Enzymatic synthesis of β-lactam antibiotics: A thermodynamic background

The equilibrium constants and the respective standard Gibbs energy changes for hydrolysis of some β-lactam antibiotics have been determined. Native and immobilized penicillin amidase (EC 3.5.1.11) from Escherichia coli has been used as a catalyst. The values of standard Gibbs energy changes corresponding to the pH-independent product of equilibrium concentrations (ΔG⁰_c = ? RT ln K_c) have been calculated. The differences in the structure of the antibiotics nucleus hardly ever affect the value of the pH-independent component of the standard Gibbs energy change (ΔG⁰_c) and value of apparent standard Gibbs energy change at a fixed pH (ΔG^0′_c). At the same time, the value of ΔG⁰_c is more sensitive to the structure of the acyl moiety of the antibiotic; when ampicillin is used instead of benzylpenicillin, ΔG⁰_c increases by ～6.3 kJ mol^?1 (1.5 kcal mol^?1). pH-dependences of the apparent standard Gibbs energy changes for hydrolysis of β-lactam antibiotics have been calculated. The pH-dependences of ΔG^0′_c for hydrolysis of all β-lactam antibiotics have a similar pattern. The thermodynamic pH optimum of the synthesis of these compounds is in the acid pH range (pH < 5.0). The breakage of the β-lactam ring leads to a sharp decrease in the ΔG^0′_c value and a change in the pattern of the pH-dependence. For example, at pH 5.0 ΔG^0′_c decreases from 14.4 kJ mol^?1 for benzylpenicillin to ?1.45 kJ mol^?1 for benzylpenicilloic acid. The reason for these changes is mainly a considerable increase in the pK of the amino group of the nucleus of the antibiotic and, as a consequence, a decrease in the component of standard Gibbs energy change, corresponding to the ionization of the system. The thermodynamic potentials of the enzymatic synthesis of semisynthetic penicillins and cephalosporins on the basis of both free acids and their derivatives (N-acylated amino acids, esters) are discussed. It is shown that with esters of the acids, a high yield of the antibiotic can, in principle, be achieved at higher pH values.     

The interaction of mequitazine with phospholipid model membranes

The effect of increasing concentrations of mequitazine, a quinuclidinylmethyl-phenothiazine, on the phase transition temperature (T_c), the broadening of the transition peak, the enthalpy and entropy of transition of dimyristoyl-, dipalmitoyl- and distearoylphosphatidylcholine (DPPC) liposomes was studied. Pest critical micelle concentrations of mequitazine (CMC = 5.23 X 10^?2M), caused broadening of the transition peak and lowering of the T_c of pure liposomes. The ratio of peak heights from the nuclear magnetic resonance (NMR) spectra of egg phosphatidycholine liposomes was used as a criterion for assessing the interaction of the drug with phospholipid membranes. Mequitazine interacts with both the polar head groups and hydrophobic membrane interior.     

Plasmon-Assisted Enhancement and Tuning of Optical Properties in <Emphasis Type="Italic">β-</Emphasis>In<Subscript>2</Subscript>S<Subscript>3</Subscript> Quantum Dots

Exciton-plasmon coupling can significantly modify the spectral response of semiconductor quantum dots in a metal nanoparticle-semiconductor complex system. β-In₂S₃ quantum dots of size ～3 nm and Ag nanospheres of size ～100 nm were synthesized by chemical route and coated over glass substrates. In the strong coupling regime, the plasmons are shown to mediate indirect Coulomb interaction between the quantum dots. In the proximity of Ag plasmons, the excitonic binding energy of the β-In₂S₃ quantum dots increases by ～500 meV, indicating that the interaction potential between the quantum dots is positive and repulsive in nature. This interaction also leads to strong coupling of the defect levels in the SQD complex. The defect emission wavelength can be enhanced by an order of 10² or shifted from red region (～650 nm) to green (～550 nm) by controlling the plasmon-induced defect level coupling. The experimental observation demonstrates one of the theoretically predicted consequences of exciton-plasmon interaction. This work demonstrates the possibility of harnessing the potential of the two complimentary systems (semiconductor quantum dots and metal nanoparticles) to achieve controllable emission and absorption properties for fabrication of nano plasmonic devices.     

