Theoretical Study of the Wavelength Selection for the Photocleavage of Coumarin-caged D-luciferin

The equilibrium structures and optical properties of the photolabile caged luciferin, (7-diethylaminocoumarin-4-yl)methyl caged D-luciferin (DEACM-caged D-luciferin), in aqueous solution were investigated via quantum chemical calculations. The probable conformers of DEACM-caged D-luciferin were determined by potential energy curve scans and structural optimizations. We identified 40 possible conformers of DEACM-caged D-luciferin in water by comparing the Gibbs free energy of the optimized structures. Despite the difference in their structures, the conformers were similar in terms of assignments, oscillator strengths and energies of the three low-lying excited states. From the concentrations of the conformers and their oscillator strengths, we obtained a theoretical UV/Vis spectrum of DEACM-caged D-luciferin that has two main bands of shape nearly identical to the experimental UV/Vis spectrum. The absorption bands with maxima ~ 384 and 339 nm were attributed to the electronic excitations of the caged group and the luciferin moiety, respectively, by analysis of the theoretical UV/Vis spectrum. Furthermore, the analysis showed that DEACM-caged D-luciferin is excited in the caged group only by light of wavelength ranging within 400-430 nm, which is in the long-wavelength tail of the 384 nm band. This should be tested to lower damage upon photocleavage.

Estradiol-tethered micropatterned surfaces for the study of estrogenic non-genomic pathways

Besides its well-known hormonal effects initiated in the nucleus, estradiol (E2) also activates non-nuclear pathways through interactions with receptors located on the cell plasma membrane. Micropatterned substrates consisting of gold dots bearing tethered E2 distributed on a cell-adhesive substrate were prepared and shown to trigger specifically E2 non-genomic effects in cells grown on the substrates.

Temperature sensitive supramolecular self assembly of per-6-PEO-β-cyclodextrin and α,ω-di-(adamantylethyl)poly(N-isopropylacrylamide) in water

The host/guest interactions in water of a star polymer consisting of a β-cyclodextrin (β-CD) core bearing six poly(ethylene oxide) arms linked to the C6 positions of β-CD (β-CD-PEO7, Mn 5000 g mol(-1)) and α,ω-di-(adamantylethyl)poly(N-isopropylacrylamide) (Ad-PNIPAM-12K, Mn 12,000 g mol(-1)) were studied by 1D and 2D (1)H and (13)C NMR spectroscopy, isothermal calorimetry (ITC), and light scattering (LS). In cold water (T < 26 °C) supramolecular "dumbbell" assemblies, consisting of PNIPAM chains with β-CD/Ad inclusion complexes at each end, formed viaβ-CD-insertion of the terminal Ads through the β-CD secondary face. Light scattering, microcalorimetry (DSC), and DOSY NMR studies indicated that mixed aqueous solutions of β-CD-PEO7 and Ad-PNIPAM-12K undergo a reversible heat-induced phase transition at ∼32 °C, accompanied by a release of a fraction of the Ad-bound β-CD-PEO7 into bulk solution and the formation of aggregated Ad-PNIPAM-12K stabilized by a β-CD-PEO7 shell.

Temperature-dependent adsorption and adsorption hysteresis of a thermoresponsive diblock copolymer

A nonionic-cationic diblock copolymer, poly(2-isopropyl-2-oxazoline)60-b-poly((3-acrylamidopropyl)trimethylammonium chloride)17, (PIPOZ60-b-PAMPTMA17), was utilized to electrostatically tether temperature-responsive PIPOZ chains to silica surfaces by physisorption. The effects of polymer concentration, pH, and temperature on adsorption were investigated using quartz crystal microbalance with dissipation monitoring and ellipsometry. The combination of these two techniques allows thorough characterization of the adsorbed layer in terms of surface excess, thickness, and water content. The high affinity of the cationic PAMPTMA17 block to the negatively charged silica surface gives rise to a high affinity adsorption isotherm, leading to (nearly) irreversible adsorption with respect to dilution. An increase in solution pH lowers the affinity of PIPOZ to silica but enhances the adsorption of the cationic block due to increasing silica surface charge density, which leads to higher adsorption of the cationic diblock copolymer. Higher surface excess is also achieved at higher temperatures due to the worsening of the solvent quality of water for the PIPOZ block. Interestingly, a large hysteresis in adsorbed mass and other layer properties was observed when the temperature was cycled from 25 to 45 °C and then back to 25 °C. Possible causes for this temperature hysteresis are discussed.

