The DMPC lipid phase transition influences differently the first and the second electron transfer reactions in bacterial reaction centers

Photoactive ultrathin molecular nanosheets with reversible lanthanide binding terpyridine centers

In recent years, functional molecular nanosheets have attracted much attention in the fields of sensors and energy storage. Here, we present an approach for the synthesis of photoactive metal-organic nanosheets with ultimate molecular thickness. To this end, we apply low-energy electron irradiation induced cross-linking of 4'-(2,2':6',2''-terpyridine-4'-yl)-1,1'-biphenyl-4-thiol self-assembled monolayers on gold to convert them into functional ∼1 nm thick carbon nanomembranes possessing the ability to reversibly complex lanthanide ions (Ln-CNMs). The obtained Ln-CNMs can be prepared on a large-scale (>10 cm²) and inherit the photoactivity of the pristine terpyridine lanthanide complex (Ln(III)-tpy). Moreover, they possess mechanical stability as free-standing sheets over micrometer sized openings. The presented methodology paves a simple and robust way for the preparation of ultrathin nanosheets with tailored photoactive properties for application in photocatalytic and energy conversion devices.

The physicochemical properties of membranes correlate with the NADPH oxidase activity

BACKGROUND

Phagocytes kill ingested microbes by exposure to high concentrations of toxic reactive species generated by NADPH-oxidases. This membrane-bound electron-transferring enzyme is tightly regulated by cellular signaling cascades. So far, molecular and biophysical studies of the NADPH-oxidase were performed over limited temperature ranges, which weaken our understanding of immune response or inflammatory events. In this work, we have inspected the influence of temperature and lipid membrane properties on the NADPH-oxidase activity using a system free of cell complexity.

METHODS

We have extended the experimental conditions of the accepted model for NADPH-oxidase activity, the so-called cell-free assay, to a large temperature range (10-40°C) using different membrane compositions (subcellular compartments or liposomes).

RESULTS

A remarkable increase of superoxide production rate was observed with rising temperature. Synchrotron radiation circular dichroism data showed that this is not correlated with protein secondary structure changes. When lipid bilayers are in fluid phase, Arrhenius plots of the oxidase activity showed linear relationships with small activation energy (Ea), while when in solid phase, high Ea was found. The sterol content modulates kinetic and thermodynamic parameters.

CONCLUSION

High temperature promotes the rate of superoxide production. The key element of this enhancement is related to membrane properties such as thickness and viscosity and not to protein structural changes. Membrane viscosity that can be driven by sterols is a paramount parameter of Ea of NADPH oxidase activity. The membrane bilayer state modulated by its sterol content may be considered locally as an enzyme regulator. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

A model for ultra-fast charge transport in membrane proteins

Isolated proteins have recently been observed to transport charge and reactivity over very long distances with extraordinary rates and near perfect efficiencies in spite of their site. This is not the case if the peptide is in water, where the efficiency of charge hopping to the next site is reduced to approximately 2%. Here, water is not an ideal solvent for charge transport. The issue at hand is how to explain such enormous charge transfer quenching in water compared to another typical medium, namely lipid. We performed molecular dynamics simulations to computationally substantiate the novel long-distance charge transfer yield of the polypeptides in lipids. This is characterized by the charge transfer persistent-distance decay constant and not by the rate, which is seldom, if ever, measured and hence not directly addressed here. This model can encompass an extremely wide range of yields over very long distances in peptides in various media. The calculations here demonstrate the good charge transport efficiency in lipids in contrast to the poor efficiency in water. The protein charge transport also exhibits a very strong anisotropic effect in lipids. The peptide secondary structure effect of charge transfer in membranes is analyzed in contrast to that in water. These results suggest that this model can be useful for the prediction of charge transfer efficiency in various environments of interest and indicate that the charge transfer is highly efficient in membrane proteins.

