Mixed Assemblies Research Articles

Assembly modulates dissociation: electrokinetic experiments reveal peculiarities of the charge formation at monolayer films

Self-assembled monomolecular films of alkanethiols chemisorbed on gold were used as a two-dimensional model system to study the effects of spatial confinement of surface functional groups on their acid-base behaviour; reduction of the surface charge density by mixed assemblies revealed an increase of the acidity of carboxy groups; surfaces with low amounts of charged groups show the interplay of dissociation of functional groups and preferential adsorption of ions in terms of surface charge formation.

Oxidation of Thioanisole by Peroxomolybdate in Assemblies of Cetylpyridinium Chloride and Methyltri-n-octylammonium Chloride

Methyltri-n-octylammonium chloride (Aliquat 336) is sparingly soluble in water but is readily soluble with a 2-fold excess of micellized cetylpyridinium chloride (CPyCl), and the mixtures show breaks in plots of surface tension or electrolytic conductance against concentration indicative of a critical micelle concentration slightly lower than that of CPyCl. Micellization markedly increases 35Cl and 14N NMR line widths of CPyCl, but addition of NaCl reduces the 35Cl line width and addition of Aliquat increases it. Mixing Aliquat and CPyCl has little effect on their 14N line widths. Ion pairing in alcohol mixtures also increases 35Cl line widths. In water these mixed assemblies behave similarly to micelles of CPyCl as regards effects on rates and equilibria of interconversion of tri- and tetraperoxomolybdate ions, and oxidation of thioanisole by the latter, although it is slightly slower than in micelles of CPyCl. Despite differences in the hydrophobic regions, and relationships between amphiphilic structures and morphologies of association colloids, assemblies of CPyCl and Aliquat behave very much like CPyCl micelles in their physical properties and effects upon reactivity. Geometrical optimization indicates that Aliquat can adopt conformations that allow intercalation with CPyCl micelles.

Narp and NP1 Form Heterocomplexes that Function in Developmental and Activity-Dependent Synaptic Plasticity

Narp is a neuronal immediate early gene that plays a role in excitatory synaptogenesis. Here, we report that native Narp in brain is part of a pentraxin complex that includes NP1. These proteins are covalently linked by disulfide bonds into highly organized complexes, and their relative ratio in the complex is dynamically dependent upon the neuron's activity history and developmental stage. Complex formation is dependent on their distinct N-terminal coiled-coil domains, while their closely homologous C-terminal pentraxin domains mediate association with AMPA-type glutamate receptors. Narp is substantially more effective in assays of cell surface cluster formation, coclustering of AMPA receptors, and excitatory synaptogenesis, yet their combined expression results in supraadditive effects. These studies support a model in which Narp can regulate the latent synaptogenic activity of NP1 by forming mixed pentraxin assemblies. This mechanism appears to contribute to both activity-independent and activity-dependent excitatory synaptogenesis.

Self-Assembled Fibrillar Networks through Highly Oriented Aggregates of Porphyrin and Pyrene Substituted by Dialkyl l-Glutamine in Organic Media

Microfibrous self-aggregation of chromophoric groups of porphyrin and/or pyrene substituted by didodecyl l-glutamic acid in organic media is confirmed by transmission electron microscopic (TEM) observation. Chromophoric probes of porphyrin and pyrene moieties enable evaluation of their assembling behavior photophysically through UV−vis, circular dichroism (CD), and fluorescence spectroscopic characterization. This spectroscopic characterization was able to compensate the lack of TEM observation for the aggregation even at a low concentration below the critical gel concentration. The temperature affects the salient features of the photophysics of porphyrin or pyrene in the microfibrous assemblies. Highly oriented network structures were formed at low temperature since the CD intensities of the porphyrin and pyrene systems increased with lowering the temperature. Fluorescence spectroscopic characterization confirmed the monomer excitation of porphyrin itself, and efficient excimer formation for the pyrene−pyrene charge transfer was detected at low temperature. In particular, we also obtained the preliminary results of fluorescence spectroscopic measurement on singlet−singlet energy migration from pyrene to porphyrin in the mixed assemblies for mimicry of the efficient energy transfer process of the photosynthetic antenna complex.

