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D Mauzerall

Publications and source records attributed to D Mauzerall.

At least 19 recordsLinked to original sources
PubMed2001

Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers: a pulsed photoacoustic study of electron transfer in photosystem I reveals a similarity to bacterial reaction centers in both volume change and entropy.

The thermodynamic properties of electron transfer in biological systems are far less known in comparison with that of their kinetics. In this paper the enthalpy and entropy of electron transfer in the purified photosystem I trimer complexes from Synechocystis sp. PCC 6803 have been studied, using pulsed time-resolved photoacoustics on the 1 micros time scale. The volume contraction of reaction centers of photosystem I, which results directly from the light-induced charge separation forming P(700+F(A)/F(B-) from the excited-state P700*, is determined to be -26 +/- 2 A3. The enthalpy of the above electron-transfer reaction is found to be -0.39 +/- 0.1 eV. Photoacoustic estimation of the quantum yield of photochemistry in the purified photosystem I trimer complex showed it to be close to unity. Taking the free energy of the above reaction as the difference of their redox potentials in situ allows us to calculate an apparent entropy change (TDeltaS) of +0.35 +/- 0.1 eV. These values of DeltaV and TDeltaS are similar to those of bacterial reaction centers. The unexpected sign of entropy of electron transfer is tentatively assigned, as in the bacterial case, to the escape of counterions from the surface of the particles. The apparent entropy change of electron transfer in biological system is significant and cannot be neglected.

Chlorophyll↗
PubMed2001

Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers: volume change, enthalpy, and entropy of electron-transfer reactions in manganese-depleted photosystem II core complexes.

We have previously reported the thermodynamic data of electron transfer in photosystem I using pulsed time-resolved photoacoustics [Hou et al. (2001) Biochemistry 40, 7109-7116]. In the present work, using preparations of purified manganese-depleted photosystem II (PS II) core complexes from Synechocystis sp. PCC 6803, we have measured the DeltaV, DeltaH, and estimated TDeltaS of electron transfer on the time scale of 1 micros. At pH 6.0, the volume contraction of PS II was determined to be -9 +/- 1 A3. The thermal efficiency was found to be 52 +/- 5%, which corresponds to an enthalpy change of -0.9 +/- 0.1 eV for the formation of the state P680+Q(A-) from P680*. An unexpected volume expansion on pulse saturation of PS II was observed, which is reversible in the dark. At pH 9.0, the volume contraction, the thermal efficiency, and the enthalpy change were -3.4 +/- 0.5 A3, 37 +/- 7%, and -1.15 +/- 0.13 eV, respectively. The DeltaV of PS II, smaller than that of PS I and bacterial centers, is assigned to electrostriction and analyzed using the Drude-Nernst equation. To explain the small DeltaV for the formation of P680+Q(A-) or Y(Z*)Q(A-), we propose that fast proton transfer into a polar region is involved in this reaction. Taking the free energy of charge separation of PS II as the difference between the energy of the excited-state P680* and the difference in the redox potentials of the donor and acceptor, the apparent entropy change (TDeltaS) for charge separation of PS II is calculated to be negative, -0.1 +/- 0.1 eV at pH 6.0 (P680+Q(A-)) and -0.2 +/- 0.15 eV at pH 9.0 (Y(Z*)Q(A-)). The thermodynamic properties of electron transfer in PS II core reaction centers thus differ considerably from those of bacterial and PS I reaction centers, which have DeltaV of approximately -27 A3, DeltaH of approximately -0.4 eV, and TDeltaS of approximately +0.4 eV.

Cyanobacteria↗
PubMed2001

Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers: volume change, enthalpy, and entropy of electron-transfer reactions in the intact cells of the cyanobacterium Synechocystis PCC 6803.

