has been calculated using 20 high-resolution (< 1.5 Å) crystal structures. The intrinsic rotational strength due to heme non-planarity was calculated using π-electron theory and time-dependent density functional theory (TDDFT). Calculations on model protoporphyrins with a planar nucleus and with various torsional angles for the 2- and 4-vinyl substituents showed maximum R of ± 0.70 Debye–Bohr magneton (1 DBM = 0.9273 · 10−38 cgs units). Viewing the heme so that the 2- and 4-vinyls are in a counter-clockwise relationship, if a vinyl points toward the viewer, it contributes positively to R. Calculations of the intrinsic R for explicit heme geometries of SW Mb, CTT Hb, and hHb gave averages of 0.40 ± 0.09, − 0.44 ± 0.04, and + 0.32 ±0.11  DBM, respectively. Coupling of the Soret transition with aromatic side-chain and peptide backbone transitions was also considered. For SW Mb, the magnitudes of the contributions decreased in the order Rint > Raro > Rpep. For CTT Hb and hHB, the orders were, respectively, Rint > Rpep > Raro and Rint > Raro ≈ Rpep. Human Hb α chains showed the same trend as CTT Hb. Only in the hHb β chains did Raro predominate, with the order Raro > Rint > Rpep. The total predicted Rtot for SW Mb, CTT Hb, and hHb averaged + 0.77 ± 0.10 (0.56–0.80), − 0.37 ± 0.12 (− 0.5), and + 0.31 ±0.17  DBM (0.23–0.50), respectively. (Values in parentheses are experimental values.) Thus, contrary to the currently accepted view, coupling with aromatic side-chain or peptide transitions is not the dominant factor in the Soret circular dichroism (CD) of these proteins. The Soret CD is dominated by intrinsic CD of the heme chromophore, of which vinyl torsion is the major determinant. This result suggests an explanation for the large effect of heme isomerism on the Soret CD of Mb and Hb. Rotation about the α-γ axis may be associated with large changes in vinyl torsion and thus substantially alter the intrinsic CD, even reversing its sign.
Key words: Hemoglobin; Myoglobin; Circular Dichroism; Soret Band; Heme Isomerism.
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