Low molecular weight chitosan conjugated with folate for siRNA delivery in vitro: optimization studies

The low transfection efficiency of chitosan is one of its drawbacks as a gene delivery carrier. Low molecular weight chitosan may help to form small-sized polymer-DNA or small interfering RNA (siRNA) complexes. Folate conjugation may improve gene transfection efficiency because of the promoted uptake of folate receptor-bearing cells. In the present study, chitosan was conjugated with folate and investigated for its efficacy as a delivery vector for siRNA in vitro. We demonstrate that the molecular weight of chitosan has a major influence on its biological and physicochemical properties, and very low molecular weight chitosan (below 10 kDa) has difficulty in forming stable complexes with siRNA. In this study, chitosan 25 kDa and 50 kDa completely absorbed siRNA and formed nanoparticles (≤220 nm) at a chitosan to siRNA weight ratio of 50:1. The introduction of a folate ligand onto chitosan decreased nanoparticle toxicity. Compared with chitosan-siRNA, folate-chitosan-siRNA nanoparticles improved gene silencing transfection efficiency. Therefore, folate-chitosan shows potential as a viable candidate vector for safe and efficient siRNA delivery.

Thermodynamic characterization of the exchange of detergents and amphipols at the surfaces of integral membrane proteins

The aggregation of integral membrane proteins (IMPs) in aqueous media is a significant concern for mechanistic investigations and pharmaceutical applications of this important class of proteins. Complexation of IMPs with amphiphiles, either detergents or short amphiphilic polymers known as amphipols (APols), renders IMPs water-soluble. It is common knowledge that IMP-detergent complexes are labile, while IMP-APol complexes are exceptionally stable and do not dissociate even under conditions of extreme dilution. To understand the thermodynamic origin of this difference in stability and to guide the design of new APols, we have studied by isothermal titration calorimetry (ITC) the heat exchanges during two reciprocal processes, the "trapping" of detergent-solubilized IMPs in APols and the "stripping" of IMP-APol complexes by detergents, using two IMPs (the transmembrane domain of porin OmpA from Escherichia coli and bacteriorhodopsin from Halobium salinarium), two APols [an anionic polymer derived from acrylic acid (A8-35) and a cationic phosphorylcholine-based polymer (C22-43)], and two neutral detergents [n-octyl thioglucoside (OTG) and n-octyltetraethylene glycol (C(8)E(4))]. In the presence of detergent, free APols and IMP-APol complexes form mixed particles, APol-detergent and IMP-APol-detergent, respectively, according to the regular mixing model. Diluting IMP-APol-detergent complexes below the critical micellar concentration (CMC) of the detergent triggers the dispersion of detergent molecules as monomers, a process characterized by an enthalpy of demicellization. The enthalpy of APol <--> detergent exchange on the hydrophobic surface of IMPs is negligibly small, an indication of the similarity of the molecular interactions of IMPs with the two types of amphiphiles. The enhanced stability against dilution of IMP-APol complexes, compared to IMP-detergent ones, originates from the difference in entropy gain achieved upon release in water of a few APol molecules (in the case of IMP-APol complexes) or several hundred detergent molecules (in the case of IMP-detergent complexes). The data account both for the stability of IMP-APols complexes in the absence of detergent and for the ease with which detergents displace APols from the surface of proteins.

