Fluorescence changes in isolated broken chloroplasts and the involvement of the electrical double layer

Role of Ions in the Regulation of Light-Harvesting

Regulation of photosynthetic light harvesting in the thylakoids is one of the major key factors affecting the efficiency of photosynthesis. Thylakoid membrane is negatively charged and influences both the structure and the function of the primarily photosynthetic reactions through its electrical double layer (EDL). Further, there is a heterogeneous organization of soluble ions (K+, Mg2+, Cl-) attached to the thylakoid membrane that, together with fixed charges (negatively charged amino acids, lipids), provides an electrical field. The EDL is affected by the valence of the ions and interferes with the regulation of "state transitions," protein interactions, and excitation energy "spillover" from Photosystem II to Photosystem I. These effects are reflected in changes in the intensity of chlorophyll a fluorescence, which is also a measure of photoprotective non-photochemical quenching (NPQ) of the excited state of chlorophyll a. A triggering of NPQ proceeds via lumen acidification that is coupled to the export of positive counter-ions (Mg2+, K+) to the stroma or/and negative ions (e.g., Cl-) into the lumen. The effect of protons and anions in the lumen and of the cations (Mg2+, K+) in the stroma are, thus, functionally tightly interconnected. In this review, we discuss the consequences of the model of EDL, proposed by Barber (1980b) Biochim Biophys Acta 594:253-308) in light of light-harvesting regulation. Further, we explain differences between electrostatic screening and neutralization, and we emphasize the opposite effect of monovalent (K+) and divalent (Mg2+) ions on light-harvesting and on "screening" of the negative charges on the thylakoid membrane; this effect needs to be incorporated in all future models of photosynthetic regulation by ion channels and transporters.

Calcium binding by spinach stromal proteins

Calcium binding to spinach (Spinacia oleracea L.) stromal proteins was examined by dual-wavelength spectrophotometry using the metallochromic indicator tetramethylmurexide. The data are consistent with the existence of at least two, probably independent, classes of binding sites. The total number of binding sites varied between 90-155 nmol·mg(-1) protein with "average" binding constants of 1.1-2.7·mM(-1). Both Mg(2+) and La(3+) inhibited calcium binding competitively, with "average" inhibitor constants of 0.26·mM(-1) and 39.4·mM(-1), respectively; an increase in the potassium concentration up to 50 mM had no effect. In a typical experiment a decrease in pH (7.8 to 7.1) resulted in a decrease in the total number of calcium binding sites from 90 to 59 nmol·mg(-1) protein, but in an increase of the "average" affinity from 2.7 to 4.5·mM(-1). Calculations, using these data and those of Gross and Hess (1974, Biochim. Biophys. Acta 339, 334-346) for binding site I of washed thylakoid membranes, showed that the free-Ca(2+) concentration in the stroma under dark conditions, pH 7.1, is higher than under light conditions, pH 7.8. The physiological relevance of the observed calcium binding by stromal proteins is discussed.

The use of fluorescein-dipalmitoylphosphatidylethanolamine for measuring pH-changes in the internal compartment of phospholipid vesicles

The synthesis and characterisation of fluorescein-phosphatidylethanolamine (FPE) is described. The effects of dielectric constant, ionic strength and ambient pH upon the optical absorbance properties of FPE are presented. It is shown that under appropriate conditions, FPE rapidly and quantitatively reports the pH of the aqueous bulk phases when incorporated into phospholipid vesicles. It is also shown that, when the external medium is highly buffered, FPE is capable of specificity reporting only the pH of the intravesicular compartment. The application of FPE for studies of intravesicular pH changes of reconstituted membranous protein systems is discussed.

Surface-charge related actions of polylysine on thylakoid membranes

Polycation binding to the negatively charged surface of chloroplast thylakoid membranes is known to cause an inhibition of photosystem I activity. It also interferes with the cation-dependent rearrangement of chlorophyll proteins in the thylakoid membrane. It was shown that added anions prevented or reversed the inhibition of photosystem I by polylysine without decreasing its binding to the membranes. Anions also caused a change in the interaction of the chlorophyll proteins in polylysine-treated thylakoids as indicated by an increase in the relative fluorescence intensity from photosystem II. In both cases, the relative effectiveness of the anions tested depended on their valence; for example, the tetravalent species Fe(CN)4-(6) was effective at concentration at least 2 orders of magnitude lower than the divalent species SO2-(4). These results suggest that anions act by screening the positive charge of the polylysine-coated membrane surface. Measurements of the response of the anionic fluorescent probe 1-anilinonapthalene-8-sulfonate to an addition of anions to polylysine-treated thylakoids supported this contention. It was concluded that the action of polylysine on photosystem I and on the chlorophyll proteins is mediated by changes of the electrical properties of the thylakoid membrane and may not involve a direct binding of the polycation to the affected membrane proteins.

