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Biomedical subjects

H Scheer

Publications and source records attributed to H Scheer.

At least 37 records · Page 2Linked to original sources
PubMed1998

Triplet energy transfer in bacterial photosynthetic reaction centres.

[3-vinyl]-132-OH-bacteriochlorophyll a has been selectively exchanged against native bacteriochlorophyll a in the monomer binding sites at the A- and B-branch of the photosynthetic reaction centre from Rhodobacter sphaeroides. Transient absorption difference measurements were performed on these samples over a temperature range from 4.2 to 300 K with 20 ns time resolution. Specifically the decay of the primary donor triplet state, 3P870, as well as the rise and decay rates of the carotenoid triplet state, 3Car (spheroidene), were measured. The observed rates revealed a thermally activated carotenoid triplet formation corresponding to the decay of the primary donor triplet state. The activation energies for the triplet energy transfer process were 100(+/-10) cm-1 for reaction centers from wild-type Rhodobacter sphaeroides 2.4.1, with and without exchange of the monomeric bacteriochlorophyll on the electron transfer-active branch, BA. For reaction centers from Rhodobacter sphaeroides R26.1 with both monomers exchanged against [3-vinyl]-132-OH-bacteriochlorophyll a, and subsequent spheroidene reconstitution the activation energy was 460(+/-20) cm-1. These activation energies correspond to the energy difference between the triplet states of the accessory BChl monomer, BB, and the primary donor when native BChl a or [3-vinyl]-132-OH-BChl a is present in the BB binding site. In all samples the 3Car formation rates were bi-phasic over a large temperature range. A fast temperature-independent rate was observed on the wavelength of the carotenoid triplet-triplet absorption which dominated at very low temperatures. Additionally, a slower temperature-independent 3Car formation rate was observed at low temperatures which could be explained with the assumption of heterogeneity in the energy barrier (3BB) and/or the primary donor triplet state (3P870). A tunneling mechanism as proposed earlier by Kolaczkowski (PhD thesis, Brown University, 1989) is not only unnecessary but also incompatible with the available experimental data.

Journal Article↗
PubMed1998

Stabilization of photosystem II reaction centers: influence of bile salt detergents and low pH.

Rapid deterioration of samples is a major obstacle in research on the isolated reaction center of photosystem II. Its stability was tested systematically using a wide range of detergents, varying pH and temperature. Stability and activity did not depend on ionic properties of detergents or on critical micellar concentration. However, both were significantly increased by bile salt detergents in the dark as well as in the light. Low pH (5.5) and low temperature further improved stability. The results suggest that in particular the zwitterionic bile salt detergent, CHAPS, in pH 5.5 buffers is a very useful detergent and even superior to dodecylmaltoside for work with photosystem II reaction centers.

Bile Acids and Salts↗
PubMed1998

Reconstitution of the B800 bacteriochlorophylls in the peripheral light harvesting complex B800-850 of rhodobacter sphaeroides 2.4.1 with BChl a and modified (bacterio-)chlorophylls

A method is described for reversibly removing bacteriochlorophyll from the B800-site of the B850-850 antenna complex from Rhodobacter sphaeroides. This method uses the oligosaccharidic detergent Triton BG-10, together with an incubation at pH 5.0. Reconstitution at the B800-site has been successfully achieved for a range of modified bacteriochlorophylls. Copyright 1998 Elsevier Science B.V. All rights reserved.

Journal Article↗
PubMed1997

Z,E isomerization of the alpha-84 phycoviolobilin chromophore of phycoerythrocyanin from Mastigocladus laminosus investigated by Fourier-transform infrared difference spectroscopy.