Lectin-mediated agglutination of liposomes containing glycophorin: Effects of acyl chain length

Glycophorin from human erythrocytes has been incorporated into liposomes of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC) and distearoylphosphatidylcholine (DSPC). The thermal properties of unsonicated liposomes with glycophorin/lipid molar ratios up to 4·10^?3 have been studied by differential scanning calorimetry and the numbers of lipids withdrawn from participation in the gel-to-lamellar phase transition were found to be 42±22 (DMPC), 197±28 (DPPC) and 240±64 (DSPC). The initial rates of agglutination of sonicated liposomes with glycophorin/lipid molar ratios up to 4·10^?3 by wheat germ agglutinin in the concentration range 0–7 μM have been measured over a range of temperature. Below the gel-to-lamellar phase transition (T_c) the rates of agglutination increase with acyl chain length in the sequence DMPC < DPPC < DSPC. Agglutination is found to be second order in liposome concentration and is completely reversed on saturation of the wheat germ agglutinin-binding sites by N-acetylglucosamine. Agglutination rates decrease with increasing temperature below T_c and are largely independent of temperature above T_c. The results are discussed in relation to the clustering of glycophorin in the phospholipid bilayers and its effect on binding and subsequent interliposomal bridge formation by wheat germ agglutinin.     

Thermotropic and barotropic phase transitions of N-methylated dipalmitoylphosphatidylethanolamine bilayers

In order to understand the effect of polar head group modification on the thermotropic and barotropic phase behavior of phospholipid bilayer membranes, the phase transitions of dipalmitoylphosphatidylethanolamine (DPPE), dipalmitoylphosphatidyl-N-methylethanolamine (DPMePE), dipalmitoylphosphatidyl-N,N-dimethylethanolamine (DPMe₂PE) and dipalmitoylphosphatidylcholine (DPPC) bilayer membranes were observed by differential scanning calorimetry and high-pressure optical methods. The temperatures of the so-called main transition from the gel (L_β) or ripple gel (P_β′) phase to the liquid crystalline (L_α) phase were almost linearly elevated by applying pressure. The slope of the temperature-pressure boundary, dT/dp, was in the range of 0.220-0.264 K MPa⁻¹ depending on the number of methyl groups in the head group of lipids. The main-transition temperatures of N-methylated DPPEs decreased with increasing size of head group by stepwise N-methylation. On the other hand, there was no significant difference in thermodynamic quantities of the main transition between the phospholipids. With respect to the transition from the subgel (L_c) phase to the lamellar gel (L_β or L_β′) phase, the transition temperatures were also elevated by applying pressure. In the case of DPPE bilayer the L_c/L_β transition appeared at a pressure higher than 21.8 MPa. At a pressure below 21.8 MPa the L_c/L_α transition was observed at a temperature higher than the main-transition temperature. The main (L_β/L_α) transition can be recognized as the transformation between metastable phases in the range from ambient pressure to 21.8 MPa. Polymorphism in the gel phase is characteristic of DPPC bilayer membrane unlike other lipid bilayers used in this study: the L_β′, P_β′ and pressure-induced interdigitated gel (L_βI) phases were observed only in the DPPC bilayer. Regarding the bilayers of DPPE, DPMePE and DPMe₂PE, the interdigitation of acyl chain did not appear even at pressures as high as 200 MPa.     

Thermodynamic and structural properties of phosphatidylserine bilayer membranes in the presence of lithium ions and protons

