PubMed Health⌕ Search

Biomedical subjects

W W Parson

Publications and source records attributed to W W Parson.

At least 37 records · Page 2Linked to original sources
PubMed1984

Nanosecond fluorescence from isolated photosynthetic reaction centers of Rhodopseudomonas sphaeroides.

The time-course of fluorescence from reaction centers isolated from Rhodopseudomonas sphaeroides was measured using single-photon counting techniques. When electron transfer is blocked by the reduction of the electron-accepting quinones, reaction centers exhibit a relatively long-lived (delayed) fluorescence due to back reactions that regenerate the excited state (P*) from the transient radical-pair state, PF. The delayed fluorescence can be resolved into three components, with lifetimes of 0.7, 3.2 and 11 ns at 295 K. The slowest component decays with the same time-constant as the absorbance changes due to PF, and it depends on both temperature and magnetic fields in the same way that the absorbance changes do. The time-constants for the two faster components of delayed fluorescence are essentially independent of temperature and magnetic fields. The fluorescence also includes a very fast (prompt) component that is similar in amplitude to that obtained from unreduced reaction centers. The prompt fluorescence presumably is emitted mainly during the period before the initial charge-transfer reaction creates PF from P*. From the amplitudes of the prompt and delayed fluorescence, we calculate an initial standard free-energy difference between P* and PF of about 0.16 eV at 295 K, and 0.05 eV at 80 K, depending somewhat on the properties of the solvent. The multiphasic decay of the delayed fluorescence is interpreted in terms of relaxations in the free energy of PF with time, totalling about 0.05 eV at 295 K, possibly resulting from nuclear movements in the electron-carriers or the protein.

Bacteriochlorophylls↗
PubMed1982

An x-ray absorption study of the iron site in bacterial photosynthetic reaction centers.

Measurements were made of the extended x-ray absorption fine structure (EXAFS) of the iron site in photosynthetic reaction centers from the bacterium Rhodopseudomonas sphaeroides. Forms with two quinones, two quinones with added o-phenanthroline, and one quinone were studied. Only the two forms containing two quinones maintained their integrity and were analyzed. The spectra show directly that the added o-phenanthroline does not chelate the iron atom. Further analysis indicates that the iron is octahedrally coordinated by nitrogen and/or oxygen atoms located at various distances, with the average value of about 2.14 A. The analysis suggests that most of the ligands are nitrogens and that three of the nitrogen ligands belong to histidine rings. This interpretation accounts for several unusual features of the EXAFS spectrum. We speculate that the quinones are bound to the histidine rings in some manner. Qualitative features of the absorption edge spectra also are discussed and are related to the Fe-ligand distance.

Iron↗
PubMed1981

Enthalpy and volume changes accompanying electron transfer from P-870 to quinones in Rhodopseudomonas sphaeroides reaction centers.

A capacitor microphone was used to measure the enthalpy and volume changes that accompany the electron transfer reactions, PQAhv leads to P+Q-A and PQAQBhv leads to P+QAQ-B, following flash excitation of photosynthetic reaction centers isolated from Rhodopseudomonas sphaeroides. P is a bacteriochlorophyll dimer (P-870), and QA and QB are ubiquinones. In reaction centers containing only QA, the enthalpy of P+Q-A is very close to that of the PQA ground state (delta Hr = 0.05 +/- 0.03 eV). The free energy of about 0.65 eV that is captured in the photochemical reaction evidently takes the form of a substantial entropy decrease. In contrast, the formation of P+QAQ-B in reaction centers containing both quinones has a delta Hr of 0.32 +/- 0.02 eV. The entropy change must be near zero in this case. In the presence of o-phenanthroline, which blocks electron transfer between Q-A and QB, delta Hr for forming P+Q-AQB is 0.13 +/- 0.03 eV. The influence of flash-induced proton uptake on the results was investigated, and the delta Hr values given above were measured under conditions that minimized this influence. Although the reductions of QA and QB involve very different changes in enthalpy and entropy, both reactions are accompanied by a similar volume decrease of about 20 ml/mol. The contraction probably reflects electrostriction caused by the charges on P+ and Q-A or Q-B.