Complexation of integral membrane proteins by phosphorylcholine-based amphipols

Amphiphilic macromolecules, known as amphipols, have emerged as promising candidates to replace conventional detergents for handling integral membrane proteins in water due to the enhanced stability of protein/amphipol complexes as compared to protein/detergent complexes. The limited portfolio of amphipols currently available prompted us to develop amphipols bearing phosphorylcholine-based units (PC). Unlike carboxylated polymers, PC-amphipols remain soluble in aqueous media under conditions of low pH, high salt concentration, or in the presence of divalent ions. The solubilizing properties of four PC-amphipols were assessed in the case of two membrane proteins, cytochrome b(6)f and bacteriorhodopsin. The protein/PC-amphipol complexes had a low dispersity in size, as determined by rate zonal ultracentrifugation. Short PC-amphipols (<M> approximately 22 kDa) of low dispersity in length, containing approximately 30 mol% octyl side groups, approximately 35 mol% PC-groups, and approximately 35 mol% isopropyl side groups, appeared best suited to form stable complexes, preserving the native state of BR over periods of several days. BR/PC-amphipol complexes remained soluble in aqueous media at pH> or =5, as well as in the presence of 1 M NaCl or 12 mM calcium ions. Results from isothermal titration calorimetry indicated that the energetics of the conversion of BR/detergent complexes into BR/amphipol complexes are similar for PC-amphipols and carboxylated amphiphols.

Enthalpy of interaction and binding isotherms of non-ionic surfactants onto micellar amphiphilic polymers (amphipols)

The interactions in water between short amphiphilic macromomolecules, known as amphipols, and three neutral surfactants (detergents), dodecylmaltoside (DM), n-octylthioglucoside (OTG), and n-octyltetraethyleneoxide (C8E4), have been assessed by static and dynamic light-scattering (SLS and DLS), capillary electrophoresis (CE), and isothermal titration calorimetry (ITC). The amphipols selected are random copolymers of the hydrophobic n-octylacrylamide (25-30 mol %), a charged hydrophilic monomer, either acrylic acid ( approximately 35 mol %) or a phosphorylcholine-modified acrylamide (40-70 mol %), and, optionally, N-isopropylacrylamide (30-40 mol %). In water, the copolymers form micelles of small size (hydrodynamic radius: approximately 5 nm). Neutral surfactants, below their critical micellar concentration (cmc), form mixed micelles with the amphipols irrespective of the chemical structure of the detergent or the polymer. The fraction of detergent in the surfactant/polymer complexes increases significantly (cooperatively) as the surfactant concentration nears the cmc. The ITC data, together with data gathered by CE, were fitted via a regular mixing model, which allowed us to predict the detergent concentration in equilibrium with complexes and the heat evolved upon transfer of detergent from water into a mixed surfactant/polymer complex. The enthalpy of transfer was found to be almost equal to the enthalpy of micellization, and the regular mixing model points to a near-ideal mixing behavior for all systems. Amphipols are promising tools in biochemistry where they are used, together with neutral surfactants, for the stabilization and handling of proteins. This study provides guidelines for the optimization of current protein purification protocols and for the formulations of surfactant/polymer systems used in pharmaceutics, cosmetics, and foodstuffs.

End-grafted low-molecular-weight PNIPAM does not collapse above the LCST

The interfacial properties of end-grafted temperature-responsive poly(N-isopropylacryamide) (PNIPAM) were quantified by direct force measurements both above and below the lower critical solution temperature (LCST) of 32 degrees C. The forces were measured between identical, opposing PNIPAM films and between a PNIPAM film and a lipid membrane. At the grafting densities and molecular weights investigated, the polymer extension did not change significantly above the LCST, and the polymers did not adhere. Below the LCST, the force-distance profiles suggest a vertical phase separation, which results in a diluter outer layer and a dense surface proximal layer. At large separations, the force profiles agree qualitatively with simple polymer theory but deviate at small separations. Importantly, at these low grafting densities and molecular weights, the end-grafted PNIPAM does not collapse above the LCST. This finding has direct implications for triggering liposomal drug release with end-grafted PNIPAM, but it increases the temperature range where these short PNIPAM chains function as steric stabilizers.

Interactions in water of alkyl and perfluoroalkyl surfactants with fluorocarbon- and hydrocarbon-modified poly(N-isopropylacrylamides)

Fluorescence spectroscopy and isothermal titration calorimetry (ITC) have been used to study the interactions in water at 25 degrees C of two anionic surfactants--sodium dodecyl sulfate (SDS) and sodium perfluorononanoate (SPFN)--with various pyrene-labeled hydrophobically modified poly(N-isopropylacrylamides) (HM-PNIPAM) grafted at random with small amounts of fluorocarbon chains (1H,1H-perfluorooctyl, CH2C7F15); (PNIPAM-F), or (n-octadecyl, C18H37) (PNIPAM-HPy) or both (PNIPAM-F/HPy). In aqueous solution, the copolymers form micellar structures consisting of a loose corona of hydrated poly(N-isopropylacrylamide) chains and a hydrophobic core rich in hydrocarbon or fluorocarbon groups. From fluorescence studies based on changes in the ratio of pyrene excimer to monomer emission intensity, it has been established (1) that mixed SDS/C18H37 clusters form along the polymer chain upon addition of SDS to either PNIPAM-HPy or PNIPAM-F/HPy and (2) that SPFN does not interact with the hydrocarbon-rich microdomains of the polymeric micelles. The conclusions were corroborated by ITC experiments, which yield the overall enthalpy change associated with polymer/surfactant interactions. They provided strong evidence (1) that SDS molecules adsorb along the PNIPAM main chain but do not mix with the fluorocarbon-rich microdomains of PNIPAM-F or PNIPAM-F/HPy and (2) that SPFN associates with the perfluorocarbon substituents of PNIPAM-F and PNIPAM-F/HPy but has a poor affinity for the polymer chain.

Promotion of U937 cell adhesion on polypropylene surfaces bearing phosphorylcholine functionalities

Phosphorylcholine (PC) groups were grafted onto ammonia plasma-treated biaxially-oriented polypropylene (BOPP) surfaces, via (a) reductive amination of phosphorylcholine glyceraldehyde and (b) a two-step procedure involving the chemical amplification of surface amine groups with tris(2-aminoethyl amine) and subsequent reductive amination of phosphorylcholine glyceraldehyde. The occurrence of grafting was ascertained by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier-transform infrared (ATR-FT-IR) spectroscopy. The wettability of PC-modified surfaces was assessed by dynamic contact-angle measurements using the Wilhelmy plate method. Human U937 macrophages adhered and proliferated to a significantly larger extent on PC-modified surfaces, compared to unmodified or ammonia plasma-modified BOPP.

Amphiphilic telechelic poly(N-isopropylacrylamide) in water: from micelles to gels

We report the first study of aqueous solutions (0.025 gL(-1) to 46 gL(-1)) of a telechelic poly(N-isopropylacrylamide) with octadecyl termini (C(18)-PNIPAM-C(18), M(w): 37000, 320 NIPAM units, M(w)/ M(n)=1.07) obtained by reversible addition-fragmentation chain transfer (RAFT) free radical polymerization of N-isopropylacrylamide. Static and dynamic light scattering measurements and fluorescence spectroscopy, using 8-anilino-1-naphthalenesulfonic acid (ANS) as probe, yielded the concentration dependence of the size and aggregation number of C(18)-PNIPAM-C(18) micelles in cold ( 20( degrees )C) dilute aqueous solutions. Concentrated solutions ( c>20 gL(-1)) form transient gels exhibiting an oscillatory shear behavior that can be approximated by a single-relaxation Maxwellian model. Aqueous solutions of C(18)-PNIPAM-C(18) undergo a phase transition upon heating to 31.5( degrees )C as determined by microcalorimetry. The heat-induced phase separation of dilute (0.025 gL(-1)) C(18)-PNIPAM-C(18) solutions yields a fluid that is colloidally stable at temperatures higher than 33( degrees )C. The overall results are consistent with a model assuming the formation of flowerlike micelles in the dilute regime and a network of micelles connected by telechelic chains in the concentrated regime.

pH-dependent mucoadhesion of a poly(N-isopropylacrylamide) copolymer reveals design rules for drug delivery