The binding of lithium ions to phosphatidylserine has been studied by differential scanning calorimetry for dialkyl and diacyl lipid forms and by X-ray diffraction for dihexadecylphosphatidylserine (DHPS). On first mixing DHPS with LiCl solutions an ordered L_β (L_c) phase is formed with a bilayer repeat distance of 5.55 nm and one strong wide-angle, chain-chain reflection at 0.405 nm (26°C), corresponding to bilayers of little, (mono)hydrated lipid with chains approximately perpendicular to the membrane surface. On heating, this phase transforms to an inverted hexagonal phase (H₁₁, H_α) with a repeat distance of 3.75 nm, at a chain-melting transition temperature of approximately 90°C (DHPS). Cooling, after equilibration of the DHPS⁻·Li⁺ sample in the fluid phase, creates a new low-temperature phase (L_c') which has a repeat distance of 4.0 nm, corresponding to strongly tilted chains (ϕ=42°). The L_c phase also transforms on heating to the H_α phase, but at a considerably lower chain-melting temperature of approx. 70°C (DHPS). The calorimetric behavior as a function of Li⁺ concentration is qualitatively very similar for the different dialkyl- and diacylphosphatidylserines studied, and is analogous to the results obtained on pH titration. After an initial small increase in transition temperature, that is caused by coulombic ion binding and concomitant surface charge neutralization, a much larger increase in the chain-melting transition temperature occurs, caused by dehydration of the lipid, as a consequence of a further stereospecific ion binding. This suggests that Li⁺ and H⁺ have similar binding sites on the PS headgroup.     

Adsorption of the polypeptides on a solid surface. III. Behavior of stiff chains in a pore

A strict analytical theory has been developed describing the behavior of a model lattice polymer chain of arbitrary stiffness in a slitlike pore at polymer–adsorbent interaction energies –ε. The thermodynamic characteristics of the system were calculated. It was shown that the transition of the macromolecule from the solution volume inside a pore occurs by the first-order phase transition with evolution of latent heat of adsorption. The transition point –ε = –ε_c is determined by the chain stiffness and is independent of the pore width D. It is shown that in the precritical range, –ε < –ε_c, the free energy ΔF of the macromolecules in the pores is adequately described by the universal dependence ΔF = ΔF(D*/A), where D* is some effective pore width depending on the value of –ε, and A is the length of the Kuhn segment. At high attraction energies, –ε ? –ε_c, the macromolecules are bonded to the pore walls by a great number of units and their free energy depends only on –ε and the chain stiffness, ΔF = ΔF(A, ε). Close to the critical energy –ε ? –ε_c (transition range), ΔF is determined by both the stiffness of the macromolecule and the pore width D: ΔF ～ A²D^?1 for fairly high values of A and D. The possibilities of using porous media as protein stabilizers are discussed, and the value of the stabilizing effect depending on the chain stiffness is estimated.     

Study on ATP concentration changes in cytosol of individual cultured neurons during glutamate-induced deregulation of calcium homeostasis

For the first time, simultaneous monitoring of changes in the concentration of cytosolic ATP ([ATP]_c), pH (pH_c), and intracellular free Ca²⁺ concentration ([Ca²⁺]_i) of the individual neurons challenged with toxic glutamate (Glu) concentrations was performed. To this end, the ATP-sensor AT1.03, which binds to ATP and therefore enhances the efficiency of resonance energy transfer between blue fluorescent protein (energy donor) and yellow-green fluorescent protein (energy acceptor), was expressed in cultured hippocampal neurons isolated from 1–2-day-old rat pups. Excitation of fluorescence in the acceptor protein allowed monitoring changes in pH_c. Cells were loaded with fluorescent low-affinity Ca²⁺ indicators Fura-FF or X-rhod-FF to register [Ca²⁺]_i. It was shown that Glu (20 μM, glycine 10 μM, Mg²⁺-free) produced a rapid acidification of the cytosol and decrease in [ATP]_c. An approximately linear relationship (r ² = 0.56) between the rate of [ATP]_c decline and latency of glutamate-induced delayed calcium deregulation (DCD) was observed: higher rate of [ATP]_c decrease corresponded to shorter DCD latency period. DCD began with a decrease in [ATP]_c of as much as 15.9%. In the phase of high [Ca²⁺]_i, the plateau of [ATP]_c dropped to 10.4% compared to [ATP]_c in resting neurons (100%). In the presence of the Na⁺/K⁺-ATPase inhibitor ouabain (0.5 mM), glutamate-induced reduction in [ATP]_c in the phase of the high [Ca²⁺]_i plateau was only 36.6%. Changes in [ATP]_c, [Ca²⁺]_i, mitochondrial potential, and pH_c in calcium-free or sodium-free buffers, as well as in the presence of the inhibitor of Na⁺/K⁺-ATPase ouabain, led us to suggest that in addition to increase in proton conductivity and decline in [ATP]_c, one of the triggering factors of DCD might be a reversion of the neuronal plasma membrane Na⁺/Ca²⁺ exchange.     