The photoreaction of the phycoviolobilin (PVB) chromophore-containing part of phycoerythrocyanin (PEC) from Mastigocladus laminosus was investigated by Fourier-transform infrared spectroscopy (FT-IR). Difference spectra between the parent states P566 and P507 were obtained in 1H2O and 2H2O for the first time. The spectra are generally characterised by large changes in the range between 1710 and 1590 cm(-1) and by a strong difference band around 1270 cm(-1). In order to study the influence on the PVB chromophore upon aggregation, spectra of the alpha-subunit and the (alphabeta)3 trimer are compared, showing distinct differences which may be of relevance for the chromophore-protein and protein-protein interactions. The difference spectra demonstrate many similarities to the spectra recently obtained for the Pr --> meta-Rc transition of phytochrome [Foerstendorf et al. (1996) Biochemistry 35, 10793-10799]. In particular, a band around 1710 cm(-1), which was tentatively assigned to the C = O stretch of ring D is also observed in the spectra of PEC. It strongly supports this identification and the deduced molecular interpretation on the protonation state of the chromophore.

Cyanobacteria↗
PubMed1997

[Laparoscopic transperitoneal inguinal hernia repair (TAPP)--complications and results of a prospective study].

Results of a prospective study of 776 TAPP procedures performed on 600 patients are reported. We found rates of 0.5% for intraoperative complications and 1.3% for early-postoperative complications. Late postoperative complications have been observed in an incidence of 4.4% for haematomas, 7.9% for scrotal discomfort, 1.2% for hyperaesthesia and trocar hernias have occurred in 0.9%. The recurrence rate was 3.9%. Results suggest that improvement of the technique of laparoscopic hernia repair may prevent complications and may lower future complication rates.

Adolescent↗
PubMed1996

Probing native-like orientation of pigments in modified reaction centers from Rhodobacter sphaeroides R26 by linear dichroism.

Site-specific pigment modifications are useful to investigate structure-function relationships in photosynthesis. In reaction centers bearing modified (bacterio)pheophytins, changed electron transfer kinetics have been related to the changed redox potentials of the pigments introduced (Huber, H. et al. (1995) Chemical Physics, Special Issue, vol 197 (Hochstrasser, R.M. and Hofacker, G.L. eds.) pp. 297-305; [1]). In order to analyze potentially interfering structural changes induced in these reaction centers by the exchange procedure, in particular mispositioning or misorientation of the pigments, low-temperature linear dichroism spectra have been measured for reaction centers from Rhodobacter sphaeroides containing modified bacteriopheophytins and bacteriochlorophylls at the sites HA,B and BA,B, respectively. They show that all modified pigments are oriented similar to the native ones, and that they do not affect significantly the linear dichroism of the monomeric bacteriocholorophylls and bacteriopheophytins or of the primary donor.

Bacteriochlorophylls↗
PubMed1996

Fourier-transform infrared spectroscopy of phytochrome: difference spectra of the intermediates of the photoreactions.

The photocycle of 124 kDa phytochrome A from Avena sativa was studied by Fourier-transform infrared spectroscopy at low temperatures. Difference spectra between the parent state Pr and the intermediates of the Pr-->Pfr pathway, i.e. lumi-R, meta-Ra, and meta-Rc, and between Pfr and the intermediates of the Pfr-->Pr pathway, lumi-F and meta-F, were obtained in 1H2O and 2H2O for the first time. Each spectrum shows characteristic spectral features which allow a clear distinction between the different intermediates. A general feature is that greater changes occur with increasing temperature, i.e. at the later steps of the photoreactions. Nevertheless, the changes in the spectral regions of the protein (amide I and amide II) were found to be surprisingly small, excluding larger conformational changes of the protein. All spectra of the intermediates are characterized by a strong negative band around 1700 cm-1. This band is tentatively assigned to the C = O stretch of ring D of the chromophore. Since it is not observed in the difference spectra between the parent states, it is concluded that ring D is located in a similar molecular environment in Pr and Pfr. In the photoproducts lumi-R and lumi-F, this band undergoes an upshift to 1720 cm-1. The high frequencies suggest that the chromophore is protonated in these intermediates as well as in Pr and Pfr.

Avena↗
PubMed1996

Origin of the two carbonyl oxygens of bacteriochlorophyll a. Demonstration of two different pathways for the formation of ring E in Rhodobacter sphaeroides and Roseobacter denitrificans, and a common hydratase mechanism for 3-acetyl group formation.

