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K Sauer

Publications and source records attributed to K Sauer.

At least 91 records · Page 5Linked to original sources
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

Linear dichroism of light harvesting bacteriochlorophyll proteins from Rhodopseudomonas sphaeroides in stretched polyvinyl alcohol films.

Light-harvesting bacteriochlorophyll-protein complexes from Rhodopseudomonas sphaeroides 2.4.1 and R-26 mutant are solubilized in sodium dodecyl sulfate and imbedded in polyvinyl alcohol. Stretching induces orientation, and the linear dichroism of visible and near infrared absorption is analyzed. Based on a simple model, angles between the particle axis and the transition dipole moments are found. In the near infrared absorption band of the R-26 light-harvesting protein the dichroic ratio varies from 1.30 to 1.57. Using the absorption curves the band is resolved into two exciton components. In the visible absorption band the dichroic ratio has a constant value of 0.43 for the R-26 protein but varies with wavelength for the wild type light-harvesting protein. This variation is attributed to an additional bacteriochlorophyll not present in the R-26 protein.

Bacterial Proteins↗
PubMed1979

Absorption changes of P-700 reversible in milliseconds at low temperature in Triton-solubilized photosystem I particles.

Triton-solubilized Photosystem I particles from spinach chloroplasts exhibit largely reversible P-700 absorption changes over the temperature range from 4.2 K to room temperature. For anaerobic samples treated with dithionite and neutral red at pH 10 and illuminated during cooling, a brief (1 microseconds) saturating flash produces absorption changes in the long wavelength region that decay in 0.95 +/- 0.2 ms from 4.2 to 50 K. Above 80 K a faster (100 +/- 30 microseconds) component dominates in the decay process, but this disappears again above about 180 K. The major decay at temperatures above 200 K occurs in about 1 ms. The difference spectrum of these absorption changes between 500 and 900 nm closely resembles that of P-700. Using ascorbate and 2.6-dichlorophenolindophenol as the reducing system with a sample of Photosystem I particles cooled in darkness to 4.2 K, a fully reversible signal is seen upon both the first and subsequent flashes. The decay time in this case is 0.9 +/- 0.3 ms.

Chlorophyll↗
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↗
PubMed1979

Triplet states in photosystem I of spinach chloroplasts and subchloroplast particles.

We report light-induced electron paramagnetic resonance triplet spectra from samples of chloroplasts or digitonin photosystem I particles that depend upon the dark redox state of the bound acceptors of photosystem I. If the reaction centers are prepared in the redox state P-700 A X- FdB-FdA-, then upon illumination at 11K we observe a polarized chlorophyll triplet species which we interpret as arising from radical pair recombination between P-700+ and A-. This chlorophyll triplet is apparently the analog of the PR state of photosynthetic bacteria [Parson, W.W. & Cogdell, R.J. (1975) Biochim. Biophys. Acta 416, 105-149]. If the reaction centers are prepared in the dark redox state P-700 A X FdB-FdA-, then upon illumination at 11K we observe a different triplet species of uncertain origin, possibly pheophytin or carotenoid. This species is closely associated with the photosystem I reaction center and it traps excitation when P-700 is oxidized.

Chloroplasts↗
PubMed1979

Photooxidation of antenna bacteriochlorophyll in chromatophores from carotenoidless mutant Rhodopseudomonas sphaeroides and the attendant loss of dimeric exciton interaction.

Intense continuous illumination of purified chromatophores from carotenoidless mutant Rhodopseudomonas sphaeroides results in progressive photooxidative loss of the near infrared absorption band near 860 nm assigned to antenna bacteriochlorophyll. The quantum yield of this reaction is low, approximately 1.7 x 10(-5). The loss in near infrared absorption is accompanied by a proportional shift in the absorption maximum to shorter wavelengths. The double circular dichroism feature in the near infrared decreases at a faster rate than does the absorbance. These results are explained by a model in which the antenna bacteriochlorophyll, initially associated as dimers (lambda(max) = 860.2 nm), is progressively converted to the monomeric state (lambda(max) = 851.9 nm). The wavelength shift is attributed to disruption of exciton coupling in the dimer. Acetone/methanol extraction indicates that the maximum molar extinction coefficients of the dimer and monomer do not differ by more than 4%. The occurrence of an absorption maximum at 852 nm for monomeric bacteriochlorophyll in a protein complex demonstrates that it is not necessary to invoke aggregation of the chromophores as the origin of the shift from 770 nm in typical organic solvents.