Influence of cholesterol on liposome stability and on in vitro drug release

Cholesterol plays a strategic role in liposome composition; however, the quantity used to achieve an appropriate formulation has not been yet clarified. Therefore, by screening arrangement of lipids and cholesterol ratio, the main aim of this study is to investigate the most suitable amount of cholesterol in lipids in order to prepare stable and controlled drug release vehicles. For the preparation of liposomes, DMPC, DPPC and DSPC phospholipids were used and combined with different molar ratios of cholesterol (e.g. 100, 80-20, 70-30, 60-40 and 50-50%). Stability studies were conducted by storing the formulations at 37 and 50 °C for 30 days and by analysing them by AFM, DLS and FT-IR. By detecting the two most stable formulations from the stability results, drug encapsulation and in vitro release studies in PBS were performed by encapsulating atenolol and quinine. The release results were validated using a simulation model to ensure the reliability and suitable interpretation of the data. The generated model showed a good correlation between the prediction and the in vitro obtained results. By using 70:30% ratio (known in literature as 2:1), it is possible to reach the most stable formulation to guarantee a controlled and reproducible release for drugs with different physicochemical characteristics and pharmaceutical applications.

Ca(2+) bridging of apposed phospholipid bilayers

In an effort to provide insight into the mechanism of Ca(2+)-induced fusion of lipid vesicles, molecular dynamics simulations in the isobaric-isothermal ensemble are used to investigate interactions of Ca(2+) with apposed lipid bilayers in close proximity. Simulations reveal the formation of a Ca(2+)-phospholipid "anhydrous complex" between apposed bilayers, whereas similar calculations performed with Na(+) display only complexation between neighboring lipids within the same bilayer. The binding of Ca(2+) to apposed phospholipids brings large regions of the bilayers into close contact (<4 Å), displacing water from phospholipid head groups in the process and creating regions of local dehydration. Dehydration of the apposed bilayers leads to ordering of the phospholipid tails, which is partially disrupted by the presence of Ca(2+)-phospholipid bridges.

Characterisation of RC-proteoliposomes at different RC/lipid ratios

Reconstitution of membrane proteins in phospholipid vesicles allows the investigation of such macromolecules in a biomimetic simplified environment. The often employed micelle-to-vesicle-transition method for proteoliposome preparation is a fast and reproducible technique. In this, communication is shown that the lipid/protein ratio influences the size of the proteoliposomes and the actual protein reconstitution. The results indicate that for photosynthetic reaction centres, the best conditions for ligand-interaction experiments are achieved with a lipid/protein value of 1000:1, while for complete protein incorporation, the 2000:1 ratio should be chosen.

Stabilization effect of single-walled carbon nanotubes on the functioning of photosynthetic reaction centers

The interaction between single-walled carbon nanotubes and photosynthetic reaction centers purified from purple bacterium Rhodobacter sphaeroides R-26 has been investigated. Atomic force microscopy studies provide evidence that reaction center protein can be attached effectively to the nanotubes. The typical diameter of the nanotube is 1-4 nm and 15 +/- 2 nm without and with the reaction centers, respectively. Light-induced absorption change measurements indicate the stabilization of the P+(Q(A)Q(B))- charge pair, which is formed after single saturating light excitation after the attachment to nanotubes. The separation of light-induced charges is followed by slow reorganization of the protein structure. The stabilization effect of light-initiated charges by the carbon nanotubes opens a possible direction of several applications, the most promising being in energy conversion and storage devices.

Enthalpy/entropy driven activation of the first interquinone electron transfer in bacterial photosynthetic reaction centers embedded in vesicles of physiologically important phospholipids

The thermodynamics and kinetics of light-induced electron transfer in bacterial photosynthetic RCs are sensitive to physiologically important lipids (phosphatidylcholine, cardiolipin and phosphatidylglycerol) in the environment. The analysis of the temperature-dependence of the rate of the P(+)Q(A)(-)Q(B)-->P(+)Q(A)Q(B)(-) interquinone electron transfer revealed high enthalpy change of activation in zwitterionic or neutral micelles and vesicles and low enthalpy change of activation in vesicles constituted of negatively charged phospholipids. The entropy change of activation was compensated by the changes of enthalpy, thus the free energy change of activation ( approximately 500 meV) did not show large variation in vesicles of different lipids.