Electron Transport↗
PubMed1981

Transient states in reaction centers containing reduced bacteriopheophytin.

Photosynthetic reaction centers isolated from Rhodopseudomonas sphaeroides strain R-26 were excited with non-saturating 7-ps, 600-nm flashes under various conditions, and the resulting absorbance changes were measured. If the quinone electron acceptor (Q) is in the oxidized state, flash excitation generates a transient state (PF), in which an electron has moved from the primary electron donor (P, a dimer of bacteriochlorophylls) to an acceptor complex involving a special bacteriopheophytin (H) and another bacteriochlorophyll (B). PF decays in 200 ps as an electron moves from H to Q and the acceptor complex are reduced photochemically before the excitation, the flash generates a different transient state of P with a high quantum yield. This state decays with a lifetime of 340 ps. There is no indication of electron transfer from P to B under these conditions, but this does not rule out the possibility that B is an intermediate electron carrier between P and H. Measurements of the yield of fluorescence from P under various conditions show that the 340 ps state is not the fluorescent excited singlet state of P. The transient state could be a triplet state, a charge-transfer state of P, or another excited singlet state that is not fluorescent.

Aerobiosis↗
PubMed1981

Temperature dependence of electron transfer between bacteriopheophytin and ubiquinone in protonated and deuterated reaction centers of Rhodopseudomonas sphaeroides.

The rate of the electron-transfer reaction between bacteriopheophytin and the first quinone in isolated reaction centers of Rhodopseudomonas sphaeroides has an unusual temperature dependence. The rate increases about threefold with decreasing temperature between 300 and 25 K, and decreases abruptly at temperatures below 25 K. Partial deuteration of the reaction centers alters the temperature dependence of the rate constant. Qualitative features of the temperature dependence can be understood in the context of a theory of nonadiabatic electron transfer (Sarai, 1980. Biochim. Biophys. Acta 589:71-83). We conclude that very low-energy (10-50 cm-1) processes, perhaps skeletal vibrations of the protein, are important to electron transfer. Higher-energy vibrations, possibly involving the pyrrolic N--H bonds of bacteriopheophytin, also are important in this process.

Chlorophyll↗
PubMed1981

Energies and kinetics of radical pairs involving bacteriochlorophyll and bacteriopheophytin in bacterial reaction centers.

Absorbance changes reflecting the formation of a transient radical-pair state, P(F), were measured in reaction centers from Rhodopseudomonas sphaeroides under conditions that blocked electron transfer to a later carrier (a quinone, Q). The temperature dependence of the absorbance changes suggests that P(F) is an equilibrium mixture of two states, which appear to be mainly (1)[P([unk])B([unk])] and (1)[P([unk])H([unk])]. P is a bacteriochlorophyll dimer, B is a bacteriochlorophyll absorbing at 800 nm, and H is a bacteriopheophytin. In the presence of Q([unk]), the energy of (1)[P([unk])B([unk])] is about 0.025 eV above that of (1)[P([unk])H([unk])], (1)[P([unk])H([unk])] can decay to a triplet state, P(R), which also is an equilibrium mixture of two states, separated by about 0.03 eV. The lower of these appears to be mainly a locally excited triplet state of P, (3)P; the upper state contains a major contribution from a triplet charge-transfer state, (3)[P([unk])B([unk])]. The temperature dependence of delayed fluorescence from P(R) indicates that (3)P lies 0.40 eV below the excited singlet state, P(*), which is about 0.05 eV above (1)[P([unk])H([unk])]. The (1,3)[P([unk])B([unk])] charge-transfer states thus appear to interact with the locally excited states of P and B to give singlet and triplet states that are separated in energy by about 0.35 eV. This is 10(6) times larger than the splitting between (1)[P([unk])H([unk])] and (3)[P([unk])H([unk])] and implies strong orbital overlap between P([unk]) and B([unk]). This is consistent with recent picosecond studies which suggest that electron transfer from P(*) to B occurs within 1 ps and is followed in 4 to 10 ps by electron transfer from B([unk]) to H.

