Hemes electronic state transitions

Ultraviolet (UV) and visible spectra, also known as electronic spectra, involve transitions between different electronic states. The accessible regions are 200-400 nm for UV and 400-750 nm for visible spectra. The groups giving rise to the electronic transitions in the accessible regions is termed chromophores, which include aromatic amino acid residues in proteins, nucleic acid bases, NAD(P)H, flavins, hemes, and some transition metal ions. Two parameters characterize an absorption band, namely the position of peak absorption Wmax) and the extinction coefficient (e), which is related to concentrations of the sample by the Beer-Lambert law ... 

To experimentally probe the electronic and thermal consequences of flash photolysis, a femtosecond time-resolved near-IR study of photoexcited Mb was undertaken (22). This study probed the spectral evolution of band III, a weak ( max 100 M-1 cm-1) near-IR charge transfer transition (14) centered near 13, 110 cm-1 that is characteristic of five-coordinate ferrous hemes in their ground electronic state (S = 2). Because band III is absent when the heme is electronically excited, the dynamics of its reappearance provides an incisive probe of relaxation back to the ground electronic state. Moreover, because the spectral characteristics of band III (integrated area center frequency line width) correlate strongly with temperature (23-26), the spectral evolution of band III also probes its thermal relaxation. 

Mott transition, 25 170-172 paramagnetic states, 25 148-161, 165-169 continuum model, 25 159-161 ESR. studies, 25 152-157 multistate model, 25 159 optical spectra, 25 157-159 and solvated electrons, 25 138-142 quantitative theory, 25 138-142 spin-equilibria complexes, 32 2-3, see also specific complex four-coordinated d type, 32 2 implications, 32 43-44 excited states, 32 47-48 porphyrins and heme proteins, 32 48-49 electron transfer, 32 45-46 race-mization and isomerization, 32 44—45 substitution, 32 46 in solid state, 32 36-39 lifetime limits, 32 37-38 measured rates, 32 38-39 in solution, 32 22-36 static properties electronic spectra, 32 12-13 geometric structure, 32 6-11 magnetic susceptibility, 32 4-6 vibrational spectra, 32 13 summary and interpretation... 

Cytochrome be, complex to cytochrome c to cytochrome oxidase Cytochrome c is a peripheral membrane protein that is loosely bound to the outer surface of the inner mitochondrial membrane. It binds to the cytochrome be, complex and accepts an electron via an Fe3 to Fe2+ transition. Then it binds to cytochrome oxidase and donates the electron, with the iron atom of the heme of cytochrome c then reverting to the Fe3+ state (Fig. 2). 

Proximal Electronic Control of Heme Reactivity ( ). The deprotonation of a coordinated imidazole results in a large reduction in CO binding rate constants k5(Im)/fc5(Im ) = 170 (Table I). However, the changes are inverse to expectation deprotonation causes the rate constant to decrease, whereas if increased electron donation by the fifth ligand was the critical factor, the rate constant should increase. It is probable that Im- as an axial ligand stabilizes the pentacoordinated ferroporphyrin and that this effect far overbalances any influence of increased charge donation by Im in the transition state on the reaction path by which the hexacoordinated Fe(P)(Im )(CO) is formed. 

If the resonant electronic transition is associated with a site of biological activity, then the technique offers a sensitive probe for structural features of the site. Heme proteins afford particularly informative resonance Raman spectra, with a rich assortment of porphyrin ring vibrations, which can be classified and analyzed via their symmetry properties [132-134], Some of these frequencies are sensitive to the structural features of spin- and oxidation-state changes of the heme group. These can be used to monitor the structural consequences of ligation or electron transfer in heme proteins [66, 135-138]. 

According to a widely accepted consensus, the complexation of 02 with heme followed by the two-step electron transition leads to the formation of the state [Fe2+02"], in which the dioxygen adopts an active form capable of hydroxylating the substrates, including saturated hydrocarbons (Coon et. al., 1981 Guengerich and Macdonald 1984). The complexation and the first electron transfer proceed rapidly and take place at atmospheric pressure of dioxygen froml0"2to 10 3s. The second electron transfer is a relatively slow reaction (for the enzyme from P. Putida, k = 4 s 1), which commonly limits the entire process. 

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