Iron Porphyrin Phenoxides: Models for Some Hemoglobin Mutants
Ainscough, E.W., Addison, A.W., David Dolphin, and James, B.R.
J. Am. Chem. Soc.
Variously substituted phenoxides (L) react with [Fe(PPIXDBE)]2O (PPIXDBE is protoporphyrin IX di-tert-Bu ester dianion) to produce 5-coordinate high-spin complexes Fe(PPIXDBE)L which display spectroscopic properties similar to those of the Met form of the α mutant chain of Hb M Boston. The addition of pyridine or 1-methylimidazole (L') to Fe(PPIXDBE)L at 77 oK produced low-spin 6-coordinate complexes Fe(PPIXDBE)LL' which were studied spectroscopically. With the strongly basic 2,6-dimethoxyphenoxide (L), the above reaction was studied at 298 oK, where for L' = 1-methylimidazole the binding constant was approximately 100 M-1 in CH2Cl2. The Fe(PPIXDBE)LL' complexes were made in an attempt to mimic the Fe(III) in the αchain of Met Hb M Iwate; however, the latter is high spin. With excess p-nitrophenoxide in CH2Cl2, Fe(PPIXDBE)(OC6H4-4-NO2) forms Fe(PPIXDBE)(OC6H4 -4-NO2)2-, which exhibits a high-spin EPR spectrum at 77 oK. Addition of phenoxides or F- to Fe(II) protoporphyrin ester systems produces species such as Fe(PPIXDBE)X22- (X = RO- or F-), similar to those found previously with MeO- and OH-. The addition of CO to a bisphenoxy species, in Me2SO, results in a splitting of the Soret band at 438 nm into 2 bands at 434 and 413 nm, which are attributed, respectively, to a carbonyl (phenoxide) species and a carbonyl species which contains no phenoxide. The visible spectral data support the view that upon reduction of Hb M Iwate at pH 6.5 by Na2S2O4 the Fe-tyrosine bond is broken.