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Biomedical subjects

R J Cogdell

Publications and source records attributed to R J Cogdell.

At least 73 records · Page 4Linked to original sources
PubMed1981

Reconstitution of carotenoids into the light-harvesting pigment-protein complex from the carotenoidless mutant of Rhodopseudomonas as sphaeroides R26.

Two carotenoids, neurosporene and spheroidene, have been successfully added to chromatophores from the carotenoidless mutant of Rhodopseudomonas sphaeroides R26. Carotenoids reconstituted in this way into the B-850 light-harvesting pigment-protein complex both sensitive bacteriochlorophyll fluorescence and protect the complex from the photodynamic reaction.

Bacterial Chromatophores↗
PubMed1981

Energy transfer between the carotenoid and the bacteriochlorophyll within the B-800-850 light-harvesting pigment-protein complex of Rhodopseudomonas sphaeroides.

Energy transfer between carotenoid and bacteriochlorophyll has been studied in isolated B-800-850 antenna pigment-protein complexes from different strains of Rhodopseudomonas sphaeroides which contain different types of carotenoid. Singlet-singlet energy transfer from the carotenoid to the bacteriochlorophyll is efficient (75-100%) and is rather insensitive to carotenoid type, over the range of carotenoids tested. The yield of carotenoid triplets is low (2-15%) but this arises from a low yield of bacteriochlorophyll triplet formation rather than from an inefficient triplet-triplet exchange reaction. The rate of the triplet-triplet exchange reaction between the bacteriochlorophyll and the carotenoid is fast (Ktt greater than or equal to 1.4 . 10(8) S-1) and also relatively independent of the type of carotenoid present.

Bacteriochlorophylls↗
PubMed1980

A comparison of the constituent polypeptides of the B-800--850 light-harvesting pigment-protein complex from Rhodopseudomonas sphaeroides.

The polypeptide composition of the B-800-850 light-harvesting pigment-protein complex from Rhodopseudomonas sphaeroides has been determined. The complex consists of equimolar amounts of two small polypeptides. The two polypeptides have very similar molecular weights and amino acid composition but are clearly separable by eithr SDS polyacrylamide gradient gel electrophoresis or isoelectric focussing.

Amino Acids↗
PubMed1980

Identification of the carotenoid present in the B-800-850 antenna complex from Rhodopseudomonas capsulata as that which responds electrochromically to transmembrane electric fields.

Mild proteolysis of Rhodopseudomonas capsulata chromatophores results in a parallel loss of the 800 nm bacteriochlorophyll absorption band a blue shift in the carotenoid absorption bands associated with the B-800-850 light-harvesting complex. Both the light-induced and the salt-induced electrochromic carotenoid band shift disappear in parallel to the loss of the 800 nm bacteriochlorphyll absorption upon pronase treatment of chromatophores. During the time required for the loss of the 800 nm bacteriochlorophyll absorption and the loss of the electrochromic cartenoid band shift photochemistry is not inhibited and the ionic conductance of the membrane remains very low. We conclude that the carotenoid associated with the B-800-850 light-harvesting complex is the one that responds electrochromically to the transmembrane electric field. Analysis of the pigment content of Rps. capsulata chromatophores indicates that all of the carotenoid may be accounted for in the well defined pigment-protein complexes.

Bacterial Chromatophores↗
PubMed1978

On the state of carotenoids bound to reaction centers of photosynthetic bacteria: a resonance Raman study.

The carotenoids bound to reaction centers of wild, Ga and GIC strains of Rhodopseudomonas spheroides, of Rhodospirrillum rubrum, strain S1 and of Rhodopseudomonas viridis, yield very similar, but unusual resonance Raman spectra. Through a comparison with resonance Raman spectra of 15,15'-cis-beta-carotene, these carotenoids are shown to assume cis conformations, while the corresponding chromatophores contain all-trans forms only. These cis conformations likely are identical for all the carotenoids studied. They remain unaffected by variations of temperature from 20 to 300 K as well as by the redox state of P-870. They are unstable, being rapidly isomerised towards the all-trans forms when extracted from the reaction centers. The possible nature of these conformers is discussed on the basis of their electronic and vibrational spectra.

Carotenoids↗
PubMed1978

Analysis of the pigment content of an antenna pigment-protein complex from three strains of Rhodopseudomonas sphaeroides.

