Lipid chain mobility in interdigitated DPPC systems

The segmental lipid chain mobility in the gel phase of dipalmitoylphosphatidylcholine (DPPC) multilayers dispersed in buffer and in the interdigitated gel phase induced by glycerol, ethylene glycol, ethanol and chaotropic salt NaClO₄ was compared by using conventional electron spin resonance (ESR) spectroscopy. The stearic acids bearing the nitroxide moiety at different positions down the acyl chain (n-NSA,n-5, 7, 10, 12 and 16) were used to characterized the chain motion, and the outer hyperfine splittings of the spectra, 2A _max, were taken as indices of the rotational mobility of the chain in the gel phase. The ESR measurements revealed a gradient of increased mobility on proceeding towards the terminal methyl end in the fully hydrated gel phase of DPPC bilayers. This gradient was reduced in the interdigitated gel phase induced by ethanol and chaotropic salt NaClO₄, whereas the rotational mobility throughout the length of the chain was comparable to that near the polar/apolar interface in the interdigitated gel phase in glycerol and ethylene glycol. Moreover, the motional anisotropy was much less affected by temperature in the interdigitated gel state of DPPC in glycerol and ethylene glycol as compared both to normal bilayer gel phase and to the other interdigitated DPPC systems. Finally, there was no evidence for chain interdigitation in the fluid phase of DPPC dispersions in any medium.     

Gravity modulation study on opposed flame spread over thin solid fuels

In this work a numerical investigation has been carried out to study the effect of g-jitter on zero-gravity (0g_e) opposed flow spreading flame over thin solid fuels. For comparison simulations have also been carried out for normal gravity (1g_e) downward spreading flames. G-jitter is emulated by gravity modulation of sinusoidal (A_ge sin(2πt/T_ge)) gravity perturbation (g-perturbation) of a particular time-period (T_ge) and amplitude (A_ge) over a selected base gravity level (0g_e or 1g_e). The response of flames at 0g_e base gravity and at 1g_e base gravity was different to the imposed g-perturbation. While at 0g_e the mean and the amplitude of the oscillatory flame spread rate (FSR) magnified with increase in the time period of g-perturbation, interestingly for the 1g_e flame a maximum mean FSR and oscillation amplitude occurs at certain perturbation time period. Further, at very small perturbation time-periods (T_ge) the FSR at 1g_e was lower than the steady state FSR. The amplitude of oscillatory FSR increased with increase in perturbation amplitude (A_ge). However, the 0g_e flame which is little affected (compared to 1g_e flame) at small perturbation amplitude (A_ge) is affected severely at large perturbation amplitude (A_ge). Both the gas phase and fuel pyrolysis (or fuel response) follow perturbation signal with a lag but fuel pyrolysis is more sluggish and lags behind gas phase. The phase lag between fuel pyrolysis and gas increases at smaller time-periods (T_ge) and tends to enhance the effect of external perturbation whereas at larger time-periods (T_ge) this lag inhibits the effect of external perturbation.     

Electrolyte interaction with DPPC vesicles: An ESR study

Summary The partition of the spin probe TEMPO between the fluid lipid phase of single-walled vesicles of dipalmitoylphosphatidylcholine and the aqueous bulk solution have been used to investigate the interaction of monovalent ions with polar head of neutral phospholipids. The study has been performed by electron spin resonance (ESR) spectroscopy in the temperature range of (20÷60)°C and in the presence of (0÷3) M 1∶1 electrolyte. In the absence of electrolyte the spin probe TEMPO reveals the characteristic order→disorder DPPC main phase transition atT _m≈37°C, while the pretransition occurs atT _p≈27.5°C. On increasing the ionic strength of the dispersion medium it results for the partition coefficient,P _C, that, at each temperature,P _C(3)>P _C(2)>P _C(1)>P _C(0). Correspondingly, the pretransition disappears and theT _m value downshifts from ≈37°C with 0 M electrolyte to ≈34°C with 3M salt in the order:T _m(3)>T _m(2)>T _m(1)>T _m(0). The results suggest an increase in the net surface charge density of vesicles due to high ionic-strength values. The alteration of the electric interactions occurring into the polar zone of DPPC bilayer reduces the hindrances which, in turn, favour the enhancement of TEMPO partitioning in the hydrophobic core of phospholipid bilayers. The authors of this paper have agreed to not receive the proofs for correction.     

