The behavior of 9-aminoacridine as an indicator of transmembrane pH difference in liposomes of natural bacterial phospholipids

Photobase effect for just-in-time delivery in photocatalytic hydrogen generation

Carbon dots (CDs) are a promising nanomaterial for photocatalytic applications. However, the mechanism of the photocatalytic processes remains the subject of a debate due to the complex internal structure of the CDs, comprising crystalline and molecular units embedded in an amorphous matrix, rendering the analysis of the charge and energy transfer pathways between the constituent parts very challenging. Here we propose that the photobasic effect, that is the abstraction of a proton from water upon excitation by light, facilitates the photoexcited electron transfer to the proton. We show that the controlled inclusion in CDs of a model photobase, acridine, resembling the molecular moieties found in photocatalytically active CDs, strongly increases hydrogen generation. Ultrafast spectroscopy measurements reveal proton transfer within 30 ps of the excitation. This way, we use a model system to show that the photobasic effect may be contributing to the photocatalytic H₂ generation of carbon nanomaterials and suggest that it may be tuned to achieve further improvements. The study demonstrates the critical role of the understanding the dynamics of the CDs in the design of next generation photocatalysts.

Analysis of light-induced transmembrane ion gradients and membrane potential in Photosystem I proteoliposomes

Photosystem I (PSI) complexes can support a light-driven electrochemical gradient for protons, which is the driving force for energy-conserving reactions across biological membranes. In this work, a computational model that enables a quantitative description of the light-induced proton gradients across the membrane of PSI proteoliposomes is presented. Using a set of electrodiffusion equations, a compartmental model of a vesicle suspended in aqueous medium was studied. The light-mediated proton movement was modeled as a single proton pumping step with backpressure of the electric potential. The model fits determinations of pH obtained from PSI proteoliposomes illuminated in the presence of mediators of cyclic electron transport. The model also allows analysis of the proton gradients in relation to the transmembrane ion fluxes and electric potential. Sensitivity analysis enabled a determination of the parameters that have greater influence on steady-state levels and onset/decay rates of transmembrane pH and electric potential. This model could be used as a tool for optimizing PSI proteoliposomes for photo-electrochemical applications.

Subunit-specific mechanisms and proton sensitivity of NMDA receptor channel block

We have compared the potencies of structurally distinct channel blockers at recombinant NR1/NR2A, NR1/NR2B, NR1/NR2C and NR1/NR2D receptors. The IC50 values varied with stereochemistry and subunit composition, suggesting that it may be possible to design subunit-selective channel blockers. For dizocilpine (MK-801), the differential potency of MK-801 stereoisomers determined at recombinant NMDA receptors was confirmed at native receptors in vitro and in vivo. Since the proton sensor is tightly linked both structurally and functionally to channel gating, we examined whether blocking molecules that interact in the channel pore with the gating machinery can differentially sense protonation of the receptor. Blockers capable of remaining trapped in the pore during agonist unbinding showed the strongest dependence on extracellular pH, appearing more potent at acidic pH values that promote channel closure. Determination of pK(a) values for channel blockers suggests that the ionization of ketamine but not of other blockers can influence its pH-dependent potency. Kinetic modelling and single channel studies suggest that the pH-dependent block of NR1/NR2A by (-)MK-801 but not (+)MK-801 reflects an increase in the MK-801 association rate even though protons reduce channel open probability and thus MK-801 access to its binding site. Allosteric modulators that alter pH sensitivity alter the potency of MK-801, supporting the interpretation that the pH sensitivity of MK-801 binding reflects the changes at the proton sensor rather than a secondary effect of pH. These data suggest a tight coupling between the proton sensor and the ion channel gate as well as unique subunit-specific mechanisms of channel block.

A quantitative model for using acridine orange as a transmembrane pH gradient probe

Monitoring the acidification of the internal space of membrane vesicles by proton pumps can be achieved easily with optical probes. Transmembrane pH gradients cause a blue-shift in the absorbance spectrum and the quenching of the fluorescence of the cationic dye acridine orange. It has been postulated that these changes are caused by accumulation and aggregation of the dye inside the vesicles. We tested this hypothesis using liposomes with transmembrane concentration gradients of ammonium sulfate as model system. Fluorescence intensity of acridine orange solutions incubated with liposomes was affected by magnitude of the gradient, volume trapped by vesicles, and temperature. These experimental data were compared to a theoretical model describing the accumulation of acridine orange monomers in the vesicles according to the inside-to-outside ratio of proton concentrations, and the intravesicular formation of sandwich-like piles of acridine orange cations. This theoretical model predicted quantitatively the relationship between the transmembrane pH gradients and spectral changes of acridine orange. Therefore, adequate characterization of aggregation of dye in the lumen of biological vesicles provides the theoretical basis for using acridine orange as an optical probe to quantify transmembrane pH gradients.

