Orientation of the pigments in Photosystem II: A low-temperature linear dichroism and polarized fluorescence emission study of chlorophyll-protein complexes isolated from Chlamydomonas reinhardtii

The orientation of the various absorbing and fluorescing dipoles in Photosystem II have been investigated by linearly polarized light spectroscopy at 5 K, performed on macroscopically oriented PS II complexes derived from Chlamydomonas reinhardtii. Linear dichroism and absorption spectra show that the Q_Y transitions of the chlorophyll molecules are mostly tilted at less than 35° from the plane of largest cross-section of the particle (which in vivo coincides with the plane of the thylakoid membrane). The chlorophyll forms absorbing at 676 and 683 nm are oriented closer to the membrane than the forms absorbing at 665 and 670 nm which are tilted at approximately 35° from the plane. A dip observed around 680 nm in the LD/absorption spectra indicates a component tilted at a larger angle away from the membrane plane than the 676 nm- and 683 nm-absorbing species. A component weakly absorbing around 693 nm and exhibiting a negative LD (tilt larger than 35°) is clearly resolved. The amplitude of the LD at 693 nm relative to that observed at the maximum (676 nm) varies from sample to sample. In the blue spectral region, two populations of carotenoids are observed; one absorbs around 460 and 490 nm, while the other absorbs around 510 nm. They are oriented out of and near to the thylakoid plane, respectively. Comparison of polarized absorption and fluorescence spectra from the same oriented samples allows the assignment of the 695 nm fluorescence emission to the dipoles responsible for the LD signal at 693 nm.     

Ultraviolet Dichroism of fd Bacteriophage

The internal structure of the bacterial virus fd was investigated by ultraviolet dichroism of virus solutions oriented by flow through a small capillary tube. The dichroism was found to be positive for wavelengths longer than 262 mμ and shorter than 239 mμ, and negative for the intermediate wavelengths. The magnitude of the effect was at all times small, with dichroic ratios of 1.22 and 0.83 at 280 mμ and 250 mμ, respectively. the intuitive interpretation that this was the result of the addition of negative DNA dichroism and positive protein dichroism was confirmed by the application of a simple theory which allowed the calculation from protein and DNA absorption data of a dichroism curve closely approximating the experimental one. The parameters arrived at by this procedure indicate a semiangle of 25° ± 5° for a cone described by the normals to the DNA base planes inside the virus. The protein absorbers tryptophan and probably tyrosine were found to be oriented on the average relatively parallel to the longitudinal axis of the virus.     

Photoselection studies on the orientation of chlorophyll aI in the functional membrane of photosynthesis

The orientation of chlorophyll a_I in the functional membrane of photosynthesis in green plants is studied by a photoselection technique. On excitation of an isotropic suspension of isolated spinach chloroplasts with a linearly polarized flash of light linear dichroism of the absorption changes of chlorophyll a_I (wavelengths 705 and 430 nm) is observed. The dichroism is maximum for excitation at wavelengths greater than 690 nm, medium at excitation into the blue band of the chloroplast absorption spectrum, and it is small if excitation goes into all red transition moments above 600 nm. This reflects the degree of order between the transition moments of the antennae system around Photosystem I. We conclude as to a higher order between the transition moments at the long-wavelength end of the spectrum in comparison with a lower degree of order between the transition moments belonging to the intervall from 600 to 680 nm. This confirms the results of other authors which were obtained with oriented chloroplasts. However, the photoselection approach avoids characteristic artifacts which may affect linear-dichroism studies with oriented membranes.A quantitative interpretation of the observed photoinduced dichroism of chlorophyll a_I to yield the orientation of the respective porphyrin rings in the membrane is not feasible yet due to the absence of specific information on the symmetry properties of the antennae system and on the geometry of the chlorophyll a_I aggregate. Under the assumption of a circular degenerate antennae system a rather flat inclination of chlorophyll a_I has to be expected.     