A respiring culture of Rhodobacter sphaeroides, grown in the dark under defined aerobic conditions, produced cells capable of immediately commencing adaptation to photosynthetic growth on exposure to light and further reduction of oxygen tension. Adaptation was complete after 12 h and the bacteriochlorophyll a content increased 10-20-fold. This adaptation was performed in the presence of either H2(18)O or 18O2. The extracted bacteriochlorophyll a was examined by mass spectrometry to determine the origin of both the 3-acetyl adn 13(1)-oxo oxygen atoms: both were derived from water. The derivation of the 13(1)-oxo group from water in R. sphaeroides indicates that the formation of isocyclic ring E from the 13-propionic acid methylester side chain of Mg(2+)-protoporphyrin IX monomethylester is an anaerobic process involving a hydratase. This is very different to the situation in higher plants and green algae where the formation of isocyclic ring E is an aerobic process in which the 13(1)-oxo group is derived from molecular oxygen via an oxygenase. In contrast to adapting R. sphaeroides cells, the 13(1)-oxo group of bacteriochlorophyll a in growing cells of the obligate aerobic chemotrophic bacterium Roseobacter denitrificans, was labelled by 18O2 and is, therefore, derived from molecular oxygen like in higher plants and green algae; however, the 3-acetyl group was not labelled by 18O2. Thus, while the 13(1)-oxo group has different origins in R. sphaeroides and R. denitrificans, the 3-acetyl group arises in both bacteria by enzymic hydration of the vinyl group of a chlorophyll a derivative.

Bacteria↗
PubMed1996

Stereoselectivity of pigment exchange with 13(2)-hydroxylated tetrapyrroles in reaction centers of Rhodobacter sphaeroides R26.

Bacteriochlorophyll a and bacteriopheophytin a carry a stereochemically labile asymmetric carbon at position C13(2). The steric requirements of photosynthetic reaction centers from Rhodobacter sphaeroides R26 have been probed by exchange experiments with the respective epimeric 13(2)-hydroxylated pigments, in which epimerisation is blocked. (13(2)S)-13(2)-Hydroxy-bacteriochlorophyll a is accepted at both monomeric binding sites, BA,B, (13(2)S)-13(2)-hydroxy-bacteriopheophytin a exclusively at the inactive site HB. The orientation of the 13(2)-COOCH3 substituents in these pigments is the same as in the native (13(2)R)-bacteriochlorophylls and (13(2)R)-bacteriopheophytins. In no cases are the unnaturally configured 13(2)-hydroxylated (13(2)R)-epimers accepted, even if a large excess (> 95%) is offered. It is concluded that the three binding sites always require the 13(2)-COOCH3 group on the opposite side of the macrocycle (down) than the 17-propionic ester side chain (up).

Circular Dichroism↗
PubMed1996

Comparison of the structural requirements for bacteriochlorophyll binding in the core light-harvesting complexes of Rhodospirillum rubrum and Rhodospirillum sphaeroides using reconstitution methodology with bacteriochlorophyll analogs.

Bacteriochlorophyll (BChl) structural requirements for formation of the core light-harvesting complex (LH1) and its structural subunit complex were examined by reconstitution with BChl analogs and the alpha- and beta-polypeptides of Rhodospirillum rubrum and Rhodobacter sphaeroides. Comparable results were obtained with most of the BChl analogs and the polypeptides of each bacterium, indicating the conservation of BChl binding sites. These systems showed the following common requirements for formation of the subunit complex and LH1: (1) Mg or a metal of similar size and coordination chemistry (e.g., Zn, Cd, Ni), (2) a bacteriochlorin oxidation state of the macrocyclic ring, (3) a 13(2)-carbomethoxy group, and (4) an intact ring V. Some structural features were not as critically important. For example, the subunit complex and LH1 could be formed with both sets of polypeptides and BChl b, as well as with analogs containing either short (ethanol) or long (phytol) esterifying alcohols. Two derivatives were identified that behave differently with the two sets of polypeptides. The 3-acetyl group is required to form LH1 in both bacteria, although a subunit-type complex was readily formed with [3-vinyl]BChl a and the polypeptides of Rs. rubrum but formed only slightly under special conditions with polypeptides of Rb. sphaeroides. [13(2)-OH]BChl a(p) formed both subunit- and LH1-type complexes with the alpha- and beta-polypeptides of Rb. sphaeroides but not with those of Rs. rubrum. Thus, some subtle differences in the BChl binding sites exist in the LH1 complexes of these two bacteria.

