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• Articles by Shmuel Malkin

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• Temperature-dependent triplet and fluorescence quantum yield...

  Temperature-dependent triplet and fluorescence quantum yields of the photosystem II reaction center described in a thermodynamic model Biophysical Journal, Volume 67, Issue 1, 1 July 1994, Pages 318-330 M.L. Groot, E.J. Peterman, P.J. van Kan, I.H. van Stokkum, J.P. Dekker and R. van Grondelle Abstract A key step in the photosynthetic reactions in photosystem II of green plants is the transfer of an electron from the singlet-excited chlorophyll molecule called P680 to a nearby pheophytin molecule. The free energy difference of this primary charge separation reaction is determined in isolated photosystem II reaction center complexes as a function of temperature by measuring the absolute quantum yield of P680 triplet formation and the time-integrated fluorescence emission yield. The total triplet yield is found to be 0.83 +/- 0.05 at 4 K, and it decreases upon raising the temperature to 0.30 at 200 K. It is suggested that the observed triplet states predominantly arise from P680 but to a minor extent also from antenna chlorophyll present in the photosystem II reaction center. No carotenoid triplet states could be detected, demonstrating that the contamination of the preparation with CP47 complexes is less than 1/100 reaction centers. The fluorescence yield is 0.07 +/- 0.02 at 10 K, and it decreases upon raising the temperature to reach a value of 0.05–0.06 at 60–70 K, increases upon raising the temperature to 0.07 at approximately 165 K and decreases again upon further raising the temperature. The complex dependence of fluorescence quantum yield on temperature is explained by assuming the presence of one or more pigments in the photosystem II reaction center that are energetically degenerate with the primary electron donor P680 and below 60–70 K trap part of the excitation energy, and by temperature-dependent excited state decay above 165 K. A four-compartment model is presented that describes the observed triplet and fluorescence quantum yields at all temperatures and includes pigments that are degenerate with P680, temperature-dependent excited state decay and activated upward energy transfer rates. The eigenvalues of the model are in accordance with the lifetimes observed in fluorescence and absorption difference measurements by several workers. The model suggests that the free energy difference between singlet-excited P680 and the radical pair state P680+l- is temperature independent, and that a distribution of free energy differences represented by at least three values of about 20, 40, and 80 meV, is needed to get an appropriate fit of the data. Abstract | PDF (1254 kb)

• Comparison of the Light-Harvesting Networks of Plant and Cya...

  Comparison of the Light-Harvesting Networks of Plant and Cyanobacterial Photosystem I Biophysical Journal, Volume 89, Issue 3, 1 September 2005, Pages 1630-1642 Melih K. Şener, Craig Jolley, Adam Ben-Shem, Petra Fromme, Nathan Nelson, Roberta Croce and Klaus Schulten Abstract With the availability of structural models for photosystem I (PSI) in cyanobacteria and plants it is possible to compare the excitation transfer networks in this ubiquitous photosystem from two domains of life separated by over one billion years of divergent evolution, thus providing an insight into the physical constraints that shape the networks’ evolution. Structure-based modeling methods are used to examine the excitation transfer kinetics of the plant PSI-LHCI supercomplex. For this purpose an effective Hamiltonian is constructed that combines an existing cyanobacterial model for structurally conserved chlorophylls with spectral information for chlorophylls in the Lhca subunits. The plant PSI excitation migration network thus characterized is compared to its cyanobacterial counterpart investigated earlier. In agreement with observations, an average excitation transfer lifetime of ∼49ps is computed for the plant PSI-LHCI supercomplex with a corresponding quantum yield of 95%. The sensitivity of the results to chlorophyll site energy assignments is discussed. Lhca subunits are efficiently coupled to the PSI core via gap chlorophylls. In contrast to the chlorophylls in the vicinity of the reaction center, previously shown to optimize the quantum yield of the excitation transfer process, the orientational ordering of peripheral chlorophylls does not show such optimality. The finding suggests that after close packing of chlorophylls was achieved, constraints other than efficiency of the overall excitation transfer process precluded further evolution of pigment ordering. Abstract | Full Text | PDF (725 kb)

• Model for the Fluorescence Induction Curve of Photoinhibited...

  Model for the Fluorescence Induction Curve of Photoinhibited Thylakoids Biophysical Journal, Volume 75, Issue 1, 1 July 1998, Pages 503-512 Dmitrii V. Vavilin, Esa Tyystjärvi and Eva-Mari Aro Abstract The fluorescence induction curve of photoinhibited thylakoids measured in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea was modeled using an extension of the model of Lavergne and Trissl (. . 68:2474–2492), which takes into account the reversible exciton trapping by photosystem II (PSII) reaction centers and exciton exchange between PSII units. The model of Trissl and Lavergne was modified by assuming that PSII consists of photosynthetically active and photoinhibited (inactive in oxygen evolution) units and that the inactive PSII units can efficiently dissipate energy even if they still retain the capacity for the charge separation reaction. Comparison of theoretical and experimental fluorescence induction curves of thylakoids, which had been subjected to strong light in the presence of the uncoupler nigericin, suggests connectivity between the photoinhibited and active PSII units. The model predicts that photoinhibition lowers the yield of radical pair formation in the remaining active PSII centers. However, the kinetics of PSII inactivation in nigericin-treated thylakoids upon exposure to photoinhibitory light ranging from 185 to 2650mol photons m s was strictly exponential. This may suggest that photoinhibition occurs independently of the primary electron transfer reactions of PSII or that increased production of harmful substances by photoinhibited PSII units compensates for the protection afforded by the quenching of excitation energy in photoinhibited centers. Abstract | Full Text | PDF (245 kb)

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Abstract

A quantitative examination of the "series" model of two distinct photosystems in photosynthesis is presented. Fractional absorptions and quantum yields of the two photosystems and energy transfer efficiency from photosystem II to photosystem I are introduced as parameters. Equations for the dependence of the enhancement functions and quantum yields on the wavelength are obtained. The predictions of the theory are compared to present literature data. There is, in general, an agreement of data to the form of the equations. Calculation of the energy transfer efficiency from photosystem II to photosystem I yields a significant value, ranging approximately from 0.5 to 1, in the different examples discussed, which include green, blue-green, and red algae.Inconsistency with data obtained in the presence of the inhibitor DCMU and its significance is also discussed.