Dichroism of Photosensitive Pigment in Rhabdoms of the Crayfish Orconectes

Microspectrophotometric measurements of isolated crayfish rhabdoms illuminated transversely show that their photosensitive absorption exhibits a dichroic ratio of 2 in situ. The major absorption axis matches the axial direction of the closely parallel microvilli comprising the receptor organelle. Since these microvilli are regularly oriented transversely in about 24 layers, with the axes of the microvilli at 90° in alternate layers, transverse illumination of a properly oriented rhabdom displays alternate dichroic and isotropic bands. Because all the microvilli from any one cell share the same orientation, the layers of microvilli constitute two sets of orthogonal polarization analyzers when illuminated along the normal visual axis. Furthermore, since the dichroic ratio is 2 and transverse absorption in isotropic bands is the same as that in the minor absorbing axis of dichroic bands, the simplest explanation of the analyzer action is that the absorbing dipoles of the chromophores, as in rod and cone outer segments, lie parallel to the membrane surface but are otherwise randomly oriented. The rhabdom's functional dichroism thus arises from its specific fine structural geometry.     

Orientational changes of the absorbing dipole or retinal upon the conversion of rhodopsin to bathorhodopsin, lumirhodopsin, and isorhodopsin.

The orientational change of the absorbing dipole of the retinal chromophore in vertebrate rhodopsin (rhodo) upon photo-excitation to bathorhodopsin (batho), lumirhodopsin (lumi) and isorhodopsin (iso), has been studied by polarized absorption and linear dichroism measurements on magnetically oriented frog rod suspensions that were blocked at liquid nitrogen temperature. Both the azimuthal component delta theta and the polar component delta theta of the total angular change were studied in separate experiments. Delta theta was estimated from polarized absorption measurements on rods oriented transversally with respect to the analyzing beam. The data show unequivocally that upon the rhodo leads to batho transition, the dipole shifts out of the membrane plane by only few degrees; delta theta congruent to -3 degree. This azimuthal shift was nearly exactly reversed upon the batho leads to lumi decay. A very small shift (delta theta less than or equal to 1 degree) toward the membrane plane was observed upon a rhodo leads to iso conversion. The polar component delta theta of the angular shift was estimated by studying the photoreversion of linear dichroism induced by photo-excitation with polarized light in rods oriented parallel to the analyzing beam. Upon the rhodo leads to batho transition, ther was a shift delta theta = 11 +/- 3 degrees. The overall angular shift upon this first photo-exciting step, which corresponded to the isomerisation of retinal, was only delta omega = 11 +/- 3 degrees. This is smaller than what may be expected for a cis-trans isomerization of a retinal molecule with one end fixed, and different from what has been previously estimated by another group. These discrepancies are discussed.     

Orientation of the hemes of high potential cytochromes relative to photosynthetic membranes, as shown by the linear dichroism of oriented preparations

The orientations of high potential cytochromes with respect to photosynthetic membranes was investigated in spinach chloroplasts and in Rhodopseudomonas viridis. The general approach consists in detection with polarized light of photoinduced absorbance changes related to the oxidation of the cytochromes. The orientation of cytochrome c-558 was measured at room temperature in chromatophores and whole cells of Rps. viridis, oriented on glass slides and in a magnetic field, respectively. The orientation of cytochrome b-559 of green plants was detected at 77 K in magnetically oriented chloroplasts. In both cases the dichroic ratio for the band shows that the heme plane makes an angle greater than 35°C with the membrane plane. Moreover, the dichroic ratio is not constant throughout the and β bands, for both cytochrome c-558 and b-559. Linear dichroism spectra of oriented pure horse heart cytochrome c and cytochrome c₂ of Rhodopseudomonas sphaeroides in stretched polyvinyl alcohol films show that the variations of the dichroic ratio in the and β bands can be explained by the occurrence of x- and y-polarized transitions absorbing at slightly different wavelengths.     

