Model compounds, hemoglobin

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

Early in the search for myoglobin or hemoglobin model compounds for study, it was learned that cobaltoheme substituted into apo—myoglobin or -hemoglobin... 

All successful myoglobin and hemoglobin model compounds provide steric bulk on the distal side of the porphyrin ring with a hydrophobic pocket for complexation of dioxygen as well as a bulky alkyl imidazole proximal ligand... 

It is proposed to polymerize the vinyl group of the hemin molecule with other vinyl comonomers to prepare model compounds to be used in hemoglobin research. Considering hemin and styrene to be species 1 and 2, respectively, use the resonance concept to rank the reactivity ratios rj and X2. 

Many analytical techniques have been used to study deoxy- and oxymyoglobin and hemoglobin and their model compounds. A summary of these techniques, adapted... 

STRUCTURE OF THE ACTIVE SITE IN MYOGLOBIN AND HEMOGLOBIN COMPARISON TO MODEL COMPOUNDS... 

Structure of the Active Site in Myoglobin and Hemoglobin Comparison to Model Compounds... 

Sharma, V. S., Traylor, T. G., Gardiner, R., and Mizukami, H. (1987). Reaction of nitric oxide with heme proteins and model compounds of hemoglobin. Biochemistry 26, 3837-3843. 

The metalloporphyrin complexes and their derivatives have been studied from the standpoint of model compounds of hemoglobin. The polymer-metalloporphyrin complexes are also formed by the reaction in Scheme 8, and a few qualitative investigations have been made with poly(L-lysine)9,10, poly(L-histidine)11, and poly(vinylimidazole)12 as the polymer ligand. Blauer9 has studied the complex formation of heme with poly(L-lysine) and has discussed the effects of the molecular weight and secondary structure of poly(L-lysine) on complex formation. 

Support for the vtew that the globin portion of the molecule produces a constraint upon the iron atom (which would otherwise move into the heme pocket) comes from the behavior of myoglobin and model compounds (such as the picket-fence corn-pounds with I- or 2-mcthylimidazole mimicking the porphyrin and histidine), which are easier to study than the more complex hemoglobin ... 

Figure 1. Coordination of the iron in (2-methylimidazole)mesotetra-phenylporphinato Fe(II). As far as can be judged at the resolution of the deoxymyoglobin and hemoglobin Fouriers, the stereochemistry of the iron is the same in these proteins as in this model compound. P is the mean plane of the porphyrin nitrogens Pc the mean plane of the porphyrin nitrogens and carbons (53).

Comparisons between R- and T-state hemoglobins on the one hand and a variety of synthetic model compounds on the other have allowed an evaluation of the possible occurrence and importance of electronic, proximal-base tension, and distal-side steric effects on the kinetics of ligation of CO and 02. Although all of these effects could influence the reactivities of hemoproteins, we conclude that hemoglobin reactivity and cooperativity are controlled predominantly by the presence or absence of proximal-base tension. 

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