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R Friesner

Publications and source records attributed to R Friesner.

7 recordsLinked to original sources
PubMed1982

Model for primary charge separation in reaction centers of photosynthetic bacteria.

We present model calculations of the dynamics of primary electron transfer (ET) in reaction centers of photosynthetic bacteria. We obtain half times of [unk]1 ps and approximately 5 ps for the first two ET processes, in excellent agreement with experimental observations. Our model is based on (i) a theoretical framework capable of describing ET in the presence of strong electronic interstate resonance coupling and (ii) energy parameters extracted from recent experimental data and molecular orbital calculations. Our analysis suggests that (i) strong electronic interstate mixing is crucial to the rapidity and efficiency of irreversible ET; (ii) possibly five rather than three electronic states participate in the transient ET prior to the reduction in vivo of the quinone complex; and (iii) conventional ET theories, which rely on weak electronic interstate mixing, are unfit for describing ET in reaction centers of photosynthetic bacteria.

Journal Article↗
PubMed1980

A new approach to the theory of linear dichroism in partially ordered systems. Application to reaction centers and whole cells of photosynthetic bacteria.

We have developed a new approach to the theory of linear dichroism in partially ordered systems. The description of the partially ordered ensemble uses a density of states function, D(theta, phi, approximately delta), which gives the probability that the direction of polarization for incident polarized light has spherical angles theta and phi in an axis system fixed with respect to the molecule; approximately delta = (delta 1, delta 2...delta n) is a set of parameters that describes the partial ordering. We derive new formulas for linear dichroism using the density of states function and then apply these formulas to the analysis of linear dichroism in reaction centers and whole cells of photosynthetic bacteria. One advantage of our approach is that the order parameter, approximately delta, provides a more complete description of the distribution function than the traditional order parameters used by other authors. Knowledge of approximately delta gives a good physical description of the partial ordering and allows one to calculate accurate limits for the range of possible orientations of the transition moments.

Bacterial Proteins↗
PubMed1979

The orientation of the primary donor in bacterial photosynthesis.

The triplet state EPR spectra of magnetically aligned whole cells of Rhodopseudomonas viridis and Rhodopseudomonas palustris display a marked dependence on the orientation of the static EPR field with respect to the alignment field direction. This observation implies that the primary donor species on which the triplets are localized are ordered within the membranes. We have developed a theoretical model for the system to enable calculation of the orientation of the magnetic axes of the primary donor species with respect to the membranes in which they reside. The triplet state spectra are generated by an ensemble of partially ordered magnetic systems and a computer simulation of the experimental results. The triplet orientation is very similar for the two organisms studied, where one axis lies predominantly in the plane of the membrane and the other two axes have approximately equal projections onto the normal to the membrane.

Bacteriochlorophylls↗
PubMed1979

Magnetophotoselection of the triplet state of reaction centers from Rhodopseudomonas sphaeroides R-26.

Reaction centers of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26, give rise to large triplet state EPR signals upon illumination at low temperature (11 K). Utilizing monochromatic polarized light to generate the EPR spectra (magnetophotoselection) we have shown that the intensities of the observed triplet signals are strongly dependent upon the wavelength and polarization direction of the excitation. These data can be used to calculate the orientations of the excited transition moments with respect to each other and with respect to the triplet state principal magnetic axes system. Our quantitative approach is to follow the procedure outlined in a previous publication (Frank, H.A., Friesner, R., Nairn, J.A., Dismukes, G.C. and Sauer, K. (1979) Biochim. Biophys. Acta 547, 484-501) where computer simulations of the observed triplet state spectra were employed. The results presented in the present work indicate that the transition moment at 870 nm which is associated with the bacteriochlorophyll 'special pair' lies almost entirely along one of the principal magnetic axes of the triplet state. Aso, the 870 nm transition moment makes an angle of approx. 60 degrees with the 546 nm transition moment which is associated with a bacteriopheophytin. This latter result is in agreement with previous photoselection studies on the same bacterial species (Vermeglio, A., Breton, J., Paillotin, G. and Cogdell, R. (1978) Biochim. Biophys. Acta 501, 514-530).

Bacterial Chromatophores↗
PubMed1979

Development of electron spin polarization in photosynthetic electron transfer by the radical pair mechanism.

We have extended the radical pair theory to treat systems of membrane-bound radicals with g tensor anisotropy. Analysis of the polarized electron paramagnetic resonance (EPR) signals of P700+, originating from photosystem I of higher plants, in terms of the radical pair mechanism provides information about the sequence of early electron acceptors. To account for the orientation dependence of the line shape and integrated area of this polarized signal, we propose the electron transfer sequence to be P700 leads to A1 leads to X leads to Fd(A, B), where A1 is a small organic molecule (possibly chlorophyll), X is the acceptor species observed recently in low-temperature EPR studies, and Fd(A, B) are the ferredoxin iron-sulfur centers A and B. Our calculations provide information about the life-times of A1-, and X-, and their exchange interactions with P700+. We also find supporting evidence for the orientation of X- in the thylakoid membrane reported recently by G. C. Dismukes and K. Sauer (Biochim. Biophys. Acta. 504:431-445.).

Chlorophyll↗