Journal Article↗
PubMed1980

Subpicosecond and picosecond studies of electron transfer intermediates in Rhodopseudomonas sphaeroides reaction centers.

The primary electron transfer processes in isolated reaction centers of Rhodopseudomonas sphaeroides have been investigated with subpicosecond and picosecond spectroscopic techniques. Spectra and kinetics of the absorbance changes following excitation with 0.7-ps 610-nm pulses, absorbed predominantly by bacteriochlorophyll (BChl), indicate that the radical pair state P+BPh-, in which an electron has been transferred from the BChl dimer (P) to a bacteriopheophytin (BPh), is formed with a time constant no greater than 4 ps. The initial absorbance changes also reveal an earlier state, which could be an excited singlet state, or a P+BChl- radical pair. The bleaching at 870 nm produced by 7 ps excitation at 530 nm (absorbed by BPh) or at 600 nm (absorbed predominantly by BChl) shows no resolvable delay with respect to standard compounds in solution, suggesting that the time for energy transfer from BPh to P is less than 7 ps. However, the bleaching in the BPh band at 545 nm following 7-ps 600-nm excitation, exhibits an 8- to 10-ps lag with respect to standard compounds. This finding is qualitatively similar to the 35-ps delay previously observed at 760 nm by Shuvalov at al. (Shuvalov, V.A., Klevanik, A.V., Sharkov, A.V., Matveetz, Y.A. and Kryukov, P.G. (1978) FEBS Lett. 91, 135-139) when 25-ps 880-nm excitation flashes were used. A delay in the bleaching approximately equal to the width of the excitation flash can be explained in terms of the opposing effects of bleaching due to the reduction of BPh, and absorbance increases due to short-lived excited states (probably of BChl) that turn over rapidly during the flash. The decay of the initial bleaching at 800 nm produced by 7-ps 530- or 600-nm excitation flashes shows a fast component with a 30-ps time constant, in addition to a slower component having the 200-ps kinetics expected for the decay of P+BPh-. the dependence on excitation intensity of the absorbance changes due to the 30-p]s component indicate that the quantum yield of the state responsible for this step is lower than that observed for the primary electron transfer reactions. This suggests that at least part of the transient bleaching at 800 nm is due to a secondary process, possibly caused by excitation with an excessive number of photons. If the 800-nm absorbing BChl (B) acts as an intermediate electron carrier in the primary photochemical reaction, electron transfer between B and the BPh must have a time constant no greater than 4 ps.

Bacteriochlorophylls↗
PubMed1979

The involvement of iron and ubiquinone in electron transfer reactions mediated by reaction centers from photosynthetic bacteria.

Reaction centers from Rhodopseudomonas sphaeroides strain R-26 were prepared with varying Fe and ubiquinone (Q) contents. The photooxidation of P-870 to P-870+ was found to occur with the same quantum yield in Fe-depleted reaction centers as in control samples. The kinetics of electron transfer from the initial electron acceptor (I) to Q also were unchanged upon Fe removal. We conclude that Fe has no measurable role in the primary photochemical reaction. The extent of secondary reaction from the first quinone acceptor (QA) to the second quinone acceptor (QB) was monitored by the decay kinetics of P-870+ after excitation of reaction centers with single flashes in the absence of electron donors, and by the amount of P-870 photooxidation that occurred on the second flash in the presence of electron donors. In reaction centers with nearly one iron and between 1 and 2 ubiquinones per reaction center, the amount of secondary electron transfer is proportional to the ubiquinone content above one per reaction center. In reaction centers treated with LiClO4 and o-phenanthroline to remove Fe, the amount of secondary reaction is decreased and is proportional to Fe content. Fe seems to be required for the secondary reaction. In reaction centers depleted of Fe by treatment with SDS and EDTA, the correlation between Fe content and secondary activity is not as good as that found using LiClO4. This is probably due in part to a loss of primary photochemical activity in samples treated with SDS; but the correlation is still not perfect after correction for this effect. The nature of the back reaction between P-870+ and Q-B was investigated using stopped flow techniques. Reaction centers in the P-870+ Q-B state decay with a 1-s half-time in both the presence and absence of o-phenanthroline, an inhibitor of electron transfer between Q-B and QB. This indicates that the back reaction between P-870+ and Q-A is direct, rather than proceeding via thermal repopulation of Q-A. The P-870+ Q-B state is calculated to lie at least 100 mV in free energy below the P-870+ Q-A state.

