The involvement of the electrical double layer in the quenching of 9-aminoacridine fluorescence by negatively charged surfaces

9-AAA inhibits growth and induces apoptosis in human melanoma A375 and rat prostate adenocarcinoma AT-2 and Mat-LyLu cell lines but does not affect the growth and viability of normal fibroblasts

The present study found that, similarly to 5-fluorouracil, low concentrations (1-10 µM) of 9-aminoacridine (9-AAA) inhibited the growth of the two rat prostate cancer AT-2 and Mat-LyLu cell lines and the human melanoma A375 cell line. However, at the same concentrations, 9-AAA had no effect on the growth and apoptosis of normal human skin fibroblasts (HSFs). The differences between the cellular responses of the AT-2 and Mat-LyLu cell lines, which differ in malignancy, were found to be relatively small compared with the differences between normal HSFs and the cancer cell lines. Visible effects on the cell growth and survival of tumor cell lines were observed after 24-48 h of treatment with 9-AAA, and increased over time. The inhibition of cancer cell growth was found to be due to the gradually increasing number of cells dying by apoptosis, which was observed using two methods, direct counting and FlowSight analysis. Simultaneously, cell motile activity decreased to the same degree in cancer and normal cells within the first 8 h of incubation in the presence of 9-AAA. The results presented in the current study suggest that short-lasting tests for potential anticancer substances can be insufficient; which may result in cell type-dependent differences in the responses of cells to tested compounds that act with a delay being overlooked. The observed differences in responses between normal human fibroblasts and cancer cells to 9-AAA show the requirement for additional studies to be performed simultaneously on differently reacting cancer and normal cells, to determine the molecular mechanisms responsible for these differences.

The ionization properties of cardiolipin and its variants in model bilayers

The anionic phospholipid cardiolipin has an unusual dimeric structure with a two-phosphate headgroup and four acyl chains. Cardiolipin is present in energy-transducing membranes that maintain electrochemical gradients, including most bacterial plasma membranes and the mitochondrial inner membrane, where it mediates respiratory complex assembly and activation, among many other roles. Dysfunctional biogenesis of cardiolipin is implicated in the pathogenesis of several diseases including Barth syndrome. Because cardiolipin is a dominant anionic lipid in energy-conserving membranes, its headgroup is a major contributor to surface charge density and the bilayer electrostatic profile. However, the proton dissociation behavior of its headgroup remains controversial. In one model, the pKa values of the phosphates differ by several units and the headgroup exists as a monoanion at physiological pH. In another model, both phosphates ionize as strong acids with low pKa values and the headgroup exists in dianionic form at physiological pH. Using independent electrokinetic and spectroscopic approaches, coupled with analysis using Gouy-Chapman-Stern formalism, we have analyzed the ionization properties of cardiolipin within biologically relevant lipid bilayer model systems. We show that both phosphates of the cardiolipin headgroup show strong ionization behavior with low pKa values. Moreover, cardiolipin variants lacking structural features proposed to be required to maintain disparate pKa values--namely the secondary hydroxyl on the central glycerol or a full complement of four acyl chains--were shown to have ionization behavior identical to intact cardiolipin. Hence, these results indicate that within the physiological pH range, the cardiolipin headgroup is fully ionized as a dianion. We discuss the implications of these results with respect to the role of cardiolipin in defining membrane surface potential, activating respiratory complexes, and modulating membrane curvature.

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.

