摘要:

AbstractFunctional and structural aspects of the mechanism of oxygenic photosynthesis are described and documented by examples: (1) Coupling of two Photosystems I and II for the electron transfer from water to NADP+. (2) Vectorial electron transfer and transmembrane charge separation as the primary act of light energy conversion. (3) Functioning of the reaction centers I and II in the photosystems and the role of the pool of plastoquinones. (4) Proton translocation and use of the proton potential for ATP formation. (5) Approaches to the mechanism of water oxidation. (6) Organization of the proteins and their cofactors as well as the molecular structure of Photosystem I. (7) Light energy migration in the antenna system.

ISSN:0005-9021    

DOI:10.1002/bbpc.19961001202

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe photobiology ofHalobacteriais reviewed here. Furthermore, the functions of halobacterial retinal proteins as ion pumps for photosynthesis and as sensors mediating phototaxis are described. The well‐characterized transport mechanism of bacteriorhodopsin allows the suggestion of a general functional model of all halobacterial retinal proteins due to their structural similarities. Experimental findings showing the respective interconversions of proton‐ and chloride (halide)‐pumps, of outward‐directed to inward‐directed pumps and of sensors to pumps confirm this model. From these findings a general concept of ion translocation is derived and described as the

ISSN:0005-9021    

DOI:10.1002/bbpc.19961001203

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe peripheral light‐harvesting complex of the photosynthetic bacteriumRhodopseudomonas acidophila(LH2) and the major plant light‐harvesting complex LHCII have a very similar function: to absorb solar photons and to transfer the electronic excitation to the pigments surrounding the reaction center, the so‐called ‘core’. Nevertheless, their structures exhibit a dramatically different arrangement of the pigments. In LH2 the bacteriochlorophyll molecules are arranged in a highly symmetric ring, while in LHCII the positioning of the chlorophylls is very irregular. In both complexes the average distance between the pigments is 1 nm or less and, as a consequence, the electronic interaction between the pigments is strong (>100 cm−1). Therefore, the excitation transport in these photosynthetic light‐harvesting systems can not be described by a simple Förster type transfer mechanism, but new or other transfer mechanisms may be operative, for instance a mechanism in which the excitation is to some extent delocalized. Crucial parameters are the strength of the electronic coupling, the amount of energetic disorder and/or heterogeneity and the nature and strength of the interactions of the pigments with the protein. Here we will discuss the current status of the field of photosynthetic energy transfer in particular for LH2. We will evaluate a few simple models that contain some of the essential ingredients to describe the process of energy transfer and finally we will discuss some of the perspectives in this sc

ISSN:0005-9021    

DOI:10.1002/bbpc.19961001204

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractTrapping and charge stabilization in intact cells ofChloroflexus aurantiacusandChlorobium limicolawere studied by means of photovoltage kinetics and fluorescence emission spectra upon excitation in the chlorosome. InChloroflexustwo electrogenic phases with time constants of 100 ps for trapping (P+H−formation) from the core‐complexes and 530 ps for charge stabilization (QAreduction) were found. InChlorobiumthere were also two electrogenic phases of 105 ps and 650 ps. The later phase most likely represents the charge stabilization on the iron‐sulfur center, Fx. A comparison of stationary fluorescence spectra of both species indicates that the trapping time from the core pigments inChlorobiumis significantly faster than inChloroflexus.This proves the existence of at least two intermediary acceptors between the primary donor (P 840) and Fxin theChlorobiumreaction center. In both species the photovoltage signal in the low excitation limit appeared after a lag phase of 55 to 65 ps, which is ascribed to the energy transfer from the chlorosome to the core pig

ISSN:0005-9021    

DOI:10.1002/bbpc.19961001205

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractFemtosecond spectroscopy is used to study the molecular mechanisms of the primary electron transfer. Data on native and mutated reaction centers ofRhodopseudomonas viridisshow, that primary electron transfer is an ultrafast stepwise reaction. The electron is transferred via a chain of pigments: In a first reaction step the electron is transported from the special pair to the accessory bacteriochlorophyll with a time constant of ≈ 3 ps. A second, faster reaction carries the electron with 0.65 ps to a bacteriopheophytin. Experiments on mutated reaction centers with strongly modified reaction times yield additional information on energetics, reorganisation energies and electronic coupling of the reaction center. A consistent theoretical treatment of the data shows that standard non‐adiabatic theory describes well the primary electron transfer process. It also reveals that the realised reaction parameters optimise the reaction centers for highest possible quantum yi

ISSN:0005-9021    

DOI:10.1002/bbpc.19961001206

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe most prominent cornerstone of reaction center function is the high quantum yield of transmembrane charge separation approaching unity in the systems studied so far. This feature necessarily implies that the deactivation rate of the excited primary donor1P*not involving electron transfer is not accessible in native systems. Therefore, reaction centers ofRhodobacter sphaeroideshave been modified via thermal exchange of the relevant primary electron acceptor BA. Substitution of BAwith 3‐vinyl‐132‐OH‐bacteriochlorophyll increases the lifetime of1P*from about 2 to 240 ps at 90 K. Transient absorption studies reveal that this increase is due to a decrease of the rate of charge separation. At 90 K this decrease causes a concomitant drop in the quantum yield of charge separation Y to 78% leading to a fast ground state P recovery of 22%. This directly translates into an internal conversion rate from1P*to the ground state ofkIC= (1.1 ns)−1at 90 K. The corresponding data at 270 K (τF(1P*) = 66 ps;Y= 91%) result inkIC= (750 ps)−1suggesting a weak thermal activation as usually observed for internal conversion processes. These internal conversion data constitute an important parameter for theoretical modelling of the primary charge separation along both, the active and inactive pigm

