Steven R. Kline

Reduction and analysis of SANS and USANS data using IGOR Pro

A software package is presented for performing reduction and analysis of small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS) data. A graphical interface has been developed to visualize and quickly reduce raw SANS and USANS data into one- or two-dimensional formats for interpretation. The resulting reduced data can then be analyzed using model-independent methods or non-linear fitting to one of a large and growing catalog of included structural models. The different instrumental smearing effects for slit-smeared USANS and pinhole-smeared SANS data are handled automatically during analysis. In addition, any number of SANS and USANS data sets can be analyzed simultaneously. The reduction operations and analysis models are written in a modular format for extensibility, allowing users to contribute code and models for distribution to all users. The software package is based on Igor Pro, providing freely distributable and modifiable code that runs on Macintosh and Windows operating systems.

Small angle neutron scattering near Lifshitz lines: Transition from weakly structured mixtures to microemulsions

We have studied the phase behavior, wetting transitions, and small angle neutron scattering (SANS) of water, n‐alkane, and n‐alkyl polyglycol ether (CiEj) systems in order to locate the transition between weakly structured mixtures and microemulsions, and to provide a measure for the transition. We first determined the wetting transition by macroscopic measurements and then measured the location of the Lifshitz lines by SANS. Starting with well‐structured mixtures (exhibiting nonwetting middle phases and well‐expressed scattering peaks, features that qualify them as microemulsions) the wetting transition was induced by increasing the chain length of the alkane or by changing the oil/water volume ratio, and then the Lifshitz line was crossed. Further, starting with systems past the disorder line (weakly structured mixtures that display wetting middle phases and no scattering peaks), local structure was induced by either increasing the surfactant concentration or decreasing the oil/water volume ratio or the...

Controlling the Self‐Assembly of Metal‐Seamed Organic Nanocapsules

The advent of modern molecular characterization techniques in the mid 20th century brought about a renaissance in our understanding of life s many processes. Notably, the structural determination of large biomolecules through the development of techniques such as NMR spectroscopy and X-ray diffraction (XRD) has given scientists valuable insight into the inner workings of cells. Many of these molecules are highly symmetrical, multicomponent entities, though determining the processes by which they assemble has been a difficult task. The biosynthesis of DNA, for example, came much later than its structural characterization by Watson and Crick. It is exactly this knowledge, however, that allows one to control the system. Supramolecular chemists have likewise endeavored to control the self-assembly processes in multicomponent entities. Although simpler than many biomolecules, the size and complexity of the macromolecules that embody this field largely preclude the use of standard mechanistic analyses that are applicable to smaller compounds. Our work has focused on metal-seamed pyrogallol[4]arene (PgC) nanocapsules. Forming rapidly through selfassembly, these large entities are composed of 2 or 6 macrocyclic units that act as chelates through their upper rims for 8, 12, or 24 metal ions (Figure 1). It is important to note that the 6-macrocycle, 24-metal ion hexameric nanocapsule results from the remarkable self-assembly of 30 entities. The dimeric or hexameric capsules are highly sym-