Interaction of small molecules with phospholipid bilayer membranes: permeability studies

The passage of a phospholipid through the gel to liquid crystal phase transition is associated with an increase in the motional freedom of its fatty acyl chains as measured by spectroscopic techniques and an essentially isothermal absorption of heat as measured by differential scanning calorimetry (DSC). In addition, bilayers formed from that phospholipid display a permeability maximum for both non-electrolytes and electrolytes in the temperature region of the phase transition. In this study the sodium (and in some cases glucose) permeabilities of liposomes composed of either dimyristoyl or dipalmitoyl phosphatidylcholine plus dicetylphosphate were measured in the presence of a group of benzene and adamantane derivatives known to increase fatty acyl chain motion below the lipid transition temperature (T_c) and in the case of the adamantanes to also lower the T_c as measured by DSC. None of these compounds change the temperature at which the permeability maximum occurs despite their lowering of the phospholipid T_c. That is, in the presence of these additives there is observed an apparent dissociation between the phase transition and the permeability maximum. It is proposed that the permeability maximum normally observed in the temperature region of the T_c is associated with the completion of the ‘melting’ process. Hence a compound could cause early ‘melting’ of the bilayer but not change its permeability properties if the temperature at which the ‘melting’ process neared completion was not changed.     

Individual heme a and heme a3 contributions to the Soret absorption spectrum of the reduced bovine cytochrome c oxidase

Bovine cytochrome c oxidase (CcO) contains two hemes, a and a₃, chemically identical but differing in coordination and spin state. The Soret absorption band of reduced aa₃-type cytochrome c oxidase consists of overlapping bands of the hemes a²⁺ and a₃²⁺. It shows a peak at ～444 nm and a distinct shoulder at ～425 nm. However, attribution of individual spectral lineshapes to hemes a²⁺ and a₃²⁺ in the Soret is controversial. In the present work, we characterized spectral contributions of hemes a²⁺ and a₃²⁺ using two approaches. First, we reconstructed bovine CcO heme a²⁺ spectrum using a selective Ca²⁺-induced spectral shift of the heme a²⁺. Second, we investigated photobleaching of the reduced Thermus thermophilus ba₃- and bovine aa₃-oxidases in the Soret induced by femtosecond laser pulses in the Q-band. The resolved spectra show splitting of the electronic B_0x-, B_0y-transitions of both reduced hemes. The heme a²⁺ spectrum is shifted to the red relative to heme a₃²⁺ spectrum. The ～425 nm shoulder is mostly attributed to heme a₃²⁺.     

Concerted involvement of cooperative proton–electron linkage and water production in the proton pump of cytochrome c oxidase

In cytochrome c oxidase, oxido-reductions of heme a/Cu_A and heme a₃/Cu_B are cooperatively linked to proton transfer at acid/base groups in the enzyme. H⁺/e⁻ cooperative linkage at Fe_a3/Cu_B is envisaged to be involved in proton pump mechanisms confined to the binuclear center. Models have also been proposed which involve a role in proton pumping of cooperative H⁺/e⁻ linkage at heme a (and Cu_A). Observations will be presented on: (i) proton consumption in the reduction of molecular oxygen to H₂O in soluble bovine heart cytochrome c oxidase; (ii) proton release/uptake associated with anaerobic oxidation/reduction of heme a/Cu_A and heme a₃/Cu_B in the soluble oxidase; (iii) H⁺ release in the external phase (i.e. H⁺ pumping) associated with the oxidative (R → O transition), reductive (O → R transition) and a full catalytic cycle (R → O → R transition) of membrane-reconstituted cytochrome c oxidase. A model is presented in which cooperative H⁺/e⁻ linkage at heme a/Cu_A and heme a₃/Cu_B with acid/base clusters, C₁ and C₂ respectively, and protonmotive steps of the reduction of O₂ to water are involved in proton pumping.     

