Dan Lundberg

Dissolution and Gelation of Cellulose in TBAF/DMSO Solutions: The Roles of Fluoride Ions and Water

Solutions of cellulose in a mixture of tetrabutylammonium fluoride and dimethyl sulfoxide (TBAF/DMSO) containing small and varying amounts of water were studied by nuclear magnetic resonance (NMR). By measuring the composition dependences of (19)F NMR and (1)H NMR chemical shifts and line widths, details on the dissolution and gelation mechanisms for cellulose in TBAF/DMSO were elucidated. Our results suggest that the strongly electronegative fluoride ions act as hydrogen bond acceptors to cellulose hydroxyl groups, thus dissolving the polymer by breaking the cellulose-cellulose hydrogen bonds and by rendering the chains an effective negative charge. It was found that the fluoride ions also interact strongly with water. Small amounts of water remove the fluoride ions from the cellulose chains and allow reformation of the cellulose-cellulose hydrogen bonds, which leads to formation of highly viscous solutions or gels even at low cellulose concentrations.

Condensation and Decondensation of DNA by Cationic Surfactant, Spermine, or Cationic Surfactant–Cyclodextrin Mixtures: Macroscopic Phase Behavior, Aggregate Properties, and Dissolution Mechanisms

The macroscopic phase behavior and other physicochemical properties of dilute aqueous mixtures of DNA and the cationic surfactant hexadecyltrimethylammounium bromide (CTAB), DNA and the polyamine spermine, or DNA, CTAB, and (2-hydroxypropyl)-β-cyclodextrin (2HPβCD) were investigated. When DNA is mixed with CTAB we found, with increasing surfactant concentration, (1) free DNA coexisting with surfactant unimers, (2) free DNA coexisting with aggregates of condensed DNA and CTAB, (3) a miscibility gap where macroscopic phase separation is observed, and (4) positively overcharged aggregates of condensed DNA and CTAB. The presence of a clear solution beyond the miscibility gap cannot be ascribed to self-screening by the charges from the DNA and/or the surfactant; instead, hydrophobic interactions among the surfactants are instrumental for the observed behavior. It is difficult to judge whether the overcharged mixed aggregates represent an equilibrium situation or not. If the excess surfactant was not initially present, but added to a preformed precipitate, redissolution was, in consistency with previous reports, not observed; thus, kinetic effects have major influence on the behavior. Mixtures of DNA and spermine also displayed a miscibility gap; however, positively overcharged aggregates were not identified, and redissolution with excess spermine can be explained by electrostatics. When 2HPβCD was added to a DNA-CTAB precipitate, redissolution was observed, and when it was added to the overcharged aggregates, the behavior was essentially a reversal of that of the DNA-CTAB system. This is attributed to an effectively quantitative formation of 1:1 2HPβCD-surfactant inclusion complexes, which results in a gradual decrease in the concentration of effectively available surfactant with increasing 2HPβCD concentration.

Surfactants Containing Hydrolyzable Bonds

There is a growing demand for hydrolyzable surfactants, i.e., surfactants that break down ina controlled way by changing the pH. Environmental concern is the main driving force behind currentinterest in these surfactants, but they are also of interest in applications where surfactants are neededin one stage but later undesirable at another stage of a process. This chapter summarizes the fieldof hydrolyzable surfactants with an emphasis on their more recent development. Surfactants that break downeither on the acid or on the alkaline side are described. It is shown that the susceptibility to hydrolysisfor many surfactants depends on whether or not the surfactant is in the form of micelles or as free unimersin solution. It is shown that whereas nonionic ester surfactants are more stable above the CMC (micellarretardation), cationic ester surfactants break down more readily when aggregated than when present as unimers(micellar catalysis).

Interactions of PAMAM Dendrimers with Negatively Charged Model Biomembranes.

We have investigated the interactions between cationic poly(amidoamine) (PAMAM) dendrimers of generation 4 (G4), a potential gene transfection vector, with net-anionic model biomembranes composed of different ratios of zwitterionic phosphocholine (PC) and anionic phospho-L-serine (PS) phospholipids. Two types of model membranes were used: solid-supported bilayers, prepared with lipids carrying palmitoyl-oleoyl (PO) and diphytanoyl (DPh) acyl chains, and free-standing bilayers, formed at the interface between two aqueous droplets in oil (droplet interface bilayers, DIBs) using the DPh-based lipids. G4 dendrimers were found to translocate through POPC:POPS bilayers deposited on silica surfaces. The charge density of the bilayer affects translocation, which is reduced when the ionic strength increases. This shows that the dendrimer-bilayer interactions are largely controlled by their electrostatic attraction. The structure of the solid-supported bilayers remains intact upon translocation of the dendrimer. However, the amount of lipids in the bilayer decreases and dendrimer/lipid aggregates are formed in bulk solution, which can be deposited on the interfacial layers upon dilution of the system with dendrimer-free solvent. Electrophysiology measurements on DIBs confirm that G4 dendrimers cross the lipid membranes containing PS, which then become more permeable to ions. The obtained results have implications for PAMAM dendrimers as delivery vehicles to cells.

Encapsulation of DNA in Macroscopic and Nanosized Calcium Alginate Gel Particles

Calcium alginate beads, which are biodegradable and biocompatible, have been widely employed as delivery matrices for biomacromolecules. In the present work, the feasibility of encapsulation of DNA (which is used as a model biomacromolecule) in calcium alginate nanobeads (sub-200 nm size), prepared using a recently developed protocol based on the phase inversion temperature (PIT) emulsification method [Machado et al. Langmuir 2012, 28, 4131-4141], was assessed. The properties of the nanobeads were compared to those of the corresponding macroscopic (millimeter sized) calcium alginate beads. It was found that DNA, representing a relatively stiff and highly charged polyanion (thus like-charged to alginate), could be efficiently encapsulated in both nanosized and macroscopic beads, with encapsulation yields in the range of 77-99%. Complete release of DNA from the beads could be accomplished on dissolution of the gel by addition of a calcium-chelating agent. Importantly, the DNA was not denatured or fragmented during the preparation and collection of the nanobeads, which are good indicators of the mildness of the preparation protocol used. The calcium alginate nanobeads prepared by the herein utilized protocol thus show good potential to be used as carriers of sensitive biomacromolecules.