Differently oriented chlorophylls in Mesotaenium caldariorum detected by microphotometrical dichroism measurements in vivo

The chloroplasts of individual cells of Mesotaenium caldariorum were examined microphotometrically under non-polarized and polarized measuring light. The measurement with non-polarized light showed different absorption bands of the thylakoids depending on the position of their surface with respect to the incident light beam: in the edge position, the absorption bands lie at 672 nm, in the face position at 678 nm. From this difference in absorption maxima, we conclude that the molecules related to the sub-bands at the two wavelengths are oriented differently. The Q_y transition of the molecules which absorb light at 678 nm must be oriented parallel to the face of the thylakoids (fraction I), while that of the molecules absorbing at 672 nm is oriented perpendicular to the face (fraction II). Measurement with polarized light leads to the same conclusion that two fractions of differently oriented chlorophylls exist: In the edge position, a very large difference between E_∥ and E_⊥ (dichroism) was found in red light, with a maximum of E_∥ lying at 675 nm and a maximum of E_⊥ at 670 nm, with a shoulder at 650 nm. In the blue region, especially in the Soret band zone, the chloroplast showed a negative dichroism in the edge position, which changes over to positive values when the wavelength exceeds 450 nm. In the face position no dichroism in red or blue light could be detected. Comparison of the ‘edge position dichroism’ in red light with that in blue light justifies the supposition that the chlorin planes of the chlorophyll molecules may be oriented perpendicular or parallel to the thylakoid face, in the case of perpendicular orientation with the Q_y transitions of fraction II and the x-transitions (B_x, Q_x) of fraction I projecting out of the plane, and for parallel orientation with all transition moments lying parallel to the plane (fraction I). The relative dichroism, $\text{(E}{}_{\mathrm{\parallel }}\text{? E}{}_{\mathrm{\perp }}\text{)}\text{(E}{}_{\mathrm{\parallel }}\text{+ E}{}_{\mathrm{\perp }}\text{)}$, measured at the edge position amounts to 0.34 (i.e., 34% of the total absorption) at 680 nm. These data probably do not reflect the total quantity of oriented chlorophyll because from the opposite orientations of the Q_y transition moments of fraction I and II pigment a partial quenching of the measurable dichroism results. The red light absorption bands of the two chlorophylls oriented in an opposite manner (fractions I and II) correspond to the known bands of Photosystem I and II.     

Aggregation of 8,12-diethyl farnesyl bacteriochlorophyll c at low temperature

The effect of temperature on the aggregation of 3^lR-8,12-diethyl farnesyl bacteriochlorophyll c in a mixture of n-pentane and methylcyclohexane (1/1, v/v) was studied by means of absorption, circular dichroism and fluorescence spectroscopy. At room temperature essentially only two aggregate species, absorbing at 702 nm (A-702) and 719 nm (A-719), were present. Upon cooling to 219 K, A-702 was quantitatively converted to A-719. Further lowering of the temperature led to the stepwise formation of larger aggregates by the conversion of A-719 to aggregate species absorbing at 743 nm (A-743) and 755 nm (A-755). All absorption changes were reversible. A-719 was highly fluorescent (maximum at 192 K: 744 nm), while A-743 and especially A-755 were weakly fluorescent. Below 130 K the mixture solidified, and no major changes in the absorption spectrum were observed upon further cooling. At 45 K, however, a relatively strong emission at 775 nm was observed. Below 200 K, the absorption, fluorescence and circular dichroism spectra resembled that of the chlorosome. These results open up the possibility to study higher aggregates of BChl c as models for the chlorosome by various methods at low temperature, thus avoiding interference by thermal processes.Abbreviations A-680, A-702, A-719, A-743 and A-755- BChl c aggregates absorbing at the wavelengths indicated - BChl- bacteriochlorophyll - R[E,E] BChl c _F- the 3¹ R isomer of 8,12-diethyl BChl c esterified with farnesol (F), analogously - M- methyl - Pr- propyl - S- stearol (see Smith 1994) - CD- circular dichroism     

On the various types of circular dichroism induced on acridine orange bound to poly(S-carboxymethyl-L-cysteine).