Estimating Orientation Factors in the FRET Theory of Fluorescent Proteins: The TagRFP-KFP Pair and Beyond

The orientation factor κ², one of the key parameters defining Förster resonance energy transfer efficiency, is determined by the transition dipole moment orientations of the donor and acceptor species. Using the results of quantum chemical and quantum mechanical/molecular mechanical calculations for the chromophore-containing pockets in selected colored proteins of the green fluorescent protein family, we derived transition dipole moments corresponding to the S_0,min → S₁ excitation for green fluorescent protein, red fluorescent protein (TagRFP), and kindling fluorescent protein, and the S_1,min → S₀ emission for TagRFP. These data allowed us to estimate κ² values for the TagRFP-linker-kindling fluorescent protein tetrameric complex required for constructing novel sensors.     

Dissimilar mechanisms of cytochrome c release induced by octyl glucoside-activated BAX and by BAX activated with truncated BID

In the present study, we compared alkali-resistant BAX insertion into the outer mitochondrial membrane, mitochondrial remodeling, mitochondrial membrane potential changes, and cytochrome c (Cyt c) release from isolated brain mitochondria triggered by recombinant BAX oligomerized with 1% octyl glucoside (BAX_oligo) and by a combination of monomeric BAX (BAX_mono) and caspase 8-cleaved C-terminal fragment of recombinant BID (truncated BID, t^cBID). We also examined whether the effects induced by BAX_oligo or by BAX_mono activated with t^cBID depended on induction of the mitochondrial permeability transition. The results obtained in this study revealed that t^cBID plus BAX_mono produced BAX insertion and Cyt c release without overt changes in mitochondrial morphology. On the contrary, treatment of mitochondria with BAX_oligo resulted in BAX insertion and Cyt c release, which were accompanied by gross distortion of mitochondrial morphology. The effects of BAX_oligo could be at least partially suppressed by mitochondrial depolarization. The effects of t^cBID plus BAX_mono were insensitive to depolarization. BAX_oligo produced similar BAX insertion, mitochondrial remodeling, and Cyt c release in KCl- and in N-methyl-d-glucamine-based incubation media indicating a non-essential role for K⁺ influx into mitochondria in these processes. A combination of cyclosporin A and ADP, inhibitors of the mitochondrial permeability transition, attenuated Cyt c release, mitochondrial remodeling, and depolarization induced by BAX_oligo, but failed to influence the effects produced by t^cBID plus BAX_mono. Thus, our results suggest a significant difference in the mechanisms of the outer mitochondrial membrane permeabilization and Cyt c release induced by detergent-oligomerized BAX_oligo and by BAX activated with t^cBID.     

Reference Phase Analysis of Free and Bound Intracellular Solutes: II. Isothermal and Isotopic Studies of Cytoplasmic Sodium, Potassium, and Water

The intracellular reference phase (RP) method and ultra-low temperature micro-dissection were used for isothermal and isotopic phase distribution studies of Na⁺, K⁺, and water in amphibian oocyte cytoplasm. One-third of the cytoplasmic water is available as solvent for [³H]sucrose. This fraction, designated c1, quantitatively coincides with the water volume in which Na⁺ and K⁺ are freely diffusible. Two-thirds of the cytoplasmic water is inaccessible to sucrose and is designated c2. The Na⁺ and K⁺ associated with c2 are extremely slowly exchanging (bound) and at different concentrations than in c1. The cations in c1 are in mass-action equilibria with those in c2, each described by an equation of the form

C^c_i = C^c1_i + C^c2_i = q_i·C^RP_i + ^maxC^c2_i·f(C^RP_i

in which C^c_i is the cytoplasmic Na⁺ or K⁺ concentration, C^c1_i is the free, and C^c2_i the bound cation concentration averaged over the cytoplasmic water. q_i is the fractional free solute space, C^RP_i the RP concentration, ^maxC^c2_i the concentration of binding sites, and the function f is satisfied by the Langmuir isotherm. Numerical values for the variables of the isotherm are determined. Activity coefficients are calculated from RP data and provide a basis for generalizing the oocyte results to other cells. The conclusion is drawn that both c1 and c2 are widely distributed in cells, and that cellular ionic activities involve two distinct systems: the cell-membrane system and an adsorbed water ion-exchange-like buffering system. Alternative explanations for the two-component cytoplasm are considered. A model is proposed in which c1 is a normal intracellular aqueous phase controlled by the plasma membrane, whereas c2 consists of water and ions adsorbed in hydrate crystalline structures. In oocytes these structures are identified with yolk platelets.

