ISSN:0021-2148    

DOI:10.1002/ijch.198800012

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractSingle crystals were obtained from the photochemical reaction center protein of the photosynthetic bacteriumRhodobacter sphaeroideswild type strain 2.4.1. At low temperatures, the crystals display pronounced anisotropy in their triplet state electron spin resonance (ESR) spectra. An analysis of the angle dependence of the resonance field positions revealed two to four magnetically nonequivalent primary donor molecules per unit cell. This is consistent with the symmetry assignment of P 2 12 12 1deduced from previous X‐ray diffraction studies (Frank, H.A.; Taremi, S.S.; Knox, J.R.J. Mol. Biol., 1987,198: 139–141). However, the ESR data could not be fit assuming that the unit cell axes coincide with the crystal morphological axes as was the case previously forRb. sphaeroidesR26 (Gast, P.; Norris, J.R.FEBS Lett., 1984,177: 277–280). In order to explain the present data, it was necessary to assume a rhombic crystal cross‐section with the unit cell axes lying along the diagonals of the rhombus. The unit cell axesbandcwere found to make angles of 60 ± 2° and 30 ± 2°, respectively, with one of the faces of the crystal. The orientation of the triplet state dipolar tensor axes relative to the unit cell axes differs slightly from that inRb. sphaeroidesR26 crystals, suggesting that theRb. sphaeroides2.4.1 reaction center is rotated by approximately 7° relative to the orientation found

ISSN:0021-2148    

DOI:10.1002/ijch.198800013

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe temperature dependence of the longwavelength absorption band of theRhodopseudomonas viridisreaction center is investigated with a thermal expansion model of the special pair. The validity of the model is demonstrated through the reproduction of optical lineshapes measured at various temperatures and through the consistency of the energetic parameters obtained in this fashion. The connection with the temperature dependence of the primary electron transfer process is briefly discussed.

ISSN:0021-2148    

DOI:10.1002/ijch.198800014

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractOligonucleotide‐mediated mutagenesis of the light‐harvesting I (LHI; B880 complex) antenna genes fromRhodobacter capsulatushas been used to study the bacteriochlorophyll‐peptide interactions in this photosynthetic membrane complex. A system of deletion strains and complementing plasmids has been used to assay the effects of changing single amino acid residues in the a structural polypeptide of LHI. Fourteen mutations have been introduced in the putative bacteriochlorophyll‐binding site: Ala‐X‐X‐X‐His. Residues with side chains smaller than valine can substitute for alanine, but the LHI complex is lost from the membrane if alanine is replaced by residues larger than valine, or if histidine is replaced by arginine, asparagine, aspartic acid, glutamine, proline, or threonine. The block in biogenesis is not transcriptional, since the associated reaction center, whose genes are known to be downstream from the LHI genes, continues to be inserted into the membrane. The membrane‐bound reaction center is photochemically active in these LHI‐depleted strains. Such strains may be useful for directly observing electron transfer properties of the reaction center in the membrane. This system is a model for studying the assembly of integral membrane proteins, since easily assigned optical transitions of the prosthetic groups sig

ISSN:0021-2148    

DOI:10.1002/ijch.198800015

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe time‐evolution of the near‐infrared absorption changes accompanying the conversion of the excited primary electron donor, P*, to the radical pair state, P+BPhL−, has been examined in detail inRhodobacter capsulatusreaction centers. In a series of spectra spanning the time interval from about 600 fs to 15 ps, two isosbestic points occurring at 765 run and 798 nm are maintained throughout. The finding of an isosbestic point at 798 nm in this series of spectra, which encompass the time during which P* decays and BPhL−forms, places severe constraints on the possible reduction of the 800‐nm‐absorbing monomeric BChlLin the initial stage of the charge‐separation process. The near‐infrared P* spectrum, which is revealed clearly only when P is excited directly in its long‐wavelength absorption band, contains bleaching only of the 855‐nm band and a broad featureless transient absorption below 810 nm. This spectrum shows that P does not contribute any significant oscillator strength to the 800‐nm ground

ISSN:0021-2148    

DOI:10.1002/ijch.198800016

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractCarotenoporphyrin‐quinone triad molecules have previously been found to successfully mimic many aspects of photosynthetic electron and energy transfer. These molecules generate long‐lived, energetic charge‐separated states via a biomimetic multistep electron transfer scheme. In many of these molecules, the overall quantum yield for charge separation has been limited by an unfavorable partitioning of an intermediate charge‐separated state between charge recombination and further reaction to yield the desired species. One strategy for overcoming this limitation is based on the fact that, in general, electron transfer rate constants do not depend linearly on reaction free energy change. In the triads, raising the energy of the intermediate charge‐separated state would be expected to speed up the desired electron transfer (“normal” Marcus behavior), but slow down charge recombination (which formally lies in the “inverted” region). Laser flash photolysis and transient fluorescence measurements on two new triad molecules reveal enhanced quantum yields of long‐lived charge‐separated states which are consistent