The pigment content of a B800-850 light-harvesting pigment-protein complex isolated from three different stains of Rhodopseudomonas sphaeroides has been determined. In each case the ratio of carotenoid to bacteriochlorophyll present is very nearly 1 : 3 an no specificity with regard to carotenoid type was observed. The fourth derivative of the infra-red absorption bands of the complex was determined and it is concluded that the minimal functional unit of B800-850 complex consists of 1 carotenoid molecule and three bacteriochlorophyll molecules. The data presented here, together with the previous study of Austin, (Austin, L.A. (1976) Ph.D. Thesis, University of California at Berkeley, Lawrence Berkeley Laboratory Report No. LBL 5512) suggest that the 800 nm absorption band represents one of these bacteriochlorophyll molecules while the remaining two bacteriochlorophylls are responsible for the 850 nm band. The absorption spectra and circular dichroism spectra of the complexes suggests that their structure has not been greatly altered during the purification.

Bacterial Chromatophores↗
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↗
PubMed1976

The type, amount, location, and energy transfer properties of the carotenoid in reaction centers from Rhodopseudomonas sphaeroides.

Analysis of photosynthetic reaction centers from Rhodopseudomonas sphaeroides strains 2.4.1 and Ga shows that each contains approx. 1 mol of a specific carotenoid per mol of reaction center. In strain 2.4.1. the carotenoid is spheroidene (1-methoxy-3,4-didehydro-1,2,7',8',-tetrahydro-psi,psi-carotene); in strain Ga, it is chloroxanthin (1-hydroxy-1, 2, 7', 8'-tetrahydro-psi,psi-carotene). The carotenoid is bound to the same pair of proteins as are the bacteriochlorophylls and bacteriopheophytins of the reaction center. This binding induces strong circular dichroism in the absorption bands of the carotenoid. The carotenoid is close enough to the other pigments of the reaction center so that light energy transfers efficiently from the carotenoid to the bacteriochlorophyll, sensitizing bacteriochlorophyll fluorescence. The fluorescence polarization spectrum of the reaction centers shows that the transition vectors for the visible absorption bands of the carotenoid lie approximately parallel to the 600 nm (Qx) transition of the bacteriochlorophyll complex.

Carotenoids↗
PubMed1975

Carotenoid triplet states in reaction centers from Rhodopseudomonas sphaeroides and Rhodospirillum rubrum.

Purified photochemical reaction centers from three strains of Rhodopseudomonas sphaeroides and two of Rhodospirillium rubrum were reduced with Na2S2O4 so as to block their photochemical electron transfer reactions. They then were excited with flashes lasting 5-30 ns. In all cases, absorbance measurements showed that the flash caused the immediate formation of a transient state (PF) which had been detected previously in reaction centers from Rps. sphaeroides strain R26. Previous work has shown that state PF is an intermediate in the photochemical electron transfer reaction in the reaction centers of that particular strain, and the present work generalizes that conclusion. In the reaction centers from two strains that lack carotenoids (Rps. sphaeroides R26 and R. rubrum G9), the decay of PF yields a longer-lived state (PR) which is probably a triplet state of the bacteriochlorophyll of the reaction center. In the R26 preparation, the decay of PF was found to have a half-time of 10 +/- 2 ns. The decay kinetics rule out the identification of PF as the fluorescent excited singlet state of the reaction center. In the reaction centers from three strains that contain carotenoids (Rps sphaeroides 2.4.1 and Ga, and R. rubrum S1), state PR was not detected, and the decay of PF generated triplet states of carotenoids. The efficiency of the coupling between the decay of PF and the formation of the carotenoid triplet appeared to be close to 100% at room temperature, but somewhat lower at 77 degrees K. Taken with previous results, this suggests that the coupling is direct and does not require the intermediate formation of state PR. This conclusion would be consistent with the view that PF is a biradical which can be triplet in character.

Carotenoids↗
PubMed1975

Some experiments on the primary electron acceptor in reaction centres from Rhodopseudomonas sphaeroides.

The bacterial reaction center absorbance change at 450 nm (A-450) assigned to an anionic semiquinone, has been suggested as a candidate for the reduced form of the primary electron acceptor in bacterial photosynthesis. In reaction centers of Rhodopseudomonas sphaeroides we have found kinetic discrepancies between the decay of A-450 and the recovery of photochemical competence. In addition, no proton uptake is measurable on the first turnover, although subsequent ones elicit one proton bound per electron. These results are taken to indicate that the acceptor reaction after a long dark period may be different for the first turnover than for subsequent ones. It is suggested that A-450 is still a likely candidate for the acceptor function but that in reaction centers, additional quinone may act as an adventitious primary acceptor when the "true" primary acceptor is reduced. Alternatively, the primary acceptor may act in a "ping-pong" fashion with respect to subsequent photoelectrons.

Cytochrome c Group↗