The volume and enthalpy changes for charge transfer in the 0.1-10 micros time window in photosynthetic reaction centers of the intact cells of Synechocystis PCC 6803 were determined using pulsed, time-resolved photoacoustics. This required invention of a method to correct for the cell artifact at the temperature of maximum density of water caused by the heterogeneous system. Cells grown under either white or red light had different PS I/PS II molar ratios, approximately 3 and approximately 1.7, respectively, but invariable action spectra and effective antenna sizes of the photosystems. In both cultures, the photoacoustic measurements revealed that their thermodynamic parameters differed strongly in the spectral regions of predominant excitation of PS I (680 nm) and PS II (625 nm). On correcting for contribution of the two photosystems at these wavelengths, the volume change was determined to be -27 +/- 3 and -2 +/- 3 A3 for PS I and PS II, respectively. The energy storage on the approximately 1 micros time scale was estimated to be 80 +/- 15% and 45 +/- 10% per trap in PS I and PS II, respectively. These correspond to enthalpies of -0.33 +/- 0.2 and -1 +/- 0.2 eV for the assumed formation of ion radical pairs P700+F(AB-) and Y(Z*)P680Q(A-), respectively. Taking the free energy of the above reactions as the differences of their redox potentials in situ, apparent entropy changes were estimated to be +0.4 +/- 0.2 and -0.2 +/- 0.2 eV for PS I and PS II, respectively. These values are similar to that obtained in vitro for the purified reaction center complexes on the microsecond time scale [Hou et al. (2001) Biochemistry 40, 7109-7116, 7117-7125]. The constancy of these thermodynamic values over a 2-fold change of the ratio of PS I/PS II is support for this method of in vivo analysis. Our pulsed PA method can correct the "cell" or heterogeneous artifact and thus opens a new route for studying the thermodynamics of electron transfer in vivo.

Calorimetry↗
PubMed1996

A simple light-driven transmembrane proton pump.

Light-induced lipophilic porphyrin/aqueous acceptor charge separation across a single lipid-water interface can pump protons across the lipid bilayer when the hydrophobic weak acids, carbonylcyanide m-chlorophenylhydrazone and its p-trifluoromethoxyphenyl analogue, are present. These compounds act as proton carriers across lipid bilayers. In their symmetric presence across the bilayer, the positive currents and voltages produced by the photogeneration of porphyrin cations are replaced by larger negative currents and voltages. The maximum negative current and voltage occur at the pH of maximum dark conductance. The reversed larger current and voltage show a positive ionic charge transport in the same direction as the electron transfer. This transport can form an ion concentration gradient. The movement of protons is verified by an unusual D2O isotope effect that increases the negative ionic current by 2- to 3-fold. These effects suggest that an interfacial pK shift of the weak acid caused by the local electric field of photoformed porphyrin cations/acceptor anions functions as the driving force. The estimated pumping efficiency is 10-30%. Time-resolved results show that proton pumping across the bilayer occurs on the millisecond time scale, similar to that of biological pumps. This light-driven proteinless pump offers a simple model for a prebiological energy transducer.

Acids↗
PubMed1996

Evidence for ion chain mechanism of the nonlinear charge transport of hydrophobic ions across lipid bilayers.

The conductivity across a lipid bilayer by tetraphenylborate anion is increased 10-fold on the photoformation of lipophilic porphyrin cations. The cations alone have negligible conductivity. This nonlinear photogenerated increase of ion conductivity is termed the photogating effect. Substitution of H by Cl in the para position of tetraphenylborate leads to a 100-fold enhancement of conductivity, whereas the dark conductivities for this and other substituted borates are the same. Moreover, the halo-substituted borates show a large enhancement of conductivity in the low concentration range (10(-8) M), whereas that of tetraphenylborate is small and space charge is negligible. The enhanced ion conductivity has great structural sensitivity to the structure of the anion, the cation, and the lipid, whereas the partition coefficient of all the borates and the concentration of photoformed cations are only slightly affected. The photogated ion transport has a twofold larger activation energy than transport in the dark. Time-resolved photocurrents and voltages demonstrate that the translocation rate of the porphyrin cation is also enhanced 100-fold by the Cl-borate anion but only 10-fold by the H-borate anion. For these reasons the nonlinear gating effect cannot be explained by electrostatics alone, but requires an ion chain or ion aggregate mechanism. Kinetic modeling of the photoinduced current with a mixed cation-anion ion chain can fit the data well. The photogating effect allows the direct study of ion interactions within the bilayer.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1996

Charge transfer across a single lipid-water interface causes ion pumping across the bilayer.

