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D Mauzerall

Publications and source records attributed to D Mauzerall.

At least 37 records · Page 2Linked to original sources
PubMed1986

Large transient nonproton ion movements in purple membrane suspensions are abolished by solubilization in Triton X-100.

Light-induced release/uptake of both protons and other ions cause transient changes in conductivity in suspensions of purple membrane (PM) fragments (Marinetti, Tim, and David Mauzerall, 1983, Proc. Natl. Acad. Sci. USA, 80:178-180). We find that the release/uptake of nonproton ions with quantum yield greater than 1 is observed at most pHs and ionic strengths. Only at both low pH and low ionic strength is the conductivity transient mostly due to protons. Our hypothesis is that during the photocycle, changes occur in the PM's dense surface charge distribution that result in changes in the number of counterions bound or condensed at the membrane surface. To test this, the PM structure was perturbed with the nonionic detergent Triton X-100. Immediately after addition, Triton does not abolish the nonproton ion movements; in fact at low detergent concentrations (0.02% vol/vol) the signal amplitudes increased considerably. However, when PM is completely solubilized into monomers in Triton, the conductivity transients are due to protons alone, though at lower quantum yield compared with native PM. These results suggest that changes in the surface charge distribution in native PM's photocycle could contribute to proton transfer between the aqueous phase and bR itself.

Bacteriorhodopsins↗
PubMed1986

Photoinitiated ion movements in bilayer membranes containing magnesium octaethylporphyrin.

A photocurrent produced by planar lipid bilayers containing Mg-octaethylporphyrin in the presence of oxygen has been investigated to determine if the current is due to movement of the MgOEP+ ion in the bilayer. Photoexcitation of the MgOEP is known to produce MgOEP+ in the bilayer when an electron acceptor is present. However, the aqueous electron acceptors ferricyanide and methyl viologen (MV+2) have opposite effects on the photocurrent. Ferricyanide decreases the photo current, even in the presence of oxygen, whereas methyl viologen increases the photocurrent, but only when oxygen is present. We attribute most of the photocurrent to the movement of superoxide anion. The difference in effect between ferricyanide and methyl viologen is attributed to the different rates of reduction of O2 by reduced MV+ (fast) vs. ferrocyanide (slow) and the known competition between ferricyanide and oxygen as the acceptor for the photoexcited porphyrin. It is inferred that most of the MgOEP is localized in the polar region of the lipid bilayer. Addition of ferrocyanide to the aqueous phase on one side of the bilayer, to trap MgOEP+ produced on the other side by MV+2, fails to increase the lifetime of the photovoltage. With a pH gradient across the bilayer, we observed only 5% of the photovoltage expected for the selective transport of H+ or OH- by MgOEP+. Thus, these measurements set the lower limit for the cross bilayer transit time of MgOEP+ or its charge in the range of 0.1-0.5 s.

Bacteriorhodopsins↗
PubMed1986

Dielectric properties of the polar head group region of zwitterionic lipid bilayers.

A theoretical model describing the dielectric properties of the lipid membrane-water interface region was developed. The rotating polar head groups (e.g. phosphatidylcholine) were simulated as a collection of interacting dipoles imbedded in a nonhomogeneous dielectric. The interactions between the nearest neighborhood were explicitly taken into account, while the other interactions were evaluated by means of the continuum theories. The values of the dielectric constant, its anisotropy and the spontaneous polarization of the interface were evaluated. As an application, we calculated the energy of interaction between an ion and the membrane polar head group region. The results indicate a small spontaneous polarization of the interface (1-1.7 Debyes per lipid molecule) due to the tilting angle of the choline residue with respect to the membrane surface. This dipolar field partially compensates that of opposite orientation originating from the ester group region, giving calculated overall dipolar potentials in better agreement with the experimental data. Our model suggests also a very strong dielectric anisotropy of the interface region, the component of the dielectric constant perpendicular to the membrane plane being much smaller than the parallel component.

Calorimetry↗
PubMed1986

Evidence for Cyclic Electron Flow around Photosystem II in Chlorella pyrenoidosa.

Electron flow around photosystem II was investigated in Chlorella pyrenoidosa. Using a bare platinum O(2) electrode, simultaneous measuremnts were made of steady-state photosynthesis in continuous light, the yield of oxygen (Y(o(2) )) produced by a superimposed saturating xenon flash, and the change in fluorescence yield of a weak flash triggered before and 70 microseconds after the saturating flash. Throughout most of the continuous photosynthesis-irradiance curve, normalized O(2) flash yields (Y(o(2) )/Y(o(2)max)) and normalized variable fluorescence yields (Deltaphi/Deltaphi') were linearly correlated with a slope of 1.0. As photosynthetic rates reached light saturation, however, the variable fluorescence yields remained relatively constant while O(2) flash yields decreased. These results strongly suggest that there is a cyclic electron flow around photosystem II in unpoisoned intact cells at light saturation and supraoptimal light intensities.