Changes of the compositional asymmetry of phospholipids associated to the increment in the membrane surface potential

The contribution of phosphatidylinositol (PI) and phosphatidylserine (PS) to the outer negative membrane surface potential was studied in normal, PS-rich and PI-rich yeast cells. Under carefully defined conditions; PS and PE were quantified by using the non-penetrating chemical probe trinitrobenzenesulfonic acid (TNBS) and the PI by degradation with a specific phospholipase C. An asymmetric distribution of phospholipids in the plasma membrane with more PS (80-90%), PI (70-85%) and PE (70-85%) in the inner leaflet was found. When compared to normal cells there were 3-times more PI and 2-times more PS in the outer leaflet of the PI-rich and PS-rich cells. These values are consistent with the two-times increased surface potential in these cells. Interestingly, the contribution of PI was around twice the contribution of PS to the surface potential in the cells studied. When compared to normal cells there was a two-times increased accessibility of PS to TNBS in the PI-rich cells and the accessibility of PI to phospholipase C was also increased two-times in the PS-rich cells, while the proportion of derivatized PE was similar in all cells. Taking into account that the amount of PI is similar in normal cells and PS-rich cells and the amount of PS is similar in PI-rich cells and normal cells, a charge driven transbilayer transport of acidic phospholipids can be proposed.

Proton-linked transport systems as sensors of changes in the membrane surface potential

The kinetic properties of proton linked transport systems and their relation to the membrane surface potential were studied in yeast cells. (1) The negative surface potential of cells rich in anionic phospholipids was found to be 2-times higher than that of control cells; in agreement with their 2-fold increase in the anionic/zwitterionic phospholipid ratio (A/Z). (2) At low external concentration of substrates (high-affinity systems), higher uptake activities were observed for the anions, glutamate, aspartate and phosphate; the zwitterion glycine and the cations lysine and arginine, in both phosphatidylserine and phosphatidylinositol rich cells when compared to control cells. (3) On the other hand, at high external concentration of substrates (low-affinity systems), lower uptake activities were observed for glutamate, aspartate, phosphate and glycine in the cells rich in anionic phospholipids. (4) A decrease in Km without significant alteration in Vmax was found in the high-affinity transport systems that can be explained by the increase in proton concentration at the interface caused by the enhancement in negative surface charge of the cells rich in anionic phospholipids. (5) The mechanisms of the high-affinity proton linked transport systems are compatible with a model which is necessarily ordered, protons before anions. The low-affinity transport systems, on the other hand, follow a random order of binding. The transport systems studied behave as sensors of the changes in surface potential. The reduction of the surface potential reversed the transport alterations with the following sequence: monovalent cations less than divalent cations less than cationic local anesthetics.

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.

Intrinsic and artifactual pH buffering in chloroplast thylakoids

After an increase in pH of the suspension medium, a pH gradient across the membrane of chloroplast thylakoids stored at pH greater than or equal to 6.5 is often maintained for several minutes. The intrinsic hydrogen ion buffering capacity of the thylakoid membranes between pH 6.5 and 8.5 is about 40 neq/mg chlorophyll, but can be artificially inflated by penetration of the external buffer into the thylakoid vesicle. A delta pH imposed across the thylakoid membrane by an acid/base transition cannot be estimated accurately by the fluorescent probe 9-aminoacridine, especially with osmotically shrunken thylakoids in which 9-aminoacridine appears to become bound or adsorbed to the membrane. This interaction may be related to the existence of the previously demonstrated special pool of slowly equilibrating, "sequestered" protons.

The influence of metal cations and pH on the heat sensitivity of photosynthetic oxygen evolution and chlorophyll fluorescence in spinach chloroplasts