ISSN:0021-2148    

DOI:10.1002/ijch.198800017

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractPhotosystem I (PS I) particles prepared from pea chloroplasts using either digitonin or Triton X‐100 as the solubilizing detergent were used to investigate the EPR characteristics of the electron acceptors A0, A1, and (Fe‐S)X. The signal from component A1appears asymmetric in both digitonin and Triton X‐100 solubilized samples. However, the difference spectrum which represents the isolated signal from component A0is symmetric in Triton X‐100 solution, but not in digitonin. Upon deuteration of digitonin solubilized PS I, the signal from A1became partially narrowed to give a spectrum comparable to that of deuterated Triton X‐100 solubilized particles. However, deuteration did not produce new spectral features in the digitonin solubilized preparation, but emphasized those already present Upon storage of either type of particles at 77 K, it was found that the signal from component A1, but not that from A0, decayed. The difference spectrum of A1obtained withg= 2.0058 and a peak‐to‐peak linewidth of 0.95 mT is compatible with the identification of A1as an anionic semiquinone. In digitonin solubilized particles, component A0and not A1affected the EPR signal of the iron‐sulphur center X, which suggests a magnetic interaction between A0and X. These results suggest that component A0is closer to X than is A1, and that A1may not be in a linear electron transport pathway between A0and the iron‐sulphur centers. They also show that Triton X‐100 perturbs the structure of

ISSN:0021-2148    

DOI:10.1002/ijch.198800018

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractWe have characterized the electron donors to Photosystem II (PS II) in an O2‐evolving reaction center core preparation from spinach (Ghanotakis, D.F.; Yocum, C.F.FEBS Lett., 1986,197: 244–248) using EPR spectroscopy of the manganese center involved in water oxidation and of the tyrosine donor responsible for signal II. Both the 16‐line and the 19‐line form of the S2multiline EPR signal can be observed in the core particles under conditions similar to those which produce these signals in PS II membranes. Consequently, the structure of the coupled cluster of three or four Mn ions remains intact in the core particles. The Kok parameters which characterize the number of reaction centers capable of advancing by 0, 1, or 2 equivalents in response to a short laser pulse are found to be the same in PS II membranes and in the core particles. Also, there is no evidence for partially inactivated centers which reach the S2or S3states, but which do not advance to O2release. Thus the new class of highly resolved O2‐evolving particles appears to be well suited for biophysical studies directed at the mechanism of water

ISSN:0021-2148    

DOI:10.1002/ijch.198800019

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe light‐induced radical cation of the primary electron donor, P+.960, in photosynthetic reaction centers (RCs) fromRhodopseudomonas viridishas been investigated by various magnetic resonance techniques (ESR, ENDOR, TRIPLE). Comparison of the measured hyperfine couplings with those of the cation radical of monomeric bacteriochlorophyllb(BChlb) and with RHF‐INDO/SP molecular orbital calculations, performed on P+.960using coordinates of an X‐ray structure analysis, consistently show an asymmetric distribution of the unpaired electron over the two BChlbmolecules which constitute P960. The derived spin density ratio is 2.8 in favor of the dimer half bound to the L protein subunit. The possible relevance of this result for the primary electron transfer step in the RC is briefly disc

ISSN:0021-2148    

DOI:10.1002/ijch.198800020

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe pathways of electron donation in Photosystem II (PS II) as studied by electron paramagnetic resonance (EPR) and time‐resolved optical spectroscopy are discussed. The EPR studies have defined two competing pathways of electron donation in PS II, from cytochromeb559and from the Mn site of the oxygen‐evolving center. The kinetics of re‐reduction of the primary electron donor of PS II (P680). as measured by optical spectroscopy, are re‐evaluated in light of the EPR results. We propose that the 35‐μs kinetic component is due to the reduction of chlorophyll, an alternate electron donor on the cytochromeb559pathway, rather than to the reduction of P680. The chlorophyll/cytochromeb559pathway has been proposed to be part of a cyclic electron transfer pathway around PS II; we suggest that photooxidation of chlorophyll is the first step leading to photoinhibition and that cytochromeb559serves to protect PS II from photoinhibition by rapidly re‐reducing the oxidized chlorophyll (Thompson, L.K.; Brudvig, G.W.Biochemistry, 1988,27: 6653). These results and proposals are summarized in an overall scheme of electron transfer pathways and ra

ISSN:0021-2148    

DOI:10.1002/ijch.198800021

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY