Action of Humicola lanuginosa lipase on long-chain lipid substrates: 1. Hydrolysis of monoolein monolayers

The hydrolysis of 1-monoolein (MO) monomolecular films by Humicola lanuginosa lipase (HLL) was followed by measuring the simultaneous decrease with time in the film area and the surface potential using a ‘zero order trough’ at constant surface pressure (Verger and de Haas, Chem. Phys. Lipids 10 (1973) 127). The decrease with time in the film area reflects both the reduction in the area per molecule as well as the solubilization of the substrate and the product molecules during the transformation of the substrate MO into product of oleic acid (OA). The surface potential changes were interpreted as the results of the accumulation at the interface of negatively charged OA. The surface concentration of OA transiently present at the interface was determined by the surface pressure and the surface potential measurements on the basis of a developed kinetic model. In the proposed model we have taken into account the product and substrate solubilization rates in the presence of β-cyclodextrin (β-CD) as well as the flux supplied progressively by the moving barrier from the reservoir to the reaction compartment in order to keep the constant surface pressure. Values of the global kinetic constant Q_m were obtained. The selective lipolytic product acceptor, β-CD, accelerated considerably the hydrolytic process.     

Calculation of van der Waals interactions involving lipid vesicles

Van der Waals energies of interaction involving vesicles and lipid layers are calculated for different geometries. Results from exact and approximate calculations are compared with some existing experimental data. It is shown that sufficient accuracy can be obtained using relatively simple ‘additivity’ approximations for the contribution of dispersion interactions. A set of calculated results is presented for a small unilamellar vesicle interacting with coated and uncoated metal and polymer surfaces. A lipid coating lowers magnitude of the interaction energy approximately two-fold. The procedure gives a simple possibility to estimate Hamaker constants (and ‘Hamaker functions’) from handbook data taking into account the existing uncertainity in the materials constants of the interacting substances.     

Membrane electrochemistry

Electrochemistry and biomembranes are interface science in that both are concerned with the phenomena at, as well as across, the interfaces. Membrane electrochemistry may be defined as the application of electrochemistry to biomembrane studies. Additionally, transport processes within the membrane are involved in biomembranes. Since biomembranes are diverse and are usually not amenable to probing by electrochemicophysical techniques, model membrane systems have been developed for their investigation.

The introduction of experimental bilayer lipid membranes (BLM) technique and its modifications have been instrumental in the development and testing of membrane transport concepts (carriers vs channels) and electronic processes in membranes. Instead merely viewing a biomembrane as a physical barrier containing carriers or channels to carry out ionic processes, an ultrathin lipid or biological membrane can also be considered as a complete ‘electrochemical cell’ with one membrane/solution interface reducing (as a cathode) and the other membrane/solution interface oxidizing (as an anode). It is now possible to understand energy transduction (charge generation, separation, and redox reactions) in terms of ultrathin lipid membranes separating two aqueous solutions.

In this paper, we shall discuss the basic principles of electrochemistry as they are applied to membrane studies. Emphasis will be on experimental bilayer lipid membranes (BLM) which have been extensively investigated as models of biomembranes.     

Behavior of pure and mixed DPPC liposomes spread or adsorbed at the air-water interface

Liposomes from pure dipalmitoylphosphatidylcholine (DPPC) and mixed DPPC: distearoylphosphatidylcholine (DSPC): soybean lecithin (SL) prepared by the Bangham method with sonication were dispersed into solution or spread at the interface and the kinetics of the surface film formation was studied by measuring and recording the evolution of superficial tension, surface potential, and superficial (¹⁴C labeled) DPPC density.A simple theoretical approach can describe these kinetics by two processes: irreversible diffusion of closed vesicles into or from the bulk phase, and irrevers ible transformation of closed spherical vesicles into destroyed ones which form the surface film. Diffusion controls the phenomenon for small initial amounts of liposomes.Transformation controls the phenomenon for important initial amounts of liposomes. The kinetic constant of the transformation,K, does not depend on the technique used to form the surface film (spreading or adsorption).The equilibrium and rheological properties of surface films formed after liposome spreading are compared to those of monolayers     