Poly(Vinyl Pyridine) as a Universal Surface Modifier for Immobilization of Nanoparticles

Metal, metal oxide, and plastic surfaces were modified with poly(vinylpyridine) using adsorption, spin-coating, and dip-coating techniques and various metal, semiconductor, and dielectric nanoparticles were subsequently attached. Prior to the modification, the substrates were treated with glow discharge plasma for cleaning purposes and to introduce surface functional groups that are capable of interaction with the polymer. UV−vis absorption spectroscopy, atomic force, and electron scanning microscopies were used to characterize polymer layers and layers of immobilized nanoparticles. The surface modification with poly(vinylpyridine) via adsorption technique is suggested as a simple, effective, and highly versatile method for the preparation of single, mixed, and multilayered assemblies of various nanoparticles on different substrates.

Mo¨ssbauer Study of the Spin Direction in Oxalate-Bridged Mn(II)-Fe(III) Complexes

Oxalate-bridged mixed metal assemblies A[Mn(II)Fe(III)(ox) 3 ] with large templating cations, A + =P(C 6 H 5 ) 4 + and As(C 6 H 5 ) 4 + , exhibited a spin direction almost perpendicular to the honeycomb layers consisting of the alternate array of Mn(II) and Fe(III) ions through oxalate ligands. On the other hand, the spins were aligned parallel to the basal layers in the compound with the small cation, NPr 4 + . A variation of ca. 70 in spin direction was observed for mixed crystals NPr 4 [Mn(II)Fe(III) x Cr(III) 1 m x (ox) 3 ]. Mössbauer spectroscopy oxalates spin direction

Investigation of intranodal depletion effects

The modeling of depletion induced intranodal effects on important neutron physical parameters in nodal diffusion theory is addressed. Consideration is given to two situations where these aspects are of particular interest, namely, in mixed oxide cores where strong interaction between uranium and plutonium mixed oxide assemblies occur, and in boiling water reactor cores where significant control rod history effects are encountered. A model based on a low order polynomial representation of intranodal cross-section spatial behaviour is considered. Two approaches for determining the constraints for the polynomial fitting procedure are applied. The first one is a conventional method employing intranodal exposure values, whereas the second model combines intranodal exposure and isotopic inventory information. Numerical studies are performed in order to evaluate the relative merits of the different models. It is demonstrated that pin power predictions are significantly influenced by intranodal effects. It is also found that the combined use of intranodal isotopic inventory and exposure distributions for estimating intranodal cross-section behaviour significantly improves the accuracy in pin powers over the more traditional approach of utilizing exposure distributions only.

Phase behavior of fully hydrated DMPC-amphiphilic cyclodextrin systems

With the aim of exploring relationships between the chemical structure and the physico-chemical properties of amphiphilic β-cyclodextrin, a reappraisal of the obtaining of pure heptakis (2,3-di-O-hexanoyl)-β-cyclodextrin (β-CDC 6) was undertaken. In this paper the chemical characterization of the newly synthesized β-CDC 6 and its ability to form mixed structures with dimyristoylphosphatidylcholine (DMPC) are reported. Miscibility of the two amphiphiles is examined: (i) in monolayers formed at the air–water interface by analyzing the surface pressure-area isotherms; and (ii) in fully hydrated mixtures by differential scanning calorimetry (DSC) and X-ray diffraction at small and wide angles. Results demonstrate that the β-cyclodextrin derivative is partially miscible to the phospholipid: intimate mixing occurs at β-CDC 6 molar ratios smaller than 7–15 mol%, depending on the dimensional scale considered, while beyond these compositions phase separation is observed. At the air–water interface, the miscibility region of the two compounds shows non-ideal behavior characterized by the non-additivity of the molecular areas in the mixed monolayers. At the three-dimension level, the formation of a β-CDC 6/DMPC mixed lamellar phase occurs except at β-CDC 6 molar ratios close to 5 mol% at which a highly ordered structure is depicted below the solid-to-liquid state transition of the DMPC hydrocarbon chains. At β-CDC 6 contents higher than 7 mol%, the mixed assemblies coexist with excess amphiphilic cyclodextrin which then forms a separated hexagonal structure.

A method for calculating, on the basis of tables for a crisis in bundles, the critical heat flux in mixed rod assemblies

A method for calculating the critical heat, flux in mixed assemblies of fuel rods, for example, RBMK, containing triangular and tetragonal as well as peripheral macrocells, is presented. The method is based on a generalization of the experimental data in the form of tables for rod bundles. It is recommended for parameter ranges which are covered by the experimental data as well as for other ranges where there are no data. The advantages of the tabular method are as follows: It can be used in very wide parameter ranges, it gives a smooth description of the dependence of the critical heat fluxes on these parameters, it is convenient. highly reliable and accurate, and it is easy to use. 1 figure, 1 table, 7 references.

