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![[Figure 2]](von5002fig2mag.jpg)
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Figure 2
An example illustrating the importance of bond orders for the chemical interpretation of compounds within a macromolecule is shown. (a) In HDD from the CCD (as of February 2024), all N atoms have two single bonds within the rings, meaning they each can carry a −1 charge. This would result in the Fe atom having a +4 charge. However, within the protein, the iron of the haem often interacts with one or two amino-acid residues that can accept one or two more electrons. In other words, within the protein, the charge of the metal atom can increase. This would imply that the iron could have more than a +4 charge, which is not very likely. (b) After correcting the bond orders, the Fe atom now has a nominal charge of +2, and within the protein it can have a +3 charge. Note that both structures could exist as resonance forms. However, the structure in (a) would have higher energy compared with the structure in (b). When representing a structure, it seems reasonable to select the most probable one. The main difference between the structures in (a) and (b) is that the structure in (a) has 14 double bonds, while the structure in (b) has 13. The two extra electrons in structure (a) come from the Fe atom, making it +4. These figures were produced by ChemDraw version 23.01. |
![[Figure 2]](von5002fig2.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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