Electrochemical behaviour of some polyoxo clusters

We describe the redox behaviour in non-aqueous solvents of some cyclopentadienyl(oxo)titanium derivatives. The derivative [Ti₄{η⁵-C₅H₄(SiMe₃)}₄(μ-O)₆] shows an electrochemically and chemically reversible le reduction process, followed by a multi-electron, chemically complicated reduction at a fairly cathodic potential. On the basis of the overall electrochemical features and the comparison with the redox behaviour of the quasi-planar compound [[Ti{η⁵-C₅H₄(SiMe₃)}Cl(μ-O)]₄] we propose an EECCEE mechanism for the first derivative, where the second electron-transfer induces a cascade of chemical reactions giving rise to irreversible cluster breakdown. The electrochemically induced fragmentation can be viewed as a retrosynthetic pathway. The heterometallic derivative [{Ti(η⁵-C₅H₄Me)₂(μ₂-MoO₄)₂}₂] shows two consecutive reduction processes; the first is chemically reversible, and the second quasi-reversible. The molybdate bridges apparently increase the stability of the electrogenerated anions. However none of these poly-oxo clusters can be considered as good models of electron ‘sinks’.     

Comparative study of lipolysis by PLA2 of DOPC substrates organized as monolayers, bilayer vesicles and nanocapsules

The water-soluble lipolytic enzymes act at the interface of insoluble lipid substrates, where the catalytical step is coupled with various interfacial phenomena as enzyme penetration, solubilization of reaction products, loss of mechanical stability of organized assemblies of phospholipids molecule, etc. One biologically relevant example is the enzymatic hydrolysis of DOPC by PLA(2), which results in cleavage of phospholipids molecules into water insoluble lipolytic products, namely oleic acid and lysophospholipid. In general, the enzymatic activity depends on the substrate organization and molecular environment of the catalytic reaction. The lipolysis by phospholipase A(2) of dioleoylphosphatidylcholine substrates organized as monolayer, bilayers vesicles and lipid nanocapsules was studied by measuring the decrease of the surface area at constant surface pressure or increase of the surface pressure at constant area at air-water interface. A kinetic model describing the coupling of the catalytic act with corresponding interfacial phenomena was developed. By using the kinetic model the values for the global hydrolytic kinetic constants were obtained. The obtained value for the monolayer is five orders of magnitude higher than this obtained with small unilamellar vesicles and six orders of magnitude higher then those obtained with lipid nanocapsules. The comparison shows that the enzymatic catalytic act occurring in the lipid environment of the monolayer is more efficacious than at the vesicle and nanocapsules interfaces.     

Progres report

This paper provides a brief resumé of our current researches in organometallic chemistry. The first deals with non-classical organolanthanoid chemistry, which has, as targets, subvalent compounds of the early 4f elements (La, Ce, Pr, Nd) and cationic organosamarium(II) and -ytterbium(II) complexes. The second is concerned with new aspects of metal amide chemistry including (a) silver(I) amides and isonitrile complexes of lithium amides; (b) derivatives of aromatic diamides such di(metalamino)cyclophanes; (c) metal silyl complexes derived from insertion of a stable silylene into M(II)---N(SiMe₃)₂ (M = Ge, Sn or Pb) bonds; and (d) reactions of M′---X compounds with 1,3,5-triazine [M′ = Li or Na and X = N(SiMe₃)₂, C(SiMe₃)₃ or CH(SiMe₃)₂] The two other present areas (on subvalent compounds of Group 14 elements and metal 1-azaallyls, β-diketiminates and 1,3-diazaallyls) are reported more cursorily. The Introduction is divided into three section entitled ‘Relationship to the Journal of Organometallics Chemistry’, ‘Reviews (from 1956 to 1995)’ and ‘An outline of principal contributions’.     

Copper(I) and mercury(II) halide complexes of 1,2-bis(diphenylthioylphosphino)hydrazine

The reaction of 1,2-bis(diphenylthioylphosphino)hydrazine (L) with copper(I) and mercury(II) halides affords the complexes, [{CuLX}₂] (X = I, Br or Cl), [HgLX₂] (X = Cl or Br) and the tetrametallic complex, [{L(HgI₂)₂}₂]. Single crystal X-ray structures have been performed on the uncoordinated ligand, L, as well as the complexes [{CuLX}₂] (X = I, Br and Cl), [HgLBr₂] and [{L(HgI₂)₂}₂. The molecules of L exist as dimers as a result of pairs of N–HS hydrogen bonds. The copper(I) complexes are centrosymmetric dimetallic species, the two copper atoms being bridged by L and the X atoms. In all cases the coordination sphere around the Cu atoms is approximately trigonal pyramidal with an ‘S₂X₂’ donor set. The complex, [HgLBr₂], is a distorted tetrahedral monomer with an ‘S₂Br₂’ donor set and L acting as a bidentate thus forming a seven-membered chelate ring. The tetramercury iodo complex, [{L(HgI₂)₂}₂], contains two ‘L(HgI₂)₂’ units linked centrosymmetrically via an I atom from each moiety. The geometry around the Hg atoms is distorted tetrahedral. The influence of hydrogen bonding between the hydrazine backbone hydrogens of L and the coordinated halide ions in for the structures of the metal complexes is discussed.     