Combined Scanning Tunneling Microscopy and Infrared Spectroscopic Characterization of Mixed Surface Assemblies of Linear Conjugated Guest Molecules in Host Alkanethiolate Monolayers on Gold

Molecular resolution scanning tunneling microscopy (STM) imaging and high sensitivity infrared reflection spectroscopy (IRS) measurements have been combined to characterize the structures of mixed self-assembled monolayers (SAMs) of fully conjugated, linear thiolate-terminated molecules and short chain (8−12) n-alkanethiolates on Au{111}. Immersion of preformed, ordered alkanethiolate SAMs into dilute solutions of the conjugated molecules results in two-dimensional matrix isolation of conjugated adsorbates in the host SAM. The post-immersion host SAM matrix shows retained alkanethiolate ordering with the guest molecules inserted both singly into boundaries between SAM structural domains and in bundles at substrate step edges. Inserted molecules of lengths in the ∼15 A range adopt surface orientations similar to those of alkanethiolate molecules at all compositions, including the pure conjugated SAMs. In contrast, the configuration of an inserted, long conjugated molecule (∼40 A), varies monotonically with...

Surface-Mediated Segregation and Transport Processes in Mixed Hydrocarbon Multilayer Assemblies

The transport and structural phase dynamics exhibited by multilayer assemblies comprised of cyclic and linear alkanes are analyzed with reflection absorption infrared (RAIR) and temperature-programmed desorption spectroscopies. Infrared spectroscopy reveals that methyl group substitutions have a significant effect on the nature of the mode softening seen in the C-H stretching region for surface-bound cyclohexanes. The magnitude of the red-shifts seen increases with the degree of methyl substitution. The energetics of the surface binding do not correlate in a simple way with the magnitude of red-shifts seen in the RAIR spectra, however. We find instead that the strengths of the surface interactions are more directly correlated with both the size and shape of the molecule (with the latter presumably reflecting its ability to form a densely packed structure). We also find that the diffusion of molecules in a mixed hydrocarbon multilayer assembly is weakly activated, with substantial interlayer mixing being seen at temperatures significantly below the threshold for the desorption of the multilayer. The mixing, while driven by mass action, shows a pronounced bias for the surface binding of n-alkanes over cycloalkanes of similar molecular weight (e.g., n-octane is more strongly bound on Pt(111) than is cis-1,3-dimethylcyclohexane). The data strongly suggest that attractive lateral interactions in the adsorbed layers lead to the biases seen in this surface-induced segregation. Thermal desorption spectra confirm this sensitivity and interestingly show multiple monolayer desorption features for cyclic alkane adsorbates when mixed in a monolayer assembly with an appropriate linear n-alkane. We suggest that the attractive lateral interactions in the monolayer lead to the formation of island domains and that the desorption kinetics appear to sensitively reflect this underlying rate/structure sensitivity.

X-ray Diffraction Study of the Effect of the Detergent Octyl Glucoside on the Structure of Lamellar and Nonlamellar Lipid/Water Phases of Use for Membrane Protein Reconstitution

The phase behavior and structural parameters of dioleoylphosphatidylethanolamine (DOPE), monoolein (MO), and dipalmitoylphosphatidylcholine (DPPC) hydrated in excess aqueous phase containing micellar octyl glucoside (OG) were investigated by means of synchrotron X-ray diffraction in a wide range of temperatures. The lipid-to-detergent molar ratios were selected to be equal to those in a recent membrane-protein-reconstitution study. It was established that lipids displaying either nonlamellar (DOPE and MO) or lamellar propensities (DPPC) form mixed supramolecular structures with the detergent OG. The phase states and structures of these mixed assemblies are essentially different from those of the pure lipid components. At full hydration and at the investigated lipid-to-detergent molar ratios, OG destabilizes the inverted-hexagonal (HII) phase organization of DOPE and the inverted cubic (Q224) structure of MO, transforming them into a lamellar liquid crystalline (Lα) phase. The fluid DOPE/OG and MO/OG bilay...