Spin probe ESR studies of PEGxLiClO4 polymer electrolyte systems

The technique of Electron Spin Resonance (ESR) is shown to be useful in the study of dynamics of solid polymer electrolytes (SPE). Through the ESR of the nitroxide radical (2,2,6,6-tetramethyl-1-piperidine-1-oxyl; TEMPO) dispersed in the SPE PEG₄₆LiClO₄ temperature dependence of correlation time is found. The glass transition temperature T_g is estimated to be −51 °C from the measurement of T_50G, the temperature at which the extrema separation 2A_ZZ becomes 50G and is found to be close to that measured using DSC (−51.7 °C). T_g for pure PEG-2000, which could not be measured from DSC because of its high crystallinity, is determined to be −72 °C by spin probe ESR. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

EPR properties of Au atoms adsorbed on various sites of the MgO(1 0 0) surface from relativistic DFT calculations

Using all electron fully relativistic DFT calculations we have computed the EPR properties of Au atoms bound to various sites of the MgO surface. Changes in g-tensor and hyperfine coupling constants provide a way to identify the gold adsorption site and to map the surface morphology by comparison of measured and calculated EPR spectra. We found a strong reduction of the isotropic hyperfine coupling constant, a_iso(Au), for adsorbed gold compared to the free atom; this reduction, which is about 45% for terrace sites, is more pronounced when Au interacts with low-coordinated sites like steps, edges and corners where it is about 60%. The reduction of a_iso(Au) is accompanied by a corresponding increase of the superhyperfine interaction with the surface oxygen sites, as measured by a_iso(¹⁷O). Large anisotropies in the g-tensor are computed for all sites.     

Studies on behaviors of dipalmitoylposphatidylcholine and bilirubin in mixed monolayer at the air/water interface

Mixed monolayers of dipalmitoylposphatidylcholine (DPPC) and bilirubin (BR) were prepared on different subphases. The properties of DPPC/BR monolayer, such as collapse pressure (π_coll), limiting area per molecule (A_lim), surface compressibility modulus, free energy (ΔG_mix) and excess free energy (ΔG_ex), were investigated based on the analysis of the surface pressure-area isotherms on pure water. The results showed that DPPC and BR were miscible and formed non-ideal mixed monolayers at the air/water interface. With the molar fraction of BR (X_BR) increasing, the LE-LC coexistence region of DPPC monolayer was eliminated gradually. The DPPC/BR complex (M_D-B) of 1:2 stoichiometry formed as a result of the strong hydrogen bonds between the polar groups of DPPC and BR. The studies of effects of pH values and calcium ions in subphase on the DPPC/BR monolayers showed that the mixed monolayer became expanded on alkali aqueous solution and on 1 mmol/L CaCl₂ aqueous solution. The orientation of DPPC and BR at air/water interface was also discussed.     

Theoretical study of hyperfine coupling constants and electron spin g factors for X2Σ+ diatomics from Groups 1 and 2