The photoformation of magnesium-porphyrin cations (P+) at a single lipid bilayer-water interface can pump lipophilic borate anions completely across the lipid bilayer and causes an actual reversal of the photovoltage. The system consists of a lipid bilayer containing magnesium octaethylporphyrin, an aqueous or interfacial electron acceptor on one side, and chloro- or fluoro-substituted tetraphenylborate in both aqueous electrolyte solutions. With 1-micros pulsed illumination, an immediate positive photovoltage is observed, which decreases on the microsecond and millisecond time scales. On the time scale of seconds, as the P+ cation concentration decays in reverse electron transfer, the voltage swings negative to a value almost equal to its initial value and finally decays with a half-time (approximately 20 s) longer than the time constant of the system (approximately 5 s). Thus, an ion gradient across the membrane is formed, trapped by the nonlinear relation between ion mobility and ion concentration. Continuous light illumination confirms that negative charge moves in the direction opposite that of the initial photoinduced electron transfer. Steady-state measurements indicate an ion pumping efficiency of approximately 30%. This simple mechanism may be a progenitor of photobiological ion pumps.

Biophysical Phenomena↗
PubMed1996

Volume and enthalpy changes in the early steps of bacteriorhodopsin photocycle studied by time-resolved photoacoustics.

We have studied the photoinduced volume changes, energetics, and kinetics in the early steps of the bacteriorhodopsin (BR) photocycle with pulsed, time-resolved photoacoustics. Our data show that there are two volume changes. The fast volume change ( < or = 200 ns) is an expansion (2.5 +/- 0.3 A3/molecule) and is observed exclusively in the purple membrane (PM), vanishing in the 3-[(3-cholamidopropyl)-dimethylammonio] -1-propane-sulfonate-sulfonate-solubilized BR sample; the slow change (approximately 1 micros) is a volume contraction (-3.7 +/- 0.3 A3/molecule). The fast expansion is assigned to the restructuring of the aggregated BR in the PM, and the 1-micros contraction to the change in hydrogen bonding of water at Asp 212 (Kandori et al. 1995. J. Am. Chem. Soc. 117:2118-2119). The formation of the K intermediate releases most of the absorbed energy as heat, with delta Hk = -36 +/- 8 kJ/mol. The activation energy of the K --> L step is 49 +/- 6 kJ/mol, but the enthalpy change is small, -4 +/- 10 kJ/mol. On the time scale we studied, the primary photochemical kinetics, enthalpy, and volume changes are not affected by substituting the solvent D2O for H2O. Comparing data on monomeric and aggregated BR, we conclude that the functional unit for the photocycle is the BR monomer, because both the kinetics (rate constant and activation energy) and the enthalpy changes are independent of its aggregation state.

Acoustics↗
PubMed1994

Synthesis of amphipathic porphyrins and their photoinduced electron transfer reactions at the lipid bilayer-water interface.

A one flask synthesis of cis-substituted amphipathic porphyrins is reported. These porphyrins were used to study electrostatic effects on photoinduced electron transfer across the lipid bilayer-water interface. A neutral porphyrin undergoes only dynamic interfacial electron transfer reactions irrespective of charge of the acceptor, although ionic strength effects indicate a negative charge on the porphyrin donor species. A dianionic porphyrin forms an interfacial static complex with a dicationic electron acceptor, methyl viologen, at low ionic strength. The electron transfer rate within the complex is slow, 10(5) approximately 10(6) s-1, which is attributed to a near orthogonal orientation between the donor and the acceptor pi orbitals.

Electrochemistry↗
PubMed1993

Photoinduced electron transport across a lipid bilayer mediated by C70.

Electron transport across a membrane is central to photosynthesis, to mitochondrial respiration and to the design of molecular systems for solar energy conversion. Relatively few synthetic molecules, however, have been shown to facilitate transport of electrons across a lipid bilayer. We report here that C70 can act as both a photosensitizer for electron transfer from a donor molecule and a mediator for electron transport across a lipid bilayer membrane. The steady-state photocurrent density obtained from the C70-bilayer system is about 40 times higher, at comparable light intensities, than that of the carotene-porphyrinquinone system, previously the most efficient artificial system. The C70-bilayer system has a quantum yield of about 0.04, while the stability (tens of minutes) and turnover number (electrons transported per C70 before decay) of 10(3) are one to three orders of magnitude greater than those of other systems. We anticipate that other higher fullerenes may also provide the basis for efficient transmembrane electron-transport systems.

Electron Transport↗
PubMed1989

Photogating of ionic currents across a lipid bilayer.

