TEMPERATURE DEPENDENCE OF DELAYED LIGHT EMISSION IN THE 6 to 340 MICROSECOND RANGE AFTER A SINGLE FLASH IN CHLOROPLASTS

Abstract. The delayed light emission decay rate (up to 120 μs) and the rise in chlorophyll a fluorescence yield (from 3 to 35 μs) in isolated chloroplasts from several species, following a saturating 10 ns flash, are temperature independent in the 0–35°C range. However, delayed light in the 120–340 μs range is temperature dependent. Arrhenius plots of the exponential decay constants are: (a) linear for lettuce and pea chloroplasts but discontinuous for bush bean (12–17°C) and spinach (12–20°C) chloroplasts; (b) unaffected by 3-(3,4 dichlorophenyl)-1,1-dimethylurea (inhibitor of electron flow), gramicidin D (which eliminates light-induced membrane potential) and glutaraldehyde fixation (which stops gross structural changes).
The discontinuities, noted above for bush bean and spinach chloroplasts, are correlated with abrupt changes in (a) the thylakoid membrane lipid fluidity (monitored by EPR spectra of 12 nixtroxide stearate, 12NS) and (b) the fluidity of extracted lipids (monitored by differential calorimetry and EPR spectra of 12 NS). However, no such discontinuity was observed in (a) chlorophyll a fluorescence intensity of thylakoids and (b) fluorescence of tryptophan residues of delipidated chloroplasts.
Microsecond delayed light is linearly dependent on light intensity at flash intensities as low as one quantum per 2 times 10⁴ chlorophyll molecules. We suggest that this delayed light could originate from a one quantum process in agreement with the hypothesis that recombination of primary charges leads to this light emission. A working hypothesis for the energy levels of Photosystem II components is proposed involving a charge stabilization step on the primary acceptor side, which is in a lipid environment.
Finally, the redox potential of P680 (the reaction center for chlorophyll of system II) is calculated to be close to 1.0–1.3 V.     

OPTICAL SPECTROSCOPY OF BILAYER MEMBRANES

Abstract— The absorption and fluorescence spectroscopy of natural and model bilayer lipid membranes is reviewed. Basic structural features of biological membranes and the relative advantages of black lipid membranes (BLM) and of liposomes are discussed. Theoretical considerations show that the wavelengths of absorption maxima are affected by the refractive index and dielectric constant of the medium surrounding the chromophore. Techniques of obtaining photoelectric action spectra, direct absorption spectra, and reflection spectra of BLM are described. Polarized spectra can give information about the orientation of membrane constituents and show, for example, that the porphyrin ring of chlorophyll in BLM is tilted at 45 ± 5° to the membrane surface. Absorption maxima of chlorophyll in BLM are compared with solution spectra of various chlorophyll adducts and aggregates. It is concluded that chlorophyll in BLM exists largely as solvated monomer and dimer (or oligomer), depending on concentration, and is not coordinated with water. From the theory of fluorescence spectroscopy it follows that aggregation and the polarity of the environment affect the fluorescence yield and lifetime of a membrane component, and also the wavelength of its emission maximum. The microviscosity of the membrane matrix affects the anisotropy of fluorescence. Techniques of steady-state fluorescence spectroscopy and of fluorescence lifetime measurements are reviewed. Examples of the use of fluorescent probes in membrane studies are given. Certain probes such as anilinonaphthalene sulfonate (ANS) preferentially bind to membrane proteins. The location of a probe in a particular membrane region can be pinpointed from its fluorescence yield and emission maximum. The orientation of the hydrocarbon chains of membrane lipids has been found, from fluorescence polarization of certain probes, to be normal to the membrane surface as postulated a priori on the basis of the lipid bilayer model. Anisotropy of fluorescence shows that elongated probe molecules rotate rapidly about their long axes when surrounded by phospholipids but become immobilized when bound to proteins. Changes in intensity and anisotropy of fluorescence as function of temperature have demonstrated the existence of phase transitions and phase equilibria of membrane lipids. Excimer fluorescence has been used as a measure of the available lipid core volume of membranes. Mechanisms of energy transfer between membrane components are reviewed. The theoretical dependence of energy transfer on distance and orientation for several rigid and fluid membrane models is discussed in terms of the structural information it can provide. Fluorescence sensitization resulting from energy transfer within and across bilayer membranes has been demonstrated in various systems. Quantitative measurement of energy transfer efficiency in BLM has shown that such transfer is about five times more efficient than in solutions at comparable donor-acceptor distances. Lipid membranes can be viewed as structures which maintain their components at high concentrations, in a reactive state, and at favourable orientations.     