This study investigated the mucoadhesive property of a hydrophobically modified copolymer N-isopropylacryamide and glycidylacrylamide NIPAM-N-Gly-(C18)2 (NIPAM-Gly). Prior studies demonstrated that the interfacial properties of this copolymer are pH dependent and that the chains form strong hydrogen bonds at pH < 7 via the carboxylic acid side chains of the glycine moieties. Mucin interactions with the copolymer brushes were investigated by surface plasmon resonance and by direct force measurements. Mucin adsorption was determined as a function of pH, ionic strength, and mucin concentration. It adsorbs to the copolymer strongly at pH 5, but the adsorption decreases with increasing pH. The adsorbed amount is also ionic-strength dependent, decreasing with increasing monovalent salt concentrations at all pH values investigated. When compared with similar investigations with poly(ethylene oxide), these results provide insights into both the chemical characteristics and the solution conditions that determine the mucoadhesive properties of polymers.

Tuning the interfacial properties of grafted chains with a pH switch

Environmentally responsive, water-soluble polymers have a wide variety of uses ranging from drug delivery to viscosity modifiers. Their utility lies in the ability to use environmental perturbations to dramatically alter the material properties. Here, we describe the interfacial properties of a hydrophobically modified copolymer of N-isopropylacrylamide and glycinylacrylamide (NIPAM-N-Gly-(C18)2), which is both temperature and pH responsive. Direct force measurements quantified the substantial pH-dependent change in the molecular properties of end-grafted NIPAM-N-Gly-(C18)2 monolayers. At pH 8.0, where the glycine side chains are ionized, the polymers exhibit stereotypical polyelectrolyte behavior. Side chain neutralization at pH 5.0 causes a substantial decrease in the film thickness, and the polymer films adhere strongly. The adhesion is presumably through H-bonding between the glycine side chains. Our findings revealed the likely molecular basis of pH-dependent changes in the copolymer films and identified clear design criteria for tuning the interfacial properties of these polymer films.

In vitro evaluation of pH-sensitive polymer/niosome complexes

Large unilamellar niosome and control liposome vesicles were rendered pH-sensitive by complexation with a hydrophobically modified pH-responsive copolymer of N-isopropylacrylamide, N-glycidylacrylamide, and N-octadecylacrylamide at a copolymer/lipid mass ratio of 0.3. The vesicles were characterized and tested for their stability and pH-sensitivity in buffer and human serum. Their in vitro cytotoxicity was evaluated as well as their ability to mediate cytoplasmic delivery of encapsulated fluorescent probe using J774 murine macrophage-like cells. At pH 7.2, vesicles were found to be stable over 90 days at 4 degrees C. At 37 degrees C, the polymer destabilized the vesicles under weakly acidic conditions. However, niosomes but not liposomes were partly destabilized in human serum at 37 degrees C. Premature leakage of niosomal contents in serum was attributed to the polymer collapse which is favored in the presence of multivalent cations. On the cellular level, niosomes were cytotoxic above 0.075 mM while no appreciable decrease in cell viability was shown for the liposomes and copolymer alone at short incubation times (< 2 days). Finally, only liposomes and not niosomes were able to release their contents in the cytoplasm after internalization by phagocytosis.

Quantitative Assays of the Amount of Diethylenetriaminepentaacetic Acid Conjugated to Water-Soluble Polymers Using Isothermal Titration Calorimetry and Colorimetry

The level of conjugation of diethylenetriaminepentaacetic acid (DTPA) to the polysaccharide sodium hyaluronan (HA) has been measured by a colorimetric assay, isothermal titration calorimetry (ITC), and (1)H NMR spectroscopy. The colorimetric assay is based on the red shift, upon complexation with gadolinium ion (Gd3+), of the wavelength of maximum absorption of the dye arsenazo III. It can be performed in a few minutes using as little as 10 microg of polymer with a detection limit of approximately 0.03 mmol of DTPA (gram of polymer)-1. The ITC measurements yield values of the amount of DTPA linked to HA identical to those obtained by colorimetry. The levels of DTPA conjugation calculated by integration of signals at 3.1-3.2 ppm (DTPA protons) and at 2.0 ppm (HA acetamide protons) in the 1H NMR spectrum of HA-DTPA are consistently overestimated by a factor of approximately 2, compared to the data obtained by ITC and colorimetry. The longer relaxation times of protons of the polymer backbone, compared to those of protons attached to the freely moving DTPA side-chains may account for the discrepancy.