Photoformation of metalloporphyrin cations in a lipid bilayer increases the ionic currents of negative and decreases those of positive hydrophobic ions. At low concentrations of the mobile hydrophobic ion, a 30% change in conductivity is observed that decreases with increasing concentration of positive tetraphenylphosphonium ion and increases drastically with increasing concentration of negative tetraphenylboride ion. In the region of saturated conductance of boride ion, the increase in conductivity is 3.6-fold. A 15-fold increase is observed with the protonophore carbonyl cyanide 3-chlorophenylhydrazone. In this case the net charge gated is 300 times greater than the photogenerated charge in the bilayer membrane. Thus there is a net gain in this organic field effect phototransistor. The gating can also be accomplished by continuous light or chemical oxidants. Photogating is explained as space charge effects inside the bilayer.

Indicators and Reagents↗
PubMed1989

Symplastic Transport of Carboxyfluorescein in Staminal Hairs of Setcreasea purpurea Is Diffusive and Includes Loss to the Vacuole.

The kinetics of symplastic transport in staminal hairs of Setcreasea purpurea was studied. The tip cell of a staminal hair was microinjected with carboxyfluorescein (CF) and the symplastic transport of this CF was videotaped and the digital data analyzed to produce kinetic curves. Using a finite difference equation for diffusion between cells and for loss of dye into the vacuole, kinetic curves were calculated and fitted to the observed data. These curves were matched with data from actual microinjection experiments by adjusting K (the coefficient of intercellular junction diffusion) and L (the coefficient of intracellular loss) until a minimum in the least squares difference between the curves was obtained. (a) Symplastic transport of CF was governed by diffusion through intercellular pores (plasmodesmata) and intracellular loss. Diffusion within the cell cytoplasm was never limiting. (b) Each cell and its plasmodesmata must be considered as its own diffusion system. Therefore, a diffusion coefficient cannot be calculated for an entire chain of cells. (c) The movement through plasmodesmata in either direction was the same since the data are fit by a diffusion equation. (d) Diffusion through the intercellular pores was estimated to be slower than diffusion through similar pores filled with water.

Journal Article↗
PubMed1989

Photoinduced electron transfer across lipid bilayers containing magnesium octaethylporphyrin.

Both photoinitiated (thermodynamically downhill) and photodriven (thermodynamically uphill) electron transfer reactions across lipid bilayers are sensitized by magnesium octaethyl porphyrin (MgOEP). It is shown that the reaction mechanism is via reduction of photoexcited MgOEP at the reducing (ascorbate) side of the bilayer and the charge carrier is likely the neutral protonated MgOEP anion. The MgOEP cation (or its neutral form) does not contribute to charge passage across the bilayer even though it is readily formed at the acceptor (ferricyanide or methyl viologen) side of the membrane. Photoelectric measurements on planar bilayers show that the time constant for reduction of excited MgOEP is about 10 microseconds with 10 mM ascorbate. The membrane transport of the mediator appears to be rate limiting when the reaction is photoinitiated and the interfacial reaction appears to be limiting when the reaction is photodriven. The quantum yield of the process is about 0.1 in the latter case and about 0.02 in the former. The former yield is increased to about 0.15 in the presence of a redox mediator, duroquinone. In these systems, the magnesium porphyrin is both sensitizer and trans membrane redox mediator.

Electron Transport↗
PubMed1988

Pulsed photoacoustic detection of flash-induced oxygen evolution from intact leaves and its oscillations.

Photoacoustic signals from intact leaves, produced upon excitation with single-turnover flashes, were shown to be dependent on their position in the flash sequence. Compared to the signal obtained from the first flash, all the others were time-shifted and had increased amplitudes. The signal from the third flash had the largest deviation, whereas that from the second flash deviated only minimally. The amplitude difference of the signals relative to that from the first flash was measured at a convenient time point (5 ms) and showed oscillations of period 4, similar to the O(2)-evolution pattern from algae. These oscillations were strongly damped, tending to a steady state from about the seventh flash on. The extra photoacoustic signal (relative to the first flash) was shown to be inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, heat treatment, or water infiltration. Its change with flash number, its saturation with increasing flash energy, and the above inhibition criteria indicate that it originates in pulsed O(2) evolution. The sound wave produced by the first flash, however, arose by a photothermal mechanism only, as shown by its linear dependence on the flash intensity and insensitivity to the above treatments. The above flash pattern demonstrates that the photocycle of the S states (i.e., positive charge accumulation before two water molecules can be oxidized in a concerted way to produce molecular oxygen) occurs in intact leaves. It proves the applicability of the photoacoustic method for mechanistic studies of O(2) evolution in leaves under physiological conditions. Water content of leaves is readily measured by this method.