Effect of cations on the linear dichroism and selective polarized light scattering of thylakoids

The effect of cations on the linear dichroism (LD) and selective polarized light scattering of higher plant thylakoids was investigated. The results show that the major change in LD signal caused by the addition of cations is due to a scattering contribution most probably resulting from thylakoid stacking. However, minor changes in the LD signal also occur on the short wavelength side of the main LD band that persist even when a large proportion of the scattering change is eliminated by increasing the refractive index of the medium. The minor changes appear to be correlated with the cation-induced increase in variable fluorescence and resolution of the spectra at 77 K reveals that the changes in dichroism are due to LD bands of pigments associated with the light-harvesting complex.

Experimental and theoretical considerations of mechanisms controlling cation effects on thylakoid membrane stacking and chlorophyll fluorescence

The roles of specific cation binding, charge neutralization and electrostatic screening mechanisms in controlling salt-induced stacking and chlorophyll fluorescence changes in thylakoid membranes are examined in the light of new experimental evidence and theoretical calculations of the forces between membrane surfaces. A comparison of the biphasic stacking and fluorescence phenomena generated by organic mono- and divalent cations known sterically to inhibit specific binding with the effects generated by inorganic mono- and divalent cations suggests that the observed salt-induced changes at pH greater than or equal to 7.5 are predominantly governed by the electrostatic screening mechanism in agreement with previous work (e.g. Barber, J., Mills, J.D. and Love, A. (1977) FEBS Lett. 74, 174-181). Detailed calculations of the coulombic double layer repulsive force between negatively charged membrane surfaces immersed in a mixed electrolyte of valence type Z1+/Z1-,Z2+/Z1- were performed both under the constraints of fixed surface charged density and fixed surface potential. From a close comparison of the theoretical results with new experimental data on salt-induced stacking and fluorescence changes and a consideration of the contributions of the 'hydration' repulsive force and the van der Waals attractive force, it is argued that a reduction in surface charge density alone by lateral diffusion is probably insufficient to realize membrane stacking and that an increase in the van der Waals attractive force is necessary to account for the experimental observations perhaps through the formation of protein rich domains. In view of the complexity of the thylakoid membranes, the conclusions are to be considered qualitative. Nevertheless, these calculations give support to a model in which the cation induced chlorophyll fluorescence and stacking changes can be explained by lateral diffusion of two types of pigment protein complexes in the lipid matrix of the membrane. Such diffusion gives rise to changes in energy transfer between Photosystem II and Photosystem I and also to the creation of domains having low and high electrical surface charge density.

Effects of bulk pH and of monovalent and divalent cations on chlorophyll a fluorescence and electron transport in pea thylakoids

Millimolar concentrations of monovalent cations enhance and divalent cations impede the redistribution (spill-over) of electronic excitation energy from Photosystem (PS) II to PS I in cation-depleted (sucrose-washed) thylakoids; this concept is based on chlorophyll a fluorescence and electron transport measurements over a narrow pH range around 7. We have tested the above concept in pea thylakoids over the pH range 5 to 9 by parallel measurements of various chlorophyll a fluorescence parameters (spectra, transients, and lifetimes at 77 K and 293 K, and polarization at 293 K) and of the rates of partial reactions of PSI and II. Our results provide the following information. (1) Mg2+ enhancement of fluorescence is maximum between 680 and 690 nm and minimum between 710 and 720 nm. (2) The optimum conditions for the observation of the Mg2+-induced enhancement of fluorescence are: wavelength of emission, 685 nm; concentratin of Mg2+, 10 mM, and pH, approximately 7.5. (3) Mg2+ decreases the efficiency of excitation redistribution from PS II to PS I over the pH range 6 to 9. (4) The antagonistic effects between Na+ and Mg2+ hold simultaneously for both the fluorescence intensity and lifetime, at physiological temperatures, only within the pH range 6 to 8. (5) Mg2+ enhances the light-limited electron transport rate through PS II in the pH range 5.4 to 8.2 and decreases that through PS I at pH 7.1 and 8.2. The % increase in PS II is, however, about twice the % decrease in PS I.

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.

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.