     

Long-Range Interactions between Ligands Bound to a DNA Molecule Give Rise to Adsorption with the Character of Phase Transition of the First Kind

Abstract

Influence of long-range interactions between ligands bound to DNA molecule on the character of their adsorption is studied using computer modeling. For this investigation, two calculation procedures are developed. They are based upon the method of the free energy minimum and on the partition function method. The both procedures demonstrate that in the case of a strong enough attraction between all the bound ligands their binding to DNA has the character of phase transition of the first kind. There is a break in the binding curve c(c₀) where c—relative concentration of bound ligands, c ₀—molar concentration of free ligands. The break occurs because there is an interval of central degrees of binding (～50% of the maximum c value) that are prohibited for individual DNA molecules. Such a transition might be caused by some types of DNA condensation. Attraction between the neighboring ligands only, adjacent or/and separated by double helix regions, does not cause this effect.     

Growth and leaf gas exchange in Populus euphratica across soil water and salinity gradients

Soil water and salinity conditions of the riparian zones along the Tarim River, northwest China, have been undergoing alterations due to water use by human or climate change, which is expected to influence the riparian forest dominated by an old poplar, Populus euphratica. To evaluate the effects of such habitat alterations, we examined photosynthetic and growth performances of P. euphratica seedlings across experimental soil water and salinity gradients. Results indicated that seedlings were limited in their physiological performance, as evidenced by decreases in their height and biomass, and the maximal quantum yield of photosystem II (PSII) photochemistry (F_v/F_m), the effective quantum-use efficiency of PSII (F_v′/F_m′), and photochemical quenching (q_P) under mild (18% soil water content, SWC; 18.3 g kg^?1 soil salt content, SSC) and moderate (13% SWC, 22.5 g kg^?1 SSC) water or salinity stress. However, seedlings had higher root/shoot ratio (R/S), increased nonphotochemical quenching (NPQ), and water-use efficiency (WUE) relative to control under such conditions. Under severe (8% SWC, 27.9 g kg^?1 SSC) water or salinity stress, P. euphratica seedlings had only a fifth of biomass of those under control conditions. It was also associated with damaged PSII and decreases in WUE, the maximal net photosynthetic rate (P _Nmax), light-saturation point (LSP), and apparent quantum yield (_α). Our results suggested that the soil conditions, where P.euphratica seedlings could grow normally, were higher than ～ 13% for SWC, and lower than ～22.5 g kg^?1 for SSC, the values, within the seedlings could acclimate to water or salinity stress by adjusting their R/S ratio, improving WUE to limit water loss, and rising NPQ to dissipate excessive excitation energy. Once SWC was lower than 8% or SCC higher than ～28 g kg^?1, the seedlings suffered from the severe stress.     

Barotropic and thermotropic bilayer phase behavior of positional isomers of unsaturated mixed-chain phosphatidylcholines