Circular dichroism and absorption spectra are measured on mixed solutions of acridine orange and poly(S-carboxymethyl-L -cysteine) at different pH and P/D mixing ratios. The observed circular dichroism spectra are classified into several types, mainly based on the number and sign of circular dichroic bands in the visible region. Three of them are associated with the absorption spectra characteristic of dimeric dye or higher aggregates of dye. Type I is observed with solutions, of which the pH is acid and P/D is higher than 4, and it has an unsymmetrical pair of positive and negative dichroic bands at 470 and 430 nm. This type is induced on the dye bound to the polymer in the β-conformation. Types II and III are considered to be characteristic of randomly coiled polymers. Type II is exhibited by solutions of P/D higher than 1 at pH 5–7 and has two dichroic bands around the same wavelengths as Type I but with opposite signs and an additional positive band at 560 nm. Type III, shown by solutions of P/D 2–0.6 at pH 6–10.5, has three dichroic bands around the same wavelengths as Type II but with signs opposite to it. The other two types of circular dichroism, induced for the solutions of P/D less than 1 at slightly acid pH, are associated with the absorption spectra of monomeric dye and are observed with disordered or randomly coiled polymer. They have a pair of dichroic bands at 540 and 425 nm, and the signs of these bands are opposite to each other in these two types.     

Complexes of aspartate aminotransferase with hydroxylamine derivatives: spectral studies in solution and in the crystalline state

Hydroxylamine and its derivatives of general formula H2NOR react with aldehydes and aldimines to produce oximes. If R corresponds to the side chain of a natural amino acid, such compounds can be thought of as analogs of the corresponding amino acids, lacking the alpha-carboxylate group. Oximes formed between such compounds and pyridoxal phosphate in the active site of aspartate amino-transferase mimic external aldimine intermediates that occur during catalysis by this enzyme. The properties of oxime derivatives of mitochondrial aspartate aminotransferase with hydroxylamine and 6 compounds H2NOR were studied by absorption spectroscopy and circular dichroism in solution and by linear dichroism in crystals. Stable oximes, absorbing at lambda max congruent to 380 nm and exhibiting a negative Cotton effect, were obtained with the carboxylate-containing compounds. The oximes formed with carboxylate-free compounds showed somewhat different properties and stability. With H-Tyr a stable complex absorbing at lambda max congruent to 370 nm rather than at 380 nm, was obtained, H-Ala and H-Phe produced unstable oximes with the initial absorption band at lambda max congruent to 380 nm that was gradually replaced by a band at lambda max congruent to 340 nm. The species absorbing at 340 nm were shown to be coenzyme-inhibitor complexes which were gradually released from the enzyme. A similar 330-340 nm absorption band was observed upon reaction of the free coenzyme with all hydroxylamine inhibitors at neutral pH-values. The results of the circular dichroism experiments in solution and the linear dichroism studies in microcrystals of mAspAT indicate that the coenzyme conformation in these inhibitor/enzyme complexes is similar to that occurring in an external aldimine analogue, the 2-MeAsp/mAspAT complex. Co-crystallizations of the enzyme with the H2NOR compounds were also carried out. Triclinic crystals were obtained in all cases, suggesting that the "closed" structure cannot be stabilized by a single carboxylate group.     

The effect of laser radiations on human ceruloplasmin. Absorption, circular dichroism and electron paramagnetic resonance study.

Laser radiations at wavelengths ranging from 514.5 to 360.0 nm decolorize human ceruloplasmin. Kinetic behavior of the two chromophores absorbing at 610 nm, as measured by absorption and circular dichroism data, indicate different quantum yields of the two type I copper ions, whose maximum lies approximatively at 400 nm. Furthermore, as Electron Paramagnetic Resonance measurements demonstrate, the photochemical process involves reduction of the two type I copper ions leaving type II copper unchanged.     