Journal Article↗
PubMed1978

The orientation of membrane bound radicals: an EPR investigation of magnetically ordered spinach chloroplasts.

The orientation of membrane-bound radicals in spinach chloroplasts is examined by electron paramagnetic resonance (EPR) spectroscopy of chloroplasts oriented by magnetic fields. Several of the membrane-bound radicals which possess g-tensor anisotropy display EPR signals with a marked dependence on the orientation of the membranes relative to the applied EPR field. The fraction of oxidized and reduced plastocyanin, P-700, iron-sulfur proteins A and B, and the X center, an early acceptor of Photosystem I, can be controlled by the light intensity during steady-state illumination and can be trapped by cooling. The X center can be photoreduced and trapped in the absence of strong reductants and high pH, conditions previously found necessary for its detection. These results confirm its role as an early electron acceptor in P-700 photo-oxidation. X is oriented with its smallest principal g-tensor axis (gx) predominantly parallel to the normal to the thylakoid membrane, the same orientation as was found for an early electron acceptor based on time-resolved electron spin polarization studies. We propose that the X center is the first example of a high potential iron-sulfur protein which functions in electron transfer in its 'superreduced' state. We present evidence which suggests that iron-sulfur proteins A and B are 4Fe-4S clusters in an 8Fe-8S protein. Center B is oriented with gy predominantly normal to the membrane plane. The spectra of center A and plastocyanin do not show significant changes with sample orientation. In the case of plastocyanin, this may indicate a lack of molecular orientation. The absence of an orientation effect for reduced center A is reconcilable with a 4Fe-4S geometry, provided that the electron obtained upon reduction can be shared between any pair of Fe atoms in the center. Orientation of the 'Rieske' iron-sulfur protein is also observed. It has axial symmetry with g parallel close to the plane of the membrane. A model is proposed for the organization of these proteins in the thylakoid membrane. A new EPR signal was observed in oriented chloroplasts. This broad unresolved resonance displays a g value of 3.2 when the membrane normal is parallel to the field. It shifts to g = 1.9 when the membrane normal is perpendicular to the field. The signal is sensitive to illumination and to washing of the thylakoid membranes of broken chloroplasts. We suggest that there is a relation between this signal and the water-oxidizing enzyme system.

Chloroplasts↗
PubMed1978

Electron acceptors associated with P-700 in Triton solubilized photosystem I particles from spinach chloroplasts.

Flash-induced absorption changes of Triton-solubilized Photosystem I particles from spinach were studied under reducing and/or illumination conditions that serve to alter the state of bound electron acceptors. By monitoring the decay of P-700 following each of a train of flashes, we found that P-430 or components resembling it can hold 2 equivalents of electrons transferred upon successive illuminations. This requires the presence of a good electron donor, reduced phenazine methosulfate or neutral red, otherwise the back reaction of P-700+ with P-430 occurs in about 30 ms. If the two P-430 sites, designated Centers A and B, are first reduced by preilluminating flashes or chemically by dithionite under anaerobic conditions, then subsequent laser flashes generate a 250 microseconds back reaction of P-700+, which we associate with a more primary electron acceptor A2. In turn, when A2 is reduced by background (continuous) illumination in presence of neutral red and under strongly reducing conditions, laser flashes then produce a much faster (3 microseconds) back reaction at wavelengths characteristic of P-700. We associate this with another more primary electron acceptor, A1, which functions very close to P-700. The organization of these components probably corresponds to the sequence P-700-A1-A2-P-430[AB]. The relation of the optical components to acceptor species detected by EPR, by electron-spin polarization or in terms of peptide components of Photosystem I is discussed. Preliminary experiments with broken chloroplasts suggest that an analogous situation occurs there, as well.

Chlorophyll↗
PubMed1978

Absorption and circular dichroism spectra of chloroplast membrane fragments from spinach, barley and a barley mutant at room temperature and liquid nitrogen temperature.

The absorption and CD spectra of chloroplast fragments from spinach, barley and a barley mutant (chlorophyll b-minus) were studied at temperatures of 23 degrees C and --196 degrees C. The CD spectrum of wild type barley and spinach at --196 degrees C showed troughs at 640, 653, 676 and 695 nm and a maximum at 667 nm. The CD spectrum of the barley mutant at --196 degrees C consisted of a large trough at 684 nm, a small trough at 695 nm and a positive peak at 670 nm. A new feature observed at --196 degrees C but not at 23 degrees C is the trough at 640 nm. This 640 nm CD signal is missing in the CD spectrum of the barley mutant. It is attributable to the light-harvesting chlorophyll a/b protein which appears to be missing in the mutant. Another new feature, the trough at 695 nm, was observed in the CD spectra of spinach, barley and the barley mutant at --196 degrees C. The 695 nm trough appears to be sensitive to detergents and it may be due to a labile chlorophyll a.protein complex. Possible interpretations of the data are discussed.

