Olafson B D, Goddard W A
Proc Natl Acad Sci U S A. 1977 Apr;74(4):1315-9. doi: 10.1073/pnas.74.4.1315.
From ab initio quality calculations on model systems, we conclude that in unliganded Fe-porphyrin the FE lies in the plane for both the high-spin (q) and intermediate-spin (t) states. Thus, the high-spin d6 Fe is not too big to fit into the porphyrin plane (as often suggested). We find the q state lower for a porphyrin hole radius greater than 1.94 A and the t state lower for smaller sizes. For the five-coordinate complex including an axial nitrogenous ligand [a model for myoglobin (Mb) and hemoglobin (Hb)], we find the ground state to be q with the Fe 0.3 A out of the plane (recent x-ray data on deoxy Mb suggests about 0.4 A). The origin of this out-of-plane displacement is the nonbonded repulsions between the axial ligand and porphyrin nitrogen orbitals. Pushing the Fe of the five-coordinate complex into the plane does not lead to a stable low-spin state (as usually suggested), the q and t states being the low-lying states. Bonding the O2 to form the six-coordinate complex stabilizes the t form of the Mb model, leading to a singlet state of MbO2 with Fe in the plane. (It has often been suggested that the Fe of MbO2 and HbO2 is low-spin Fe2+; however, we find this not to be the case.) The bonding in the MbO2 model confirms the ozone model of the bonding, leading to a structure consistent with the Pauling model (our calculated FeOO bond angle is 119 degrees). The total charge transfer to the O2 is 0.10 electron, in disagreement with the Weiss model. Molecular orbital calculations (Hartree-Fock) incorrectly lead to septet ground state (S = 3) for the MbO2 model. The implications for the cooperative O2 binding in hemoglobin and protein modifications of the energetics of the active site are considered. Use of our calculated force constants for displacement of Fe perpendicular to the heme plane suggests that the movement of the Fe upon a change in the quaternary structure from the T to the R form is only about 0.04 A toward the heme plane.
通过对模型体系的从头算质量计算，我们得出结论：在无配体的铁卟啉中，无论是高自旋（q）态还是中间自旋（t）态，铁都位于卟啉平面内。因此，高自旋的d6铁并不太大以至于无法嵌入卟啉平面（正如通常所认为的那样）。我们发现，对于卟啉孔半径大于1.94 Å的情况，q态能量更低；对于较小尺寸的情况，t态能量更低。对于包含轴向含氮配体的五配位配合物[肌红蛋白（Mb）和血红蛋白（Hb）的模型]，我们发现基态为q态，铁原子位于平面外0.3 Å处（最近关于脱氧肌红蛋白的x射线数据表明约为0.4 Å）。这种平面外位移的起源是轴向配体与卟啉氮轨道之间的非键排斥作用。将五配位配合物的铁原子推到平面内并不会导致稳定的低自旋态（如通常所认为的那样），q态和t态是低能态。将O2结合形成六配位配合物会使肌红蛋白模型的t形式稳定，导致MbO2的单重态，铁原子位于平面内。（人们经常认为MbO2和HbO2中的铁是低自旋Fe2+；然而，我们发现并非如此。）MbO2模型中的键合证实了键合的臭氧模型，导致了与鲍林模型一致的结构（我们计算的FeOO键角为119度）。向O2的总电荷转移为0.10电子，这与魏斯模型不一致。分子轨道计算（哈特里 - 福克）错误地导致MbO2模型的基态为七重态（S = 3）。我们考虑了其对血红蛋白中O2协同结合以及活性位点能量学中蛋白质修饰的影响。使用我们计算的垂直于血红素平面的铁原子位移的力常数表明，当四级结构从T态转变为R态时，铁原子向血红素平面的移动仅约为0.04 Å。
