Ted Mar

Induction kinetics of delayed light emission in spinach chloroplasts

Induction curves of the delayed light emission in spinach chloroplasts were studied by measuring the decay kinetics after each flash of light. This study differs from previous measurements of the induction curves where only the intensities at one set time after each flash of light were recorded. From the decay kinetics after each flash of light, the induction curves of the delayed light emission measured 2 ms after a flash of light were separated into two components: one component due to the last flash only and one component due to all previous flashes before the last one. On comparing the delayed light induction curves of the two components with the fluorescence induction curves in chloroplasts treated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and in chloroplasts treated with hydroxylamine and 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the component due to the last flash only is found to be dependent on the concentration of open reaction centers and the component due to all previous flashes except the last is dependent on the concentration of closed reaction centers. This implies that the yield of the fast decaying component of the delayed light emission is dependent on the concentration of open reaction centers and the yield of the slow decaying component is dependent on the concentration of closed reaction centers.

Photodichroic studies of the photoreaction center from Rhodospirillum Rubrum. I. Attribution of P870 to two non parallel dipoles

A randomly oriented sample of photoreaction center prepared from Rhodospirillum rubrum was excited at 77 degrees K by an actinic linearly polarized light of 870 nm. Under such conditions, only those chromophores with components of their absorption dipoles oriented parallel to the polarization of the actinic light are bleached. The change in absorbance at 900 nm of this photoselected sample was observed while varying the angle of polarization of a weak measuring light. The polarization of the absorbance change was thus evaluated as 0.25. This value is interpreted to mean that P870 is attributable to two absorption dipoles forming an angle included between 35.75 degrees and 90 degrees. Comparison with the p value of 0.5 obtained on a similar preparation by polarization of fluorescence (Ebrey, T. G. and Clayton, R.K. (1969) Photochem. Photobiol. 10, 109-117) leads to the conclusion that either these two dipoles emit fluorescence without being coupled by singlet-singlet energy transfer or that only one of them is a fluorescence emitter in the absence of reversible singlet-singlet energy transfer.

Relative phototrapping rates of the two bacteriopheophytins in the photoreaction center of Ectothiorhodospira sp.

The photoreaction center of Ectothiorhodospira sp. includes a bound c-type cytochrome containing four hemes. When reduced, this cytochrome competes with the reduced primary electron acceptor for charge recombination with the oxidized primary donor. This allows the phototrapping of reduced intermediary species. While only the A arm bacteriopheophytin anion was phototrapped at temperatures above 250 K, both bacteriopheophytin anions were phototrapped at 219 K. The trapping and decay rate constants of the two bacteriopheophytin anions were obtained from the kinetics were obtained from the kinetics of their light-induced absorbance change and EPR signal. The bacteriopheophytin situated in the A arm was phototrapped 274-times faster than the one in the B arm. Both phototrapped anions decay by charge recombination with the oxidized cytochrome.

An asymmetric dimer exciton model. Application to the primary electron donor of bacterial photoreaction center

Abstract An asymmetric dimer excition theory is developed that takes into account both environmental and vibronic effects on the electronic transition energies. Explicit equations are presented for the transition energies, the dipole moments, the angle between the dipole moments and the dipole and rotational strengths for the electronic transitions in this asymmetric dimer. This model is proposed to describe the structure of the special pair of bacteriochlorophyll a molecules believed to constitute the primary electron donor of bacterial photoreaction center. The model is found to be consistent with most of the spectroscopic properties of the photoreaction center. We used the equations derived from the asymmetric model along with absorption and circular dichroism spectroscopy data to predict a geometrical structure for the primary electron donor.

Different temperature dependencies of the charge recombination reaction in photoreaction centers isolated from different bacterial species

Abstract We compared the temperature dependency of the rate of the charge recombination reaction in photoreaction centers isolated from Ectothiorhodospira sp. and from Rhodospirillum rubrum G9. We also examined the temperature dependency of the bandwidth and peak wavelength of their far-red absorption band. In both preparations, the peak wavelength and the bandwidth vary monotonically with temperature between 80 and 300 K. However, the rate of the charge recombination reaction has a quite different temperature dependency. In the preparation from R. rubrum, the reaction is accelerated 5-fold in a typical sigmoidal fashion as the temperature is lowered from 300 to 80 K. In the preparation from Ectothiorhodospira sp., the reaction is accelerated monotonically only about 1.5-fold in the same temperature range. At temperatures below 100 K, the rates are similar in the two preparations. We interpret the temperature dependency of the charge recombination reaction in terms of an activationless electron-transfer model formulated by Jortner (Jortner, J. (1980) Biochim. Biophys. Acta 394, 193–230). The minimal model provides a good fit for the temperature dependency of charge recombination in the preparation from Ectothiorhodospira sp. However, to fit the temperature dependency of the R. rubrum preparation with the same model, we must further postulate that the electronic coupling factor varies with temperature in this preparation. We find that, in both preparations, the temperature dependency of the far-red absorption bandwidth is consistent with the assumption that similar vibrational modes are involved in electron transfer and in electronic excitation.