The ESR parameters of the cations Be⁺ ₂, Mg⁺ ₂, Ca⁺ ₂, BeMg⁺, BeCa⁺, MgCa⁺ and the mixed radicals ZBe, ZMg, ZCa (Z = Li, Na, K), all having a X ²Σ⁺ _u(1σ² _g1σ_u)/X ²Σ⁺(1σ²2σ) ground state, have been studied theoretically. The A _iso and A _dip constants have been calculated with UHF, CISD, MP2, B3LYP, PW91PW91 wavefunctions, and 6–311+G(2df) basis sets. The electron spin g factors (magnetic moment μ_s) have been evaluated from correlated (MRDCI) wavefunctions, using a Hamiltonian based on Breit-Pauli theory with perturbation expansions up to second order, and 6–311 + G(2d) basis sets. As expected for s-rich radicals, the hyperfine spectra are governed by the A _iso terms. Both Δg∥ and Δg⊥ values are negative, but Δg∥ lies close to zero. For Δg⊥, the coupling with 1 ²Π_(u) dominates the sum-over-states expansions. Although the singly occupied MOs (SOMO) are mostly of s character, the |Δg⊥| are relatively large, up to 5200 ppm for cationic, and up to 7850 ppm for neutral radicals. These large values are caused by low excitation energies and high magnetic transition moments, the latter due to the fact that the σ?(s-s) SOMO has the same nodal properties as a pσ orbital. Of the radicals considered here, an ESR spectrum is available only for Mg⁺ ₂. Our theoretical A _iso of ?287 MHz reproduces well the matrix result (-291 MHz). Calculated values of ?10 ppm for Δg∥ and of ?1280 ppm for Δg⊥ give an average 〈Δg〉 = ?860 ppm that lies within the experimental range of ?600(±300) ppm in Ne, and of ?1300(±500) ppm in Ar matrices.     

Raman scattering from soft phonons in NdP5O14

Raman spectra of NdP₅O₁₄ were recorded between 4.2 and 600 K and analyzed. The monoclinic-orthorhombic phase transition temperature was determined to be 420 K by measuring the energy of an A_g-B_2g and a B_g-B_3g soft optic phonon mode as a function of temperature (symmetries refer to the monoclinic and orthorhombic phase, respectively). Coupling of these optic phonons to the soft acoustical phonon which is associated with the phase transition is discussed.     

Raman study of phase transformation of TiO2 rutile single crystal irradiated by infrared femtosecond laser

Titanium dioxide (TiO₂) rutile single crystal was irradiated by infrared femtosecond (fs) laser pulses with repetition rate of 250 kHz and phase transformation of rutile TiO₂ was observed. Micro-Raman spectra show that the intensity of E_g Raman vibrating mode of rutile phase increases and that of A_1g Raman vibrating mode decreases apparently within the ablation crater after fs laser irradiation. With increasing of irradiation time, the Raman vibrating modes of anatase phase emerged. Rutile phase of TiO₂ single crystal is partly transformed into anatase phase. The anatase phase content transformed from rutile phase increased to a constant with increasing of fs pulse laser irradiation time. The study indicates the more stable rutile phase is transformed into anatase phase by the high pressure produced by fs pulse laser irradiation.     

Influence of (phospho)lipases on properties of mica supported phospholipid layers

The effect of enzymes: lipase from Candida cylindracea (L_Cc), phospholipase A₂ from hog pancreas (PLA₂) and phospholipase C from Bacillus cereus (PLC) to modulate wetting properties of solid supported phospholipid bilayers was studied via advancing and receding contact angle measurements of water, formamide and diiodomethane, and calculation of the surface free energy and its components from van Oss et al. (LWAB) and contact angle hysteresis (CAH) approaches. Simultaneously, topography of the studied layers was determined by Atomic Force Microscopy (AFM). The investigated lipid bilayers were transferred on mica plates from subphase of pure water by means of Langmuir-Blodgett and Langmuir-Schaefer techniques. The investigated phospolipid layers were: saturated DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), unsaturated DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine), and their mixture DPPC/DOPC. The obtained results revealed that the lipid membrane degradation by the enzymes caused increase in its surface free energy due to the amphiphilic hydrolysis products, which may accumulate in the lipid bilayer. In result activity of the enzymes may increase and then break down the bilayer structure takes place. It is likely that after dissolution of the hydrolysis reaction products in the bulk phase, patches of bare mica surface are accessible, which contribute to the apparent surface free energy changes. Comparison of AFM images and the free energy changes of the layers gives better insight into changes of their properties. The observed gradual increase in the layer surface free energy allows controlling of the hydrolysis process to obtain the surfaces of defined properties.     

