Reaction center absorption spectra

In the meantime, Fritsch, Buchanan and Michel examined heme orientation in the crystals of Rp. viridis reaction centers. Absorption spectra of the crystals poised at different redox potentials were obtained using plane-polarized light. The midpoint potentials were quantitatively determined by fitting the absorbances at 552, 553, 556 and 559 nm to Nemst functions with two exponential terms. Based on the known orientations of the hemes in the reaction-center crystal, the calculaed potentials were found to correspond to the specific hemes in the following sequence ... 

Field effects in the Reaction Center absorption spectrum... 

The intermediary acceptor I is thought to be a complex involving a BPh and one of the monomeric BChls (18,30-34) This view is based on the observation that the reduction of I causes absorption changes in regions of the spectrum attributable to BPh and also in regions normally ascribed to BChl It has been proposed from nanosecond studies on reaction centers having Q reduced that P" is a thermal mixture of P BPh and P BChl , with about 60% of the added electron density on the BPh at room temperature (33) Our recent kinetic measurements (22) and low-temperature picosecond photodichroism stu-... 

One approach for identifying the type of iron center in a protein has been to remove the center, intact, by ligand exchange between the protein and an exogenous acceptor ligand (reviewed by Berg and Holm, 1982). The removed, or extruded, center is identified by comparison of its spectral properties (usually absorption spectrum) to known model compounds. Alternatively, the extruded center can be inserted into a second apoprotein which has been previously determined to accept only one type of iron center. The reconstructed standard protein is then analyzed by EPR. The latter method, interprotein cluster transfer, requires that acceptor apoproteins for all known classes of centers are included in the reaction mixture and that the reconstituted reporter ... 

Reaction of PCTFE with a stoichiometric amount of chromium hexacarbonyl in DMF at 95°C for 5 days under a nitrogen atmosphere, followed by hydrolysis results in the formation of a brown-black polymer. Analysis of the infrared data indicates that carbonylation does indeed occur (Equation 11). The infrared absorption spectrum shows a large decrease in the C-Cl stretch at 970 cm-1 with a concomitant appearance of a very strong band in the carbonyl stretching region centered at 1680 cm1. There is also a broad band centered at 3490 cm-1 in the hydroxyl stretching region and two bands of moderate intensity... 

Formation of an amide is also indicated in the reaction of PCTFE with Cr(CO)6 and the primary amine, benzylamine. The infrared absorption spectrum shows an N-H stretch centered at 3400 cm, aromatic C-H stretches at 3063 and 3030 cm1, aliphatic C-H stretches at 2933 and 2876 cm1, a broad amide I/amide II band ranging from 1680-1580 cm1, and a C-N stretch at 1454 cm1. The C-Cl stretch at 970 cm1 also shows a significant decrease in... 

The UV-visible absorption spectrum of the CdS nanotubes given in Fig. 2a shows a blue-shift in the excitonic absorption band to 460 nm. The blue-shift from the bulk value of 515 nm [12] is due to quantum confinement effects in the CdS nanotubes, the inner diameter of the nanotubes being less than the Bohr-exciton diameter of CdS (6 nm). Xiong et al. [13] have reported an absorption band at 459 nm for CdS nanotubes with an inner diameter 5 nm prepared by an in situ micelle-template-interface reaction. An absorption maximum around 450 nm has been reported in nanoparticles and hollow spheres of CdS [12,14], In Fig. 2b, we show the photoluminescence (PL) spectrum of the CdS nanotubes prepared by us, revealing a band centered at 610 nm. This band is due to charge carriers trapped at surface defects of the nanotubes [15,16],... 

In chromatophores, the of Qa decreases by 59 mV/pH unit as the pH is raised, up to an apparent p A that is between 7.8 and 9.8, depending on the species [16,30,35,36]. The pA A probably reflects the binding of a proton to a group other than the quinone itself, because the absorption spectrum and EPR spectrum of Qa match those expected for an anionic semiquinone [31,37-40]. The EN-DQR spectrum of Qa suggests that the quinone is hydrogen-bonded to a histidine residue of the protein [41]. The E value of about -0.18 V measured above the p/ A may be the most relevant value when Qa is photoreduced, because Qa probably transfers an electron to Qg before proton uptake occurs. In isolated reaction centers of Rb. sphaeroides, the E j of Qg is about 0.07 V more positive than that of Qa [29,34,42- 4]. The difference between the two E values appears... 

The optical absorption spectrum of reaction centers isolated from the carotenoidless strain R-26 of Rb. sphaeroides has major bands near 530, 545, 600, 760, 800 and 880 nm (Fig. 5). There also is a set of strong, overlapping absorption bands... 

Fig. 5. Absorption spectrum of reaaion centers from Rb. sphaeroides strain R-26, measured at 5 K with a film of reaction centers in polyvinylalcohol. Redrawn from Kirmaier et al. [76],...

If isolated reaction centers from Rb. sphaeroides or Rp. viridis are excited with a subpicosecond flash, the transfer of an electron from P to BPhL occurs with a time constant of 3 to 4 ps [72,131,132,147,148]. The kinetics can be measured by following the bleaching of the BPh s absorption bands at 545 and 760 (or 800 nm for the latter in Rp. viridis) and the appearance of broad absorption bands due to BPh and P at 760 and 1250 nm (1325 nm in Rp. viridis). Prior to the reduction of the BPh, P can be detected by its broad absorption bands in the visible and near-IR regions of the spectrum, and by its stimulated emission (fluorescence) at 920 or 1000 nm. The stimulated emission from P decays with kinetics that match the formation of BPh . 

The transient absorption spectrum of the OH adduct of dimethylaniline has a broad band centered at ca 380 nm. Decay of this species results in a growth of absorption in the 450 nm region where the radical cation has a strong absorption. Assignment of the radical cation spectrum was supported by eomparison with the spectra obtained in the eleetron-transfer reaction of sulfate radicals with DMA, and with DMA+ spectra generated by flash photolysis and eleetroehemical methods [101, 102]. The spectral characteristics of the observed transient species are summarized in Table 2. 

The uv absorption spectrum of simple aliphatic nitrocompounds consists of an extended band centered at 2700-2800 A and is most probably associated with an n 7t transition involving oxygen-atom lone-pair electrons . Both the photochemical and thermal decompositions of nitrocompounds are extremely complex, in part owing to secondary reactions between products and substrate although the overall mechanisms are still unclear in many cases, the primary modes of decomposition of some molecules have been established. 

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