The heat-sensitivity of photosynthetic oxygen evolution of thylakoids isolated from spinach increases by increasing the pH above neutral value. The temperature for inactivation (transition temperature) is lowered from about 45° C (pH 6.0-7.4) to 33°C (pH 8.5). Similar results are obtained with intact chloroplasts. At pH 7.0 the transition temperature of washed thylakoids decreases by lowering the salt concentration below 20 mM with monovalent cations (Li(+), Na(+), K(+)) and below 3-4 mM with divalent cations (Mg(2+), Ca(2+), Sr(2+)). Illumination decreases the heat-sensitivity of oxygen evolution in intact chloroplasts, but even increases the heat-sensitivity in uncoupled chloroplasts. In intact chloroplasts the transition temperature of the heat-induced rise in chlorophyll fluorescence yield (Fo; see Schreiber and Armond 1978) decreases from 44° C to 38° C when the pH of the suspending medium is increased from 6.5 to 8.5. At 20° C, Fo is almost insensitive to pH (6.0-8.5). At 40° C, however, Fo is constant between 6.0 and 7.0, but strongly increases by increasing the pH above neutral value. The results are discussed in terms of a close relation between electrostatic forces at the thylakoid membrane and thermal sensitivity of photosynthetic apparatus. It is suggested that the heat-sensitivity of the photosystem II complex partially depends on the ionization state of fixed groups having alkaline pK. The "packed volume" of thylakoids suspended in a low salt medium increases when the temperature is increased above 30° C (pH 7.0) and above 20° C (pH 8.0), respectively. This result suggests a heat-induced increase in surface charge density of the thylakoid membrane.

Binding and screening by cations and the effect on exogenous NAD(P)H oxidation in Neurospora crassa mitochondria

1. The uncoupled oxidation of exogenous NADH by mitochondria from Neurospora crassa has a pH optimum at 7.0. In the presence of EDTA (1 mM) the optimum is at pH 6.5; maximal inhibition (65%) occurs at pH 7.2. This is comparable to the results with higher plant mitochondria. 2. The corresponding pH optima for NADPH oxidation are 7.75 (control), 7.0 (+ EDTA) and 8.0 (effect of EDTA), respectively. NADPH oxidation is completely inhibited by EDTA at pH 8.0. These pH optima are all about 1 higher than observed in mitochondria from higher plants. 3. The inhibition of NADH oxidation by EDTA is shown to be due to the removal of Mg2+ bound to the mitochondrial membranes. 4. It is shown that 9-aminoacridine can be used to monitor the surface potential of the membranes of Neurospora mitochondria. 5. Cations stimulate NADH oxidation by Neurospora mitochondria in a manner consistent with the theory of the diffuse layer. Quantitatively, the results suggest that Neurospora mitochondria contain fewer charges per mass of protein than Jerusalem artichoke (Helianthus tuberosus) mitochondria but more than mitochondria from Arum maculatum spadices. 6. A good correlation is found between the effect of La3+ on the fluorescence of 9-aminoacridine in the presence of mitochondria and on the oxidation of NADH by the mitochondria. La3+ has different effects on mitochondria from Neurospora, Jerusalem artichoke tubers and Arum spadices. THe results indicate that the fluorescence of 9-aminoacridine can be used to monitor binding sites on biological membranes.

Further studies of the relationship between cation-induced chlorophyll fluorescence and thylakoid membrane stacking changes

Salt-induced changes in thylakoid stacking and chlorophyll fluorescence do not occur with granal membranes obtained by treatment of stacked thylakoids with digitonin. In contrast to normal untreated thylakoids, digitonin prepared granal membranes remain stacked under all ionic conditions and exhibit a constant high level of chlorophyll fluorescence. However, unstacking of these granal membranes is possible if they are pretreated with either acetic anhydride or linolenic acid. Trypsin treatment of the thylakoids inhibits the salt induced chlorophyll fluorescence and stacking changes but stacking of these treated membranes does occur when the pH is lowered, with the optimum being at about pH 4.5. This type of stacking is due to charge neutralization and does not require the presence of the 2000 dalton fragment of the polypeptide associated with the chlorophyll a/chlorophyll b light harvesting complex and known to be lost during treatment with trypsin (Mullet, J.E. and Arntzen, C.J. (1980) Biochim. Biophys. Acta 589, 100-117). Using the method of 9-aminoacridine fluorescence quenching it is argued that the surface charge density, on a chlorophyll basis, of unstacked thylakoid membranes is intermediate between digitonin derived granal and stromal membranes, with granal having the lowest value. The results are discussed in terms of the importance of surface negative charges in controlling salt induced chlorophyll fluorescence and thylakoid stacking changes. In particular, emphasis is placed on a model involving lateral diffusion of different types of chlorophyll protein complex within the thylakoid lipid matrix.