Amino Acid Sequence↗
PubMed1996

Triplet state energy transfer between the primary donor and the carotenoid in Rhodobacter sphaeroides R-26.1 reaction centers exchanged with modified bacteriochlorophyll pigments and reconstituted with spheroidene.

The dynamics of triplet energy transfer between the primary donor and the carotenoid were measured on several photosynthetic bacterial reaction center preparations from Rhodobacter sphaeroides: (a) wild-type strain 2.4.1, (b) strain R-26.1, (c) strain R-26.1 exchanged with 13(2)-hydroxy-[Zn]-bacteriochlorophyll at the accessory bacteriochlorophyll (BChl) sites and reconstituted with spheroidene and (d) strain R-26.1 exchanged with [3-vinyl]-13(2)-hydroxy-bacteriochlorophyll at the accessory BChl sites and reconstituted with spheroidene. The rise and decay times of the primary donor and carotenoid triplet-triplet absorption signals were monitored in the visible wavelength region between 538 and 555 nm as a function of temperature from 4 to 300 K. For the samples containing carotenoids, all of the decay times correspond well to the previously observed times for spheroidene (5 +/- 2 microseconds). The rise times of the carotenoid triplets were found in all cases to be biexponential and comprised of a strongly temperature-dependent component and a temperature-independent component. From a comparison of the behavior of the carotenoid-containing samples with that from the reaction center of the carotenoidless mutant Rb. sphaeroides R-26.1, the temperature-independent component has been assigned to the buildup of the primary donor triplet state resulting from charge recombination in the reaction center. Arrhenius plots of the buildup of the carotenoid triplet states were used to determine the activation energies for triplet energy transfer from the primary donor to the carotenoid. A model for the process of triplet energy transfer that is consistent with the data suggests that the activation barrier is strongly dependent on the triplet state energy of the accessory BChl pigment, BChlB.

Bacteriochlorophylls↗
PubMed1995

The derivation of the oxygen atoms of the 13(1)-oxo and 3-acetyl groups of bacteriochlorophyll a from water in Rhodobacter sphaeroides cells adapting from respiratory to photosynthetic conditions: evidence for an anaerobic pathway for the formation of isocyclic ring E.

Using mass spectrometry, we have demonstrated 18O-labelling of both the 13(1)-oxo and 3-acetyl groups of newly-formed bacteriochlorophyll a synthesized by Rhodobacter sphaeroides cells during adaptation from respiratory to photosynthetic conditions in the presence of H218O. This derivation of the 13(1)-oxo group of bacteriochlorophyll a from water provides a stark contrast with that of chlorophylls in higher plants where ring E formation is an aerobic process in which the 13(1)-oxo group arises from molecular oxygen via an oxygenase activity. The formation of the 3-acetyl group of bacteriochlorophyll a, however, is consistent with the enzymic hydration of the 3-vinyl group of a derivative of chlorophyll a.

Anaerobiosis↗
PubMed1995

Femtosecond infrared spectroscopy of reaction centers from Rhodobacter sphaeroides between 1000 and 1800 cm-1.

Time-resolved pump-and-probe experiments of reaction centers of the purple bacterium Rhodobacter sphaeroides (R26) in the mid-IR region between 1000 and 1800 cm-1 are recorded with a time resolution of 300-400 fs. The difference spectra of the states P*, P+HA-, and P+QA- with respect to the ground state P predominantly reflect changes of the special pair. They show positive and negative bands due to changes of distinct vibrational modes superimposed on a broad background of enhanced absorption. A number of certain bands can be assigned to the special pair P, to the bacteriopheophytin HA, and to the quinone QA. The temporal evolution of the IR absorbance changes is well described by the time constants known from femtosecond spectroscopy of the electronic states. Differences occur only at very early times, which are indicative of fast vibrational relaxation with a time constant of a few hundred femtoseconds.

