State ligation

The above procedure readily yields Fig. 9.2. For estimating the excess chemical potential in the protein, again we need to know the ligation state of the metal ion. This is well known for the zinc-finger case. So we followed the above procedure, deciding what clusters to study quantum mechanically, and then composing the free energies as above. 

The metaiioporphyrins form a diverse class of molecules exhibiting complex and varied photochemistries. Until recently time-resolved absorption and fluorescence spectroscopies were the only methods used to study metailoporphyrln excited state relaxation in a submicrosecond regime. In this paper we present the first picosecond time-resolved resonance Raman spectra of excited state metaiioporphyrins outside of a protein matrix. The inherent molecular specificity of resonance Raman scattering provides for a direct probe of bond strengths, geometries, and ligation states of photoexcited metaiioporphyrins. 

An all-or-none treatment of cooperative hgand binding, wherein a macromolecule with n sites for ligand exists in two states (a) the uncomplexed form M and (b) the fully ligated state ML . 

The H NMR and X-ray crystallography results discussed in Section III have clearly shown that the environments of the heme pockets of the a and P chains of Hb as manifested by the conformations of proximal histidine (F8), distal histidine (E7), and distal valine (Ell), as well as the conformation of several porphyrin protons and the electronic structure of the heme group in both deoxy and ligated states, are not equivalent. It has not been clear what this difference in structure in the a and p heme pockets means in terms of differences in function, because there are discrepancies in the published results regarding the relative ligand affinities of the a and P chains of Hb A. 

Fig. 60. Representation of the 10 ligation states of hemoglobin X denotes ligation of a given chain or a metal substitution for Fe in a metal hybrid hemoglobin. Index numbers provide species designation. Subunit positions are shown in species 01. [Adapted from Ackers and Smith (1987)].

Therefore, it appears that the redox properties of the metallo-porphyrin are required only for the initiation step in these free-radical autoxidations and that the porphyrin is not a stoichiometrically significant catalyst (21, 22, 23). The failure of these simple approaches to a reaction iron-porphine oxide or its equivalent could indicate that the ligation state of iron in the protein, presumably including an axial thiolate, is crucial to the oxygen-transfer properties of P 450. Support... 

NMR spectroscopy is uniquely effective in probing the oxidation state, spin state, and ligation state of iron porphyrins (47, 48) and iron N-alkylporphyrins (22). For iron-tetraarylporphyrins the /3-pyrrole proton resonance is most indicative of the electronic structure of the metal... 

Catalysis Assembly), (7) (Figure 8), consists of two [Fe4S3] cubane structures linked by a hexa-coordinated sulfur atom ( J,6-S) and two bridging snlfur atoms of Cys residnes (tt2-Scys) The cluster is attached at the terminus by four additional thiolates of Cys residnes. Some have specnlated that the core itself is unstable and cannot exist ontside the protein environment in a sulfur-ligated state. To this date, there have been attempts by several laboratories to synthesize a model of this complex with varying degrees of incomplete success. 

The ranges in which Ect falls for these and other ligation states are illustrated in Fig. 9, which summarizes the experimental and the predicted energies of the mggor peaks of the porphyrin-to-Fe(III) CT bands. The experimental data, plotted as solid-line rectangles, come... 

By the judicious use of Ect values together with EPR g values and where possible the utilization of additional constraints from, for example, protein sequence data, the axial ligation states can usually be... 

FIGURE 15.8 Structures of cobalamin derivatives and the various ligation states. (From Banerjee, Gherasim, Padovani, 2009. Copyright 2009 with permission from Elsevier. )... 

The high-frequency shifted His imidazole ring NH proton signals in a variety of oxidation/ligation states of Mb are compared in Fig. 16. Observations of... 

HisBS N H proton shift spans 0.33 ppm at 5 °C among the Mb complexes (Table 2), indicating that the AspBl—HisBS hydrogen bond depends on the heme ligation state. Since the alteration of the electronic structure of either a hydrogen-donor or -acceptor influences the NH proton signal, the HisBS N H... 

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