Characterization of 9-aminoacridine interaction with chromatophore membranes and modelling of the probe response to artificially induced transmembrane delta pH values

We analyze the adsorption of the fluorescent monoamine 9-aminoacridine to the membrane phase of photosynthetic chromatophores, in the physiological interval of pH values ranging from 5.5 to 8.5 and at ionic strengths of 0.005 and 0.150 M. The interaction of the probe with the membrane phase is described with S-shaped isotherms of the Hill type and is modulated by electrostatic effects as modelled with the Gouy-Chapman-Boltzman theory. This description is consistent with different values of the surface change density of the chromatophore membranes decreasing from about 1.3 x 10(-3) to about 0.5 x 10(-3) e-/A2, on changing the pH from 8.5/7.5 to 6.5/5.5, respectively. Furthermore we show that, when the free concentrations of the probe in the inner and outer vesicle compartments are computed from the adsorbing isotherms at the proper pH values, the model considering the equilibrium distribution of the neutral monoamine following the onset of a delta pH is sufficient to describe the dependence of the artificially induced transmembrane delta pH values on the observed quenching of the probe fluorescence.

delta pH-induced fluorescence quenching of 9-aminoacridine in lipid vesicles is due to excimer formation at the membrane

The fluorescence of 9-aminoacridine (9-AA) is quenched in vesicular suspensions containing negatively charged lipid headgroups (e.g., phosphatidylserine) upon imposition of a transmembrane (inside acidic) pH-gradient. It is shown that this fluorescence loss is accompanied by the formation of 9-AA dimers that undergo a transition in the dimer excited state to a dimer-excimer state. This result has been obtained on the basis of the specific dimer fluorescence excitation and hypochromic absorbance spectra that are redshifted by maximally 275 cm-1 (4.4 nm) with respect to the corresponding monomer spectra, as well as by the detection of the characteristic broad excimer emission band, centered at 560 nm. The existence of the spectrally distinct dimer-excimer is further corroborated by fluorescence life-time measurements that indicate an increased lifetime of up to 24 ns for this complex as compared with the normal monomer fluorescence lifetime of 16 ns. The formation of this dimer-excimer complex from the monomers can be reversed completely and the original monomeric spectral properties restored after the abolishment of the electrochemical proton gradient. In addition to the delta pH-induced dimer redshift in absorbance and fluorescence excitation, a further small redshift in monomer absorbance, fluorescence excitation, and emission spectra is observed due solely to the presence of the negatively charged phospholipid headgroups.

The 'delta pH'-probe 9-aminoacridine: response time, binding behaviour and dimerization at the membrane

The fluorescence quenching of 9-aminoacridine (9-AA) after imposition of a transmembrane pH gradient (inside acidic) in liposomes has been investigated for a number of different lipid systems. The initial fluorescence decrease after a rapid pH jump, induced in the extravesicular medium by a stopped-flow mixing technique, was ascribed to a response of 9-AA to the imposed pH gradient and not to changes in the vesicular system itself. Time constants for this fluorescence quenching are in the range of several hundred milliseconds at 25 degrees C. Fluorescence recovery which should be correlated to the dissipation of the pH gradient occurs in the 100 s time range and is 10-30-times faster than the delta pH decay monitored with the entrapped hydrophilic pH-indicator dye pyranine. The quenching was severely hindered below the lipid phase transition of dipalmitoylphosphatidylglycerol. No delta pH-induced quenching was obtained in lipid vesicles containing only zwitterionic, net uncharged phosphatidylcholine headgroups. For the occurrence of quenching, the presence of negatively charged headgroups, i.e. phosphatidylglycerol or phosphatidylserine, was necessary. The extent of quenching, at a specific pH difference applied, had a cooperative dependency (Hill coefficient approximately 2) on the number of negative headgroups in the membrane and on the concentration of unquenched (unbound) 9-AA molecules. The concentration of quenched 9-AA molecules was furthermore proportional to the number of dimer-excimer complexes of 9-AA which are formed during the quenching process.