Reconstitution of photosynthetic energy conservation. II. Photophosphorylation in liposomes containing photosystem-I reaction center and chloroplast coupling-factor complex

Photophosphorylation has been reconstituted in a liposomal system containing reaction centers of photosystem I and coupling-factor complex, both highly purified from spinach chloroplasts. This energy-converting model system was put together by diluting the preparation of the coupling-factor complex with an aqueous suspension of proteolipid vesicles, preformed from photosystem-I reaction centers and soybean phospholipids by sonication. In the presence of reduced N-methyl-phenazonium methosulfate the system catalyzed photophosphorylation with rates up to 50 mumol ATP formed x mg chlorophyll-1 x h-1, which was sensitive to uncouplers and to N,N'-dicyclohexyl-carbodiimide. The properties of the system in comparison to chloroplasts is discussed.

Regulation of electron transport in photosystem-II fragments by magnesium ions

In Photosystem-II reaction-center particles (TSF-IIa) fractionated from spinach chloroplasts by Triton X-100 treatment, divalent cations appear to regulate electron-transport reactions. Oxidation of cytochrome b-559 after illumination of the particles was accelerated by the presence of Mg2+, whereas photoreduction of 2,6-dichlorophenolindophenol (DCIP) by diphenyl carbazide was inhibited, both at a half-effective concentration of Mg2+ of approx. 0.1 mM. The site of regulation was shown to be on the oxidizing side of Photosystem II, near P-680, based on the effects of actinin-light intensity and nature of the electron donors on DCIP photoreduction. Mg2+ was effective in quenching chlorophyll fluorescence in TSF-IIa particles, but the quenching was sensitive to the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea. In the reaction-center (core) complex of Photosystem II, where the light-harvesting chlorophyll-protein complex is absent, there seems to be no regulation by Mg2+ on excitation-energy distribution.

Salt-dependent changes of 9-aminoacridine fluorescence as a measure of charge densities of membrane surfaces

1. When negatively-charged membranes or particles are added to a solution containing 9-aminoacridine and only low concentrations of salts, fluorescence from the dye molecules is decreased. The quenching mechanism is a result of an increase in concentration of the positively charged dye molecule at the surface (Searle, G.F.W. and Barber, J. (1978) Biochim. Biophys. Acta 502, 309-320). 2. Fluorescence quenching is released on adding salts, the effectiveness being generally dependent on the valency of the action used: C3+ > C2+ > C+, in line with a decrease in the surface potential. 3. The differential effect of cations is analyzed according to the Gouy-Chapman theory to obtain estimates for sigma, the net charge per unit area on a number of different surfaces. 4. It was found that in some cases the estimated value of sigma was not constant for a particular membrane system, but increased with salt concentration. The variation was much diminished, though not eliminated, when more rigid surfaces were examined. 5. An alternative method based on the distribution of a divalent cation (methyl viologen) in the diffuse part of the double layer was also used to estimate the overall charge density. This technique gave values lower than those obtained from 9-aminoacridine fluorescence changes. 6. It is argued that 9-aminoacridine cations distribute near localized, charged areas of surfaces, and that the salt-dependent estimates of sigma partly reflect charge redistribution accompanying changes in electrostatic screening by cations. It appears that 9-aminoacridine is a convenient probe to monitor changes in the heterogeneity of charged membranes.