Linear dichroism of electric field oriented bacteriochlorophyll a-protein from green photosynthetic bacteria

Bacteriochlorophyll a-protein from Prosthecochloris aestuarii strain 2K was oriented in a pulsed electric field. The room temperature linear dichroism spectrum of the oriented protein in the Q_y region of the bacteriochlorophyll a absorption exhibits a single asymmetrical peak at 813 nm with a shoulder extending to the blue. The ≈12 nm fullwidth of the linear dichroism peak is only about half that of the 300 K absorption spectrum. The linear dichroism at 813 nm was not saturated at field strengths of up to 15 kV/cm. The time dependence of the linear dichroism suggests that the orienting particles are aggregates of at least some tens of bacteriochlorophyll a-protein trimers. The linear dichroism peak coincides in wavelength with the 813-nm peak of the 300 K, 4th derivative absorption spectrum of the protein and is therefore attributed to the bacteriochlorophyll a Q_y exciton transition observed in absorption at the same wavelength.     

Anisotropy of the emission and absorption bands of spinach chloroplasts measured by fluorescence polarization and polarized excitation spectra at low temperature

Spectra of fluorescence polarization were measured between 4 and 120 K of spinach chloroplasts, oriented in a magnetic field. At least seven emission bands were observed. The well known bands near 685 nm (‘F-685’) and 735–740 nm (‘F-735’) and the band near 680 nm (‘F-680’) were strongly polarized parallel to the plane of the thylakoid membrane, whereas emission bands near 695 nm (‘F-695’), 710, 730–735 and 760 nm showed perpendicular polarization. Assuming perfect orientation of the thylakoid membranes, we calculated orientation angles of 64, 47 and 66.5° for the emission dipoles of F-685, F-695 and F-735, respectively, with respect to the normal of the membrane. Excitation spectra of F-695 and F-735 in polarized light at 4 K provided information about the orientation of the absorption dipoles of chlorophylls a and b. The spectra thus obtained were in very good agreement with the linear dichroism spectrum. Moreover, they allowed us to distinguish between the pigments associated with Photosystems I and Ii, which is not possible from measurement of linear dichroism alone. The results indicate that a high degree of orientation is not confined to the long-wave absorbing bands, but also bands at shorter wavelength show a clear anisotropy. The calculated orientations were in quantitative agreement with the hypothesis that F-685 and F-735 are associated with chlorophylls absorbing at 676 and 710–715 nm, respectively.     

Circular dichroism of the alkyl amino acids in the vacuum ultraviolet

The circular dichroism and absorption for five alkyl amino acids as zwitterions has now been measured to 160 nm. Two bands are found, the nπ* at long wavelengths and ππ* at short wavelengths. In an effort to extract conformal information from the circular dichroism measurmenths, the circular dichroism spectra for these molecules is calculated using an independent systems apporach. It is found that the signs calculated using these methods are quite reliable, but that reliable magnitudes must await more accurate data to use in the calculations. Comparing the signs for both the measured and calculated circular dichriosm it if possible to determine the region inhabited by the carboxylate anion chromophore of these amino acid zwitterions. The greatest failure of the calculations is that thay are unable to explain the sigmoidal shape of the nπ* transition in proline. The problems facing workers trying to make calculations to relate circular dichroism measurements to conformation are discussed.     

Orientation of the pigments in Photosystem II: low-temperature linear-dichroism study of a core particle and of its chlorophyll-protein subunits isolated from Synechococcus sp.