The role of membrane surface charge in the control of photosynthetic processes and the involvement of electrostatic screening

Calculations of changes of the integrated space charge density within the diffuse layer adjacent to a negatively charged membrane surface have been made using analytical expressions derived from the full non-linear Poisson-Boltzmann equation of the Gouy-Chapman theory. This electrostatic screening parameter has been examined for mixed electrolytes of valency type Z1+/Z1- and Z2+/Z1- and concentration ranges were chosen so as to compare with experimental data obtained with thylakoid membranes. The results of the analysis are consistent with previous arguments (Barber, J., Mills, J.D. and Love, A. (1977) FEBS Letts. 74, 174-181) that this screening parameter is involved in the control of salt induced chlorophyll fluorescence and thylakoid stacking changes. Phenomenological equations suggesting the origin of the variations in the integrated space charge density for various salt conditions are presented. Overall the integrated space charge density (sigma chi) is shown to be a more satisfactory measure of both short and long range effects associated with electrostatic screening and double layer repulsion of charged surfaces than the planar space charge density (rho chi).

Electrostatic control of chloroplast coupling factor binding to thylakoid membranes as indicated by cation effects of electron transport and reconstitution of photophosphorylation

1. Increase in electron transport rate and the decay rate of the 518 nm absorption change, induced by EDTA treatment, is prevented by cations. The order of effectiveness is C3+ > C2+ > C+. 2. In this respect methyl viologen is an effective divalent cation in addition to its action as an electron acceptor. 3. Complete cation irreversible EDTA-induced uncoupling occurs in the dark in 2 min. Light greatly stimulates the rate of uncoupling by EDTA. It is concluded that the uncoupling is due to release of coupling factor I from the thylakoid membrane. 4. Binding of purified coupling factor I to coupling factor I-depleted thylakoids can be achieved with any cation. The order of effectiveness is C3+ > C2+ > C+, reconstituted thylakoids are active in photophosphorylation regardless of the cation used for coupling factor I binding. 5. The marked difference in the concentration requirements for cation effects on 9-aminoacridine fluorescence yield and for prevention of uncoupling by EDTA indicate that coupling factor I and its binding site have a lower surface charge density than the net surface charge density of the thylakoid membrane. 6. It is concluded that coupling factor I binding only occurs when negative charges on coupling factor I and its binding site are electrostatically screened by cations. 7. Previously reported examples of uncoupling by low ionic conditions are discussed in relation to the basic concepts of diffuse electrical layer theory.

Reversible uncoupling of energy transfer between phycobilins and chlorophyll in Anacystis nidulans: light stimulation of cold-induced phycobilisome detachment

Phycobilin fluorescence of Anacystis nidulans grown at 28 degrees C increases substantially upon cooling below 10 degrees C. A maximal increase is found around -5 degrees C and amounts to 300%, with almost complete reversibility upon re-warming. Illumination with actinic light leads to considerable stimulation of the cold-induced phycobilin fluorescence increase. Analysis of the light stimulation phenomenon reveals: (1) Actinic illumination shifts the fluorescence-temperature characteristic by about 3 degrees C upwards on the T-axis. At temperatures below 5 degrees C the light stimulating effect becomes smaller again and fluorescence-temperature characteristics measured at high and low light intensity converge around -5 degrees C. (2) In the 13-8 degrees C region a large (up to 100%) light-induced phycobilin fluorescence increase is observed, while only negligible changes occur in the dark. (3) 3-(3,4-Dichlorophenyl)-1,1-dimethyl urea (DCMU) as well as uncouplers inhibit the light stimulation, which hence depends on coupled electron transport. In agreement with previous work (Schreiber, U. (1979) FEBS Lett. 107, 4-9) it is concluded that illumination enhances cold-induced phycobilisome detachment by increasing the net negative charge at the outer surface of the thylakoid membrane. The possible role of a fluid leads to ordered transition of membrane lipids (Murata, N. and Fork, D.C. (1975) Plant Physiol. 56, 791-796) is discussed.