Photoexcitable polymer membranes. Photoinduced membrane potential across poly(vinyl chloride) membrane doped with a photosensitive crown ether having lipophilic side chain

Photoirradiation induced potential changes of 10–20 mV across the poly(vinyl chloride) membranes doped with a photosensitive lipophilic crown ether, p-[3,4-(1,4,7,10,13-pentaoxatridecane-1,13-diyl)phenylazo]hexadecyloxybenzene was studied. The photoresponse of the membrane was highly improved, presumably due to the lipophilic nature of the crown ether. The photoresponse was explained in terms of the charge density change on the membrane surface. The electric double layer theory was applied to estimate the values of the photoinduced change of the charge density.     

EVIDENCE THAT CHARGE MOTION WITHIN BACTERIORHODOPSIN DEPENDS ON SOLVENT VISCOSITY

Abstract— It is shown that glycerol in the bathing solutions of a bacteriorhodopsin model membrane slows the photoresponse. The early stage of the photoresponse is only slightly affected while the later stage is appreciably retarded. These results are in agreement with previous measurements by other investigators on the temporal behavior of the photoreaction cycle in viscous bathing solutions. Data is fit to an equation of the form τ= A η^m exp(Δ/ kT ) where t is a time constant characterizing the photoresponse, τ is the viscosity of the bathing solution and Δ is the activation energy for the thermally driven process.     

Effects of lysophospholipids on membrane order of phosphatidylcholine

Lysophospholipids are known to play a role in a wide range of cellular processes involving membrane–protein or membrane–membrane interactions; however lysolipids–lamellar lipids interactions remain unclear. The effects of lysolipids on membrane order and dynamics were examined using optical birefringence and fluorescence techniques. We found that lysophosphatidic acid (LPA) induces a considerable disorder in chain orientation for synthetic lipid of dimyristoyl-phosphatidylcholines (DMPC), whereas a slight order for natural lipid of egg yolk phosphatidylcholine (Egg-PC), e.g. the chain order decreases by 10% at 0.1 mole ratio for DMPC in comparison with the membranes without LPA and increases by 3.4% at 0.09 mole ratio for Egg-PC. Also, membrane fluidity corresponds with the change in the chain disorder, namely, the fluidity increases for DMPC membranes, while decreases for Egg-PC membranes by addition of LPA. The difference in the effects of LPA is interpreted by a difference in the chain packing between the synthetic and the natural lipid bilayers. LPA can be incorporated into natural lipid membranes without disturbance, and readjusts itself to a more favorable hydrophobic match with the bilayers. Lysophophatidylcholine (LPC) also induces a disorder in DMPC membranes, but the decrease in chain order is only half compared with that for LPA.     

ELECTRIC LINEAR DICHROISM OF P700 CHLOROPHYLL U-PROTEIN COMPLEXES

Abstract— The P₇₀₀ chlorophyll a -protein complex (CPI) isolated from green plants was oriented in aqueous solutions using pulsed electric fields of up to 6700 V cm^-1. The electric linear dichroism spectrum is reported in the range of 400–720nm. Positive peaks in the linear dichroism Δ A = A _I - A ₁ (where A_I and A₁ are the absorbance components in which the polarizer orientation is parallel and perpendicular with respect to the electric field. respectively) are observed at 443 and 686 nm. The ΔA signal at 686 nm is discussed in terms of either a specialized chlorophyll form absorbing at 686 nm. or due to an exciton component absorbing at the same wavelength.     