Influence of salicylic acid on the biophysical properties of dipalmitoyl phosphatidylcholine vesicles

The effect of the keratolytic drug salicylic acid (SA) on the thermotropic behaviour, and dynamics of dipalmitoyl phosphatidyl choline (DPPC)–water/buffer pH?7.4 vesicles was studied using DSC and ¹H NMR. In both systems, incorporation of SA in DPPC bilayer causes a significant depression in the transition temperature of both the pre-transition (PT) and the gel-to-liquid crystalline (CM) transition. The presence of the drug reduces the cooperativity of both the PT and CM transitions. These findings indicate that SA is bound strongly to the lipid bilayer leading to increased membrane fluidity. The DPPC vesicles incorporated with high drug concentration show phase segregation. One of the interesting findings in this study is the formation of a more ordered high temperature gel (L_β2) phase when the SA-doped DPPC dispersion is prepared at physiological pH. The effect of inclusion of cholesterol in the SA-free and SA-doped DPPC dispersion was also studied.     

Local31P and distant31P and195Pt ENDOR studies of powdered samples of Bis(O,O′-disubstituted-dithiophosphato)copper(II) complexes magnetically diluted in the corresponding Pd(II) and Pt(II) host lattices

ENDOR studies on bis(dithiophosphato)copper(II) complexes magnetically diluted in the corresponding Pd(II) and Pt(II) host lattices performed with the use of the method of “polycrystalline ENDOR crystallography” are reported. In these samples well resolved local³¹P (A_iso ca. 29 MHz and A_aniso ca. 0.9 MHz) as wel1 as distant³¹P (A _iso ca. 1.3 and 5.5 MHz and A_aniso ca. 0.2 and 0.4 MHz) and¹⁹⁵Pt (A_iso ca. 3.1 MHz and A_aniso ca. 0.2 MHz) ENDOR transitions are recorded. It is shown that the directions of the main values of A_aniso for all local³¹P and distant³¹P and¹⁹⁵Pt ENDOR transitions are parallel. Comparison of these results with those reported about the same complexes but diluted in other host lattices shows that there is no difference in the EPR and local³¹P ENDOR parameters thus suggesting that the structure of the paramagnetic complex remains unchanged. It is found that distant ENDOR transitions appear only in the case when the host lattice contains one molecule in the unit cell, i.e., when all molecules are parallel to each other. Using the obtained data from local and distant³¹P ENDOR transitions some structural features of Cu(dtp)₂ as well as Pd(dtp)₂ and Pt(dtp)₂ complexes are found for the first time.     

Monolayer characteristics of stereoregular PMMA at the air/water interface

Behaviors of isotactic, atactic and syndiotactic poly(methyl methacrylate) (PMMA) monolayers at the air/water interface were investigated. The monolayer characteristics of the three stereoisomers were studied and compared in terms of surface pressure-area per molecule (π-A) isotherm, hysteresis and relaxation phenomena, and the phase images observed from Brewster angle microscopy (BAM). The results show that iPMMA monolayer has a more extended and compressible property, and exhibits pronounced hysteresis and relaxation phenomena among the three tactic PMMAs. Such characteristics can reasonably be attributed to the higher flexibility of chains segments of iPMMA as estimated from its lower T_g. On the other hand, sPMMA, the one with the highest T_g, forms a more condensed monolayer due to the high cohesive interaction among polymer chains. The BAM images show that PMMA molecules are not well extended on the subphase and thus, aggregative phases were observed even in a state corresponding to the gas phase. The aggregative structures are especially significant on sPMMA monolayer, which is consistent with the results estimated from the π-A isotherms. When the temperature is elevated from 25 to 40 °C, all the monolayers of the three stereoisomers become more expanded and compressible due to the increase in chain flexibility.     

