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J Barber

Publications and source records attributed to J Barber.

At least 19 recordsLinked to original sources
PubMed1992

Subpicosecond equilibration of excitation energy in isolated photosystem II reaction centers.

Photosystem II reaction centers have been studied by femtosecond transient absorption spectroscopy. We demonstrate that it is possible to achieve good photoselectivity between the primary electron donor P680 and the majority of the accessory chlorins. Energy transfer can be observed in both directions between P680 and these accessory chlorins depending on which is initially excited. After excitation of either P680 or the other chlorins, the excitation energy is observed to equilibrate between the majority of these pigments at a rate of 100 +/- 50 fs-1. This energy-transfer equilibration takes place before any electron-transfer reactions and must therefore be taken into account in studies of primary electron-transfer reactions in photosystem II. We also show further evidence that the initially excited P680 excited singlet state is delocalized over at least two chlorins and that this delocalization lasts for at least 200 fs.

Chloroplasts↗
PubMed1992

Observation of pheophytin reduction in photosystem two reaction centers using femtosecond transient absorption spectroscopy.

Photosystem two reaction centers have been studied using a sensitive femtosecond transient absorption spectrometer. Measurements were performed at 295 K using different excitation wavelengths and excitation intensities which are shown to avoid multiphoton absorption by the reaction centers. Analyses of results collected over a range of time scales and probe wavelengths allowed the resolution of two exponential components in addition to those previously reported [Durrant, J. R., Hastings, G., Hong, Q., Barber, J., Porter, G., & Klug, D. R. (1992) Chem. Phys. Lett. 188, 54-60], plus the long-lived radical pair itself. A 21-ps component was observed. The process(es) responsible for this component was (were) found to produce bleaching of a pheophytin ground-state absorption band at 545 nm and the simultaneous appearance of a pheophytin anion absorption band at 460 nm resulting in a transient spectrum which was that of the radical pair P680+Ph-. This component is assigned to the production of reduced pheophytin. A lower limit of 60% of the final pheophytin reduction was found to occur at this rate. Despite subtle differences in transient spectra, the lifetime and yield of this pheophytin reduction are essentially independent of excitation wavelength within the signal to noise limitations of these experiments. A long-lived species was also observed. This species is produced by those processes which result in the 21-ps component, and it has a spectrum which is found to be independent of excitation wavelength. This spectrum is characteristic of the primary radical pair state P680+Ph-. In addition, a 200-ps component was found which is tentatively assigned to a slow energy-transfer/trapping process. This component was absent if P680 was excited directly and is therefore not integral to primary radical pair formation. Overall, it is concluded that the rate of pheophytin reduction is limited to (21 ps)-1, even when P680 is directly excited.

Fabaceae↗
PubMed1992

Two sites of primary degradation of the D1-protein induced by acceptor or donor side photo-inhibition in photosystem II core complexes.

Depending on experimental conditions we have found that photo-inhibitory treatment of photosystem II (PSII) core complexes, isolated from wheat, can generate two fragments of about 23-24 kDa that contain either the C-terminal or N-terminal regions of the D1-protein. A 24 kDa C-terminal fragment appears when the water splitting reaction is not functional and an electron acceptor is present. This 'donor'-side inhibition also generates an N-terminal fragment of about 10 kDa and is suggested to be due to the cleavage of a peptide bond in the region connecting transmembrane segments I and II of the D1-protein. In contrast, an N-terminal 23 kDa D1-protein fragment is detected when the water splitting reactions of the isolated complex are active, and occurs in the absence of an added electron acceptor. This 'acceptor'-side photo-inhibition also generates a C-terminal fragment of about 10 kDa.

Amino Acid Sequence↗
PubMed1992

Characterisation of photoinduced breakdown of the D1-polypeptide in isolated reaction centres of Photosystem II.

When the isolated reaction centre of Photosystem II, reconstituted with the quinone, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), is exposed to photoinhibitory illumination, a D1-polypeptide breakdown product of 24 kDa is detected by immunoblotting. In addition, weaker bands are also detected at 17, 13 and 10 kDa. It is suggested that the 24 kDa D1-polypeptide breakdown product is the same as that first observed in vivo by Greenberg et al. (1987) EMBO J. 6, 2865-2869. Its appearance in isolated Photosystem II reaction centres requires the presence of an electron acceptor, but occurs under both aerobic and anaerobic conditions. In our in vitro experimental system the photoinduced degradation of the D1-polypeptide to the 24 kDa fragment was related to the functional activity of the reaction centre and the enzymatic nature of the proteolysis was characterised by a pH optimum of about 8.0 and by inhibition with proteinase inhibitors, especially the serine-type soybean trypsin inhibitor. The results support our earlier findings (Shipton and Barber (1991) Proc. Natl. Acad. Sci. USA 88, 6691-6695) that the appearance of the light-induced D1-polypeptide breakdown pattern of fragments occurs as a consequence of donor side photoinhibition when highly oxidising species accumulate in the reaction centre and bring about pigment oxidation and degradation. We suggest that it is this selective loss of pigments that induces a conformational change in the D1-polypeptide which triggers its autoproteolytic cleavage.