Photosynthetic reaction center functionalized nano-composite films: effective strategies for probing and exploiting the photo-induced electron transfer of photosensitive membrane protein

Photosynthetic reaction center (RC), a robust transmembrane pigment-protein complex, works as the crucial component participating the primary event of the photo-electrochemical conversion in bacteria. Sparked by the high photo-induced charge separation yield (ca. 100%) of RC, great interests have been aroused to fabricate versatile RC-functionalized nano-composite films for exploring the initial photosynthetic electron transfer (ET) of RC, and thus exploiting well-designed bio-photoelectric converters. In this review, we classify and summarize the current status about the concepts and methods of constructing RC-immobilized nano-composite films or devices for probing the photo-induced ET, and applying to novel bioelectronics if it is possible.

Protein/Lipid Interaction in the Bacterial Photosynthetic Reaction Center: Phosphatidylcholine and Phosphatidylglycerol Modify the Free Energy Levels of the Quinones

The role of characteristic phospholipids of native membranes, phosphatidylcholine (PC), phosphatidylglycerol (PG), and cardiolipin (CL), was studied in the energetics of the acceptor quinone side in photosynthetic reaction centers of Rhodobacter sphaeroides. The rates of the first, k(AB)(1), and the second, k(AB)(2), electron transfer and that of the charge recombination, k(BP), the free energy levels of Q(A)(-)Q(B) and Q(A)Q(B)(-) states, and the changes of charge compensating protein relaxation were determined in RCs incorporated into artificial lipid bilayer membranes. In RCs embedded in the PC vesicle, k(AB)(1) and k(AB)(2) increased (from 3100 to 4100 s(-1) and from 740 to 3300 s(-1), respectively) and k(BP) decreased (from 0.77 to 0.39 s(-1)) compared to those measured in detergent at pH 7. In PG, k(AB)(1) and k(BP) decreased (to values of 710 and 0.26 s(-1), respectively), while k(AB)(2) increased to 1506 s(-1) at pH 7. The free energy between the Q(A)(-)Q(B) and Q(A)Q(B)(-) states decreased in PC and PG (DeltaG degrees (Q)A-(Q)B(-->)(Q)A(Q)B- = -76.9 and -88.5 meV, respectively) compared to that measured in detergent (-61.8 meV). The changes of the Q(A)/Q(A)(-) redox potential measured by delayed luminescence showed (1) a differential effect of lipids whether RC incorporated in micelles or vesicles, (2) an altered binding interaction between anionic lipids and RC, (3) a direct influence of PC and PG on the free energy levels of the primary and secondary quinones probably through the intraprotein hydrogen-bonding network, and (4) a larger increase of the Q(A)/Q(A)(-) free energy in PG than in PC both in detergent micelles and in single-component vesicles. On the basis of recent structural data, implications of the binding properties of phospholipids to RC and possible interactions between lipids and electron transfer components will be discussed.

Kinetics of the quinone binding reaction at the QB site of reaction centers from the purple bacteria Rhodobacter sphaeroides reconstituted in liposomes

Transmembrane proton translocation in the photosynthetic membranes of the purple bacterium Rhodobacter sphaeroides is driven by light and performed by two transmembrane complexes; the photosynthetic reaction center and the ubiquinol-cytochrome c oxidoreductase complex, coupled by two mobile electron carriers; the cytochrome and the quinone. This paper focuses on the kinetics and thermodynamics of the interaction between the lipophylic electron carrier ubiquinone-10 and the photosynthetic enzyme reconstituted in liposomes. The collected data were simulated with an existing recognized kinetic scheme and the kinetic constants of the uptake (7.2 x 107 M(-1) x s(-1)) and release (40 s(-1)) processes of the ligand were inferred. The results obtained for the quinone release kinetic constant are comparable to the rate of the charge recombination reaction from the state D(+)QA(-). Values for the kinetic constants are discussed as part of the overall photocycle, suggesting that its bottleneck may not be the quinone uptake reaction in agreement with a previous report.