Calibration of the response of 9-amino acridine fluorescence to transmembrane pH differences in bacterial chromatophores

The spectral characteristics of absorption and fluorescence emission of 9-amino acridine are not altered by the interaction with bacterial chromatophores, except for the attenuation of both the absorption and emission following the formation of a protonic gradient. The lifetime of fluorescence of the dye is significantly affected in the presence of membranes, and even more following illumination. The shortening of the lifetime induced by light is reversible and prevented by nigericin and K+. The onset kinetics of the fluorescence quenching following the generation of an artificial transmembrane pH difference is temperature dependent, with an activation energy of 17 +/- 3 kcal/mol. The effect of pH on the rate constants is consistent with a model assuming that the diffusion of the unprotonated species is the limiting step in the quenching phenomenon. The response of 9-amino acridine to artificially imposed delta pH's has been utilized as a calibration method for the measurements of the light-induced protonic gradient. The apparent inner volume of chromatophores, evaluated from the extraplation of the response at delta pH = 0, was found to be much larger (15- to 40-fold) than the true osmotic volume, indicating that most of the dye is bound to the membrane when accumulated into the inner lumen.

Gradation of the magnitude of the electrochemical proton gradient in Mycoplasma cells

The results presented show that in Mycoplasma mycoides var. Capri, regulation of glucose uptake by its non-metabolizable analogue methyl alpha-D-glucoside, can be used to control intracellular ATP content. This in turn leads to a control of the rate of proton extrusion catalysed by the Mg2+-dependent ATPase (phi (cHxN)2C H+) and the respective amplitudes of the components of delta mu H+. When Mycoplasma cells are incubated with 10 mM methyl alpha-D-glucoside, the amplitude of phi (cHxN)2C H+, of the electrical potential delta psi and of the chemical gradient delta pH become continuous functions of external glucose concentration within the limits of the non-energized and fully energized states. Analysis of the relationships between graduated amplitudes of delta psi, delta pH and phi (cHxN) 2C H+ show that the primary form of energy stored by a delta mu H+ generator is the electrical potential.

Measurement of chloroplast internal protons with 9-aminoacridine. Probe binding, dark proton gradient, and salt effects

A defined ratio, gamma, of the total proton uptake to the concentration change of free internal H+ is observed for illuminated envelope-free chloroplasts (Haraux, F. and de Kouchkovsky, Y. (1979) Biochim. Biophys. Acta, 546, 455-471). Proton uptake is measured by the external pH shift, free internal H+ by 9-aminoacridine fluorescence quenching. Extension of this work leads to the following conclusions, which, in the case of 9-aminoacridine behaviour, should apply to any kind of diffusible protonizable delta pH probe: 1. The gamma constancy is preserved when the internal volume (Vi) is modulated by chlorophyll and osmolarity changes: thus, 9-aminoacridine behaves as expected from the delta pH distribution of an amine of high pK; previous doubts on this point are attributed to the lack of control of the external proton uptake. 2. With variable 9-aminoacridine concentration, however, some variation of gamma confirms the existence of slight light-induced probe-membrane interactions. 3. According to the diffuse layer theory, salts decrease the negative potential at the 'plane of closest approach' of the thylakoids, thereby releasing the excess 9-aminoacridine in this diffuse layer, which increases its fluorescence. Although of equal valency, NH4+ is more potent than K+, suggesting competition between amines for specific anionic binding sites. 4. Two categories of membrane modifications are induced by salts: in addition to the above-mentioned electrical effect, mono- and divalent cations at high concentration increase the chloroplast proton binding capacity. La3+ is only able to release the excess dye in the diffuse layer and leaves gamma unchanged. Therefore the probe-membrane interactions should have limited importance for steady-state delta pH measurement. 5. A Donnan-type dark pH difference, which could seriously bias these delta pH estimates, is found experimentally to be less than 2 (no significant gamma change when Vi varies) and even theoretically less than 1 (on the basis of the concentration of the non-diffusible internal protonizable groups). Similarly, the predictable errors of Vi and its possible light-induced variations must have a small effect on delta pH under present experimental conditions.

Measurement of net proton-hydroxyl permeability of large unilamellar liposomes with the fluorescent pH probe, 9-aminoacridine

The fluorescent probe 9-aminoacridine was used to measure the rate of decay of experimentally established pH gradients across liposome membranes. From the rate of decay, separate permeability coefficients for protons (PH) and hydroxyls (POH) were calculated and summed to yield the net proton-hydroxyl permeability (Pnet). The net permeability of protons and hydroxyls was found to be approximately 10(-4) cm/s, six orders of magnitude greater than that measured for sodium and pyrophosphate ions under similar conditions. This suggests that protons and/or hydroxyls cross lipid bilayers by a different mechanism than do other monovalent cations and anions. In addition, the measurements provide a standard for net proton-hydroxyl permeability in pure phospholipid bilayers for comparison with biological membranes.