The bilayer phase transitions of six kinds of mixed-chain phosphatidylcholines (PCs) with an unsaturated acyl chain in the sn-1 or sn-2 position, 1-oleoyl-2-stearoyl- (OSPC), 1-stearoyl-2-oleoyl- (SOPC), 1-oleoyl-2-palmitoyl- (OPPC), 1-palmitoyl-2-oleoyl- (POPC), 1-oleoyl-2-myristoyl- (OMPC) and 1-myristoyl-2-oleoyl-sn-glycero-3-phosphocholine (MOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light transmittance measurements. Bilayer membranes of SOPC, POPC and MOPC with an unsaturated acyl chain in the sn-2 position exhibited only one phase transition, which was identified as the main transition between the lamellar gel (L_β) and liquid crystalline (L_α) phases. On the other hand, the bilayer membranes of OSPC, OPPC and OMPC with an unsaturated acyl chain in the sn-1 position exhibited not only the main transition but also a transition from the lamellar crystal (L_c) to the L_β (or L_α) phase. The stability of their gel phases was markedly affected by pressure and chain length of the saturated acyl chain in the sn-2 position. Considering the effective chain lengths of unsaturated mixed-chain PCs, the difference in the effective chain length between the sn-1 and sn-2 acyl chains was proven to be closely related to the temperature difference of the main transition. That is, a mismatch of the effective chain length promotes a temperature difference of the main transition between the positional isomers. Anomalously small volume changes of the L_c/L_α transition for the OPPC and OMPC bilayers were found despite their large enthalpy changes. This behavior is attributable to the existence of a cis double bond and to significant inequivalence between the sn-1 and sn-2 acyl chains, which brings about a small volume change for chain melting due to loose chain packing, corresponding to a large partial molar volume, even in the L_c phase. Further, the bilayer behavior of unsaturated mixed-chain PCs containing an unsaturated acyl chain in the sn-1 or sn-2 position was well explained by the chemical-potential diagram of a lipid in each phase.     

Two-dimensional infrared correlation spectroscopy study of the interaction of oxidized and reduced cytochrome c with phospholipid model membranes

We have used two-dimensional infrared correlation spectroscopy (2D-IR) to study the interaction and conformation of cytochrome c in the presence of a binary phospholipid mixture composed of a zwitterionic perdeuterated phospholipid and a negatively-charged one. The influence of the main temperature phase transition of the phospholipid model membranes on the conformation of cytochrome c has been evaluated by monitoring both the Amide I′ band of the protein and the CH₂ and CD₂ stretching bands of the phospholipids. Synchronous 2D-IR analysis has been used to determine the different secondary structure components of cytochrome c which are involved in the specific interaction with the phospholipids, revealing the existence of a specific interaction between the protein with cardiolipin-containing vesicles but not with phosphatidic acid-containing ones. Interestingly, 2D-IR is capable of showing the existence of significant changes in the protein conformation at the same time that the phospholipid transition occurs. In summary, 2D-IR revealed an important effect of the phospholipid phase transition of cardiolipin on the secondary structure of oxidized cytochrome c but not to either reduced cytochrome c or in the presence of phosphatidic acid, demonstrating the existence of specific intermolecular interactions between cardiolipin and cytochrome c.     

Experimental investigation of X-ray lasing in a capillary argon discharge

Results are presented from experiments on the laser generation of X-ray radiation at the wavelength λ=469 ? (ε=26.4 eV) on the 3p(J=0)−3s(J=1) transition of Ne-like Ar ions. Experiments were carried out on the SIGNAL electrophysical facility with a 3.1-mm-diameter 157-mm-long Al₂O₃ ceramic capillary filled with argon at a pressure of 0.2–1.0 Torr. The discharge current amplitude was I ∼ 25–40 kA, the current rise rate being dI/dt ∼ 10¹² A/s. By a vacuum X-ray diode tuned to detect X-ray photons with energies in the range 10–40 eV, laser pulses with a duration of t ₁ ∼ 1 ns and maximum energy of E _1,max ∼ 1 μJ were recorded. The pulses were generated 35 ns after the discharge current was switched on. The line spectra in the wavelength range of 150–500 ? showed the bright λ=469 ? line. The angular divergence of the generated X-ray laser beam was estimated to be Δϑ ∼ 2 mrad. Original Russian Text ? O.N. Gilev, V.I. Afonin, V.I. Ostashev, V.Yu. Politov, A.M. Gafarov, A.L. Zapysov, A.V. Andriyash, é.P. Magda, L.N. Shamraev, A.A. Safronov, A.V. Komissarov, N.A. Khavronin, N.A. Pkhaĭko, L.V. Antonova, L.N. Shushlebin, 2006, published in Fizika Plazmy, 2006, Vol. 32, No. 2, pp. 160–165.