Molecular Orientation in Quantasomes: III. A Flow Dichroism Apparatus and Its Application to the Study of the Structure of Spinach Quantasomes

Linear dichroism spectroscopy of nucleic acids

This review will consider solution studies of structure and interactions of DNA and DNA complexes using linear dichroism spectroscopy, with emphasis on the technique of orientation by flow. The theoretical and experimental background to be given may serve, in addition, as a general introduction into the state of the art of linear dichroism spectroscopy, particularly as it is applied to biophysical problems.

Linear dichroism of microalgae, developing thylakoids and isolated pigment-protein complexes in stretched poly(vinyl alcohol) films at 77 K

A variety of unicellular algae, thylakoids from higher plants in different stages of maturity and isolated pigment-protein complexes were oriented in stretched polyvinyl alcohol films. Low temperature linear dichroism (LD) spectra of Chlorella pyrenoidosa and higher plant thylakoids in the films were very similar to those obtained after orientation of similar samples using magnetic or electric fields. Positive LD bands corresponding to Chl a (670) and (682) and negative bands due to Chl a (658) and Chl b(648) were resolved in spectra of the light harvesting Chl a/b protein. Chl b (648) and Chl a (658) and (670) were not seen in the LD spectrum of thylakoids from plants grown in intermittent light, the Chl b-less mutant of barley, Euglena gracilis or the cyanobacteria, Phormidium luridum and Anacystis nidulans, but did appear upon chloroplast maturation in Romaine lettuce and during the greening of etiolated and intermittent light plants. The highly oriented long wavelength Chl a (682) in the light-harvesting complex may represent residual PS II whose peak dichroism is centered at 681 nm. The PS I preparation had a Chl a/b ratio of approx. 6 and the LD spectrum was positive with a maximum at 690-694 nm and a band of lower amplitude at 652 nm. The minor LD band was not observed in PS I preparations from organisms that lack chl b such as the cyanobacteria, intermittent light plants and the Chl b-less mutant of barley. We suggest that the 652 nm band is due to Chl b molecules associated with the antenna of PS I and are distinct from those on the light harvesting complex whose orientation is different. We also conclude that all the Chl a forms are oriented and that the long geometric axes of the pigment-protein complexes, as deduced from the configuration they assume in the stretched films, are axes that normally lie parallel to the plane of the native thylakoid.

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.

Orientation of chromophores in reaction centers of Rhodopseudomonas sphaeroides. Evidence for two absorption bands of the dimeric primary electron donor

Chromatophores from Rhodopseudomonas sphaeroides were oriented by allowing aqueous suspensions to dry on glass plates. Orientation of reaction center pigments was investigated by studying the linear dichroism of chromatophores in which the absorption by antenna bacteriochlorophyll had been attenuated through selective oxidation. Alternatively the light-induced absorbance changes, in the ranges 550-650 and 700-950nm, were studied in untreated chromatophores. The long wave transition moment of reaction center bacteriochlorophyll (P-870) was found to be nearly parallel to the plane of the membrane, whereas the long wave transition moments of bacteriopheophytin are polarized out of this plane. For light-induced changes the linear dichroic ratios, defined as deltaav/deltaah, are nearly the same for untreated and for oxidized chromatophores. Typical values are 1.60 at 870 nm, 0.80 at 810nm, 1.20 at 790 nm, 0.70 at 765 nm, 0.30 at 745 nm , and 0.50 at 600 nm. The different values for the absorbance decrease at 810 nm (0.80) and the increase at 790 nm (1.20) are incompatible with the hypothesis that these changes are due to the blue-shift of a single band. We propose that the decreases at 870 and 810 nm reflect bleaching of the two components of a bacteriochlorophyll dimer, the "special pair" that shares in the photochemical donation of a single electron. The increase at 790 nm then represents the appearance of a monomer band in place of the dimer spectrum, as a result of electron donation. This hypothesis is consistent with available data on circular dichroism. It is confirmed by the presence of a shoulder at 810 nm in the absorption spectrum of reaction centers at low temperature; this band disappears upon photooxidation of the reaction centers. For the changes near 760 nm, associated with bacteriopheophytin, the polarization and the shape of the "light-dark" difference spectrum (identical to the first derivative of the absorption spectrum) show that the 760 nm band undergoes a light-induced shift to greater wavelengths.

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 degrees and -170 degrees 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 degrees 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.