Journal Article↗
PubMed1985

Distributed kinetics of decay of the photovoltage at the lipid bilayer-water interface.

The decay kinetics of the photovoltage formed on pulsed illumination of a chlorophyll a- (chl a-) containing lecithin-bilayer adjacent to a ferricyanide solution on one side show characteristics of a system with distributed rate constants, i.e., the decay approaches linearity in log of time. The kinetics can be explained by a distribution of the chl cation over a few angstroms depth in the interfacial region of the bilayer and a rate constant exponentially dependent on distance as expected from tunneling theory. Addition of the donor ferrocyanide both increases the average rate and sharpens the distribution. There is a competitive inhibition by ferricyanide of the reaction of pigment cation with ferrocyanide. Removal of oxygen increases the rate of decay when an acceptor, methyl viologen or anthraquinone-2-sulfonate, forms oxygen-sensitive radicals. The cation charge does not cross the bilayer on a time scale of less than 0.01 s. These data define a reaction localized precisely in the finite interfacial region of the lipid bilayer-water interface.

Chlorophyll↗
PubMed1985

The effect of oxygen on the amplitude of photodriven electron transfer across the lipid bilayer-water interface.

The surprisingly small effect of oxygen on photoelectron transfer in pigmented lipid bilayers is traced to a short lifetime of the excited states. Decreasing the oxygen concentration by greater than 100-fold decreases the half saturating concentration of acceptor by only threefold and has no effect on the maximum photovoltage observed at acceptor saturation. This holds true for both magnesium octaethylporphyrin and chlorophyll with both ferricyanide and methyl viologen as acceptors. Since oxygen quenches excited states at near the encounter limit, the lifetime of reactive state must be short, less than 100 ns. About 100-fold higher concentrations of acceptor are required to quench the fluorescence (in liposomes) than to saturate the photoeffect. Thus the reactive state is most likely the triplet. The short life of the excited state is caused by concentration quenching, i.e., their reaction with ground state molecules. The increase of photovoltage with increasing pigment concentration shows that this quenching in a condensed form of the pigment produces ions that lead to the observed photovoltage by interfacial reaction of the anion with acceptor.

Half-Life↗
PubMed1983

Absolute quantum yields and proof of proton and nonproton transient release and uptake in photoexcited bacteriorhodopsin.

Using a sensitive differential ac conductance apparatus, we have measured transient ion movements in and the heating of bacteriorhodopsin suspensions after a light flash. The signal from the heating serves as an internal calibration of the absorbed photons and therefore the method gives the absolute quantum yield (phi) from a single measurement. At pH 4, H+ uptake precedes release, with phi = 0.4. By varying the buffer composition, we can prove that this signal is due to protons. At pH 8, however, the transient conductance increase is virtually independent of the buffer composition, showing that ions other than H+ are first released and then taken up by the purple membrane. If these ions are typical monovalent cations such as Na+ (lambda = 50 ohm-1 X cm2 X equiv-1), this process has a quantum yield of 2 or more at high salt concentrations.

Bacteriorhodopsins↗
PubMed1981

The potential span of photoredox reactions of porphyrins and chlorophyll at the lipid bilayer-water interface.

Lipid bilayers containing chlorophyll (Chl) or magnesium octaethylporphyrin (MgOEP) and separating solutions containing varying amounts of differing acceptors are illuminated by a dye laser pulse (FWHM 0.3 microseconds) at 590 mm. Interfacial charge transfer is measured at the first current peak in a voltage clamp circuit. The constants describing the hyperbolic saturations of the charge transferred by differing acceptors are only weakly related to the redox potential of the acceptors. An assymetric molecule, anthraquinone-2-sulfonate, is over 20 times as effective in accepting the electron as is the symmetrical anthraquinone-2,6-disulfonate. In contrast to this variable effectiveness, the maximum amount of charge transferred as a function of acceptor redox potential is constant up to a cut-off value: -0.6 V (vs. standard hydrogen electrode) for MgOEP and -0.5 V for Chl. The reversible redox potential of MgOEP in the bilayer was determined by following both the decrease in photoactivity and the transmembrane potential as a function of aqueous redox potential. It is +0.77 V for MgOEP and approximately 0.7 V for Chl (limited by stability). Thus, a total of 1.4 V of reversible redox potential (free energy) is obtained from 1.8 eV (internal energy) of the triplet excited state of MgOEP.

Chlorophyll↗
PubMed1981

Apparent inhibition of photoredox reactions of magnesium octaethylporphyrin at the lipid bilayer-water interface by neutral quinones.