Dynamical and morphological studies on the adsorption and penetration of human serum albumin into phospholipid monolayers at the air/water interface

The dynamic adsorption and penetration of human serum albumin (HSA) into the monolayers of five biologically important surfactants—DSPC, DPPC, DMPC, DMPE and DMPA—were systematically studied using Brewster angle microscopy, film balance and pendent drop techniques. Isotherms after different adsorption times show that the presence of HSA changed the monolayer phase behavior (e.g. the shifts of the LE→LC phase transition in the mixed phospholipid/HSA monolayers). Apparent inhomogeneous phases—‘honey-comb’ (J. Mol. Liq., 2001, 90, 149), ‘block’ or ‘stripe’ shape phases are formed due to the adsorption and penetration of HSA into these phospholipid monolayers at the air/water interface. Both the phase behavior changes and the morphological changes were confirmed by our recent structure studies in DPPA/HSA and DPPS/HSA monolayers using X-ray diffraction at grazing incidence, which directly shows that HSA penetration can change the tilt angle of phospholipids. It was found that the adsorption and penetration of HSA strongly depends on the phospholipid head-group structure and the physical state of the phospholipid films. The latter played a dominant role by providing enough space for the penetration of HSA and affecting the hydrophobic interactions of HSA with the aliphatic chains of phospholipids in monolayers at the air/water interface. In general, HSA penetrates more efficiently and quickly into monolayers of phospholipids in liquid state (e.g. DMPC compared to DSPC) and with unprotected charges (e.g. PA compared to PE and PC).     

Conformational features of calix[4]arenes with alkali metal cations: A quantum chemical investigation with density functional theory

A variety of conformations for three model calix[4]arenes with 8 or 12 OH groups have been investigated by calculations at density functional (RI-BP86) and RI-MP2 level of approximation. The calixarenes form stable complexes with the alkali metal cations of lithium up to caesium. For the investigations all-valence electron basis sets as well as various effective core potentials were probed. The stabilities of complexes were analysed in comparison with the corresponding benzene complexes, M⁺·C₆H₆. The formation of the calixarene metal complexes is considered in two steps, (a) in a distortion from the equilibrium conformation of the free calixarenes and (b) subsequent complexation. The distortion energies are small for the ‘crown’ and larger for the ‘boat’ conformations. On the other hand the latter are more stabilized by significant interaction energy of the cation with two adjacent π-systems of the aromatic rings. As a result, these two conformations are of similar stabilities for K⁺ to Cs⁺ complexes with resorc[4]arenes, with a slight advantage for the ‘boat’ structure. The most stable conformation for the coordination products of these cations with the calix[4]arene with 12 OH groups is a slightly flattened ‘crown’ that is derived by maximum hydrogen bonding of the OH-groups and the most effective cation-π interactions. Special cases are complexes of Li⁺ and Na⁺ which in most instances prefer the coordination on the oxygen atoms of the upper rim of the calixarene cavities and thus form ‘boat’-like structures.     

Activity coefficients in concentrated electrolytes: a comparison of the Brunauer-Emmett-Teller (BET) model with experimental values

Electrolyte mean ionic activity coefficients, γ_±, are calculated from the Stokes-Robinson application of the Brunauer-Emmett-Teller (BET) adsorption model for seven electrolytes (NaOH, HCl, KOH, CaCl₂, LiCl, LiBr and Ca(NO₂)₃). Only two model parameters are needed, which are derived from water vapor pressure measurements. Results were compared with experimental mean ionic activity coefficients (γ_±). Because the standard state for the BET model is the anhydrous electrolyte rather than the infinitely dilute solution, it is necessary to adjust for the differing standard states by comparing the BET and experimental γ_± values at one ‘anchoring’ concentration. The higher this ‘anchoring’ concentration, the better is the agreement between the BET and experimental mean ionic activity coefficients over the entire concentration (molality, mol kg⁻¹) range except at the two extremes of nearly pure water and nearly pure electrolyte. This is because the BET model lacks a finite limit at infinite dilution, and the experimental data are referenced to the infinite dilute solution. The BET model is well-behaved and in good agreement with experimental data on γ_±.     