Examining cationic polysaccharide deposition onto keratin surfaces through biopolymer fluorescent labeling

Fluorescein-labeled polyquaternium-10 and guar hydroxypropyltrimonium chloride were employed to study the deposition behavior of these cationic polymers onto hair from a surfactant system. The influence of the covalently attached fluorescein dye on labeled polyquaternium-10 was examined through rheological studies and comparative studies against data previously obtained from radiolabeled polyquaternium-10. A quantitative method for analyzing the amount of cationic polymer that deposits onto hair during a standard shampooing process has been developed using the labeled cationic polymers. The technique requires digestion of the hair and analysis of the resulting solutions against known standardization curves. It has been found that the molecular weight of the cationic polymers plays the most significant role in influencing the deposition of the polymers from surfactant, a far greater role than either cationic charge or washing cycles. The technique also allows for determination of polymer deposition at various tress locations, allowing for the study of the influence of tress age (i.e., damage) on polymer deposition. The use of fluorescein-labeled polyquaternium-10 also provides a unique opportunity to visualize the deposited polymers on individual hair fibers via fluorescent or confocal microscopy.

Interaction of water-soluble collagen with poly(acrylic acid)

Interactions between poly(acrylic acid) labeled with pyrene (PAA-Py) and succinylated calfskin collagen (type I) (SCSC) were studied by fluorescence spectroscopy. PAA-Py exhibits a strong emission from pyrene monomer (intensity, I(M)) when it exists in an extended conformation. It exhibits another broad emission from pyrene excimer (intensity, I(E)) when it adopts a collapsed globule conformation. At pH 3, a value that is lower than the isoelectric point of SCSC, the ratio I(E)/I(M) value decreased cooperatively with increasing concentration of SCSC at constant PAA-Py concentration, under salt-free condition. On the other hand, this effect was not observed in the presence of 0.1 M NaCl. At pH 7, a value higher than the isoelectric point of SCSC, the ratio I(E)/I(M) was not affected by the presence of SCSC in the absence and presence of salt. From electrophoretic light scattering experiments, it was found that at pH 3 PAA-Py was negatively charged, while SCSC had a positive charge. Thus it is strongly suggested that the two polymers interact by electrostatic attraction at low pH where they are oppositely charged, and that PAA-Py adopts an extended conformation in the complex formed with SCSC. Similar interactions are believed to occur between dentinal collagen and the polycarboxylate component of glass-ionomer cements.

Modification of liposomes with N-substituted polyacrylamides: identification of proteins adsorbed from plasma

Liposomes prepared from DMPC (80%) and cholesterol (20%) were modified with a series of hydrophobically modified N-substituted polyacrylamides, namely, poly[N-isopropylacrylamide] (PNIPAM), poly[N,N-bis(2-methoxyethyl) acrylamide] (PMEAM), and poly[(3-methoxypropyl)acrylamide] (PMPAM). The hydrophobic group, N-[4-(1-pyrenylbutyl)-N-n-octadecylamine was attached to one end of the polymer chains to serve as an anchor for incorporation into the liposome bilayer. Liposome-polymer interactions were confirmed using fluorescence spectroscopy and chemical analysis. Microscopy revealed differences in aggregation tendency between unmodified and polymer-modified liposomes. Proteins adsorbed to liposome surfaces during exposure to human plasma were identified by immunoblot analysis. It was found that both unmodified and polymer-modified liposomes adsorb a wide variety of plasma proteins. Contact phase coagulation proteins, complement proteins, cell-adhesive proteins, serine protease inhibitors, plasminogen, antithrombin III, prothrombin, transferrin, alpha(2)-microglobulin, hemoglobin, haptoglobin and beta-lipoprotein as well as the major plasma proteins were all detected. Some differences were found between the unmodified and polymer-modified liposomes. The unmodified liposomes adsorbed plasminogen mainly as the intact protein, whereas on the modified liposomes plasminogen was present in degraded form. Also, the liposomes modified with PNIPAM in its extended conformation (below the lower critical solution temperature) appeared to adsorb less protein than those containing the 'collapsed' form of PNIPAM (above the LCST).