A Diazirine-Modified Membrane Lipid to Study Peptide/Lipid Interactions – Chances and Challenges

Although incorporation of photo-activatable lipids into membranes potentially opens up novel avenues for investigating interactions with proteins, the question of whether diazirine-modified lipids are suitable for such studies, remains under debate. Focusing on the potential for studying lipid/peptide interactions by cross-linking mass spectrometry (XL-MS), we developed a diazirine-modified lipid (DiazPC), and examined its behaviour in membranes incorporating the model α-helical peptide LAVA20. We observed an unexpected backfolding of the diazirine-containing stearoyl chain of the lipid. This surprising behaviour challenges the potential application of DiazPC for future XL-MS studies of peptide and protein/lipid interactions. The observations made for DiazPC most likely represent a general phenomenon for any type of membrane lipids with a polar moiety incorporated into the alkyl chain. Our finding is therefore of importance for future protein/lipid interaction studies relying on modified lipid probes.     

Photo-switchable ion and enzyme sensors. Photoinduced potentiometric response of glassy carbon electrode coated with polymer or polymer/enzyme dual membrane

Photo-switchable ion and enzyme sensors were fabricated by the use of glassy carbon electrode coated with nonactindoped or enzyme modified poly(vinyl chloride) (PVC) membranes. The ion sensor with nonactin-doped PVC membrane, which contained spirobenzopyran as the photosensitive dye, exhibited a potentiometric photoresponse to NH4+ ion in the solution. The dynamic range of the NH4+ ion sensor was 10(-7)--10(-3) M. Urea, adenosine, and asparagine sensors were prepared by coating the surface of the NH4+-ion sensor with urease, adenosine deaminase, and asparaginase membranes, respectively. These enzyme sensors could be used for determining the substrates at the micro mole level. The performance characteristics of these sensors were compared with those previously prepared membrane electrode sensors.     

Role of bound water in biological membrane structure: fluorescence and infrared studies.

Bound water is a major component of biological membranes and is required for the structural stability of the lipid bilayer. It has also been postulated that it is involved in water transport, membrane fusion, and mobility of membrane proteins and lipids. We have measured the fluorescence emission of membrane-bound 1-anilino-8-naphthalenesulfonate (ANS) and the infrared spectra of membranes, both as a function of hydration. ANS fluorescence is sensitive to polarity and fluidity of the membrane-aqueous interface, while infrared absorption is sensitive to the hydrogen bonding and vibrational motion of water and membrane proteins and lipids. The fluorescence results provide evidence of increasing rigidity and/or decreasing polarity of the membrane-aqueous interface with removal of water. The membrane infrared spectra show prominent hydration-dependent changes in a number of bands with possible assignments to cholesterol (vinyl CH bend, OH stretch), protein (amide A, II, V), and bound water (OH stretch). Further characterization of the bound water should allow its incorporation into current models of membrane structure and give insight into the role of membrane hydration in cell surface function.     

Cardiolipin, a key component to mimic the E. coli bacterial membrane in model systems revealed by dynamic light scattering and steady-state fluorescence anisotropy

The phase transition temperatures of several lipidic systems were determined using two different techniques: dynamic light scattering (DLS) and steady-state fluorescence anisotropy, using two fluorescent probes that report different membrane regions (TMA-DPH and DPH). Atomic force microscopy (AFM) was used as a complementary technique to characterize different lipid model systems under study. The systems were chosen due to the increased interest in bacterial membrane studies due to the problem of antibiotic drug resistance. The simpler models studied comprised of mixtures of POPE and POPG lipids, which form a commonly used model system for Escherichia coli membranes. Given the important role of cardiolipin (CL) in natural membranes, a ternary model system, POPE/POPG/CL, was then considered. The results obtained in these mimetic systems were compared with those obtained for the natural systems E. coli polar and total lipid extract. DLS and fluorescence anisotropy are not commonly used to study lipid phase transitions, but it was shown that they can give useful information about the thermotropic behaviors of model systems for bacterial membranes. These two techniques provided very similar results, validating their use as methods to measure phase transitions in lipid model systems. The temperature transitions obtained from these two very different techniques and the AFM results clearly show that cardiolipin is a fundamental component to mimic bacteria membranes. The results suggest that the less commonly used ternary system is a considerably better mimic for natural E. coli membranes than binary lipid mixture.     