Effect of ionophores on carrier-mediated electron translocation in ferricyanide-containing liposomes

Ferricyanide-containing liposomes were used as a system to compare the electron- and proton-translocating properties of six redox reagents commonly used as electron donors for biochemical systems. The effects of different ionophore combinations on the ferricyanide-reduction rate were generally consistent with the expected proton- and electron-translocating properties of the mediators. The transmembrane pH gradient produced by hydrogen carriers was demonstrated. Nigericin or valinomycin plus carbonyl cyanide p-trifluoromethoxyphenylhydrazone are capable of collapsing this gradient and of stimulating ferricyanide reduction mediated by this type of carrier. No pH gradient is produced with the electron carrier 1,1'-dibutylferrocene. In the presence of tetraphenylboron anion, which is needed for this carrier to act as an efficient mediator, addition of valinomycin alone is sufficient to obtain full stimulation of ferricyanide reduction. NNN'N'-Tetramethyl-p-phenylenediamine does not behave as a simple electron carrier. During NNN'N'-tetramethyl-p-phenylenediamine-mediated ferricyanide reduction protons are translocated across the membrane and accumulated in the vesicles. This is not due to the presence of demethylated impurities in the NNN'N'-tetramethyl-p-phenylenediamine sample, but may be the result of an accumulation of oxidation products other than the Wurster's Blue radical. These results suggest a reconsideration of studies on protonmotive forces across membranes where NNN'N'-tetramethyl-p-phenylenediamine is used as a mediator.

Quantitative estimation of the photosynthetic proton binding inside the thylakoids by correlating internal acidification to external alkalinisation and to oxygen evolution in chloroplasts

The external alkalinisation delta pHe, or the rate of oxygen evolution vO2, of a suspension of envelope-free chlorplasts was correlated with their internal acidification, estimated from the transmembrane delta pHei. Knowing the external buffer value, the concentration of the total protons moved Hi was calculated from the delta pHe, measured with a glass electrode ([Hi] was also obtained from vO2), and the free proton concentration [Hi+] was determined from delta pHei, measured with 9-aminoacridine. This gives a ratio gamma i = theta [Hi]/theta [Hi+], which is independent of the thylakoids internal volume. Within a large pHi range, scanned by varying the light intensity, gamma i was kept reasonably constant; it was hardly sensitive to pHi. This apparent invariability implies a continuous change of the internal buffer value beta i with pHi, since beta i/gamma i = -2.3.....10pHi, a relationship which inlcudes neither the total concentration of protonizable groups [Ai] nor pKi. As gamma i approximately Ki[Ai]/(Ki + [Hi+i]2, to keep gamma i constant when pHi drops, pKi and [Ai] must increase. This may be achieved by a progressive unmasking of anionic functions, initially inaccessible in the membrane. The relative slowness of this process may explain why gamma i calculated from the initial kinetics was sometimes smaller in high than in low light, where it always equalled that measured from the steady-state amplitude at all intensities. A small deficit of [Hi+] deduced from what could have been expected from delta pHe may reflect a limited binding of protons in the membrane itself, about 1 H+ for 30--130 chlorophylls (gamma i could be between 70 and 240, more frequently around 100); these numbers varied depending on the samples, but were constant for a given preparation.

The plastoquinone pool as possible hydrogen pump in photosynthesis

The function of the plastoquinone pool as a possible pump for vectorial hydrogen (H+ + e-) transport across the thylakoid membrane has been investigated in isolated spinach chloroplasts. Measurements of three different optical changes reflecting the redox reactions of the plastoquinone, the external H+ uptake and the internal H+ release led to the following conclusions: (1) A stoichiometric coupling of 1 : 1 : 1 between the external H+ uptake, the electron translocation through the plastoquinone pool and the internal H+ release (corrected for H+ release due to H2O oxidation) is valid (pHout = 8, excitation with repetitive flash groups). (2) The rate of electron release from the plastoquinone pool and the rate of proton release into the inner thylakoid space due to far-red illumination are identical over a range of a more than 10-fold variation. These results support the assumption that the protons taken up by the reduced plastoquinone pool are translocated together with the electrons through the pool from the outside to the inside of the membrane. Therefore, the plastoquinone pool might act as a pump for a vectorial hydrogen (H+ + e-) transport. The molecular mechanism is discussed. The differences between this hydrogen pump of chloroplasts and the proton pump of Halobacteria are outlined.