Chloroplasts↗
PubMed1978

Electron spin polarization in photosynthesis and the mechanism of electron transfer in photosystem I. Experimental observations.

Transient electron paramagnetic resonance (EPR) methods are used to examine the spin populations of the light-induced radicals produced in spinach chloroplasts, photosystem I particles, and Chlorella pyrenoidosa. We observe both emission and enhanced absorption within the hyperfine structure of the EPR spectrum of P700+, the photooxidized reaction-center chlorophyll radical (Signal I). By using flow gradients or magnetic fields to orient the chloroplasts in the Zeeman field, we are able to influence both the magnitude and sign of the spin polarization. Identification of the polarized radical and P700+ is consistent with the effects of inhibitors, excitation light intensity and wavelength, redox potential, and fractionation of the membranes. The EPR signal of the polarized P700+ radical displays a 30% narrower line width than P700+ after spin relaxation. This suggests a magnetic interaction between P700+ and its reduced (paramagnetic) acceptor, which leads to a collapse of the P700+ hyperfine structure. Narrowing of the spectrum is evident only in the spectrum of polarized P700+, because prompt electron transfer rapidly separates the radical pair. Evidence of cross-relaxation between the adjacent radicals suggests the existence of an exchange interaction. The results indicate that polarization is produced by a radical pair mechanism between P700+ and the reduced primary acceptor of photosystem I. The orientation dependence of the spin polarization of P700+ is due to the g-tensor anisotropy of the acceptor radical to which it is exchange-coupled. The EPR spectrum of P700+ is virtually isotropic once the adjacent acceptor radical has passed the photoionized electron to a later, more remote acceptor molecule. This interpretation implies that the acceptor radical has g-tensor anisotropy significantly greater than the width of the hyperfine field on P700+ and that the acceptor is oriented with its smallest g-tensor axis along the normal to the thylakoid membranes. Both the ferredoxin-like iron-sulfur centers and the X- species observed directly by EPR at low temperatures have g-tensor anisotropy large enough to produce the observed spin polarization; however, studies on oriented chloroplasts show that the bound ferredoxin centers do not have this orientation of their g tensors. In contrast, X- is aligned with its smallest g-tensor axis predominantly normal to the plane of the thylakoid membranes. This is the same orientation predicted for the acceptor radical based on analysis of the spin polarization of P700+, and indicates that the species responsible for the anisotropy of the polarized P700+ spectrum is probably X-. The dark EPR Signal II is shown to possess anisotropic hyperfine structure (and possibly g-tensor anisotropy), which serves as a good indicator of the extent of membrane alignment.

Chlorella↗
PubMed1977

Analysis of the subunit structure of protochlorophyllide holochrome by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

The subunit structures of protochlorophyllide holochrome (PCH) and chlorophyllide holochrome (CH) were studied by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. PCH from leaves of dark-grown (Phaseolus vulgaris var. red kidney) is a polymeric pigment-protein complex of approximately 600,000 daltons. It is composed of 12 to 14 polypeptides of 45,000 daltons, when examined prior to and immediately following photoconversion. The protochlorophyllide or chlorophyllide pigment molecules are associated with these polypeptides. Subsequent to photoconversion, the absorption maximum of newly formed chlorophyllide shifts from 678 nm to 674 nm upon standing in darkness. Following the 678 to 674 spectral shift, the chlorophyllide is associated with a polypeptide with a molecular weight of 16,000 daltons. In addition, sucrose gradient centrifugation of PCH and CH under nondenaturing conditions indicates that during the course of the dark spectroscopic shift, the 600,000 dalton CH undergoes dissociation into a small chlorophyllide protein. The dissociation of CH, the change in the molecular weight of the chlorophyllide polypeptide from 45,000 to 16,000 daltons, as well as the dark spectroscopic shift are temperature-dependent and blocked below 0 C. It was also found that each holochrome molecule of 600,000 daltons contains at least four protochlorophyllide pigment molecules.

Journal Article↗