Postcolumn fluorescence as an alternative to evaporative light scattering detection for ceramide analysis with gradient elution in non-aqueous reversed-phase liquid chromatography

Ceramide analysis was developed with gradient elution in non-aqueous reversed-phase liquid chromatography with evaporative light scattering detection (ELSD) or postcolumn fluorescence detection. Fluorescence detection (excitation, 360 nm; emission, 425 nm) after postcolumn formation of mixed assemblies between eluted ceramides and 1,6-diphenyl-1,3,5-hexatriene was developed. In comparison with ELSD, fluorescence detection allows a better detection of the minor species ceramide from ceramide type III (commercial mixture of non-hydroxy fatty acid–sphingosine) and appears to be more sensitive for quantitation of ceramides at low concentrations. The fluorescence response is linear over a wide range of injected amount of ceramide III (expressed as stearoyl-phytosphingosine): 10 ng to 1000 ng. The response of ELSD is non linear but can be linearized in double logarithmic coordinates for calculations over a narrow range, e.g. between 10 to 350 ng ceramide III injected. The lower quantitation limits of these two detectors are similar: 5 ng ceramide III was injected.

Order/disorder gradients of n-alkanethiols on gold

This paper explores the interfacial properties of one-dimensional molecular gradients of alkanethiols (HS–(CH 2) n –X) on gold. The kinetics and thermodynamics of monolayer formation are important issues for these types of mixed molecular assemblies. The influence of chain length difference on the contact angles with hexadecane (HD), θ a and θ r, and the hysteresis, has been studied by employing alkanethiols HS–(CH 2) n –CH 3, with n=9, 11, 13, 15 and 17, in the preparation of the self-assembled monolayers (SAM) gradients. The contact angles with hexadecane, at the very extreme ends of the gradients, show characteristic values of a highly ordered CH 3-like assembly: θ a=45–50°. In the middle of the gradients θ a drops noticeably and exhibits values representative for CH 2-like polymethylenes, θ a=20–30°, indicating a substantial disordering of the protruding chains of the longer component in the gradient assembly. As expected, the exposure of CH 2-groups to the probing liquid increases with increasing differential chain length of the two n-alkanethiol used, in this case eight methylene units. However, the contact angles always display a non-zero value which means that even at a chain length difference of eight methylene units there is a substantial exposure of methyl (CH 3) groups to the probing liquid. With infrared reflection-absorption spectroscopy (IRAS) we have monitored the structural behavior of the polymethylene chains along the gradient. We find complementary evidence for disordered chains in the gradient region, and the IRAS results correlate well with the contact angle measurements.

Evidence That Intramolecular Associations between Presenilin Domains Are Obligatory for Endoproteolytic Processing

Mutations in genes encoding presenilins (PS1 and PS2) cosegregate with the majority of early onset cases of familial Alzheimer's disease. PS1 and PS2 are polytopic membrane proteins that undergo endoproteolytic cleavage to generate stable NH2- and COOH-terminal derivatives (NTF and CTF, respectively). Several lines of evidence suggest that the endoproteolytic derivatives are likely the functional units of PS in vivo. In the present report, we examine the disposition of PS NTF and CTF assemblies in stable mouse N2a neuroblastoma cell lines expressing human PS polypeptides. We show that exogenous expression of PS1 NTFs neither assemble with endogenous CTF nor exhibit dominant negative inhibitory effects on the endogenous PS1 cleavage and the accumulation of derivatives. In cells co-expressing PS1 and PS2, PS1- and PS2-derived fragments do not form mixed assemblies. In contrast, cells expressing a chimeric PS1/PS2 polypeptide form stable PS1 NTF-PS2 CTF assemblies. Moreover, expression of chimeric PS1/PS2 polypeptides harboring a familial early onset AD-linked mutation (M146L) elevates the production of Abeta42 peptides. Our results provide evidence that assembly of structural domains contained within NH2- and COOH-terminal regions of PS occur prior to endoproteolytic cleavage.

Simulation of Energy Transfer in Photosynthetic Antenna Systems with Monolayer Assemblies

To optimize the energy transfer (ET) from a mixture of 10-(1-(8-hexyl)pyrenyl)-decanoic acid, 10-(1-(6-hexyl)-pyrenyl)decanoic acid, and 10-(1-(3-hexyl)-pyrenyl)decanoic acid (HPyDA) to a mixture of 10-(3-(9-hexyl)perylenyl)-10-oxodecanoic acid and 10-(3-(10-hexyl)perylenyl)-10-oxodecanoic acid (HPeDA) in HPyDA-HPeDA mixed monolayer assemblies, fluorescence spectra of the mixed monolayers containing various compositions of HPyDA and HPeDA were measured. Efficient ET was observed in the mixed monolayers of HPeDA:HPyDA molar ratios of 1:3 - 1:300.