Further studies of the thylakoid membrane surface charges by particle electrophoresis

1. Above pH 4.3 the outer surface of thylakoid membranes isolated from pea chloroplasts is negatively charged but below this value it carries an excess of positive charge. 2. Previously the excess negative charge has been attributed to the carboxyl groups of glutamic and aspartic acid residues (Nakatani, H.Y., Barber, J. and Forrester, J.A. (1978), Biochim. Biophys. Acta 504, 215-225) and in this paper it is argued from experiments involving treatments with 1,2-cyclohexanedione that the positive charges are partly due to the guanidino group of arginine. 3. The electrophoretic mobility of granal (enriched in chlorophyll b and PS II activity) and stromal (enriched in PS I activity) lamellae isolated by the French Press technique were found to be the same. 4. Treatment of the pea thylakoids with trypsin or pronase, sufficient to inhibit the salt induced chlorophyll fluorescence changes, increased their electrophoretic mobility indicating that additional negative charges had been exposed at the surface. 5. Polylysine treatment also inhibited the salt induced chlorophyll fluorescence changes but unlike trypsin and pronase, decreased the net negative charge on the surface. 6. The isoelectric point defined as the pH which gave zero electrophoretic mobility (about 4.3) was independent of the nature of the cations in the suspending medium (monovalent vs. divalent).

Viability control and special properties of isolated rat hepatocytes

The need for quick viability tests is stressed. Aas these should achieve more than statically categorizing dead or non-dead cells, several procedures are suggested that picture the energetic state of the cells. The almost classical criterion of this category, namely stimulation of respiration by succinate, must be questioned on the basis of the present results. It is shown, that restricted respiration by succinate is not due to limited permeability of the plasma membrane, but to competition by endogenous substrates for uptake into mitochondria. Distribution equilibria for succinate appear to be according to (delta pH)2 with regard to cytoplasm. They are attained within 5-20 s or faster. Uptake is in part regulated by the surface charge density. Permeability changes caused by effectors of surface charge, such as amphiphilic ions, are examplified for succinate, chloride, phosphate, Na+, K+, and Ca2+. Such changes repeatedly also occur after pulses of BSP. They are counterregulated by the cell within a minute in a manner dependent on BSP concentration and the state of the cells. During the preincubation phase, that is the time of readaptation after transfer of cells from 0 degree C to higher temperature, a special labile state transiently occurs, where cyclic permeability changes for Ca2+, Na+, K+ can be caused by substrate addition, especially succinate, and/or ATP. The extent of these changes and their sequence again depend on the energetic state of the cells. In a probably narrow energetic window a sequence of cation movements reminding of that after depolarization of an excitable cell, is observed. Manipulation of the Na+/K+-ratio by variation of preincubation time and by ouabain shows that this is not simply the denominator for reversible calcium uptake. As the surface charge appears to reflect the energetic state, ANS fluorescence is applied to monitor the state of the plasma membrane, though difficulties arising from a slow ANS permeation are not yet solved.

9-Aminoacridine fluorescence changes as a measure of surface charge density of the thylakoid membrane

1. When suspended in a low cation-containing medium, chloroplast thylakoid membranes and carboxymethyl-cellulose particles quench the fluorescence from 9-aminoacridine (Searle, G.F.W. and Barber, J. (1978) Biochim. Biophys. Acta 502, 309--320). 2. Relief of this quenching is achieved by adding cations to the suspension medium with the order of effectiveness being C3+ greater than C2+ greater than C+, indicating that the fluorescence acts as an indicator of the surface electrical potential. 3. Using the Gouy-Chapman theory, the differential effect of divalent (methyl viologen) and monovalent (K+) cations has been used to calculate surface charge densities. 4. The calculations indicate that the surface charge density on the thylakoids significantly increases when cations are added to the low cation-containing medium. Under the same conditions the surface charge density of glutaraldehyde-fixed thylakoids and carboxymethyl-cellulose particles remained essentially constant. 5. It is argued that the 9-aminoacridine technique is able to probe localized areas on the membrane surface and that the variability of the surface charge density of untreated thylakoids may be due to redistribution of charges associated with membrane stacking as suggested by Barber and Chow (Barber, J. and Chow, W.S. (1979) FEBS Lett. 105, 5--10).