Cyclohexane Rings Reduce Membrane Permeability to Small Ions in Archaea‐Inspired Tetraether Lipids

Extremophile archaeal organisms overcome problems of membrane permeability by producing lipids with structural elements that putatively improve membrane integrity compared to lipids from other life forms. Herein, we describe a series of lipids that mimic some key structural features of archaeal lipids, such as: 1) single tethering of lipid tails to create fully transmembrane tetraether lipids and 2) the incorporation of small rings into these tethered segments. We found that membranes formed from pure tetraether lipids leaked small ions at a rate that was about two orders of magnitude slower than common bilayer‐forming lipids. Incorporation of cyclopentane rings into the tetraether lipids did not affect membrane leakage, whereas a cyclohexane ring reduced leakage by an additional 40 %. These results show that mimicking certain structural features of natural archaeal lipids results in improved membrane integrity, which may help overcome limitations of many current lipid‐based technologies.     

Enzymatic Insertion of Lipids Increases Membrane Tension for Inhibiting Drug Resistant Cancer Cells

Although lipids contribute to cancer drug resistance, it is challenging to target diverse range of lipids. Here, we show enzymatically inserting exceedingly simple synthetic lipids into membranes for increasing membrane tension and selectively inhibiting drug resistant cancer cells. The lipid, formed by conjugating dodecylamine to d -phosphotyrosine, self-assembles to form micelles. Enzymatic dephosphorylation of the micelles inserts the lipids into membranes and increases membrane tension. The micelles effectively inhibit a drug resistant glioblastoma cell (T98G) or a triple-negative breast cancer cell (HCC1937), without inducing acquired drug resistance. Moreover, the enzymatic reaction of the micelles promotes the accumulation of the lipids in the membranes of subcellular organelles (e.g., endoplasmic reticulum (ER), Golgi, and mitochondria), thus activating multiple regulated cell death pathways. This work, in which for the first time membrane tension is increased to inhibit cancer cells, illustrates a new and powerful supramolecular approach for antagonizing difficult drug targets.     

Synthesis of lipidated green fluorescent protein and its incorporation in supported lipid bilayers

Herein we report a semisynthetic method of producing membrane-anchored proteins. Ligation of synthetic lipids with designed anchor structures to proteins was performed using native chemical ligation (NCL) of a C-terminal peptide thioester and an N-terminal cysteine lipid. This strategy mimics the natural glycosylphosphatidylinositol (GPI) linkage found in many natural membrane-associated proteins; however, the synthetic method utilizes simple lipid anchors without glycans. Synthetically lipidated recombinant green fluorescent protein (GFP) was shown to be stably anchored to the membrane, and its lateral fluidity was quantitatively characterized by direct fluorescence imaging in supported membranes. Circumventing the steps of purification from native cell membranes, this methodology facilitates the reconstitution of membrane-associated proteins.     