Efficient kinetic resolution of a racemic 7-membered biaryl lactone: enantioselective synthesis of 2,2′-dihydroxymethyl-1,1′-binaphthyl

A novel pathway to enantiomerically pure 2,2′-dihydroxymethyl-1,1′-binaphthyl is described, through kinetic resolution of its racemic 7-membered biaryl lactone precursor. Oxazaborolidine-assisted borane reduction of this lactone proceeds with very high enantioselectivity (k_rel=50). For optimum results on a preparative scale, a ‘three fractions strategy’ is suggested, which combines kinetic resolution and fractional crystallization and leads to both atropisomers in high enantiomeric excess.     

Hanasanagin: a new antioxidative pseudo-di-peptide, 3,4-diguanidinobutanoyl-DOPA, from the mushroom, Isaria japonica

The ‘Hanasanagitake’ mushroom, Isaria japonica, is a folk medicine and a traditional health food. Fractionation of the 60% ethanol extract of the mushroom, guided by the antioxidant activity test, led to the isolation of a new pseudo-di-peptide, and it was called ‘hanasanagin’. Spectral analysis and chemical transformation determined the structure of hanasanagin as 3,4-diguanidinobutanoyl-DOPA.     

Agglutination of like-charged red blood cells induced by binding of beta2-glycoprotein I to outer cell surface

Plasma protein-mediated attractive interaction between membranes of red blood cells (RBCs) and phospholipid vesicles was studied. It is shown that beta(2)-glycoprotein I (beta(2)-GPI) may induce RBC discocyte-echinocyte-spherocyte shape transformation and subsequent agglutination of RBCs. Based on the observed beta(2)-GPI-induced RBC cell shape transformation it is proposed that the hydrophobic portion of beta(2)-GPI molecule protrudes into the outer lipid layer of the RBC membrane and increases the area of this layer. It is also suggested that the observed agglutination of RBCs is at least partially driven by an attractive force which is of electrostatic origin and depends on the specific molecular shape and internal charge distribution of membrane-bound beta(2)-GPI molecules. The suggested beta(2)-GPI-induced attractive electrostatic interaction between like-charged RBC membrane surfaces is qualitatively explained by using a simple mathematical model within the functional density theory of the electric double layer, where the electrostatic attraction between the positively charged part of the first domains of bound beta(2)-GPI molecules and negatively charged glycocalyx of the adjacent RBC membrane is taken into account.     

Aurophilic attraction: the additivity and the combination with hydrogen bonds

The coexistence of gold–gold contacts and hydrogen bonding is studied in the model system [H₂P(OH)AuCl]₂ and [H₂P(OH)AuPH₂(O)]₂. The two interactions are found to be comparable. The possible non-additivity of the aurophilic, Au(I)–Au(I) interaction is studied at MP2 level for the pentagonal [Au(SH)₂(AuSH)₅)]⁻ and hexagonal [Au(SH)₂(AuSH)₆]⁻ clusters. The possibilities of ‘mechanical cooperativity’ between different aurophilic attractions and of Au…S attractions are also considered.     

Solvent‐induced crystallization in poly(ethylene terephthalate) during mass transport

Solvent transport in poly(ethylene terephthalate) (PET) and related phase transformation were investigated. The data of mass sorption were analyzed according to Harmon's model for Case I (Fickian), Case II (swelling), and anomalous transport. This transport process in PET is accompanied by the induced crystallization of the original amorphous state. The transformation was examined by wide‐angle X‐ray scattering, small‐angle X‐ray scattering, differential scanning calorimetry, and Fourier transform infrared spectroscopy. During this process, the matrix is under a strain state that causes different kinetic paths of crystallization as compared with that by thermal annealing. This state of strain assists the development of the solvent‐induced crystallization. The model regarding crystallization was proposed in terms of the study of long period L, the crystal thickness l_c, and the thickness of amorphous layer l_a obtained from the one‐dimensional correlation function and interface distribution function. Different kinetic paths were discovered for different crystallization processes. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1444–1453, 2002     