One-gluon-exchange effects on semileptonic decay of baryon octet

We study one-gluon-exchange (OGE) effects ong _A /g _V of weak semileptonic decay processes of theSU(3) baryon octet. We especially investigate the systematic behavior of OGE effects on all the possible decay processes and search for better overall agreements between theory and experiment. We adopt the MIT bag model to describe quarks confined in a baryon. We also use a formulation of the perturbative QCD in which gluon field is also confined in the bag. Our calculations respect the boundary conditions which achieve the confinement of each field within the bag. The magnitudes of our corrections tog _A /g _V have a variation of 1 ～30% depending on the decay, processes. They have process-dependence quite different from that of the uncorrected values ofg _A /g _V primarily governed by theSU(6) symmetry. These OGE effects ong _A /g _V reflectSU(6) asymmetric spin-spin correlation between quarks due to the color magnetic interactions. Especially, the OGE corrections tog _A /g _V are very important in the processes, \(\Sigma ^ - \to n + l^ - + \bar v_l ,\Sigma ^ - \to p + l^ - + \bar v_l \) and \(\Xi ^ - \to \Xi ^0 + e^ - + \bar v_e \) . Consequently, we clarify that the OGE effects can sizably improve theoretical values ofg _A /g _V in favor of overall agreements with the present experimental data of semileptonic decays of the baryon octet, at least, in the MIT bag model.     

Applications of photoelectron spectroscopy to molecular propertles: XVIII. The vertlcal ionization potentials of diphosphene as calculated by perturbation corrections to koopmans' theorem

The vertlcal ionization potentials (VIP's) of dimethyl diphosphene have been computed using perturbation corrections to Koopmans' theorem. These corrections allow an estimate of the effects of polarization and correlation, and show for this molecule an important contribution of the specific correlation of the ion for the ²A_g and ²B_u states. The results predict for the first three VIP's the ordering ²A_g, ²A_u, ²B_u, in agreement with our previous assignment.     

Defect model and spin-Hamiltonian parameters for the tetragonal Nd center in the tetragonal phase of SrTiO3 crystal

The spin-Hamiltonian parameters (g factors g_∥, g_⊥ and hyperfine structure constants ¹⁴³A_∥, ¹⁴³A_⊥, ¹⁴⁵A_∥ and ¹⁴⁵A_⊥) of the tetragonal Nd³⁺ center in the low-temperature (T≈4.2 K) tetragonal phase of SrTiO₃ are calculated from a diagonalization (of energy matrix) method. In the method, the Zeeman and hyperfine interaction terms are attached to the conventional Hamiltonian and a 52×52 energy matrix concerning the ground term ⁴H_J (J=9/2, 11/2, 13/2, 15/2) is constructed. The Nd³⁺ center is attributed to Nd³⁺ occupying the 12-fold coordinated Sr²⁺ site in SrTiO₃. Differing from the defect model assumed in the previous paper that the tetragonal distortion of this Nd³⁺ center is due to the association of one interstitial oxygen ion at a nearest neighborhood of Nd³⁺ and the Nd³⁺ displacement Δz along C₄ axis, we suggest that it is due to the distortion of SrTiO₃ lattice in the tetragonal phase. The calculated g factors g_∥ and g_⊥ show good agreement with the experimental values, suggesting that our defect model of Nd³⁺ center in SrTiO₃ is reasonable. The experimental hyperfine structure constants were not reported and so our calculated results remain to be checked by EPR experiment.     