Destabilization of cationic lipid vesicles by an anionic hydrophobically modified poly(N-isopropylacrylamide) copolymer: a solid-state 31P NMR and 2H NMR study

The effect of binding PNIPAM-Py-Gly, a copolymer of N-isopropylacrylamide, N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamide and N-glycydyl-acrylamide, on membrane stability in cationic multilamellar vesicles (MLVs) was examined using solid-state phosphorus (31P) and deuterium (2H) nuclear magnetic resonance (NMR) spectroscopy. For MLVs of composition n-octadecyldiethylene oxide (ODEO)+cholesterol (CHOL)+1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)+dimethyldioctadecylammonium bromide (DODAB) (molar ratios 75:10.5:10.5:4), PNIPAM-Py-Gly induced a complete conversion from a bilayer-type 31P NMR spectrum to one characteristic of lipids undergoing isotropic motional averaging, indicating the existence of regions of high local membrane curvature. This response was sustained even at elevated temperatures. For MLVs of composition POPC+1,2-dioleoyloxy-3-(trimethylammonio)-propane (DOTAP), only at high levels of DOTAP and ionic strength did PNIPAM-Py-Gly induce even a partial conversion to an isotropic-type 31P NMR spectrum. At lower pH this effect was diminished. Raising the temperature eliminated the isotropic 31P NMR spectral component, and this effect was not reversible upon returning to room temperature. 2H NMR spectroscopy of headgroup-deuterated DOTAP and POPC confirmed the 31P NMR results, but did not provide specific surface electrostatic information. We conclude that the binding of PNIPAM-Py-Gly to phospholipid-based vesicles is dominated by electrostatic attraction between cationic lipids and the polymer's glycine residues. At high binding levels, the polymer assumes a collapsed conformation at the surface, resulting in regions of high local curvature of the lipid assembly. For ODEO-based liposomes, these effects are magnified by the additional contribution of hydrogen bonding to the strength of polymer binding.

Mechanism of the interaction of hydrophobically-modified poly-(N-isopropylacrylamides) with liposomes

Interactions of hydrophobically-modified poly-(N-isopropylacrylamides) (HM PNIPAM) with phospholipid liposomes were studied as a function of the lipid type, the lipid bilayer fluidity, and the polymer conformation. Fluorescence experiments monitoring non-radiative energy transfer (NRET), between naphthalene attached to the HM PNIPAM and 1,6-diphenyl-1,3,5-hexatriene (DPH) incorporated into the lipid bilayer, confirmed the direct penetration of hydrophobic anchor groups linked to the polymer into the liposome hydrophobic core. Contraction of the polymer backbone above the lower critical solution temperature (LCST) resulted in a partial withdrawal of the anchor groups from the lipid bilayer. Analysis of polymer/lipid mixtures by centrifugation and quasi-elastic light scattering (QELS) revealed the polymer-induced fission of liposomes in the liquid-crystalline state, resulting in the formation of vesicles 150-230 nm in diameter. The process is reversible and upon transition of the bilayer into the gel state these vesicles are converted into larger aggregates. According to the results of gel-filtration experiments the HM PNIPAM is in dynamic exchange between the liquid-crystalline lipid bilayer and the water solution, while the binding to the bilayer in the gel state is more static in nature. The binding constant for mixture of HM PNIPAM with DMPC liposomes, evaluated from the centrifugation experiments, was found to be 120 M(-1).