Neutral quinones rapidly equilibrate across the lipid bilayer, hereby rendering the photoeffects seen in pigmented bilayers sensitive to the redox properties at both interfaces. The lack of photoeffect by quinones themselves and their apparent quenching reactions with aqueous acceptors is thus explained. An aqueous donor is needed on one side to break the symmetry and to allow vectorial electron transfer to be recorded. It is concluded that the neutral quinone accumulates on the polar side of the interface with respect to the hydrophobic pigment. The system may allow the study of kinetics of proton transfer accompanying the redox reactions of the quinones.

Electron Transport↗
PubMed1979

Diurnal variation in situ of photosynthetic capacity in ulva is caused by a dark reaction.

Ulva lactuca (sea lettuce) undergoes large diurnal oscillations of light-saturated photosynthetic O(2) evolution in situ. Freshly collected samples from Great Harbor, Woods Hole, Massachusetts, had a maximum white light-saturated rate at noon that was 2.5-fold higher than the rate of matched samples collected at midnight. When kept under constant low level illumination, the cycle persisted for at least 36 hours, and after 2 weeks damped out to a constant level that was halfway between the minimum and maximum rates. The cyclic oscillations were apparent whether expressed on a weight or chlorophyll content basis, occurred in both lightly and heavily pigmented samples, and were not attributable to changes in chloroplast shading due to variations in chloroplast orientation within the frond cells. There were no cyclic variations in the initial slopes of the light saturation curves, in photosynthetic unit size, or in relative quantum efficiency. Measurement of the "fast" turnover time of photosynthesis by the delayed dual flash technique revealed no diurnal variations of this parameter. These results indicate that the cyclic variations in photosynthetic activity are modulated by a dark reaction at a step occurring after reduction of plastoquinone by electrons from photosystem II.

Journal Article↗
PubMed1978

Photo-initiated ion formation from octaethyl-porphyrin and its zinc chelate as a model for electron transfer in reaction centers.

Ion formation from the reaction of triplet (T) and ground state (P) octaethyl-porphyrin (OEP) and zinc octaethyl porphyrin (ZnOEP) and the corresponding cross-reactions have been measured in dry acetonitrile. A uniquely sensitive and fast conductance apparatus and a pulsed dye laser allowed the measurements to be made at the necessarily very low concentrations of T. The hemogeneous reaction of T (ZnOEP) and P (ZnOEP) occurs with rat constant k(1) = 2.0 x 10(8) M(-1)s(-1) and an ion yield of 67%. The similar homogeneous reaction of OEP has k(2) = 1.3 x 10(8)M(-1)s(-1) but an ion yield of only 3%. The cross-reaction of T (OEP) with P (ZnOEP) has k(3) = 1.5 x 10(8) M(-1)s(-1) and an ion yield of 27%, while the inverse cross-reaction of T (ZnOEP) with P (OEP) has k(4) = 3 x 10(8) M(-1)s(-1) and an ion yield of 20%. Thus, the rate constants are only slightly affected but the yields are sensitive to the porphyrin. The possible formation of the heterogeneous ions ZnOEP+ + OEP-, thermodynamically favored by 0.3 V over the homogeneous ions, has little influence on the observed yields. The data are explained by electron transfer and Coulomb field-electon spin-controlled escape of the initial ion-pair.

Electric Conductivity↗
PubMed1976

Multiple excitations in photosynthetic systems.

The yield of fluorescence in Chlorella from a 7 ns pulse of light is found to decrease gradually as a function of the number of hits in the photosynthetic units. The fivefold decrease in yield is spread over some three orders of magnitude of pulse energy and strongly suggests another random process in addition to that of photon absorption. Evidence supports the view that this random process is not in the time but in the spatial domain. The model used to fit the data is that of a unit with multiple traps for the singlet excitation. An excitation is captured by an open trap or destroyed by a filled trap with equal probability. These studies give evidence for the connectivity of the photosynthetic energy transfer apparatus on the short time scale. The short fluorescence lifetimes following picosecond pulse excitation of photosynthetic systems reported by several laboratories may be explained by the effect of multiple excitations.

Chlorella↗
PubMed1974

Interfacial photoreactions and chemical capacitance in lipid bilayers.

The electrical response of a pigmented lipid bilayer to a short laser pulse is measured by a tunable voltage clamp method. In this method, a variable access impedance permits "tuning" of the observed relaxations for optimal measurements. Analysis of the data so obtained leads to an equivalent circuit that contains a novel chemical capacitance charged by the specific photoreaction across a single membrane-water interface. This chemical capacitance is distinct from the ordinary membrane capacitance. The intrinsic chemical rate constant obtained from the equivalent circuit analysis is shown to be the pseudo-first-order rate constant of the reverse dark reaction of the reduced acceptor and the oxidized pigment. The tunable voltage clamp method of measurement and analysis allows unambiguous separation of this rate constant into resistive and capacitative elements, which are interpreted in molecular terms.

Cell Membrane↗