Studies of lattice dynamics in 2HTaS2 by Raman scattering

Raman spectra are presented in the high and low temperature phases of 2H-TaS₂. In the normal phase at 380 K one A_1g- and two E_2g-modes have been observed at 400, 27 and 286 cm^-1, respectively. In the charge density wave state two extra peaks appear with strong scattering intensity in E_2g- symmetry and weak in A_1g-symmetry. The E_2g peak grows and hardens toward 50 cm^-1 with decreasing temperature. The temperature dependence is very similar to the E_2g-mode in 2H-TaSe₂ of similar 3a₀ × 3a₀ superstructure. Other peaks observed in 2H-TaSe₂ in the commensurate phase are not observed. This suggests the incommensurability of the charge density wave state in 2H-TaS₂.     

Impurity in three substitutional locations: Single-crystal EPR study of VO(II) in zinc maleate tetrahydrate

Single-crystal EPR study of VO(II)-doped zinc maleate tetrahydrate has been carried out at room temperature. Four types of impurities have been identified in the single-crystal spectra with intensity ratio of 11:5:2:1. However, the analysis has been done only for the three most intense resonances. The evaluated spin Hamiltonian parameters (A in units of mT) from single-crystal rotations are site I: g_xx=1.981, g_yy=1.971, g_zz=1.939; A_xx=7.43, A_yy=7.70, A_zz=18.7; site II: g_xx=1.973, g_yy=1.966, g_zz=1.939; A_xx=7.08, A_yy=7.22, A_zz=18.5; site III: g_xx=1.978, g_yy=1.977, g_zz=1.951; A_xx=7.11, A_yy=7.32, A_zz=18.3.The powder spectrum gives only one set of g and A values, confirming the presence of only one chemically equivalent site. The paramagnetic impurity, VO(II), has entered the lattice substitutionally for all the three sites. The superhyperfine structure, in the intensity ratio of 1:4:6:4:1, arising from the protons of the water ligands, has been found in one site. The admixture coefficients have been calculated and the complex is found to be fairly covalent in nature.     

Lattice vibrations in the orthorhombic commensurate charge density wave phase of 2H-TaSe2

Lattice vibrations in 2H-TaSe₂ have been reinvestigated by Raman scattering in view of the recent orthorhombic structure of the commensurate charge density wave (CCDW) phase. The renormalization of six charge density waves on two layers gives four A_g modes and two B_1g modes in the orthorhombic CCDW phase, in place of two A_1g modes and two E_2g modes in the hexagonal CCDW phase which had been believed. The splitting of the E_2g modes to the A_g and B_1g modes in the orthorhombic symmetry depends on the interlayer interaction. The observed small splitting less than 3 cm^-1 shows the weak interlayer interaction. The energies of the four A_g modes and the two B_1g modes are presented as a function of temperature.     

Coexistence of one-electron- and monoprotonated two-electron-reduced species of the K5[PMo2VW9O40] · 24H2O heteropolyoxometalate identified by EPR

The simulation of the room-temperature experimental electron paramagnetic resonance spectrum of K₅[PMo₂VW₉O₄₀] · 24 H₂O heteropolyoxometalate indicates the presence of equal amounts of a one-electron-reduced species (g _∥ = 1.922,g _⊥ = 1.972,A _∥ = 181 G,A _⊥ = 63 G) and a monoprotonated two-electron-reduced species with mixed-valence V^IV, Mo^V and Mo^VI ions (g _iso = 1.972, ΔB _iso(p-p) = 450 G). Two unprotonated one-electron-reduced isomers are identified in dimethylsulfoxide-H₂O solution of the sample atT = 100 K (g _∥¹ = 1.929,g _⊥¹ = 1.990,A _∥¹ = 179 G,A _⊥¹ = 65 G andg _∥² = 1.918,g _⊥² = 2.000,A _∥² = 187 G,A _⊥² = 80 G, respectively). The values of the in-plane π(V−O) bond π₂² coefficient for the one-electron-reduced species (0.87 at room temperature and 0.83 and 0.74 for the species in frozen solution) suggests the delocalization of the vanadium unpaired electron towards the molybdenum ions via O_b atoms.