Pluronic copolymer liquid crystals: unique, replaceable media for capillary gel electrophoresis.

Liquid crystalline solutions of Pluronic copolymers are versatile alternatives to solutions of entangled, random coil polymers as replaceable media for capillary gel electrophoresis (CGE). Pluronic copolymers are tri-block polymers of poly(ethylene oxide) [(EO)x] and poly(propylene oxide) [(PO)y] with the general formula (EO)x(PO)y(EO)x. Large micelles form in aqueous solutions in which central, hydrophobic cores of (PO)y segments are surrounded by "brushes" of hydrated (EO)x tails. Solutions of Pluronic F127 (BASF Performance Chemicals) in a concentration range of about 18-30% are liquids at refrigerator temperatures (< or = 5 degrees C) and are easily introduced into capillaries. A self-supporting, gel-like liquid crystalline phase is formed as the temperature is raised to > or = 20 degrees C. This liquid crystalline phase consists of spherical micelles with diameters of 17-18 nm which pack with local cubic symmetry. CGE in Pluronic F127 liquid crystals separates species within several chemical classes as varied as nucleoside monophosphates and organic dyes, oligonucleotides of 4-60 nucleotides, DNA fragments of 50-3000 base pairs (bp), and supercoiled plasmid DNAs of 2000-10,000 bp. Mechanisms of molecular sieving in polymer liquid crystals must differ in fundamental ways from separations in random polymer gels because molecules move around uncrosslinked obstacles that are larger than the smallest dimensions of typical analytes. Molecular sieving in Pluronic liquid crystals is envisioned to occur as molecules squeeze between hydrated (EO)x strands of micelle brushes, or through brushtips and interstitial spaces between micelles. Small molecules such as nucleotides appear to separate by a different mechanism involving partitioning between hydrophilic and hydrophobic environments. This process is termed "hydrophobic interaction electrophoresis". The unique structures of Pluronic copolymers and their liquid crystalline phases provide new challenges and opportunities in separations science.

Scaling analysis of polyacrylamide gel surfaces synthesized in the presence of surfactants.

Surfaces of polyacrylamide hydrogels synthesized in the presence of surfactants were imaged by atomic force microscopy (AFM), and the surface morphology was studied by numerical scaling analysis. The gels were formed by polymerizing acrylamide plus a cross-linker in the presence of surfactants, which were then removed by soaking in distilled water. Gels formed in the presence of over 20% surfactant (by weight) formed clear, but became opaque upon removal of the surfactants. Other gels formed and remained clear. The surface morphology of the gels was studied by several one- and two-dimensional numerical scaling methods. The surfaces were found to be self-affine on short length scales, with a roughness (Hurst) exponent in the range from 0.85 to 1, crossing over to a constant root-mean-square surface width at long scales. Both the crossover length between these two regimes and the saturation value of the surface width increased significantly with increasing surfactant concentration, coincident with the increase in opacity. We propose that the changes in the surface morphology are due to a percolation transition in the system of voids formed upon removal of the surfactants from the bulk.

Templated pores in hydrogels for improved size selectivity in gel permeation chromatography.

The pore structures of cross-linked polyacrylamide gels can be altered by polymerizing in the presence of high concentrations of unreactive, micellar surfactant cosolutes which act as "templates". Removal of surfactant after polymerization is expected to leave pores with the approximate shape and dimensions of the surfactant micelles. A simple model was developed to simulate gel permeation chromatography (GPC) separations of globular proteins on templated gels. The model assumes that the partition coefficient for sieving of a protein is equal to the fraction of gel volume accessible to a sphere with a radius equal to the protein Stokes radius. The total gel volume is considered to include a fraction that is a conventional, random gel matrix and a remaining fraction contributed by templated pores. The pore size distribution of the conventional gel was estimated using the Ogston equation, which approximates the matrix as a random collection of long, thin, rigid fibers. Templated pores were assumed to have a Gaussian distribution of radii centered about some mean determined by the micelle radius. In comparison to conventional media, gels with templated pores are predicted to exhibit more sharply defined exclusion limits and improved resolution over a narrow size range centered on the mean templated pore size. Selectivity and resolution are expected to increase as the volume fraction of templated pores is increased and as the dispersion of templated pore radius is decreased. Small changes in template radius lead to large changes in the molecular weight range of optimal separation of globular proteins. It should be possible to create a series of GPC media that collectively offer high resolution over the molecular weight range of most globular proteins of interest.

Electrophoresis in lyotropic polymer liquid crystals.

Excellent electrophoretic separations of a variety of biological molecules can be accomplished by using uncharged, triblock copolymers as the "gel" media. These copolymers form uncrosslinked, lyotropic liquid crystalline phases of large micelles between which molecules must travel. Unlike crosslinked hydrogels in common use, these alternative media have highly ordered internal structures. Pluronic F127, representative of the copolymer class, contains poly(ethylene oxide) (EO) and poly(propylene oxide) (PO) units with an approximate molecular formula (EO)106(PO)70(EO)106. Concentrated (18-30%) solutions of Pluronic F127 are freely flowing liquids at low temperature (0-5 degrees C) but form gel-like, cubic liquid crystals of large, spherical micelles when warmed. The utility of these media is illustrated by separations of linear, double-stranded DNA up to 3,000 bp long by conventional electrophoresis, and of single-stranded DNAs from 4 to 60 nt long by capillary electrophoresis. Extraordinary separations of supercoiled DNAs were also obtained by capillary electrophoresis. The versatility, availability, and ease of use of Pluronic polymers offer major advantages over conventional media for preparative and high performance analytical separations of nucleic acids and other biomolecules. Mechanisms of molecular transport and separation operating in polymer liquid crystals must differ in fundamental ways from those in crosslinked gels. Lyotropic polymer liquid crystals are unique systems for elucidating mechanisms of macromolecule migration in ordered, dense media, and provide opportunities in separations science.

Observation of a metastable periodic structure for the (001) surface of KTaO3 after cleaving in situ.

Helium atom diffraction experiments carried out under ultrahigh vacuum conditions on a freshly cleaved (001) surface of KTaO3 reveal metastable features which decay over a period of several hours. The initial He diffraction pattern contains large scattering intensity satellite peaks very close to the specular reflection beam. As time from cleaving elapses, the satellite intensities diminish virtually to zero while the specular intensity increases, and the diffraction pattern evolves into one consistent with the (1x1) bulk termination surface. The data are compared with model calculations for scattering from a series of terraces at two heights with a distribution of terrace lengths [Surf. Sci. 384, 15 (1997)].