Neural cell pattern formation on glass and oxidized silicon surfaces modified with poly(N-isopropylacrylamide)

Control over the adsorption of proteins and over the adsorption and spatial orientation of mammalian cells onto surfaces has been achieved by modification of glass and other silicon oxide substrates with poly(N-isopropylacrylamide) (PNIPAM). The functionalization of the substrates was achieved either by a polymer-analogous reaction of aminosilanes with reactive N-(isopropylacrylamide) (NIPAM)-copolymers and by copolymerization of NIPAM with surface-bound methacrylsilane. The obtained coatings were characterized by FT-1R, ellipsometry, and surface plasmon resonance measurements. The adsorption of two proteins-fibrinogen and ribonuclease A-on these surfaces was studied in situ by real time surface plasmon resonance measurements. The PNIPAM-grafted surfaces prepared by either chemical procedure inhibited the adsorption of both proteins. More importantly they prevented the adhesion of neuroblastomaXglioma hybrid cells cultured either in serum-free medium or in a medium containing serum proteins. Deep-UV irradiation was used to perform ablation processes and to create patterns permitting the examination of spatially controlled adhesion and growth of cells. This study showed that patterned ultrathin polymer films on glass are suitable substrates for controlling the interactions of cells with surfaces and are capable of directing the attachment and spreading of cells.

Interactions of liposomes and hydrophobically-modified poly-(N-isopropylacrylamides): an attempt to model the cytoskeleton

The interactions of small unilamellar vesicles (SUV) and water-soluble copolymers were studied by fluorescence spectroscopy, differential scanning calorimetry (DSC) and quasi-elastic light scattering (QELS). The anchoring onto liposomal bilayer membranes of copolymers of N-isopropylacrylamide, N-(2-(1-naphthyl)ethyl)-N-n-octadecylacrylamide and or N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamide (0.5 mol% of the octadecylacrylamide comonomer) was monitored by non-radiative energy transfer between excited naphthalene and pyrene. The anchoring process occurred on zwitterionic lecithin liposomes and on negatively charged phosphatidic acid liposomes, whether the bilayer was in the crystalline or the liquid-crystalline phase. Insertion of the copolymer octadecyl groups within crystalline bilayers was attributed to the presence of packing defects. Aqueous solutions of poly-(N-isopropylacrylamide) and of its hydrophobically-modified copolymers exhibit a lower critical solution temperature (LCST). The coil to globule collapse of the polymer chains which is known to occur as the aqueous solution is heated through the LCST, also took place when the copolymers were anchored onto vesicular bilayers. The copolymers remained anchored during this collapse and the liposomes were not destroyed. The process was thermo-reversible. Detailed aspects of the reversibility of the phenomenon depended on the relative values of the phase transition temperatures of the liposomes and of the polymer LCST.

Syntheses of model oligosaccharides of biological significance. Synthesis of methyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-d-mannopyranoside and the corresponding mannobiosides

Methyl 2-O-allyl-4,6-O-benzylidene-3-O-(2,3,4,6-tetra-O-acetyl-alpha-D-mannopyranosyl) -alpha-D-mannopyranoside (12) was prepared in 90% yield by Helferich glycosylation of methyl 2-O-allyl-4,6-O-benzylidene-alpha-D-mannopyranoside (9) with tetra-O-acetyl-alpha-D-mannopyranosyl bromide (11). Removal of the benzylidene group and second Helferich glycosylation with 11 led to methyl 2-O-allyl-3,6-di-O-(2,3,4,6-tetra-O-acetyl-alpha-D-mannopyranosyl)-alpha-D-mannopyranoside (14) which, after deallylation and Zemplén deacetylation, gave the title compound 5. The disaccharides methyl 3-O-(alpha-D-mannopyranosyl)-alpha-mannopyranoside (7) and methyl 6-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside (6) have also been synthesized. Complete assignments of the 1H-n.m.r. spectra of the compounds 5, 6, and 7 are given.

Determination of the Structure of four glycopeptides from hen ovalbumin using 360-MHz proton magnetic resonance spectroscopy

Four glycopeptides have been purified by Dowex and Bio-Gel P2 chromatography from Pronase digests of hen ovalbumin. The high-resolution proton magnetic resonance spectra of these glycopeptides and various products of their enzymatic digestion have been obtained at 360 MHz. By use of information derived from the spectra of a number of model compounds, an unambiguous assignment of all C1-H and Man C2-H resonances in the spectra can be made. On this basis structures are proposed for the four glycopeptides which are identical with those structures previously deduced from destructive chemical methods.