Hydrophobically driven attachments of synthetic polymers onto surfaces of biological interest: Lipid bilayers and globular proteins

This paper gives a brief overview of the consequences of associations between amphiphilic water-soluble polymers and small colloidal particles of biological interest: proteins and vesicles. Typical structures of water-soluble synthetic polymers containing hydrophobic groups are presented. The segregation between polar and apolar units in these polymers induces self-organisation in micro-domains despite the lack of specific primary structure. In the presence of other amphiphilic particles like proteins and vesicles, mixed assemblies are formed. Examples of polymer associations with vesicles or globular proteins mainly focused on the acrylic derivatives, bring out common features in these mixtures. When the size of the polymer is of the same order of magnitude as that of the particle, adsorption of polymer chains creates a protective layer around each individual particle. Depending on the hydrophobicity of the partners, the association can stabilise the dispersion of unmodified particles or induce structural changes (membrane disruption, leakage). When small particles are added to solutions of long polymers, multimolecular complexation occurs. In this case, the size of the resulting aggregates depends on the concentrations. It goes from the size of one polymer molecule up to formally infinity as revealed by gelation. The identification of non-specific association modes between biological nanoparticles and macromolecules might be revealed by the general behaviour of these synthetic mixed systems.

Preparation and Characterization of Mixed Monolayer Assemblies Composed of Thiol Analogues of Cholesterol and Fatty Acid

Mixed self-assembled monolayers provide an attractive model system for investigating the role of different molecules in biological membranes. This paper describes the preparation and characterization of a novel type of mixed monolayer assemblies composed of thiol analogues of cholesterol and fatty acid. The mixed monolayers are prepared by coadsorbing 11-mercaptoundecanoic acid (MUA) and thiocholesterol (cholest-5-ene-3β-thiol, TC) from solution directly onto evaporated gold surfaces. The influence of TC on the molecular composition and conformation in the mixed monolayer is analyzed by using a combination of infrared reflection−absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS), ellipsometry, contact angle measurement, and cyclic voltammetry. The results indicate that the TC molecules maintain their conformation in the mixed monolayers, whereas the MUA molecules display a significantly more disordered conformation as compared to the MUA molecules in the pure monolayer. Cyclic voltamme...

Electrochemical characterisation of mixed monolayer assemblies of thiol analogues of cholesterol and fatty acids on gold

Abstract A self-assembled monolayer (SAM) on gold prepared from a binary mixture of a thiol analogue of cholesterol (thiocholesterol, TC) and a functionalised alkanethiol (11-mercaptoundecanoic acid, MUA) has been investigated by voltammetry. The voltammetric results are in agreement with previously reported spectroscopic data and show that the geometric arrangement and composition of the molecules in the mixed monolayer controls the heterogeneous electron transfer process of Fe(CN)63− across the assembly. The quantitative description of the influence of TC on the electron transfer rate constant is given through Tafel plots. At the oure MUA SAM electrode, the electron transfer is governed by penetration through the monolayer. The introduction of TC into the SAMs creates defects giving rise to diffusion controlled electron transfer in addition to penetration. By raising the TC content the electron transfer rate constant increases due to diffusion. This behaviour can be explained by a model in which the assembly goes from a penetrative but defect-free film barrier (pure MUA SAM) via a structure with defects in the mixed composition regime to a defect-rich structure consisting of an array of ultramicroelectrodes (pure TC SAM).

Simultaneous Control of Surface Potential and Wetting of Solids with Chemisorbed Multifunctional Ligands

Homogeneously mixed molecular assemblies of defined stoichiometry were created by adsorption of asymmetric, trifunctional ligands on gold and CuInSe2 (CISe). The ligands rely on cyclic disulfide groups for binding to the substrate and can in addition possess two different substituents, one polar substituent (p-cyanobenzoyl or anisoyl) and one long-chain, aliphatic residue (palmitoyl). Because the substituents are covalently connected, no phase segregation will occur upon surface binding. Adsorption of these ligands on conducting surfaces changed both the surface potential (because of the polar substituent) and hydrophobicity (because of the aliphatic residue). Larger changes of surface potential were obtained by adsorption of the symmetric, dipolar ligands than by adsorption of the asymmetric ligands, and larger changes occurred on gold than on CuInSe2 (up to 1.2 V between extreme modifications on Au and 0.3 V on CISe). The magnitude and direction of the observed contact potential difference changes were found to depend on the extent of coverage (as derived from electrochemical and contact angle measurements) and on the orientation of the ligands (estimated from ellipsometry and FTIR data) and could also be reconstructed using a simple, electrostatic model. These findings demonstrate that the present methodology enables simultaneous grafting of two desired properties onto solid surfaces and illustrate the predictive power of a simple, electrostatic model for molecule-controlled surface engineering.