Blotting, Western↗
PubMed1992

Too much of a good thing: light can be bad for photosynthesis.

Even though light is the ultimate substrate for photosynthetic energy conversion, it can also harm plants. This toxicity is targeted to the water-splitting photosystem II and leads to damage and degradation of the reaction centre D1-polypeptide. The degradation of this very important protein appears to be a direct consequence of photosystem II chemistry involving highly oxidizing radicals and toxic oxygen species. The frequency of this damage is relatively low under normal conditions but becomes a significant problem for the plant with increasing light intensity, especially when combined with other environmental stress factors. However, the plant survives this photoinhibition through an efficient repair system which involves an autoproteolytic activity of the photosystem II complex, D1-polypeptide synthesis and reassembly of active complexes.

Amino Acid Sequence↗
PubMed1992

Longterm drug therapy for rheumatoid arthritis in seven rheumatology private practices: I. Nonsteroidal antiinflammatory drugs.

The probability of continuation of a particular nonsteroidal antiinflammatory drug (NSAID) over 5 years was estimated for 1,775 courses taken by 532 patients with rheumatoid arthritis treated in 7 rheumatology private practices. Similar results were seen for 15 different NSAID--48% of courses were continued at 12 months, 36% at 24 months, and 20% at 60 months. Only acetylated salicylates, other than plain aspirin, were continued significantly longer than any of the other NSAID. The probability of continuation of plain aspirin was similar to other NSAID, including nonacetylated salicylates and nonsalicylate NSAID. The first NSAID taken by an individual patient was continued only marginally longer than the 4th NSAID taken by the same patient. While most NSAID courses were not continued for long periods, 20% were continued for longer than 5 years, suggesting effective longterm results in this minority of courses.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed1991

New evidence suggests that the initial photoinduced cleavage of the D1-protein may not occur near the PEST sequence.

When isolated reaction centres of photosystem 2 from pea or wheat are exposed to photoinhibitory illumination in the presence of an electron acceptor, breakdown products of the D1-protein are observed having molecular masses ranging from about 24 to 10 kDa. By using antibodies raised to the C-terminal or N-terminal portions of D1 it was shown that the major breakdown fragment of 24 kDa was derived from the C-terminus. This conclusion was supported by phosphorylation studies and from the digestion pattern obtained by lysine specific endoprotease-induced proteolysis. The complementary N-terminal breakdown fragment was found to have an apparent molecular mass of 10 kDa. The implications of these data are discussed in terms of the possible relationship between the 24 kDa C-terminal fragment and the 23.5 kDa breakdown fragment detected in vivo by Greenberg et al. [1987, EMBO J. 6, 2865-2869] and it is suggested, based on limited proteolysis using papain, that the latter may not be derived from the N-terminus as previously thought but also originates from the C-terminus.

Blotting, Western↗
PubMed1991

Photoinduced degradation of the D1 polypeptide in isolated reaction centers of photosystem II: evidence for an autoproteolytic process triggered by the oxidizing side of the photosystem.

When the isolated D1/D2/cytochrome b559 complex was exposed to bright light, a distinctive pattern of D1 polypeptide fragments was observed under both aerobic and anaerobic conditions. The major degradation product had an apparent molecular mass of 24 kDa, while other fragments were detected at 17, 14, and 10 kDa by immunoblotting. This pattern was observed when the electron acceptors 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone or silicomolybdate were present during illumination. It is known that these conditions stabilize P680+ chlorophyll and bring about the photooxidation and destruction of pigments in the reaction center, particularly chlorophyll absorbing at 670 nm and beta-carotene. When P680+ was not allowed to accumulate, either by omission of an electron acceptor or by addition of both an electron donor (Mn2+) and an acceptor, no breakdown fragments were observed. In the former case, however, some degradation of the D1 and D2 polypeptides did occur. Under conditions that gave rise to the characteristic D1 breakdown pattern, the D2 polypeptide was also degraded to specific fragments detected at about 29 and 21 kDa by immunoblotting. The results indicate that the photoinduced degradation of D1 (and D2) does not involve exogenous proteases but is most likely an autoproteolytic process. Moreover, our data indicate that the photochemical damage giving rise to D1 and D2 degradation occurs on the oxidizing rather than the reducing side of photosystem II and involves photooxidation of the accessory pigments. The results are discussed in terms of D1 and D2 turnover and photoinhibition.

