Soret absorption band

Structural formulas of the ZP and ZP-I dyads are shown in Fig. 1. The absorption spectra of ZP-I dyads indicate that the excitation energies of the ZP S2 and S, states are practically the same throughout the ZP-I series. The peak position of the Soret absorption band of the ZP-I dyads are only slightly red shifted from that of free ZP, and are practically the same as that of ZnAr3P. These results indicate a rather weak D-A electronic interaction in the ground state of ZP-I systems despite of their directly linked structure, which can be ascribed to the nearly perpendicular conformation between ZP and I molecular planes [2],... 

Closer inspection of the reduced P-450 spectrum in the light of these observations shows that there are additional anomalies relative to the supposedly simpler model systems. The extinction coefficient of the Soret band is only about 50—70 X 103, compare Table 9 where data for model complexes are presented, and there is very high absorption around 440— 460 nm. This could arise from a very broad Soret absorption band or, more likely, it could be that there is more than one type of simple reduced P-450 and that one form of it absorbs at around 450 nm. The absorption in the visible region is also very broad indeed. We return to these features later. 

Further evidence for ground-state complexation as the cause of the special catalysis was obtained by a spectroscopic study in a model system. Such complexes are typically characterized by a shift of the maximum of the porphyrin Soret absorption band relative to that of the non-complexed porphyrin. In the presence of a high concentration of tetramethy 1-1,2-dioxetane, used as a model for the co-ordinating ability of [21], the absorption maximum of ZnTPP was determined to be shifted by 1.2 nm. 

Figure 10.30 shows the absorption and fluorescence spectra of 5,10,15,20-tetraphenyl-21H,23H-porphine (TPP).17 The chemical structure of TPP is inset. The porphyrin Soret band is seen as a strong, narrow absorption peak at 418 nm. The weaker absorption peaks at 512 and 550 nm are the Q bands, also typical of porphyrins. The fluorescence spectrum has peaks at 653 and 714 nm and the fluorescence quantum yield has been measured to be 11 % in solution exposed to air.66 The Soret absorption band has strong overlap with the fluorescence spectrum of PFO, resulting in efficient Forster transfer. The Forster-transfer radius has been calculated from the relative overlap of these spectra to be 4.8 nm. 

Compared to other hemoproteins the spectra are unu >ial with respect to the blue-shift and absorption decrease upon reduction. Addition of carbon monoxide to the reduced hemoprotein results in formation of the 447 nm Soret absorption band which appears at 450 nm in a difference spectrum and gave its name to this cytochrome. 

Such carbene complexes may also be formed reductively by cytochrome P450 of liver microsomes from geminal polyhalogenated hydrocarbons. In the case of halothane a complex with a Soret absorption band at 470 nm is formed which was explained by a carbene formation according to the equations given in Fig. 

Fig. 13. Cytochrome P450 excitation profiles for the 1488 cm porphyrin mode (O) and the 351 cm Fe-S stretching mode (A). The former maximizes in the Soret absorption band of the heme (thin solid line), but the latter shows shorter wavelength maxima, which are attributed to S - Fe charge transfer transitions, for which evidence can be seen in the z-polarized single-crystal absorption spectrum (thick solid line). ...

Fig. 2.10 ER spectrum (real part) for cytochrome Cj, a tetra-heme protein, adsorbed on a bare Au electrode surface at the macroscopic formal potential (solid line) and difference absorption spectrum (dotted line), where A,ed is the absorption spectrum of the reduced form and A>x is that of the oxidized form. The two spectra are normalized at the positive-going Soret absorption band peak for comparison.

For many years it was assumed tiiat hone protons v/en nonfluorescoit This was a reas< able assum < given that die intense Soret absorption band of die heme groups is expected to result in Ftirster distances (Ro) for trp-to-... 

A similar approach was applied in the study of holo-myoglobin [41], In both cases, the lifetimes are in good agreement with those obtained by alternative methods. Acid-induced denaturation of holo-myoglobin on a pH jump (6.5 to 3.2) coupled with ESI-MS led to observations of various protein conformations [41], The MS signals were monitored on a timescale of a few seconds with sub-second to a few seconds intervals. Some of the charge states represent lifetimes shorter than 1 s. The lifetimes obtained by TRMS and stopped-flow measurements of the Soret absorption band are convergent [41], This work pointed to... 

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