Adsorption kinetics of some carotenoids at the oil/water interface

The kinetic analysis of the adsorption of two carotenoids (i.e., ethyl ester of β-apo-8′-carotenoic acid and β-carotene, all trans-isomers) from n-hexane solutions at the oil/water interface is presented for several carotenoid concentrations in the oil phase. A new kinetic approach is developed and it addresses the diffusion adsorption associated with a reversible interfacial reaction, which describes the reorientation of surfactant molecules between two conformations. This approach leads to a general analytical expression that contains four physical parameters and describes with high accuracy the experimental dynamic interfacial tensions for the two carotenoids, which independently adsorb from n-hexane phase to the n-hexane/water interface. The calculations give the characteristic times for the carotenoid adsorption at the oil/water interface in terms of diffusion relaxation and kinetic relaxation times. The results explain the long time effects on the adsorption of these carotenoids at the oil/water interface. The data are in substantial agreement with the molecular structure of these carotenoids and with the earlier data recorded for cholesterol adsorption at the n-heptane/water interface. Based on these findings, we propose a molecular mechanism for the interfacial transformation of carotenoid molecules at a hydrophobic/hydrophilic interface.     

PEG-covered lipid surfaces: bilayers and monolayers

In this review paper we survey the ways in which various micropipet techniques have been used to study the mechanochemical and interactive features of lipid bilayer vesicles and monolayer-coated gas bubbles. Special emphasis will be made on characterizing the barrier properties of grafted PEG layers and how a hierarchical approach that uses a short barrier and extended ligand allows us to start to mimic nature's own solution to the problem of ubiquitous repulsion and specific attraction. The information gained from such studies not only characterizes the membrane and other lipid surfaces and their intersurface interactions from a fundamental materials science perspective, but also provides essential materials property data that are required for the successful design and deployment of lipid-based carriers and other capsules in applications involving this so-called ‘stealthy’ surface.     

Reorganization of lipid nanocapsules at air-water interface: Part 2. Properties of the formed surface film

The state, electrical and dilatational rheological properties of surface films formed at air–water interface from lipid nanocapsules (LNC) with various compositions as well as model monolayers formed by the LNC constituents—Labrafac^®, Solutol^® and Lipoid^® are investigated. These nanocapsules constitute potential drug delivery systems where lypophilic drug will be loaded in their core. The study of the model Labrafac^®/Solutol^® (Lab/Sol) mixed monolayers shows behavior close to the ideal. Small negative deviations in the mean molecular areas a and dipole moments μ are observed. All studied monolayers have elastic behavior during the small continuous compressions. The comparison between the properties of surface films formed from LNC with those of the model monolayers confirms the idea developed in the kinetic study [I. Minkov, Tz. Ivanova, I. Panaiotov, J. Proust, P. Saulnier, Reorganization of lipid nanocapsules at air–water interface: 1. Kinetics of surface film formation, J. Colloids Surf. B: Biointerfaces, submited for publication.] that the surface films formed after a rapid disaggregation of the unstable nanocapsule fraction (LNC I) contains mainly Labrafac and Solutol. The Labrafac molar part (x_Lab) in the formed Lab/Sol mixed layer is established.     

Isothermal kinetic analysis of the thermal decomposition of magnesium hydroxide using thermogravimetric data

In the present study, the kinetics of the thermal decomposition of magnesium hydroxide is investigated, using isothermal methods of kinetic analysis. For this purpose, experiments in thermogravimetric analyser were carried out in standard values of temperature (350°, 400°, 450° and 500°C) which resulted in weight loss percent as a function of time. The data were further modified to give fraction reacted ‘' versus time to be tested in various forms of ‘' functions. In order to determine the mechanism of the magnesium hydroxide decomposition and the form of the conversion function which governs the dehydroxylation of Mg(OH)₂, four different methods of isothermal kinetic analysis were used. Applying each of these methods to the data, it was concluded that the nucleation mechanism predominates the Mg(OH)₂, decomposition for all values of temperature tested; at 350°C the kinetic model which represents the experimental data is that of reaction at phase boundaries (random nucleation), F₁: ln(1−)=kt) while for the higher temperatures 400°, 450° and 500°C the kinetic equation of nucleation and development in two dimensions, A₂: [−ln (1−)]^1/2=kt was found to fit better the experimental results. The activation energy was evaluated applying two alternative methods; the Arrhenius plot, using maximum rates of reaction, from which the activation energy was evaluated to be 20.54 kcal/mol. An alternative method based on plots of ln t versus 1/T corresponding to the same value of ‘' gave values of 10.72, 13.82 and 16.31 kcal/mol for ‘' values of 0.25, 0.50 and 0.75, respectively.