連接不同軸向配體鐵卟啉體系的自旋非極化和自旋極化密度泛函活性理論研究（英文）

吳文杰　張曉青　惠華英　等

摘要血紅素在許多生化反應中起著至關重要的作用，且血紅素的核心為卟啉環配位鐵離子.文獻采用密度泛函活性理論及其自旋極化方法對卟啉環連接的金屬離子的選擇性進行了研究，發現卟啉環連接鐵離子時其結構和活性與連接其他金屬離子的體系有很大的差異.實驗研究表明，軸向連接不同配體對體系的結構和活性有顯著影響.本文采用密度泛函活性理論及其自旋極化方法對鐵卟啉體系中鐵離子軸向連接不同配體的體系進行系統探究.軸向配體包括 SMe， SHMe， 1H咪唑及衍生物， OH， H2O， H2O2， CO， NO， O2， 呋喃，異吲哚，吡咯和吡啶.通過對全局和局域化學活性描述符的計算分析發現，當配體是CO時，體系化學性質穩定、反應活性也低；在眾多種體系中H2O和SHMe的得失質子對體系活性的影響最大.這些計算結果對更深入了解血紅素及其類似體系的活性和作用機理有重要意義.

關鍵詞卟啉環； 血紅素； 密度泛函活性理論； 自旋密度泛函活性理論

Scheme 1LFe（Ⅱ）porphyrin systems investigated in this work， with L can be replaced by fractional groups in amino acid that usually bonded with heme in nature： from cysteine： MeS-， SHMe， from histidine： 1Himidazole， imidazol1ide， from water： O2-， HO-， H2O， H2O2， and small gaseous molecules： CO， NO， O2， and also some other common ligands： furan， isoindole， pyrrole， pyridineHeme is a metalbinding porphyrin consisting of a heterocyclic organic ring made from four pyrrole subunits linked via methine bridges （Scheme 1）. As the core cofactor of hemoproteins， it serves as a prosthetic group for many biological processes including oxidative metabolism[13]， xenobiotic detoxification， synthesizing and sensing of diatomic gases， cellular differentiation， gene regulation at the level of transcription， protein translation and targeting， and protein stability. Therefore， hemoproteins such as hemoglobin[4]， myoglobin[5]， hemocyanin[67] and neuroglobin[8] are abundance in nature and play essential roles in physiological processes as sensors， activators， and carriers of gaseous molecules. Hemoglobin is most commonly found in its oxygenbinding state where the bonded metal cation is a divalent iron， Fe（Ⅱ）. When in its resting or functioning state， up to two axial ligands are required to bond with the metal cation in the metalporphyrin complex to carry out the catalytic process. The most common axial residues in hemoproteins are histidine and cysteine.

湖南師范大學自然科學學報第38卷第5期吳文杰等：連接不同軸向配體鐵卟啉體系的自旋非極化和自旋極化密度泛函活性理論研究Density functional reactivity theory （DFRT） and spinpolarized DFRT （SPDFRT） have recently been applied to understand the metalbinding specificity of porphyrin[12]， where it was found that both structure and electronic properties of the metal cation play important roles in differentiating its metalbinding specificity. Differences in metalbinding specificity of metalporphyrin complexes can result from different causes. For example， no Caporphyrin or Cdporphyrin complex exists in nature because Ca（Ⅱ） and Cd（Ⅱ） cations are too big to fit into the inner cavity of the porphyrin ring. For other complexes such as Zn（Ⅱ） and Mn（Ⅱ）， it was concluded that electronic properties， as shown from the secondorder perturbation theory analysis of the Fock matrix in the NBO basis and chemical reactivity descriptors such as hardness， electrophilicity， and dual descriptors， dictate their metalbinding specificity. It has also been revealed that the iron complex differs from other metal ion complexes in bonding and reactivity properties （i.e. charge distribution， stability， and nucleophilicity） when two axial bonds are formed， enhancing and facilitating the role of the iron cation as the center of the catalytic process and thus implying that the number and nature of the axial ligands also play an important role in the catalytic process. Another conclusion from those previous studies in the literature is that Fe（Ⅱ） and Ru（Ⅱ） complexes show similar reactivity from the spin unresolved DFRT perspective， but from the spinpolarized version of DFRT， the Feporphyrin complex stands up and often possesses the smallest value for each of the SPDFRT quantities in most cases， indicating that the Fe system is most likely to increase the total spin number， and the spin resolution showed a remarkable difference between Ru and Fe complexes. These results suggest that the specificity difference of porphyrin between Fe and Ru cations is originated from both the electron and spin properties and that to differentiate their behaviors one has to resort to spin resolved reactivity indices.