Interactions between DNA and Nonionic Ethylene Oxide Surfactants are Predominantly Repulsive

In the present work, the interactions between double-stranded (ds) or single-stranded (ss) DNA and nonionic ethylene oxide (EO) surfactants, with special attention to the possible contributions from hydrophobic interactions, have been investigated using a multitechnique approach. It was found that the presence of ss as well as dsDNA induces a slight decrease of the cloud point of pentaethylene glycol monododecyl ether (C(12)E(5)). Assessment of the partitioning of DNA between the surfactant-rich and surfactant-poor phases formed above the cloud point showed that the polymer was preferably located in the surfactant-poor phase. Surface tensiometry experiments revealed that neither of the DNA forms induced surfactant micellization. Finally, it was shown by DNA melting measurements that another EO surfactant (C(12)E(8)) did not affect the relative stabilities of ss and dsDNA. To summarize, all experiments suggest that the net interaction between DNA and nonionic surfactants of the EO type is weakly repulsive, which can be attributed mainly to steric effects. In general, the results were practically identical for the ds and ss forms of DNA, except those from the cloud point experiments, where the decrease of the cloud point was less pronounced with ssDNA. This finding indicates the presence of an attractive component in the interaction, which can reasonably be ascribed to hydrophobic effects.

Phase Behavior and Coassembly of DNA and Lysozyme in Dilute Aqueous Mixtures: A Model Investigation of DNA−Protein Interactions

Results from an experimental investigation of the phase behavior of an aqueous system of DNA from salmon testes and the protein lysozyme are presented. At very low concentrations of either or both of the macromolecular components, wormlike assemblies with a width of the order of 10 nm are formed. There are strong indications that direct interactions between the protein units are instrumental both in driving the phase separation and in controlling the morphology of the formed assemblies.

Interactions between Cationic Lipid Bilayers and Model Chromatin

Complexes formed in mixtures of cationic liposomes of varying charge density and nucleosome core particles (NCPs) or nucleosome arrays have been characterized. Under most of the conditions studied, the lipids and NCPs or arrays formed lamellar structures similar to those obtained with the liposomes and pure DNA. Thus, the dissociation of DNA from the NCP or nucleosome array and the formation of a DNA-lipid complex is thermodynamically favored, which can likely be ascribed mainly to the gain in entropy on release of the small counterions. Only at very low liposome charge densities are there indications that the NCPs/arrays do not dissociate upon interaction with the lipid bilayers. The reported results can serve as a valuable reference point in investigations of biologically more relevant systems.

Self-assembly of cationic surfactants that contain thioether groups in the hydrophobic tails.

Self-assembly in aqueous solutions of cationic surfactants that carry thioether groups in their hydrophobic tails has been investigated. Of particular interest was the identification of possible changes in the aggregate structure due to the presence of sulfur atoms. Solutions of four different compounds [CH(3)CH(2)S(CH(2))(10)N(CH(3))(3)(+)Br(-) (2-10), CH(3)(CH(2))(5)S(CH(2))(6)N(CH(3))(3)(+)Br(-) (6-6), CH(3)(CH(2))(7)S(CH(2))(6)N(CH(3))(3)(+)Br(-) (8-6), and CH(3)(CH(2))(7)S(CH(2))(8)N(CH(3))(3)(+)Br(-) (8-8)] were characterized by (1)H NMR, (13)C NMR, NMR diffusometry, and conductivity measurements. In addition to investigating aqueous solutions containing each of the thioethers present as the sole solute, mixtures of 2-10 or 6-6 with dodecyltrimethylammonium bromide (DTAB) were studied. The addition of a sulfide group to the hydrophobic tail causes an increase in the critical micelle concentration but has a limited effect on the aggregate structure. Micelles are formed at a well-defined concentration for all of the investigated surfactants and surfactant mixtures. However, a comparison of the behavior of concentrated solutions of 8-8 to that of solutions of hexadecyltrimethylammonium bromide (CTAB) of similar concentrations suggests that the presence of a sulfur atom decreases the tendency for micellar growth. This may be a consequence of a slightly higher preference for the micellar surface of a sulfur atom as compared to that of a methylene group in a similar position, an idea that is also supported by results for the surfactant mixtures.

Structural Evolution of Oleyl Betainate Aggregates: In Situ Formation of Small Unilamellar Vesicles

Betaine esters prepared from long-chain alcohols are a class of hydrolyzable cationic surfactants that is interesting both because the compounds can be designed to give harmless products on degradation and that the hydrolysis products can induce potentially useful changes in the properties of systems where such surfactants are present. In this work, the evolution in structure of aggregates formed by oleyl betainate during hydrolysis of the compound has been investigated using (1)H NMR and cryo-transmission electron microscopy (cryo-TEM). With an increasing extent of hydrolysis, and thus an increasing fraction of oleyl alcohol in the aggregates, the aggregate structure changes in a sequence consistent with an increase in the average packing parameter of the surfactant-alcohol mixture, from spherical micelles, via wormlike micelles, to vesicles. An important result from this work is that it demonstrates a means of in situ production of small unilamellar vesicles with a rather narrow size distribution.