The orientation of the pigments in the Photosystem II core particle isolated from the thermophilic cyanobacterium Synechococcus sp. has been investigated by linear dichroism spectroscopy at 10 K of macroscopically oriented samples. The absorbance (A), linear dichroism (LD) and LD/A spectra are remarkably similar to those previously reported for a core complex isolated from Chlamydomonas reinhardtii (Biochim. Biophys. Acta 850 (1986) 156–161). The spectra of the Synechococcus core particle are compared to the corresponding spectra obtained on its two main constituent chlorophyll-protein complexes CP2-b (photochemically active) and CP2-c (photochemically inactive). The various features seen in the spectra of the core particle appear well segregated into the spectra of one or the other of the two subparticles without significant loss of orientation of the pigments. The orientation of the chlorophyll macrocycles, with the Y and X optical axis preferentially parallel and perpendicular to the plane of largest cross-section of the particle, respectively, is very similar in the two subparticles. CP2-b contains mainly the beta-carotene pool absorbing around 505 and 470 nm, which is oriented close to the membrane plane, while CP2-c contains the beta-carotene pool absorbing around 495 and 465 nm and oriented closer to the normal to the membrane plane. A shoulder at 682 nm in the absorbance and linear dichroism spectra of the core complex is fully segregated in the spectra of CP2-c, thus excluding the possibility that this spectral feature could be assigned to the primary donor of PS II. A negative linear dichroism component peaking around 691 nm (LD 691) in the core particle is mainly segregated in CP2-b together with the photoactive pheophytin acceptor molecule responsible for the 544 nm positive linear dichroism signal (LD 544). While the ratio of the amplitudes LD 691/LD 544 is approximately the same for the core particle and for the CP2-b complex, the amplitude of LD 691 is significantly reduced in CP2-b compared to the core particle.     

Trapping at low temperature of oriented chloroplasts: Application to the study of antenna pigments and of the trap of photosystem-1

A technique is described for the preparation of oriented samples from spinach chloroplasts whose linear dichroism is then studied by (flash) absorption spectroscopy. The chloroplasts are suspended in a glycerol-containing medium, oriented in a magnetic field, and slowly cooled in the magnet until the medium is rigid enough to avoid disorientation effects. The absorption spectra in polarized light have been measured at ?50° and ?170°C. They allow the orientation of chlorophyll b to be resolved, and the red transition moment is found to be tilted out of the membrane plane. A study of the flash-induced absorption changes linked to Photosystem-1 activity reveals a progressive evolution of the difference spectra and of the linear dichroism with decreasing temperatures. At ?170°C, the difference spectrum of P700 in the red is well resolved. All transition moments are found to be largely parallel to the membrane plane. The potential use of the technique for other experiments by differential absorption spectroscopy and by EPR techniques is discussed.     

On the mutual orientation of pigments in Photosystem I particles from green plants

The mutual orientation of pigments in Photosystem I reaction centers from spinach is evaluated by polarized photochemistry. The photoinduced linear dichroism of the absorption changes of chlorophyll a₁ at 701 nm is studied as function of the excitation wavelength. The Photosystem I reaction center particles contain about 100 and if depleted about 40 chlorophylls, respectively. To prevent their rapid Brownian rotation they were immobilized on DEAE-Sephadex.The excitation spectrum of the linear dichroism reveals a high degree of order between the long axis of β-carotene and the Q_y transition moments of those chlorophyll a molecules absorbing at the red end of the spectrum. The latter are the most endangered ones for destructive oxidation via their triplet state. Hence, the location of β-carotene in close proximity to and in parallel with these chlorophylls seems to be most favourable for the protective role of β-carotene within the antennae system I. It is observed that the dichroic ratio of the absorption changes of chlorophyll a₁ does not exceed a figure of $\text{4}\text{3}$, which characterizes a circularly degenerate system, even at far red excitation (724 nm). This will hit selectively those few chlorophyll a molecules with their peak absorption at about 700 nm (including the photooxidizable dimer). We conclude, if the dimer is the only species peaking at 700 nm then the two chlorophyll a within the dimer have their y-axes oriented perpendicular to each other. If there are some antennae in addition to the dimer, the y-axes of all chlorophyll-a peaking at 700 nm form a star which accounts for the circular degeneracy of absorption.     