Electrophoresis, Polyacrylamide Gel↗
PubMed1991

Observation of multiple radical pair states in photosystem 2 reaction centers.

Charge recombination of the primary radical pair in D1/D2 reaction centers from photosystem 2 has been studied by time-resolved fluorescence and absorption spectroscopy. The kinetics of the primary radical pair are multiexponential and exhibit at least two lifetimes of 20 and 52 ns. In addition, a third lifetime of approximately 500 ps also appears to be present. These multiexponential charge-recombination kinetics reflect either different conformational states of D1/D2 reaction centers, with the different conformers exhibiting different radical pair lifetimes, or relaxations in the free energy of the radical pair state. Whichever model is invoked, the free energies of formation of the different radical pair states exhibit a linear temperature dependence from 100 to 220 K, indicating that they are dominated by entropy with negligible enthalpy contributions. These results are in agreement with previous determinations of the thermodynamics that govern primary charge separation in both D1/D2 reaction centers [Booth, P.J., Crystall, B., Giorgi, L. B., Barber, J., Klug, D.R., & Porter, G. (1990) Biochim. Biophys. Acta 1016, 141-152] and reaction centers of purple bacteria [Woodbury, N.W.T., & Parson, W.W. (1984) Biochim. Biophys. Acta 767, 345-361]. It is possible that these observations reflect structural changes that accompanying primary charge separation and assist in stabilization of the radical pair state thus optimizing the efficiency of primary electron transfer.

Cytochrome b Group↗
PubMed1991

Chlorophyll levels in the pigment-binding proteins of photosystem II. A study based on the chlorophyll to cytochrome ratio in different photosystem II preparations.

The chlorophyll levels in pigment proteins of photosystem II were investigated by using photosystem II preparations with different levels of complexity. Based on the assumption that there is 1 cytochrome b559 per reaction centre it has been found that oxygen-evolving complexes containing CP26 and CP29 bind 42 chlorophyll molecules. When CP26 and CP29 are stripped away, the resulting PSII cores bind 30 chlorophyll molecules while CP43-less cores bind approximately 18 chlorophylls. It is therefore concluded that CP47 and CP43 bind 9-12 molecules of chlorophyll a and the D1/D2 complex binds 6 chlorophylls. Taken together CP26 and CP29 bind about 12 chlorophyll molecules.

Chlorophyll↗
PubMed1991

Protein secondary structure of the isolated photosystem II reaction center and conformational changes studied by Fourier transform infrared spectroscopy.

The secondary structure of the photosystem II (PSII) reaction center isolated from pea chloroplasts has been characterized by Fourier transform infrared (FTIR) spectroscopy. Spectra were recorded in aqueous buffers containing H2O or D2O; the detergent present for most measurements was dodecyl maltoside. The broad amide I and amide II bands were analyzed by using second-derivative and deconvolution procedures. Absorption bands were assigned to the presence of alpha-helices, beta-sheets, turns, or random structure. Quantitative analysis revealed that this complex contained a high proportion of alpha-helices (67%) and some antiparallel beta-sheets (9%) and turns (11%). An irreversible decrease in the intensity of the band associated with the alpha-helices occurs upon exposure of the isolated PSII reaction center to bright illumination. This loss of alpha-helical content gave rise to an increase in other secondary structures, particularly beta-sheets. After similar pretreatment with light, sodium dodecyl sulfate polyacrylamide gel electrophoresis reveals lower mobility and solubility of constituent D1 and D2 polypeptides of the PSII reaction center. Some degradation of these polypeptides also occurs. In contrast, there is no change in the mobility of the two subunits of cytochrome b559. In the absence of illumination, the PSII reaction center exchanged into dodecyl maltoside shows good thermal stability as compared with samples in Triton X-100. Only at a temperature of about 60 degrees C do spectral changes take place that are indicative of denaturation.

Buffers↗
PubMed1991

The link between fluid intake and weight gain in psychosis.

Diurnal weight gain was found to be abnormal among 44 of 77 institutionalized chronically psychotic patients. All patients were weighed and urine samples obtained weekly for 3 weeks at 7 A.M. and 4 P.M. Diurnal weight gain was normalized as a percentage by subtracting the 7 A.M. weight from the 4 P.M. weight, multiplying the difference by 100, and then dividing the result by the 7 A.M. weight. Normalized diurnal weight gain (NDWG) was 2.504 +/- 1.266% for the 44 study patients with abnormal findings, .631 +/- .405% for the 16 acutely psychotic controls, and .511 +/- .351% for 29 normals. Urine excretion was related (r = .476, p = .001) to NDWG in the subgroup of study patients with abnormal NDWG, consistent with the observation that their fluid intake exceeded fluid excretion in the afternoon.

Adult↗