Myoglobin model compounds

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. 

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

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

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

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... 

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 ... 

A number of model compounds have been synthesized which have Fe(ll)-porphyrin rings carrying a side chain with histidine arranged to be able to coordinate with the metal on one side. Several of these substances show promise as oxygen carriers with properties similar to myoglobin. 

Some reviews [5-7] have appeared on NCE-electrospray ionization-mass spectrometry (NCE-ESI-MS) discussing various factors responsible for detection. Recently, Zamfir [8] reviewed sheathless interfacing in NCE-ESI-MS in which the authors discussed several issues related to sheathless interfaces. Feustel et al. [9] attempted to couple mass spectrometry with microfluidic devices in 1994. Other developments in mass spectroscopy have been made by different workers. McGruer and Karger [10] successfully interfaced a microchip with an electrospray mass spectrometer and achieved detection limits lower than 6x 10-8 mole for myoglobin. Ramsey and Ramsey [11] developed electrospray from small channels etched on glass planar substrates and tested its successful application in an ion trap mass spectrometer for tetrabutylammonium iodide as model compound. Desai et al. [12] reported an electrospray microdevice with an integrated particle filter on silicon nitride. 

Several of the proteins with ferryl intermediates have been crystalised at sufficient resolution to allow the elucidation of their 3-dimensional structure. These include cytochrome c peroxidase [95], horseradish peroxidase [96], catalase [97], myeloperoxidase [98], ribonucleotide reductase [99], cytochrome P-450 [100] and myoglobin [101]. Of these only cytochrome c peroxidase has proved stable enough to crystallise with the iron in the ferryl form [26]. High-resolution structures exist for small FeIV model compounds, both in the presence [102] and absence [7,8] of an Fe=0 bond. These compounds can have sulphur, nitrogen and chloride ligation to the iron and the iron can be five [7,8] or six [8] coordinate. 

Mossbauer spectra has been extensively used to probe the structure of the iron nucleus in biological FeIV=0 compounds. These include horseradish peroxidase compoundl[134,180,181], horseradish peroxidase compound II [182,183], horseradish peroxidase compound X [181], Japanese-radish peroxidase compounds I and II [184], chloroperoxidase compound I [185], cytochrome c peroxidase compound I [186] and ferryl myoglobin [183]. Examples of Mossbauer spectra attributed to non-porphyrin-bound FeIV are only available from synthetic model compounds. These include compounds with [130] and without [4-8] an FeIV=0 bond. 

These structures, which have been formulated by assuming that one of the unshared electron pairs on O2 forms the initial bond to the metal, are expected to lead to an angular geometry which has been observed in X-ray structures of model compounds, in oxy-myoglobin (Fig. 16-10), hi82 oxyhemoglobin. ... 

Indirect observation of transition metal chemical shifts via C was reported for the determination of 5( Fe) in a variety of ferrocenes and ferrocenyl carbenium ions, as well as in a natural myoglobin carbonyl complex and two synthetic model compounds. All measurements were made by selective double-resonance methods using either [ Fe]-labelled or... 

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