Alternative Fe-0(2) bond lengths in 0(2) adducts of iron porphyrins: Implications for hemoglobin cooperativity.

The extended fine structure (EXAFS) of the Fe K edge x-ray absorption spectrum has been analyzed for 0(2) adducts of "picket fence" iron(II) porphyrin complexes with hindered bases: Fe (0(2))(TpivPP)(2-MeIm).C(2)H(5)OH and Fe(0(2))(TpivPP)(1,2- Me(2)Im), TpivPP = 5, 10, 15, 20-(a,a,a,a,)-(0-pivaloylamidophenyl)porphina te, 2-MeIm = 2-methylimidazole, 1,2-Me(2)Im = 1,2- dimethylimidazole,).