The relationship between thylakoid stacking and salt induced chlorophyll fluorescence changes

Salt induced chlorophyll fluorescence increase and thylakoid stacking have been measured under various conditions. 1. Aging of pea chloroplasts led to a loss of salt induced chlorophyll fluorescence increase and thylakoid stacking which is suggested to be due to a decrease in membrane fluidity as measured by 1,6-diphenylhextriene fluorescence polarization. 2. The aging treatment was accompanied by a decreased in surface charge density as indicated by chloroplast electrophoretic mobility measurements. 3. Lowering of the temperature to about 0 degrees C retarded the time courses of salt induced stacking and chlorophyll fluorescence increase. 4. Like aging, addition of linolenic acid led to an inhibition of the salt induced fluorescence and stacking phenomena but in this case there was a concomitant increase in electrophoretic mobility without any detectable change in the polarization of 1,6-diphenylhextriene fluorescence. 5. Maximum stacking occurred in both aged and fresh chloroplasts in a low salt medium at about pH 4.3 and the time course for the pH induced process was rapid and relatively temperature insensitive when compared with salt induced stacking. 6. The chlorophyll a/chlorophyll b ratio was lower for salt induced 'grana' than for pH induced 'grana'. 7. The results are discussed in terms of the hypothesis that changes in the lateral interaction of membrane pigment-protein complexes underlie the salt induced chlorophyll fluorescence increase and thylakoid stacking. It is argued that electrostatic screening by cations leads to the formation of domains of low-charge, fluorescent pigment-protein complexes, seggregated from domains of high-charge, quenching complexes, resulting in a increase in chlorophyll fluorescence yield and stacking at low-charge regions on adjacent membranes. In contrast to this, it is argued that the pH induced stacking occurs because of electrostatic neutralization, a mechanism which would not be expected to induce domain formation and associated chlorophyll fluorescence changes.

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).

Correlated influence of cation concentration and excitation intensity on PS II activity-I. Influence of high salt concentration on spinach chloroplast activity

A correlated influence of cation concentration and excitation energy level on PS II Abbreviations: AMPD = 2-amino-2-methyl-1,3-propanediol; DCIP = 2-6 dichlorophenol indophenol; DCIPH2 = DCIP in reduced state; DCIPr = rate of DCIP-reduction; DCMU = 3-(3,4 dichlorophenyl)1-1 dimethyl-urea; Fv = variable fluorescence; Fcat = Fv in the presence of added cations; Ft = Fv without added cations; HEPES = N-2-hydroxyethylpiperazine N\t'2-sulfonic acid; PS II = photosystem II; Vcat = DCIPr in presence of added cations; Vt = DCIPr without added cations. activity is demonstrated.In low light conditions (under 60 Wm(-2)) Mg(++) effect on DCIP reduction rate (DCIPr) saturates at rather low concentrations (2-10 mM). Higher concentrations induce a quenching of the effect, as already observed by several authors. In high light conditions (1000 Wm(-2)) however, Mg(++) is increasingly effective on DCIPr up to concentrations of 200 mM.Na(+) induced variations of DCIPr are weak in low light conditions and slightly positive for 100-600 mM in strong light; no quenching occurs.Modifications in variable fluorescence do not follow those of DCIPr in all cases, especially in high light.These results allow us to distinguish three different effects of cations on the photochemistry of PS II: one on the spill-over, another on the turnover rate of the centers and the last on the cation exchange through the thylakoid membrane.

Correlated influence of cation concentration and excitation intensity on PS II activity-II. Comparative study between green plant and brown-alga chloroplasts

A preparation of photochemically active chloroplasts of Fucus was added to a low-salt medium with high osmolarity (HEPES AMPD buffer, 1M sorbitol). The rate of DCIP reduction (DCIPr) and the variable fluorescence (Fv) of these phaeoplasts were measured and compared with the same activities in spinach chloroplasts. A study of the influence of mono- and divalent-cations showed that salt effects on PS II activity also exist in Fucus. (i) Mg(++) action on Fv is similar, although noticeably weaker in Fucus than in spinach chloroplasts. (ii) Na(+) has no effect on Fv of Fucus chloroplasts, but its influence on DCIPr is more pronounced than in spinach. (iii) Mg(++) influence on DCIPr is largely dependent upon excitation energy. In subsaturating light (100\2-1000 W m(\t-2)), Mg(++) stimulation increases up to 100 mM, almost doubling the level. In very low wight conditions (3Wm(\t02)), however, this stimulation saturates at about 10 mM; higher concentrations are no longer effective but do not quench DCIPr noticeably, unlike the case in spinach. Therefore, cations act through similar pathways on Fucus as on spinach isolated chloroplasts but the effects on PS II centers are preponderant in Fucus whereas the modifications in non-radiative decay or pigment array size are weaker.

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).