Bacteriochlorophylls↗
PubMed1979

The quantum yield of flash-induced proton release by bacteriorhodopsin-containing membrane fragments.

The quantum yield of proton release by bacteriorhodopsin was measured from volume changes after excitation of purple membrane fragments by short flashes. At low ionic strengths, about 0.25 mol of protons is released per einstein absorbed. This agrees well with quantum yields reported recently for the conversion of bacteriorhodopsin into a metastable state (M) that absorbs near 412 nm. However, the quantum yield of proton release increases gradually with increasing ionic strength; it plateaus with a value of 0.43 +/- 0.03 at ionic strengths above 200 mM. Changing the ionic strength has no detectable effect on the quantum yield of formation of the M spectral state. It thus appears that as many as two protons can be released and rebound in each photochemical cycle at high ionic strengths. The quantum yield of proton release is essentially independent of pH over the range 6.0-8.75. The quantum yield decreases with increasing flash strength, apparently due to photoreversal of the initial photochemical reaction.

Bacteriorhodopsins↗
PubMed1979

Enthalpy changes during the photochemical cycle of bacteriorhodopsin.

We have used a capacitor microphone calorimeter to measure rapid enthalpy changes that occur when bacteriorhodopsin-containing membrane fragments are excited with short flashes of light. We resolved the enthalpy changes into three phases. At about 100 microsecond after the flash, the bacteriorhodopsins converted into metastable states have an enthalpy about 15-20 kcal mol-1 greater than the enthalpy before excitation. Some of this energy (approximately 10 kcal) is then released to the surroundings as the membrane fragments release protons to the solution. After proton release and before proton rebinding, a large amount of heat is released to the surroundings, equivalent to about 40-45 kcal/mol of bacteriorhodopsin reacting. At this point the energy of the system is about 35 kcal/mol less than it was before the flash; i.e., the system has released all of the energy of the photon (49 kcal/E) plus an additional 35 kcal/mol. Nevertheless, the free energy of the system must still be greater than it was originally, because relaxation to the original state occurs spontaneously. An entropy decrease of at least 125 cal/mol per deg is required to compensate for the heat release. An entropy decrease of this magnitude implies a major increase in molecular order in the purple membrane.

Bacteriorhodopsins↗
PubMed1978

Primary photochemical processes in isolated reaction centers of Rhodopseudomonas viridis.

Picosecond and nanosecond spectroscopic techniques have been used to study the primary electron transfer processes in reaction centers isolated from the photosynthetic bacterium Rhodopseudomonas viridis. Following flash excitation, the first excited singlet state (P*) of the bacteriochlorophyll complex (P) transfers an electron to an intermediate acceptor (I) in less than 20 ps. The radical pair state P+I-) subsequently transfers an electron to another acceptor (X) in about 230 ps. There is an additional step of unknown significance exhibiting 35 ps kinetics. P+ subsequently extracts an electron from a cytochrome, with a time constant of about 270 ns. At low redox potential (X reduced before the flash), the state P+I- (or PF) lives approx. 15 ns. It decays, in part, into a longer lived state (PR), which appears to be a triplet state. State PR decays with an exponential time of approx. 55 microseconds. After continuous illumination at low redox potential (I and X both reduced), excitation with an 8-ps flash produces absorption changes reflecting the formation of the first excited singlet state, P*. Most of P* then decays with a time constant of 20 ps. The spectra of the absorbance changes associated with the conversion of P to P* or P+ support the view that P involves two or more interacting bacteriochlorophylls. The absorbance changes associated with the reduction of I to I- suggest that I is a bacteriopheophytin interacting strongly with one or more bacteriochlorophylls in the reaction center.

Bacteriochlorophylls↗
PubMed1977

Magnetic field effects on radical pair intermediates in bacterial photosynthesis.

