Magnetic field dependence of radical-pair decay kinetics and molecular triplet quantum yield in quinone-depleted reaction centers

Radical-pair decay kinetics and molecular triplet quantum yields at various magnetic fields are reported for quinone-depleted reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides R26. The radical-pair decay is observed by picosecond absorption spectroscopy to be a single exponential to within the experimental uncertainty at all fields. The decay time increases from 13 ns at zero field to 17 ns at 1 kG, and decreases to 9 ns at 50 kG. The orientation averaged quantum yield of formation of the molecular triplet of the primary electron donor, ³P, drops to 47% of its zero-field value at 1 kG and rises to 126% at 50 kG. Combined analysis of these data gives a singlet radical-pair decay rate constant of 5 · 10⁷s^?1, a lower limit for the triplet radical-pair decay rate constant of 1 · 10⁸s^?1 and a lower limit for the quantum yield of radical-pair decay by the triplet channel of 38% at zero field. The upper limit of the quantum yield of ³P formation at zero field is measured to be 32%. In order to explain this apparent discrepancy, decay of the radical pair by the triplet channel must lead to some rapid ground state formation as well as some ³P formation. It is proposed that the triplet radical pair decays to a triplet charge-transfer state which is strongly coupled to the ground state by spin-orbit interactions. Several possibilities for this charge-transfer state are discussed.     

Proton-induced membrane fusion role of phospholipid composition and protein-mediated intermembrane contact

Glycolipid-phospholipid vesicles containing phosphatidate and phosphatidylethanolamine were found to undergo proton-induced fusion upon acidification of the suspending medium from pH 7.4 to pH 6.5 or lower, as determined by an assay for lipid intermixing based on fluorescence resonance energy transfer. Lectinmediated contact between the vesicles was required for fusion. Incorporation of phosphatidylcholine in the vesicles inhibited proton-induced fusion. Vesicles in which phosphatidate was replaced by phosphatidylserine underwent fusion only when pH was reduced below 4.5, while no significant fusion occured (pH ? 3.5) when the anionic phospholipid was phosphatidylinositol. It is suggested that partial protonation of the polar headgroup of phosphatidate and phosphatidylserine, respectively, causes a sufficient reduction in the polarity and hydration of the vesicle surface to trigger fusion at sites of intermembrane contact.     

Magnetic separation in biotechnology

New developments in magnetic labelling techniques for cells and microspheres have extended the useful range of magnetic separation, particularly high gradient magnetic separation, into biotechnical areas. The basic magnetic principles involved are reviewed and representative samples of labelling techniques and results drawn from the past three years are presented. Illustrative examples of large scale operation in other industries are also presented, demonstrating the potential of the biotechnological applications.     

Use of proton Nuclear Overhauser Effects for the determination of the conformations of amino acid residues in oligopeptides

The use of the Nuclear Overhauser Effect to determine backbone and side-chain conformations of oligopeptides is discussed. The distance between the H^α proton of a given residue and the amide proton of the following residue depends only on the dihedral angle ψ. A calibration curve is given for the determination of ψ from the Nuclear Overhauser Effect involving these protons. In amino acids with branched side chains, e.g., threonine, isoleucine, and valine, the Nuclear Overhauser Effect involving the H^β proton and the amide proton in either the same or the following residue gives limited information about both χ¹ and either or ψ. The Nuclear Overhauser Effect involving the H^α and H^γ protons in leucine gives information about χ¹ and χ².     

High resolution microanalysis for phosphorus in Golgi complex beads of insect fat body tissue by electron spectroscopic imaging

Golgi complex beads are 10 nm particles arranged in rings on the smooth forming face of the Golgi complex that stain specifically with bismuth in arthropod cells. In vitro experiments with biological molecules spotted on to cellulose acetate strips indicated that bismuth bound to the beads through phosphate groups. We could detect a weak phosphorus signal from the beads using a new technique called electron spectroscopic imaging that is capable of very high spatial resolution (0.3–0.5 nm) and sensitivity (50 atoms of phosphorus). Detection was not obscured by tissue staining with bismuth or uranyl acetate or by using an inorganic buffer (Na cacodylate). Localization of phosphorus was greatly improved by using colour-enhanced computer pictures of the electron spectroscopic images and quantitating the images. The results indicate that the phosphorus content of the beads is large enough to account for their bismuth reactivity.     

Paramagnetic probes in NMR and EPR studies of membrane enzymes

The applications of paramagnetic probes to problems of structure and mechanism are discussed from the point of view of the membrane enzymologist. Problems unique to membrane systems are discussed, and a variety of nuclear and paramagnetic probes are evaluated. Three membrane ATPase (kidney (Na+ + K+)-ATPase, Ca2+-ATPase from sarcoplasmic reticulum and Mg2+-ATPase from kidney) are used to describe the types of experiments which can be done, the information which can be obtained and the limitations involved. Nuclear relaxation studies employing 1H, 7Li+, 31P and 205Tl+ nuclei are described. The advantages and disadvantages of Mn2+, Gd3+ and Cr3+ as paramagnetic probes are discussed in terms of the three ATPases. The theory and interpretation of Mn2+ and Gd3+ EPR spectra are evaluated in studies with the (Na+ + K+)-ATPase and Ca2+-ATPase, respectively.     

The fluidity of chloroplast thylakoid membranes and their constituent lipids: A comparative study by ESR

Comparative measurements were made of the fluidity of chloroplast thylakoids, total membrane lipids and polar lipids utilizing the order parameter and motion of spin labels.No significant differences were found in the fluidity of membranes or total membrane lipids from a wild type and a mutant barley (Hordeum vulgare chlorina f₂ mutant) which lacks chlorophyll $\text{b}$ and a 25 000 dalton thylakoid polypeptide. Redistribution of intrinsic, exoplasmic face (EF) membrane particles by unstacking thylakoid membranes in low salt medium also had no effect on membrane fluidity. However, heating of isolated thylakoids decreased membrane fluidity.The fluidity of vesicles composed of membrane lipids is much greater than that of the corresponding membranes. Fluidity of the membranes, however, increased during greening indicating that the rigidity of the membranes, compared with that of total membrane lipids, is not caused by chlorophyll or its associated peptides. It is concluded that the restriction of motion in the acyl chains in the thylakoids is not caused by chlorophyll or the major intrinsic polypeptide but by some other protein components.