Responses of Transmembrane Peptide and Lipid Chains to Hydrophobic Mismatch

Hydrophobic mismatch between the hydrophobic length of membrane proteins and hydrophobic thickness of membranes is a crucial factor in controlling protein function and assembly. We combined fluorescence with circular dichroism(CD) and attenuated total reflection infrared(ATR-IR) spectroscopic methods to investigate the behaviors of the peptide and lipids under hydrophobic mismatch using a model peptide from the fourth transmembrane domain of natural resistance-associated macrophage protein 1(Nramp1), the phosphatidylcholines(PCs) and phosphatidylglycerols(PGs) with different lengths of acyl chains(14:0, 16:0 and 18:0). In all PG lipid membranes, the peptide forms stable a-helix structure, and the helix axis is parallel to lipid chains. The helical span and orientation hardly change in varying thickness of PG membranes, while the lipid chains can deform to accommodate to the hydrophobic surface of embedded peptide. By comparison, the helical structures of the model peptide in PC lipid membranes are less stable. Upon incorporation with PC lipid membranes, the peptide can deform itself to accommodate to the hydrophobic thickness of lipid membranes in response to hydrophobic mismatch. In addition, hydrophobic mismatch can increase the aggregation propensity of the peptide in both PC and PG lipid membranes and the peptide in PC membranes has more aggregation tendency than that in PG membranes.     

STUDY OF THE EFFECTS OF ULTRAVIOLET LIGHT ON BIOMEMBRANES—IV. THE EFFECT OF OXYGEN ON UV-INDUCED HEMOLYSIS AND LIPID PHOTOPEROXIDATION IN RAT ERYTHROCYTES AND LIPOSOMES

Abstract— Hemolysis induced by irradiation with ultraviolet (UV) light at 254 nm showed a pronounced oxygen effect: under irradiation in vacuum, the rate of hemolysis was decreased by an order of magnitude. Irradiation at 254 nm in air but not under vacuum caused the peroxidation of erythrocyte membrane lipids. These results suggest that membrane lipid photoperoxidation is one of the causative factors of UV hemolysis. Irradiation at different wavelengths showed that UV-induced lipid photoperoxidation in erythrocyte membranes developed while the antioxidant α-tocopherol was directly photooxidized. It is shown that the process of lipid photolysis in erythrocyte membranes involves sensitization, possibly by protoporphyrin, whose presence in liposomes accelerates the photoperoxidation at 254 and 365 nm of unsaturated fatty acid residues in lecithin. Possible mechanisms of photochemical damage to erythrocyte membranes are discussed.     

484—Photosensitive lipid membranes: Part II. The collodion-oleic acid,methylene blue sensitized membrane in aerated KCl solutions. Evidence for the intervention of singlet oxygen in the photoreaction mechanism

The photoresponse of lipid membranes of oleic acid, sensitized with methylene blue and immobilized by collodion, was examined in acrated, aqueous KCl solutions. Deposited on suitable metal electrodes, these membranes can reach considerable photovoltages, of the order of ?360 mV, depending mainly on the electrolyte concentration, the highest values being obtained with concentrated (2M) KCl solutions.In the presence of a typical singlet oxygen quencher, such as N₃Na, the photoresponse is diminished in a quantitative manner, whereas the presence of D₂O, as singlet oxygen lifetime promoter, increases the photoresponse in KCl 1 × 10^?1M solutions. Thus, the intervention of singlet oxygen in the photoreaction mechanism is indirectly proved.     

The influence of zwitterionic lipids on the electrostatic adsorption of macroions onto mixed lipid membranes

Charged lipid membranes commonly consist of a mixture of charged and zwitterionic lipids. We suggest a model that characterizes the influence of the dipolar nature of the zwitterionic lipid species on the electrostatic adsorption of macroions onto mixed membranes in the fluid state. The model is based on Poisson-Boltzmann theory which we have modified so as to account for the dipolar character of the zwitterionic lipids. In addition the membrane lipids are allowed to adjust their lateral distribution upon macroion adsorption. We consider and compare two experimentally relevant scenarios: cationic macroions adsorbed onto anionic membranes and anionic macroions adsorbed onto cationic membranes. We show that in the former case the adsorption strength is slightly weakened by the presence of the headgroup dipoles of the zwitterionic lipids. Here, macroion-induced lipid demixing is more pronounced and the lipid headgroups tilt away from a cationic macroion upon adsorption. In contrast, for the adsorption of anionic macroions onto a cationic membrane the zwitterionic lipids strongly participate in the electrostatic interaction between membrane and macroion, thus enhancing the adsorption strength significantly (we predict up to 20%). Consistent with that we find less lateral demixing of the charged lipids and a reorientation of the dipoles of the zwitterionic headgroups towards the anionic macroions. Our results may be of importance to understand the differences in the electrostatic adsorption of proteins/peptides onto cellular membranes versus complex formation between cationic membranes and DNA.     

