Chemical models of manganese metalloenzymes

• Main Author: Libby, Eduardo
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1990
• Subjects: Biochemistry; Chemistry; Inorganic chemistry
• ISBN: 9798207401874

Several mononuclear, dinuclear and tetranuclear manganese compounds that, respectively, are synthetic analogues of the active sites of the manganese metalloenzymes superoxide dismutase, catalase and the oxygen evolving center (OEC) of Photosystem II were prepared and studied. The complexes were characterized by spectroscopic, magnetochemical and electrochemical techniques and by X-ray crystallography. The properties of the complex Mn$\sb2$O$\sb2$(pic)$\sb4$ were compared with those of Mn$\sb2$O(O$\sb2$CMe)(bpy)$\sb2$Cl$\sb2$, Mn$\sb2$O(O$\sb2$CMe)$\sb2$(Cl)$\sb2$(bpy)$\sb2$, Mn(pic)$\sb3$ $\cdot$ H$\sb2$O, (Mn(dipic)$\sb2$) $\sp-$ (bpy = 2,2$\sp\prime$-dipyridyl, pic = picolinate, dipic = 2,6-pyridine dicarboxylate) and other compounds reported in the literature. A correlation of redox potentials with the number of oxygen donors was observed. The data for mononuclear and dinuclear complexes was compared to the data on the manganese enzymes. Reaction of trinuclear basic Mn(III) carboxylates with Me$\sb3$SiCl produced the Mn$\sp{\rm III}\sb3$Mn$\sp{\rm IV}$ cubane complexes (Mn$\sb4$O$\sb3$Cl$\sb4$(O$\sb2$CR)$\sb3$(L)$\sb3$) (R = Me, Et; L = pyridine, imidazole), (Mn$\sb4$O$\sb3$Cl$\sb7$(O$\sb2$CMe)$\sb3$) $\sp{3-}$ and other derivatives. Their structures model some of the properties observed for the Mn aggregate of the OEC in the S$\sb2$ state. Magnetochemical data show that the cubanes have an S = 9/2 ground state and that the energy levels are sensitive to the peripheral ligand environment. Methods for the preparation of Mn$\sp{\rm III}\sb4$ complexes with the formulation NR$\sb4$ (Mn$\sb4$O$\sb2$(O$\sb2$CR$\sp\prime$)$\sb7$(L$\sp\prime$)$\sb2$) (R = $\sp{\rm n}$Bu, Et, Me; R$\sp\prime$ = H, Me, Ph; L$\sp\prime$ = picolinate, quinaldate, oxinate) from Mn(O$\sb2$CR$\sp\prime$)$\sb2$ and NR$\sb4$MnO$\sb4$ were developed. The complexes were studied as analogues of the S$\sb1$ state of the OEC. Spectroscopic evidence showed that the tetranuclear complexes retain their structure in solution. The factors that determine the stereochemistry of the (Mn$\sb4$O$\sb2$) unit were studied. Reactivity studies demonstrated the controlled dimerization of NBu$\sb4\sp{\rm n}$ (Mn$\sb4$O$\sb2$(O$\sb2$CR)$\sb7$(pic)$\sb2$), by reaction with Me$\sb3$SiCl, to form (Mn$\sb4$O$\sb2$(O$\sb2$CMe)$\sb6$(pic)$\sb2$) $\sb2$ when R = Me. When larger carboxylates were used (R = Ph), (Mn$\sb4$O$\sb2$(O$\sb2$CPh)$\sb6$(pic)$\sb2$(MeCN)$\sb2$) formed suggesting that the aggregation can be controlled by steric factors.