Kinetics↗
PubMed1994

The derivation of the formyl-group oxygen of chlorophyll b in higher plants from molecular oxygen. Achievement of high enrichment of the 7-formyl-group oxygen from 18O2 in greening maize leaves.

The mechanism of formation of the formyl group of chlorophyll b has long been obscure but, in this paper, the origin of the 7-formyl-group oxygen of chlorophyll b in higher plants was determined by greening etiolated maize leaves, excised from dark-grown plants, by illumination under white light in the presence of either H2(18)O or 18O2 and examining the newly synthesized chlorophylls by mass spectroscopy. To minimize the possible loss of 18O label from the 7-formyl substituent by reversible formation of chlorophyll b-7(1)-gem-diol (hydrate) with unlabelled water in the cell, the formyl group was reduced to a hydroxymethyl group during extraction with methanol containing NaBH4: chlorophyll a remained unchanged during this rapid reductive extraction process. Mass spectra of chlorophyll a and [7-hydroxymethyl]-chlorophyll b extracted from leaves greened in the presence of either H2(18)O or 18O2 revealed that 18O was incorporated only from molecular oxygen but into both chlorophylls: the mass spectra were consistent with molecular oxygen providing an oxygen atom not only for incorporation into the 7-formyl group of chlorophyll b but also for the well-documented incorporation into the 13(1)-oxo group of both chlorophylls a and b [see Walker, C. J., Mansfield, K. E., Smith, K. M. & Castelfranco, P. A. (1989) Biochem. J. 257, 599-602]. The incorporation of isotope led to as much as 77% enrichment of the 13(1)-oxo group of chlorophyll a: assuming identical incorporation into the 13(1) oxygen of chlorophyll b, then enrichment of the 7-formyl oxygen was as much as 93%. Isotope dilution by re-incorporation of photosynthetically produced oxygen from unlabelled water was negligible as shown by a greening experiment in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. The high enrichment using 18O2, and the absence of labelling by H2(18)O, unequivocally demonstrates that molecular oxygen is the sole precursor of the 7-formyl oxygen of chlorophyll b in higher plants and strongly suggests a single pathway for the formation of the chlorophyll b formyl group involving the participation of an oxygenase-type enzyme.

Chlorophyll↗
PubMed1993

The accessory bacteriochlorophyll: a real electron carrier in primary photosynthesis.

The primary electron transfer in reaction centers of Rhodobacter sphaeroides is studied by subpicosecond absorption spectroscopy with polarized light in the spectral range of 920-1040 nm. Here the bacteriochlorophyll anion radical has an absorption band while the other pigments of the reaction center have vanishing ground-state absorption. The transient absorption data exhibit a pronounced 0.9-ps kinetic component which shows a strong dichroism. Evaluation of the data yields an angle between the transition moments of the special pair and the species related with the 0.9-ps kinetic component of 26 +/- 8 degrees. This angle compares favorably with the value of 29 degrees expected for the reduced accessory bacteriochlorophyll. Extensive transient absorbance data are fully consistent with a stepwise electron transfer via the accessory bacteriochlorophyll.

Journal Article↗
PubMed1993

Bacteriochlorophyll aggregates in positively charged micelles.

Micellar complexes were prepared from bacteriochlorophyll a and bacteriopheophytin a with the cationic detergents, cetyltrimethyl ammonium bromide and cetylpyridinium chloride. These complexes have spectroscopic properties (absorption, circular dichroism) which are very different from the ones formed with non-ionic detergents like Triton X-100, and also with anionic detergents. Bacteriochlorophyll a forms two complexes: One is blue-shifted and has excitonically coupled Qy transitions. The second one is extremely red-shifted. The unusual properties are suggested to result from interactions of the positively charged head-group of the detergent with the tetrapyrrole.

Bacteriochlorophylls↗