Influence of lipid composition on membrane activity of antimicrobial phenylene ethynylene oligomers

Host defense peptides (HDPs), part of the innate immune system, selectively target the membranes of bacterial cells over that of host cells. As a result, their antimicrobial properties have been under intense study. Their selectivity strongly depends on the chemical and mostly structural properties of the lipids that make up different cell membranes. The ability to synthesize HDP mimics has recently been demonstrated. To better understand how these HDP mimics interact with bilayer membranes, three homologous antimicrobial oligomers (AMOs) 1-3 with an m-phenylene ethynylene backbone and alkyl amine side chains were studied. Among them, AMO 1 is nonactive, AMO 2 is specifically active, and AMO 3 is nonspecifically active against bacteria over human red blood cells, a standard model for mammalian cells. The interactions of these three AMOs with liposomes having different lipid compositions are characterized in detail using a fluorescent dye leakage assay. AMO 2 and AMO 3 caused more leakage than AMO 1 from bacteria membrane mimic liposomes composed of PE/PG lipids. The use of E. coli lipid vesicles gave the same results. Further changes of the lipid compositions revealed that AMO 2 has selectively higher affinity toward PE/PG and E. coli lipids than PC, PC/PG or PC/PS lipids, the major components of mammalian cell membranes. In contrast, AMO 3 is devoid of this lipid selectivity and interacts with all liposomes with equal ease; AMO 1 remains inactive. These observations suggest that lipid type and structure are more important in determining membrane selectivity than lipid headgroup charges for this series of HDP mimics.     

Dramatic destabilization of transmembrane helix interactions by features of natural membrane environments

Membrane proteins have evolved to fold and function in a lipid bilayer, so it is generally assumed that their stability should be optimized in a natural membrane environment. Yet optimal stability is not always in accord with optimization of function, so evolutionary pressure, occurring in a complex membrane environment, may favor marginal stability. Here, we find that the transmembrane helix dimer, glycophorin A (GpATM), is actually much less stable in the heterogeneous environment of a natural membrane than it is in model membranes and even common detergents. The primary destabilizing factors are electrostatic interactions between charged lipids and charged GpATM side chains, and nonspecific competition from other membrane proteins. These effects overwhelm stabilizing contributions from lateral packing pressure and excluded volume. Our work illustrates how evolution can employ membrane composition to modulate protein stability.     

The effect of phenyltin chlorides on osmotically induced erythrocyte haemolysis

The toxicity of many amphiphilic compounds may result from their effect on the lipid phase of biological membranes. Upon incorporation such compounds may change the properties of membranes in general and in particular alter the organization of membrane lipids. These changes should affect, among other things, the mechanical properties of membranes. We selected two amphiphilic compounds, diphenyltin dichloride (Ph₂SnCl₂) and triphenyltin chloride (Ph₃SnCl), which are known to be located at different regions of the lipid bilayer and to be toxic. As a model biological membrane the erythrocyte plasma membrane was used. Analysis of the haemolysis kinetics showed differences between the effect of the compound studied on mechanical properties at so‐called non‐lytic concentrations. Diphenyltin dichloride showed a limited effect on erythrocyte haemolysis, whereas triphenyltin chloride affected all the parameters measured (extent of initial haemolysis, extent of final haemolysis and membrane mechanical strength). We correlated these effects with the location of the investigated compounds in liposomes. The presented data show that triphenyltin chloride reduces the erythrocyte plasma membrane mechanical strength and increases the extent of haemolysis under osmotic stress conditions. Copyright © 2005 John Wiley & Sons, Ltd.