Rotational mobility of the photoreaction center in chromatophore membranes of Rhodospirillum rubrum

Abstract We investigated the rotational mobility of the photoreaction center in chromatophores of Rhodospirillum rubrum by studying the photoinduced linear dichroism of absorption changes at 865 nm. The study was carried out in suspensions of chromatophores treated with ferricyanide in order to bleach their antenna bacteriochlorophyll and thus minimize depolarization by energy transfer. Very little depolarization of the photoinduced absorbance change at 865 nm was observed at room temperature for chromatophores immersed in a highly viscous medium over the time range 0–10 ms following an exciting light flash. In the light of independent evidence for transmembrane arrangement of the photoreaction center, we conclude that the photoreaction center protein is immobilized in the chromatophore membrane for at least 10 ms.

A kinetic model of the primary back reaction in photosynthesis of green plants

A model is proposed to explain the nonexponential decay kinetics of the primary back reaction of photosystem II and of delayed light in green plants. This model assumes that the primary back reaction is due to the recombination of the primary oxidized photoproduct Z + and the primary reduced photoproduct Q − which are embedded in a fixed position in the reaction center. The formation of a singlet exciton in the bulk chlorophyll molecules as a reaction intermediate is assumed possible. The basic assumption made in this model is that Q − and Z + can cause changes in its surrounding environment to such an extent that it can affect the structure of its neighboring centers. Using this model, the rate of the primary back reaction was calculated and was found to agree well with the experimental kinetic data of Bennoun (1970) . Also, theoretical curves of the kinetics of delayed light in the seconds region, of thermoluminescene and of chemiluminescence were calculated and were also found to agree well with experimental data. Glow curves and the fluorescence versus excitation light intensity curve were also discussed as further means of studying the primary back reaction.

Circular dichroism spectra of oriented photoreaction center from Rhodospirillum rubrum

The circular dichroism spectra of oriented and unoriented photoreaction centers of Rhodospirillum rubrum are compared. Orientation is achieved by pressing photoreaction center suspended in polyacrylamide gel. The biphasic bands at 870 and 810 nm and at 630 and 600 nm undergo a rotatory strength decrease when measured in the direction of the pressure, but not when measured in the direction normal to the pressure. Such a decrease in oriented photoreaction center is consistent with the model according to which these bands are dimer exciton bands of the special pair bacteriochlorophyll.

Measurement of mitochondrial volume independent of refractive index by light scattering

Using latex spheres of accurately determined dimensions, it was found that, although the scattered light intensity at an angle within the forward scattering lobe varies with the refractive index of the medium, the ratio of the scattered light intensity at a pair of angles is independent of the refractive index. This finding is consistent with calculations from the Mie theory by Hodkinson that the shape of the forward scattering lobe depends only on the size of the sphere and is independent of the refractive index of the sphere or of the suspending medium. This method was applied to the measurement of volume changes in mitochondria induced either by suspending the mitochondria in solutions of different osmolality or by addition of ionophores to stimulate ion uptake. The volume changes measured were independent of the refractive index of the suspending medial. It is shown that measuring the ratio of the scattered light intensity at selected pair of angles within the forward scattering lobe is a good method to measure the average size of the mitochondria.

Evidence for the photoreductive trapping of doubly reduced bacteriopheophytin in the photoreaction center of Ectothiorhodospira sp.

Continuous near-infrared illumination of Ectothiorhodospira sp. phototreaction centers in the presence of dithionite resulted in the bleaching of the entire bacteriopheophytin Qx and Qy absorption bands. We identified the two bacteriopheophytin molecules by the bandshift of their Qx transitions, ΦA being red shifted and ΦB being blue shifted. ΦA was the first to be bleached, as it was phototrapped in the short-lived Φ−A state. Further illumination led to a decreased amount of Φ−A, concomitant with the phototrapping of very long-lived bleached ΦA and ΦB species. These new species were very stable under anaerobic conditions but introduction of oxygen restored the absorption bands of bacteriopheophytin. They displayed neither the electron paramagnetic resonance signal nor the broad 645 nm absorption band characteristic of singly reduced bacteriophophytin. They are therefore suggested to be doubly reduced. Doubly reduced ΦB was phototrapped before doubly reduced ΦA with an activation energy of 2.76 kcal · mol−1 for the former and of 8.99 kcal · mol−1 for the latter. A large component of these activation energies being negentropic is suggested to indicate conformational changes induced by the doubly charged species.