Orientation and linear dichroism characteristics of porphyrin-DNA complexes

The linear dichroism spectra of complexes of tetrakis(N-methyl-4-pyridinio)prophine (H2TMpyP) and its zinc(II) derivative (ZnTMpyP) with DNA oriented in a flow gradient have been investigated. The dichroism of H2TMpyP determined within the Soret band and the Qy band system is consistent with an intercalative conformation in which the plane of the porphyrin ring system is nearly parallel to the planes of the DNA bases. In the case of ZnTMpyP on the other hand, the porphyrin ring system is inclined at angles of 62-67 degrees with respect to the axis of the DNA helix. The pyridyl groups in both cases are characterized by a low degree of orientation with respect to the axis of the helix. In contrast to H2TMpyP which does not significantly affect the degree of alignment of the DNA in the flow gradient, the binding of ZnTMpyP causes a significant decrease (about 50% for a base pair/ZnTMpyP ratio of 20) in the intrinsic dichroism at 260 nm due to the oriented DNA bases; the binding of ZnTMpyP to DNA either gives rise to regions of higher flexibility or causes bends or kinks at the binding sites. Increasing the ionic strength has little influence on the linear dichroism of the ZnTMpyP-DNA complexes, but the number of molecules bound at intercalation sites diminishes in the case of the H2TMpyP-DNA complexes; the accompanying changes in the linear dichroism characteristics suggest that external H2TMpyP complexes are formed at the expense of intercalation complexes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antenna organization and evidence for the function of a new antenna pigment species in the green photosynthetic bacterium Chloroflexus aurantiacus

Whole cells and isolated chlorosomes (antenna complex) of the green photosynthetic bacterium Chloroflexus aurantiacus have been studied by absorption spectroscopy (77 K and room temperature), fluorescence spectroscopy, circular dichroism, linear dichroism and electron spin resonance spectroscopy. The chlorosome absorption spectrum has maxima at 450 (contributed by carotenoids and bacteriochlorophyll (BChl) a Soret), 742 (BChl c) and 792 nm (BChl a) with intensity ratios of 20:25. The fluorescence emission spectrum has peaks at 748 and 802 nm when excitation is into either the 742 or 450 nm absorption bands, respectively. Whole cells have fluorescence peaks identical to those in chlorosomes with the addition of a major peak observed at 867 nm. The CD spectrum of isolated chlorosomes has an asymmetric-derivative-shaped CD centered at 739 nm suggestive of exciton interaction at least on the level of dimers. Linear dichroism of oriented chlorosomes shows preferential absorption at 742 nm of light polarized parallel to the long axis of the chlorosome. This implies that the transition dipoles are also oriented more or less parallel to the long axis of the chlorosome. Treatment with ferricyanide results in the appearance of a 2.3 G wide ESR spectrum at g 2.002. Whole cells grown under different light conditions exhibit different fluorescence behavior when absorption is normalized at 742 nm. Cells grown under low light conditions have higher fluorescence intensity at 748 nm and lower intensity at 802 nm than cells grown under high light conditions. These results indicate that the BChl c in chlorosomes is highly organized, and transfers energy from BChl c (742 nm) to a connector of baseplate BChl B792 (BChl a) presumably located in the chlorosome baseplate adjacent to the cytoplasmic membrane.     

Orientation of Photosystem-I pigments: low temperature linear dichroism spectroscopy of a highly-enriched P700 particle isolated from spinach

The linear dichroism of Photosystem I particles containing 10 chlorophylls per P700 has been investigated at 10 K. The particles were oriented by uniaxial squeezing of polyacrylamide gels. The oxidation state of P700 was altered either by incubation of the gels with redox mediators or by low temperature illumination. The Q_Y transitions of the primary electron donor P700, of the remaining unoxidized chlorophyll in P700⁺ and of a chlorophyll molecule absorbing at 686 nm, which presumably corresponds to the primary electron acceptor A₀, are all preferentially oriented perpendicular to the gel squeezing direction. The Q_Y transition of the chlorophyll forms absorbing at 670 and 675 nm appear tilted at 40 ± 5° from this orientation axis. This orientation of the various chlorophylls is compared to that previously reported for more native Photosystem I particles.Abbreviations PSI Photosystem I - P700 primary electron donor of PSI - A₀ primary electron acceptor of PSI