We have investigated the effects of magnetic fields on the formation and decay of excited states in the photochemical reaction centers of Rhodopseudomonae sphaeroides. In chemically reduced reaction centers, a magnetic field decreases the fraction of the transient state PF that decays by way of the bacteriochlorophyll triplet state PR. At room temperature, a 2-kG field decreases the quantum yield of Pr by about 40%. In carotenoid-containing reaction centers, the yield of the carotenoid triplet state which forms via PR is reduced similarly. The effect of the field depends monotonically on field-strength, saturating at about 1 kG. The effect decreases at lower temperatures, when the yield of PR is higher. Magnetic fields do not significantly affect the formation of the triplet state of bacteriochlorophyll in vitro, the photooxidation of P870 in reaction centers at moderate redox potential, or the decay kinetics of states PF and PR. The effect of magnetic fields support in view that state PF is a radical pair which is born in a singlet state but undergoes a rapid transformation into a mixture of singlet and triplet states. A simple kinetic model can account for the effects of the field and relate them to the temperature dependence of the yield of PR.

Free Radicals↗
PubMed1977

Light collection and harvesting processes in bacterial photosynthesis investigated on a picosecond time scale.

Fluorescence lifetimes have been determined for four strains of Rhodopseudomonas sphaeroides. Chromatophore samples were excited with a single picosecond flash, and the fluorescence was detected with a streak camera. The decay times are 100 psec in strains 2.4.1 and Ga, and 300 psec in the carotenoidless strain R-26. These times are related to the transfer of energy from the light-harvesting antenna pigment molecules to the photochemical reaction center. In strain PM-8 dpl, which lacks reaction centers, the lifetime is 1.1 nsec. In addition, we have obtained curves relating the quantum yield of fluorescence to the photon density of the excitation pulse. These curves can be fit with a simple model that relates excitonic processes to properties of the photosynthetic unit and that qualitatively describes differences between the mutant strains.

Journal Article↗
PubMed1976

Triplet states of bacteriochlorophyll and carotenoids in chromatophores of photosynthetic bacteria.

Chromatophores from photosynthetic bacteria were excited with flashes lasting approx. 15 ns. Transient optical absorbance changes not associated with the photochemical electron-transfer reactions were interpreted as reflecting the conversion of bacteriochlorophyll or carotenoids into triplet states. Triplet states of various carotenoids were detected in five strains of bacteria; triplet states of bacteriochlorophyll, in two strains that lack carotenoids. Triplet states of antenna pigments could be distinguished from those of pigments specifically associated with the photochemical reaction centers. Antenna pigments were converted into their triplet states if the photochemical apparatus was oversaturated with light, if the primary photochemical reaction was blocked by prior chemical oxidation of P-870 or reduction of the primary electron acceptor, or if the bacteria were genetically devoid of reaction centers. Only the reduction of the electron acceptor appeared to lead to the formation of triplet states in the reaction centers. In the antenna bacteriochlorophyll, triplet states probably arise from excited singlet states by intersystem crossing. The antenna carotenoid triplets probably are formed by energy transfer from triplet antenna bacteriochlorophyll. The energy transfer process has a half time of approx. 20 ns, and is about 1 X 10(3) times more rapid than the reaction of the bacteriochlorophyll triplet states with O2. This is consistent with a role of carotenoids in preventing the formation of singlet O2 in vivo. In the absence of carotenoids and O2, they decay half times of the triplet states are 70 mus for the antenna bacteriochlorophyll and 6-10 mus for the reaction center bacteriochlorophyll. The carotenoid triplets decay with half times of 2-8 mus. With eak flashes, the quantum yields of the antenna triplet states are in the order of 0.02. The quantum yields decline severely after approximately one triplet state is formed per photosynthetic unit, so that even extremely strong flashes convert only a very small fraction of the antenna pigments into triplet states. The yield of fluorescence from the antenna bacteriochlorophyll declines similarly. These observations can be explained by the proposal that single-triplet fusion causes rapid quenching of excited single states in the antenna bacteriochlorophyll.

Bacterial Chromatophores↗