Journal Article↗
PubMed1988

Channel-forming properties of cecropins and related model compounds incorporated into planar lipid membranes.

Cecropins, positively charged antibacterial peptides found in the cecropia moth, and synthetic peptide analogs form large time-variant and voltage-dependent ion channels in planar lipid membranes in the physiological range of concentration. Single-channel conductances of up to 2.5 nS (in 0.1 M NaCl) were observed, which suggests a channel diameter of 4 nm. Channels formed by the peptides cecropin AD and MP3 had a permeability ratio of Cl-/Na+ = 2:1 in 0.1 M NaCl. A comparative study of the three cecropins, cecropins A, B, and D, and of six synthetic analogs allowed determination of structural requirements for pore formation. Shorter amphipathic peptides did not form channels, although they adsorbed to the bilayer. A flexible segment between the N-terminal amphipathic region and the C-terminal more hydrophobic region of the peptide was required for the observation of a time-variant, voltage-dependent conductance. Cecropin AD was the most effective voltage-dependent pore-forming peptide and was also the most potent antibacterial peptide against several test organisms. A positive surface charge or cholesterol in the bilayer reduced the conductances caused by cecropin AD or MP3 by at least 5-fold. This behavior is consistent with the known insensitivity of eukaryotic cells to cecropins. Our observations suggest that the broad antibacterial activity of cecropins is due to formation of large pores in bacterial cell membranes.

Amino Acid Sequence↗
PubMed1988

Photoreduction of carbon dioxide by aqueous ferrous ion: An alternative to the strongly reducing atmosphere for the chemical origin of life.

We have shown that ferrous ion at neutral pH photoreduces water to hydrogen with a high quantum yield on excitation with near-ultraviolet light. This simple system also efficiently reduces carbon dioxide (bicarbonate ions) to formaldehyde. Overall, these reactions offer a solution to a dilemma confronting the standard or Oparin-Urey model of the origin of life. If carbon dioxide was the main form of carbon on the primitive earth, the ferrous photoreaction would have provided the reduced carbon necessary to form amino acids and other biogenetic molecules. We believe this system may have been the progenitor to the biological photosynthetic systems.

Journal Article↗
PubMed1988

Photophysical properties of Sn-porphyrins: potential clinical implications.

The photophysical properties of Sn-protoporphyrin and two of its synthetic analogues, Sn-mesoporphyrin and Sn-diiododeuteroporphyrin, were examined. All three compounds are potent competitive inhibitors of heme oxygenase, the rate-limiting enzyme in the catabolism of heme to bilirubin, and can suppress completely or diminish significantly experimentally induced or naturally occurring forms of jaundice in animals or man. The results of these studies show that all three compounds have long-lived triplet states which are quenched by molecular oxygen both in solution and when incorporated in liposomes. However, the addition of quenching groups such as iodine to the porphyrin macrocycle results in a marked (approximately 60%) decrease in the triplet yield and a threefold decrease in the triplet lifetime. The triplet yield was shown to be independent of the excitation wavelength, and as a result, the metalloporphyrins were extremely poor photosensitizers when excited in the spectral region commonly used in phototherapy. In the presence of serum albumin, the triplet state of Sn-protoporphyrin was not quenched by oxygen. These results indicate that Sn-porphyrins can be custom designed with considerably reduced photosensitizing properties for potential clinical use as inhibitors of bilirubin production.

Humans↗
PubMed1987

Kinetics of charge transfer at the lipid bilayer-water interface on the nanosecond time scale.

Advances in instrumentation allow electrical measurements across the planar lipid bilayer to be made with nanosecond time resolution. The electron transfer reaction between photoexcited magnesium octaethylporphyrin in the lipid to a variety of ionically charged acceptors in the water is found to be purely dynamic over a wide range of concentrations of acceptors and up to the time constant of the apparatus, 4 ns. The saturation of the amplitude of the photovoltage with increasing concentration of acceptor is caused by the finite lifetime of the excited state, not by formation of a static pigment-acceptor complex. The reactions are an excellent probe of the lipid-water interface over an extended time scale. No appreciable barrier to reaction exists at this interface beyond the 5-ns time. That is, any water or choline group structure may be evanescent on this time scale. Electrostatic interactions indicate that the acceptor molecules penetrate to the level of the phosphocholine groups with differing orientations. It will be possible to extend the time scale into the picosecond range by decreasing the response time and by deconvolutions.

Electric Conductivity↗