Spectrophotometric measurements of transmembrane potential and pH gradients in chromaffin granules

The electrical potential (delta psi) and proton gradient (alpha pH) across the membranes of isolated bovine chromaffin granules and ghosts were simultaneously and quantitatively measured by using the membrane-permeable dyes 3,3'dipropyl-2,2'thiadicarbocyanine (diS-C3-(5)) to measure delta psi and 9-aminoacridine or atebrin to measure delta pH. Increases or decreases in the delta psi across the granular membrane could be monitored by fluorescence or transmittance changes of diS-C3-(5). Calibration of the delta psi was achieved by utilization of the endogenous K+ and H+ gradients, and valinomycin or carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP), respectively, with the optical response of diS-C3-(5) varying linearly with the Nernst potential for H+ and K+ over the range -60 to +90 mV. The addition of chromaffin granules to a medium including 9-aminoacridine or atebrin resulted in a rapid quenching of the dye fluorescence, which could be reversed by agents known to cause collapse of pH gradients. From the magnitude of the quenching and the intragranular water space, it was possible to calculate the magnitude of the alpha pH across the chromaffin granule membrane. The time-course of the potential-dependent transmittance response of diS-C3-(5) and the delta pH-dependent fluorescence of the acridine dyes were studied simultaneously and quantitatively by using intact and ghost granules under a wide variety of experimental conditions. These results suggest that membrane-permeable dyes provide an accurate method for the kinetic measurement of delta pH and delta psi in an amine containing subcellular organelle.

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 interaction of an amphipathic fluorescence probe, 2-p-toluidinonaphthalene-6-sulphonate, with isolated chloroplasts

The amphipathic fluorescence probe, 2-p-toluidinonaphthalene-6-sulphonate has been used to investigate the surface electrical properties of chloroplast thylakoid membranes. The fluorescence yield of 2-p-toluidinonaphthalene-6-sulphonate in aqueous solution increases on addition of hypotonically shocked chloroplast, and the emission maximum shifts towards the blue to 440 nm, although the emission spectrum is somewhat distorted by chloroplast pigment absorption. The intensity of 2-p-toluidinonaphthalene-6-sulphonate fluorescence is further increased on adding salts to the membrane suspension, and changes of greater than 100% are routinely observed. Similar observations have also been made with soya bean phospholipid (azolectin) liposomes. The magnitude of the fluorescence increase is dependent on membrane concentration, being more pronounced at high surface area/suspending volume ratios. The effect of salt addition appears to be that of shielding the fixed negative charges on the membrane surface, thus increasing the fraction of 2-p-toluidinonaphthalene-6-sulphonate molecules at the surface, where the 2-p-toluidinonaphthalene-6-sulphonate has a higher fluorescence yield than in free aqueous solution. This concept is supported by the fact that the effectiveness of salts in increasing 2-p-toluidinonaphthalene-6-sulphonate fluorescence is as predicted by classical electrical double layer theory: governed mainly by the charge carried by the cation with an order of effectiveness C3+ greater than C2+ greater than C+, and not by the chemical nature of the cation or by the nature of its co-ion. It has been argued that the chlorophyll fluorescence yield, controlled by the cation composition of the suspending medium follows the total diffusible positive charge density at the thylakoid membrane surface (Barber, J., Mills, J. and Love, A. (1977) Febs. Lett. 74, 174--181). Although the cation induced 2-p-toluidinonaphthalene-6-sulphonate and chlorophyll fluorescence yield changes show similar characteristics, there are also distinct differences between the two phenomena particularly when cations are added to chloroplasts initially suspended in a virtually cation-free medium. Therefore it is concluded that although both 2-p-toluidinonaphthalene-6-sulphonate and chlorophyll fluorescence yields are governed by the electrical properties of the thylakoid membrane surface, the mechanism controlling their cation sensitivity is not the same.