Effect of surface potential on the intramembrane electrical field measured with carotenoid spectral shift in chromatophores from Rhodopseudomonas sphaeroides

Changes in the surface potential, the electrical potential difference between the membrane surface and the bulk aqueous phase were measured with the carotenoid spectral shift which indicates the change of electrical field in the membrane. Chromatophores were prepared from a non-sulfur purple bacterium, Rhodopseudomonas sphaeroides, in a low-salt buffer. Surface potential was changed by addition of salt or by pH jump as predicted by the Gouy-Chapman diffuse double layer theory. When a salf was added at neutral pH, the shift of carotenoid spectrum to shorter wavelength, corresponding to an increase in electrical potential at the outside surface, was observed. The salts of divalent cations (MgSO4, MgCl-2, CaCl2) were effective at concentrations lower than those of monovalent cation salts (NACl, KCl, Na2SO4) by a factor of about 50. Among the salts of monoor divalent cation used, little ionic species-dependent difference was observed in the low-concentration range except that due to the valence of cations. The pH dependence of the salt-induced carotenoid change was explained in terms of the change in surface charge density, which was about 0 at pH 5--5.5 and had negative values at higher pH values. The dependence of the pH jump-induced absorbance change on the salt concentration was also consistent with the change in the charge density. The surface potential change by the salt addition, which was calibrated by H+ diffusion potential, was about 90 mV at the maximum. From the difference between the effective concentrations with salts of mono- and divalent cations at pH 7.8, the surface charge density of (-1.9 +/- 0.5) . 10(-3) elementary charge per A2, and the surface potential of about -100 mV in the presence of about 0.1 mM divalent cation of 5 mM monovalent cation were calculated.

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.

The influence of the thylakoid membrane surface properties on the distribution of ions in chloroplasts

Thylakoid membranes isolated from peas have been subjected to ionic analyses using the technique of neutron activation. This has allowed the analyses of K+, Na+, Mg2+, Ca2+ and Cl- to be measured simultaneously on the same sample. By varying the ionic composition of the suspending medium it has been shown that these chloroplast membranes have no obvious chemical specificity for the inorganic cations studied and that the major controlling factor is the electrostatic neutralization of the surface negative charges. In agreement with the Gouy-Chapman theory and for the conditions used, divalent cations were preferentially attracted to the membrane surface. This finding, together with the ionic analysis of the unwashed thylakoids and of isolated intact chloroplasts, indicated that the major physiological surface cation is Mg2+ and that K+ is probably the main inorganic cation of the stroma. This conclusion is discussed in terms of counterion movement in response to light induced proton pumping at the thylakoid membrane.

Surface charges on chloroplast membranes as studied by particle electrophoresis

1. Particle microelectrophoresis mobility studies have been conducted with chloroplast thylakoid membranes and with isolated intact chloroplasts. 2. The pH dependence of the electrophoretic mobility indicated that at pH values above 4.3 both membrane systems carry a net negative charge. 3. Chemical treatment of thylakoids has shown that neither the sugar residues of the galactolipids in the membrane nor the basic groups of the membrane proteins having pK values between 6 and 10 are exposed at the surface. 4. However, treatment with 1-ethyl-3(3-dimethylaminopropyl)carbodiimide, together with glycine methyl ester, neutralized the negative charges on the thylakoid membrane surface indicating the involvement of carboxyl groups which, because of their pH sensitivity, are likely to be the carboxyl groups of aspartic and glutamic acid residues. 5. The nature of the protein giving rise to the negative surface charges on the thylakoids is not known but is shown not to involve the coupling factor or the light harvesting chlorophyll a/chlorophyll b pigment . protein complex. 6. No significant effect of light was observed on the electrophoretic mobility of either thylakoids or intact chloroplasts. 7. The striking difference in the ability of divalent and monovalent cations to screen the surface charges was demonstrated and explained in terms of the Gouy-Chapman theory. 8. Calculations of the zeta-potentials for thylakoid membranes gave values for the charge density at the plane of shear to be in the region of one electronic charge per 1500--2000 A2. 9. The significance of the results is discussed in terms of cation distribution in chloroplasts and the effect of cations on photosynthetic phenomena.

Energy transfer and its dependence on membrane properties

With isolated chloroplasts variations in the degree of energy transfer between light-harvesting chlorophyll-protein complexes can be induced by changing the cation content of the suspending medium. The changes can be observed by measuring chlorophyll-fluorescence yields and lifetimes and are probably brought about by conformatial changes in the thylakoid membrane. Detailed studies of the properties of cation-induced changes in chlorophyll fluorescence indicated that the alterations in pigment organization are due to variations in the density of positive charges immediately adjacent to the surface of the thylakoid membrane, being in qualitative agreement with predictions based on the Gouy-Chapman theory of diffuse double layers. Possible mechanisms for the membrane structural changes controlling energy transfer are given.