ISSN:0005-9021    

DOI:10.1002/bbpc.19961001207

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe effects of zwitterionic detergents on the spectroscopic properties of the primary electron donor P of photosynthetic reaction centers (RCs) from the purple nonsulfur bacteriumRhodobacter sphaeroidesare investigated. When RCs are solubilized with N‐(n‐alkyl)‐N,N‐dimethyl‐3‐ammonio‐1‐propanesulfonates (sulfobetaines) the maximum of the characteristic near‐infrared absorption of P, λmax, is shifted to lower wavelengths at room temperature as compared with nonionic detergents. While the range of the blueshift (λmax= 850 – 862 nm) is identical for different sulfobetaines, the dependence of the shift on the detergent/RC ratio varies systematically with the number of carbon atoms in then‐alkyl‐chain. N‐lauryl‐N,N‐dimethyl‐amine‐N‐oxide (LDAO), a zwitterionic detergent with a less polar head group, is only able to induce blueshifts of λmaxdown to 859 nm at much higher detergent/RC ratios. ENDOR experiments performed on the primary donor radical cation P+•reveal that two distinct states of P exist in such samples. Thus the optical bandshifts are caused by transitions between two spectroscopic species, called P866and P850. We interpret the detergent effect as an electrostatic influence of the detergent head groups on the equilibrium between two protein conformations which in turn a

ISSN:0005-9021    

DOI:10.1002/bbpc.19961001208

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractIn order to identify amino acids that could serve as ligands to the manganese cluster, which comprises the active site of the oxygen‐evolving complex (OEC), and the calcium ion that affects the stability of the OEC, we have generated a series of site‐directed mutants in the D1 polypeptide of the PS II reaction center. Our approach has been to emphasize characterization of mutants with reduced PS II activity rather than those that lack PS II activity altogether in order to address questions about the effects of the mutations on the functional properties of the Mn ensemble. A number of mutants have been isolated with impaired PS II activity and a characterization ofin vivophotosynthesis from mutant and WT strains is presented. All of the mutants described in this report produce nearly WT levels of PS II in the thylakoid membrane. Non‐conservative mutation at Asp‐103 (DN103D1), Glu‐104 (EQ104D1) or Glu‐333 (EQ333D1) results in moderate (∼ 50%) inhibition of oxygen evolution, whereas mutation at Glu‐65 (EQ65D1) or His‐337 (HF337D1) results in cells that retain only 10 to 20% of WT activity.In vivofluorescence induction kinetics are consistent with a lesion on the donor side of PS II in each of the mutants tested. We conclude that Glu‐65 and His‐337 are directly involved in binding manganese or calcium, and that Asp‐103 and Glu‐104 may be involved in calcium binding but

ISSN:0005-9021    

DOI:10.1002/bbpc.19961001209

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe oxygen evolving complex (OEC) of photosystem II (PS II) incorporates a tetra Mn‐cluster, tyrosine (Yz) and probably one histidine residue (X) as redox cofactors. Four quanta of light drive the OEC through the increasingly oxidized states S0⟹S1⟹S2⟹S3⟹S4to yield O2during S4→S0. It has been speculated that some oxidized cofactor abstracts hydrogen from bound water. This implies that its oxidoreduction is electroneutral and linked to its deprotonation. To identify such steps we investigated the rates of electron transfer and proton release as function of the D2O/H2O ratio, the pH, and the temperature in thylakoids and PS II core particles. Upon oxidation of X on S2⟹S3, a rise of the pH from 5 to 8 increased the rate of the electron transfer to Yzby a factor of 2.5 and substitution of D2O for H2O gave an isotopic ratio of 2.1. Contrastingly, during all other transitions, including the O2‐evolving step S4→S0, the electron transfer rate was much less sensitive to these parameters (factors of ≤ 1.4). These results suggest a kinetical steering role of proton transfer only during S2⟹S3. We propose that X*(His*?) serves as a hydrogen acceptor for bou

ISSN:0005-9021    

DOI:10.1002/bbpc.19961001210

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe catalytic center of photosynthetic water oxidation is a tetra‐nuclear manganese complex which presumably is covalently bound to the polypeptides of the Photosystem II (PS II) protein complex. Extended X‐ray Absorption Fine Structure (EXAFS) spectra were simulated for various likely structural elements of the PS II manganese complex. The influence of multiple scattering contributions to the EXFAS spectra is assessed. It is found that for distances below 3.5 Å multiple scattering contributions to the EXAFS spectrum of the PS II manganese complex are presumably negligible. However, for distances above 3.5 Å multiple scattering contributions to the EXAFS spectrum of the PS II manganese complex are likely to be of relevance. We conclude that the amino acid residues (glutamate, aspartate, histidine) which are ligands of the PS II manganese complex could give rise to the EXAFS Fourier peak at abou

ISSN:0005-9021    

DOI:10.1002/bbpc.19961001211

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY