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Biomedical subjects

W L Butler

Publications and source records attributed to W L Butler.

At least 37 records · Page 2Linked to original sources
PubMed1977

Energy transfer and the distribution of excitation energy in the photosynthetic apparatus of spinach chloroplasts.

Equations are derived from our model of the photochemical apparatus of photosynthesis to show that the yield of energy transfer from Photosystem II to Photosystem I, phi T(II leads to I), can be obtained from measurements on an individual sample of chloroplasts frozen to -196 degrees C by comparing the sum of two specifically defined fluorescence excitation spectra with the absorption spectrum of the sample. Then, given that value of phiT(II leads to I), the fraction of the quanta absorbed by the photochemical apparatus which is distributed initially to Photosystem I, alpha, can be determined as a function of the wavelength of excitation from the same fluorescence excitation spectra. The results obtained in this study of individual samples of chloroplasts frozen to -196 degrees C in the absence of divalent cations, namely, that phi T(II leads to I)varies from a minimum value of 0.10 when the Photosystem II reaction centers are all open to a maximum value of 0.25 when the centers are all closed and that alpha has a value of about 0.30 which is almost independent of wavelength for wavelength shorter than 675 nm (alpha increases rapidly toward unity at wavelength longer than 675 nm), agrees quite well with results obtained previously from comparative measurements of chloroplasts frozen to -196 degrees C in the presence and absence of divalent cations.

Chloroplasts↗
PubMed1977

Tripartite model for the photochemical apparatus of green plant photosynthesis.

Equations for fluorescence and the rates of photochemistry of photosystem I and photosystem II are derived from a photochemical model for the photosynthetic apparatus that includes the various interactions of the light-harvesting chlorophyll a/b complex with photosystem I and photosystem II as specific photochemical rate constants. The degree of coupling between photosystem II and the chlorophyll a/b complex which is expressed as a product of two probability terms plays a central role in this three-pigment system. The cycling of excitation energy back and forth between photosystem II and the chlorophyll a/b complex increases the exciton density in both arrays of chlorophyll according to a simple analytical expression in the equations. These equations of the tripartite model provide new and credible insights into the photochemical apparatus of photosynthesis.

Mathematics↗
PubMed1977

Isolation and Function of Allophycocyanin B of Porphyridium cruentum.

Allophycocyanin B was purified to homogeneity from the eukaryotic red alga Porphyridium cruentum. This biliprotein is distinct from the allophycocyanin of P. cruentum with respect to subunit molecular weights, and spectroscopic and immunological properties. The purified allophycocyanin B has a long wavelength absorption maximum at 669 nm at room temperature and at 675 nm at -196 C while the fluorescence emission maximum is at 673 nm at room temperature and 679 nm at -196 C. The emission spectrum of allophycocyanin shifted only 1 nm, from 659 to 660 nm, on cooling to -196 C, and was the same with allophycocyanin crystals as it was with pure solutions of the pigment. Phycobilisomes from P. cruentum have a major fluorescence emission band at 680 nm at -196 C which emanates from the small amount of allophycocyanin B present in the phycobilisomes. Light energy absorbed by the bulk of the biliprotein pigments is transferred to allophycocyanin B with high efficiency.

Journal Article↗
PubMed1976

Energy transfer in the photochemical apparatus of flashed bean leaves.

Fluorescence and energy transfer properties of bean leaves greened by brief, repetitive xenon flashes were studied at -196 degrees C. The bleaching of P-700 has no influence on the yield of fluorescence at any wavelength of emission. The light-induced fluorescence yield changes which are observed in both the 690 and 730 nm emission bands in the low temperature fluorescence spectra are due to changes in the state of the Photosystem II reaction centers. The fluorescence yield changes in the 730 nm band are attributed to energy transfer from Photosystem II to Photosystem I. Such energy transfer was also confirmed by measurements of the rate of photooxidation of P-700 AT -196 degrees C in leaves in which the Photosystem II reaction centers were either all open or all closed. It is concluded that energy transfer from Photosystem II to Photosystem I occurs in the flashed bean leaves which lack the light-harvesting chlorophyll a/b protein.

Cytochromes↗
PubMed1976

A demonstration of energy transfer from photosystem II to photosystem I in chloroplasts.

Photosystem I activity of Tris-washed chloroplasts was measured at room temperature as the rate of photoreduction of NADP and as the rate of oxygen uptake mediated by methyl viologen in both cases using dichlorophenolindophenol plus ascorbate as the source of electrons for Photosystem I. With both assay systems the rate of electron transport by Photosystem I was stimulated approx. 20% by the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea which caused the Photosystem II reaction centers to close. Photosystem I activity of chloroplasts was measured at low temperature as the rate of photooxidation of P-700. Chloroplasts suspended in the presence of hydroxylamine and 3-(3,4-dichlorophenyl)-1,1-dimethylurea were frozen to -196 degrees C after adaptation to darkness or after a preillumination at room temperature. The Photosystem II reaction centers of the frozen dark-adapted sample were all open; those of the preilluminated sample were all closed. The rate of photooxidation of P-700 at -196 degrees C with the preilluminated sample was approx. 25% faster than with the dark-adapted sample. We conclude from both the room temperature and the low temperature experiments that there is greater energy transfer from Photosystem II to Photosystem I when the Photosystem II reaction centers are closed and that these results are a direct demonstration of spillover.

Chloroplasts↗
PubMed1976

Microencapsulation of chloroplast particles.

Chloroplast and photosystem I particles were encapsulated in small spheres (about 20 mum diameter) with an artificial membrane built up by cross-linking amino groups of protamine with toluenediisocyanate. The artificial membrane was permeable to small substrate and product molecules but not to soluble proteins. Photosystem I activity was retained by the encapsulated chloroplast particles. Washed photosystem I particles were encapsulated with the soluble proteins, ferredoxin, and ferredoxin-NADP oxidoreductase, and the microcapsules photoreduced NADP using ascorbate plus dichlorophenolindophenol as the electron donor. The photosystem I particles were also encapsulated with hydrogenase from Chromatium and a very low rate of photoevolution of hydrogen was obtained. The results show that chloroplast membrane fragments can be encapsulated with soluble proteins that couple transfer reactions to the primary photochemical apparatus.

Journal Article↗
PubMed1976

Correlation of absorbance changes and thylakoid fusion with the induction of oxygen evolution in bean leaves greened by brief flashes.

Dark-grown bean leaves (Phaseolus vulgaris) which had been greened for several days in a repetitive series of brief xenon flashes were studied during the initial induction period when O(2) evolution first appears. The induction of O(2) evolution requires actinic irradiation (e.g. 2 mw/cm(2) of red light) and goes to completion in about 8 minutes with a half-time just under 3 minutes. Absorbance measurements on the intact leaves showed that a change of a carotenoid pigment, monitored at 505 nm, was closely correlated with the rate of O(2) evolution during the induction period. Inhibitor studies, however, showed that the absorbance change persisted in the presence of a number of inhibitors which blocked O(2) evolution. Electron microscopy revealed that the primary thylakoids which were unfused in the flashed leaves before induction became fused in pairs or groups of three during the 8-minute induction period. It is postulated that the 505-nm absorbance change of the carotenoid pigment is correlated more directly with the fusion process than with O(2) evolution. Heat treatment (45 C for 5 min) or infiltration with 0.8 m tris, which prevented the fusion process, also prevented the absorbance change.If the leaves were preilluminated for 8 minutes with very weak red light (20 muw/cm(2)) which induced no O(2) evolution, absorbance change, or thylakoid fusion, there was an immediate burst of O(2) evolution at the onset of actinic irradiation and the induction period, as noted by O(2) evolution or by the 505-nm absorbance change, was reduced to 2 minutes (half-time of 40 seconds). It is concluded that the electron transport system in the flashed leaves is blocked at the Mn site between water and photosystem II and that the photoactivation of Mn into the thylakoid membranes occurs during the low light, photoactivation process. After the electron transport chain is thus repaired, ion-pumping mechanisms driven by actinic light may lead to steady-state photosynthesis as well as to thylakoid fusion.

Journal Article↗
PubMed1976

Development of the Primary Photochemical Apparatus of Photosynthesis during Greening of Etiolated Bean Leaves.

Seven-day-old dark-grown bean leaves were greened under continuous light. The amount of chlorophyll, the ratio of chlorophyll a to chlorophyll b, the O(2) evolving capacity and the primary photochemical activities of Photosystem I and Photosystem II were measured on the leaves after various times of greening. The primary photochemical activities were measured as the photo-oxidation of P(700), the photoreduction of C-550, and the photo-oxidation of cytochrome b(559) in intact leaves frozen to -196 C. The results indicate that the reaction centers of Photosystem I and Photosystem II begin to appear within the first few minutes and that Photosystem II reaction centers accumulate more rapidly than Photosystem I reaction centers during the first few hours of greening. The very early appearances of the primary photochemical activity of Photosystem II was also confirmed by light-induced fluorescence yield measurements at -196 C.

Journal Article↗
PubMed1975

Excitation spectra for photosystem I and photosystem II in chloroplasts and the spectral characteristics of the distributions of quanta between the two photosystems.

The parameters listed in the title were determined within the context of a model for the photochemical apparatus of photosynthesis. The fluorescence of variable yield at 750 nm at -196 degrees C is due to energy transfer from Photosystem II to Photosystem I. Fluorescence excitation spectra were measured at -196 degrees C at the minimum, FO, level and the maximum, FM, level of the emission at 750 nm. The difference spectrum, FM-FO, which represents the excitation spectrum for FV is presented as a pure Photosystem II excitation spectrum. This spectrum shows a maximum at 677 nm, attributable to the antenna chlorophyll a of Photosystem II units, with a shoulder at 670 nm and a smaller maximum at 650 nm, presumably due to chlorophyll a and chlorophyll b of the light-harvesting chlorophyll complex. Fluoresence at the FO level at 750 nm can be considered in two parts; one part due to the fraction of absorbed quanta, alpha, which excites Photosystem I more-or-less directly and another part due to energy transfer from Photosystem II to Photosystem I. The latter contribution can be estimated from the ratio of FO/FV measured at 692 nm and the extent of FV at 750 nm. According to this procedure the excitation spectrum of Photosystem I at -196 degrees C was determined by subtracting 1/3 of the excitation spectrum of FV at 750 nm from the excitation spectrum of FO at 750 nm. The spectrum shows a relatively sharp maximum at 681 nm due to the antenna chlorophyll a of Photosystem I units with probably some energy transfer from the light-harvesting chlorophyll complex. The wavelength dependence of alpha was determined from fluorescence measurements at 692 and 750 nm at -196 degrees C. Alpha is constant to within a few percent from 400 to 680 nm, the maximum deviation being at 515 nm where alpha shows a broad maximum increasing from 0.30 to 0.34. At wavelengths between 680 and 700 nm, alpha increases to unity as Photosystem I becomes the dominant absorber in the photochemical apparatus.

Chloroplasts↗
PubMed1975

Energy transfer between photosystem II and photosystem I in chloroplasts.

A model for the photochemical apparatus of photosynthesis is presented which accounts for the fluorescence properties of Photosystem II and Photosystem I as well as energy transfer between the two photosystems. The model was tested by measuring at - 196 degrees C fluorescence induction curves at 690 and 730 nm in the absence and presence of 5mMMgCl2 which presumably changes the distrubution of excitation energy between the two photosystems. The equations describing the fluorescence properties involve terms for the distribution of absorbed quanta, alpha, being the fraction distributed to Photosystem I, and beta, the fraction to Photosystem II to Photosystem I, KT(II yields I). The data, analyzed within the context of the model, permit a direct comparison of alpha and kt(II yields I) in the absence (minus) and presence (+) of Mg-2+ :alpha minus/alpha-+ equals 1.2 and k-minus t)II yields I)/K-+T(II yields I) equal to 1.9. If the criterion that alpha + beta equal to 1 is applied absolute values can be calculated: in the presence of Mg-2+, alpha-+ equal to 0.27 and the yield of energy transfer, phi-+ t(II yields I) varied the presence of Mg-2+, alpha-+ equal to 0.27 and the yield of energy transfer, phi-+ t(II yields I) varied from 0.065 when the Photosystem II reaction centers were all open to 0.23 when they were closed. In the absence of Mg-2+, alpha-minus equal to 0.32 and phi t(II yields I) varied from 0.12 to 0.28. The data were also analyzed assuming that two types of energy transfer could be distinguished; a transfer from the light-harvesting chlorophyll of Photosystem II to Photosystem I, kt(II yields I), and a transfer from the reaction centers of Photosystem II to Photosystem I, kt(II yields I). In that case alpha-minus/alpha+ equal to 1.3, k-minus t(II yields I)/k+ t(II yields I)equal to 1.3 and k-minus t(II yields I) equal to 3.0. It was concluded, however, that both of these types of energy transfer are different manifestations of a single energy transfer process.

Chloroplasts↗
PubMed1975

Quenching of chlorophyll fluorescence and primary photochemistry in chloroplasts by dibromothymoquinone.

The quenching action of dibromothymoquinone on fluorescence and on primary photochemistry was examined in chloroplasts at minus 196 degrees C. Both the initial (F0) and final (FM) levels of fluorescence as well as the fluorescence of variable yield (FV equals FM minus FO) were quenched at minus 196 degrees C to a degree which depended on the concentration of dibromothymoquinone added prior to freezing. The initial rate of photoreduction of C-550 at minus 196 degrees C, which was assumed to be proportional to maximum yield for primary photochemistry, phipo, was also decreased in the presence of dibromothymoquinone. Simple theory predicts that the ratio FV/FM should equal phipo. Excellent agreement was found in a comparison of relative values of phipo with relative values of FV/FM at various degrees of quenching by dibromothymoquinone. These results are taken to indicate that FO and FV are the same type of fluorescence, both emanating from the bulk chlorophyll of Photosystem II. Dibromothymoquinone appears to create quenching centers in the bulk chlorophyll of Photosystem II which compete with the reaction centers for excitation energy. The rate constant for the quenching of excitation energy by dibromothymoquinone is directly proportional to the concentration of the quencher. Rate constants for the de-excitation of excited chlorophyll molecules by fluorescence, kF, by nonradiative decay processes, kD, by photochemistry, kP, and by the specific quenching of dibromothymoquinone, kQ, were calculated assuming the absolute yield of fluorescence at FO to be either 0.02 or 0.05.

Bromine↗
PubMed1975

Fluorescence quenching in photosystem II of chloroplasts.

A simple photochemical model for the photosynthetic units of Photosystem II based on first-order rate constants for de-excitation of excited chlorophyll molecules is presented in the form of equations which predict the yields of fluorescence (i.e. at the FO level, at the maximal FM level and the fluorescence of variable yield, FV equals FM minus FO). Two types of quenching mechanisms are recognized: (1) increasing nonradiative decay processes in the bulk chlorophyll by creating quenching centers which complete with the reaction centers for the excitation energy (this mechanism quenches both FO and FV) and (2) increasing nonradiative decay of the excited reaction center chlorophyll (this mechanism quenches FV but not FO). Quenching in the bulk chlorophyll preserves the relationship that Fv/FM is equal to the maximum yield of photochemistry; quenching at the reaction center chlorophyll decreases FV/FM substantially (since FV is quenched specifically) but may have very little effect on the yield of photochemistry. Estimates are made of the relative magnitudes of the rate constants for de-excitation of the excited reaction center chlorophyll by photochemistry, kp, by nonradiative decay processes, kd, and by energy transfer back to the bulk chlorophyll, kt. Fluorescence is assumed to emanate only from the bulk chlorophyll. Energy transfer from Photosystem II to Photosystem I may occur from either the excited bulk chlorophyll or from the excited reaction center chlorophyll. The model is valid for any degree of energy transfer between Photosystem II units.

Bromine↗
PubMed1975

Photoreceptor Pigment for Blue Light in Neurospora crassa.

Irradiating the mycelium of Neurospora crassa with moderate intensities of blue light causes a reversible photoreduction of a b-type cytochrome. The action spectrum for the photoreduction of cytochrome b is very similar to the absorption spectrum of flavin pigments. Prolonged irradiation of the mycelium with strong blue light irreversibly bleaches flavin-like pigments and as these pigments are bleached the photoresponse of cytochrome b is lost. We conclude from these and other data that a flavin is the photoreceptor pigment for the photoreduction of cytochrome b. The close similarity between the action spectrum for the photoreduction of cytochrome b and action spectra for a number of physiological photoresponses suggests that this photoreceptor pigment controls a wide variety of photobiological processes in a wide diversity of organisms.

Journal Article↗