Ferrocene Species Included within a Pyrogallol[4]arene Tube

Research in host–guest complexes with ferrocene as a guest continues to attract attention. Macrocyclic hosts spanning from curcubiturils and cyclodextrins to resorcinarenes have been used to both encapsulate ferrocene and use as a component in nanometric frameworks. C-alkylpyrogallol[4]arenes (PgCs) are bowl-shaped compounds that are commonly used as building blocks in the construction of larger entities, such as capsules and nanotubes. Our work with C-methyl and C-heptylpyrogallol[4]arene has likewise shown that these compounds can function as hosts for ferrocene. The host–guest complex thus formed is a dimeric capsule with the enclosed and highly ordered ferrocene located between two PgC hemispheres. In addition to such capsular motifs, the conical shape of the calixarenes, resorcinarenes, and pyrogallolarenes can likewise lead to the formation of tubular solid-state structures. These often incorporate large nonsolvent molecules as part of the tubular framework. An excellent example of a PgC-based tubular framework that accommodates large nonsolvent molecules is the host–guest complex of Chexylpyrogallol[4]arene (PgC6) with pyrene. [9] In this complex, tetramers of PgC6 associate with one another through hydrogen bonding, whereas the pyrene molecules intercalate between the C-hexyl pendant arms of the PgC. This leads to two distinct regions within the structure: a hydrophilic tube that encloses guest solvents along with a hydrophilic tube that accommodates the pyrene. Herein, we describe a second host–guest complex of C-methylpyrogallol[4]arene (PgC1) and ferrocene that conforms to a tubular structural motif. In contrast to the capsular motif, a tubular hydrophobic cavity, rather than a capsular cage, is responsible for incarceration of the guest, whereas the hydroxyls of the PgC1 complexes along with polar solvent molecules form the long-range hydrogen-bonding superstructure. Slow changes in concentration of a PgC1 and ferrocene solution caused by evaporation led to the crystallization of this unique architecture. Methanolic solutions containing various ratios of PgC1 to ferrocene (with the concentration of ferrocene set at 10 3 molL ) were allowed to evaporate until crystallization was evident. At a 1:1 PgC1/ferrocene ratio, crystals of the previously reported dimeric product were the sole product. However, at ferrocene ratios of 6:1 or higher, two different crystal habits formed were found, with green needle-like crystals accompanying the dark blue prisms of the ferrocene dimer. X-ray diffraction analysis of the single crystal showed the dark green needles to be a novel tubular motif 1 featuring ferrocene “beads” in a hydrophobic cylinder of repeating trimers of PgC1. The tubular structure 1 (Figure 1) displays a complicated hydrogen-bonding arrangement of PgC1 complexes. Each tube consists of alternating units of 3 PgC1 complexes rotated by 608 relative to one another along the crystallographic C axis and a single ferrocene guest. The overall structure thus closely resembles a family of resorcinarene-based nanotubes described by Rissannen et al. However, in contrast to both the resorcinarene tubes and our previously reported

Solution-phase structures of gallium-containing pyrogallol[4]arene scaffolds.

The variations in architecture of gallium-seamed (PgC4Ga) and gallium-zinc-seamed (PgC4GaZn) C-butylpyrogallol[4]arene nanoassemblies in solution (SANS/NMR) versus the solid state (XRD) have been investigated. Rearrangement from the solid-state spheroidal to the solution-phase toroidal shape differentiates the gallium-containing pyrogallol[4]arene nanoassemblies from all other PgCnM nanocapsules studied thus far. Different structural arrangements of the metals and arenes of PgC4Ga versus PgC4GaZn have been deduced from the different toroidal dimensions, C–H proton environments and guest encapsulation of the two toroids. PGAA of mixed-metal hexamers reveals a decrease in gallium-to-metal ratio as the second metal varies from cobalt to zinc. Overall, the combined study demonstrates the versatility of gallium in directing the selfassembly of pyrogallol[4]arenes into novel nanoarchitectures.

Determination of the persistence length of helical and non-helical polypeptoids in solution

Control over the shape of a polymer chain is desirable from a materials perspective because polymer stiffness is directly related to chain characteristics such as liquid crystallinity and entanglement, which in turn are related to mechanical properties. However, the relationship between main chain helicity in novel biologically derived and inspired polymers and chain stiffness (persistence length) is relatively poorly understood. Polypeptoids, or poly(N-substituted glycines), constitute a modular, biomimetic system that enables precise tuning of chain sequence and are therefore a good model system for understanding the interrelationship between monomer structure, helicity, and persistence length. The incorporation of bulky chiral monomers is known to cause main chain helicity in polypeptoids. Here, we show that helical polypeptoid chains have a flexibility nearly identical to an analogous random coil polypeptoid as observed via small angle neutron scattering (SANS). Additionally, our findings show that polypeptoids with aromatic phenyl side chains are inherently flexible with persistence lengths ranging from 0.5 to 1 nm.

Formation of kinetically trapped nanoscopic unilamellar vesicles from metastable nanodiscs.

Zwitterionic long-chain lipids (e.g., dimyristoyl phosphatidylcholine, DMPC) spontaneously form onion-like, thermodynamically stable structures in aqueous solutions (commonly known as multilamellar vesicles, or MLVs). It has also been reported that the addition of zwitterionic short-chain (i.e., dihexanoyl phosphatidylcholine, DHPC) and charged long-chain (i.e., dimyristoyl phosphatidylglycerol, DMPG) lipids to zwitterionic long-chain lipid solutions results in the formation of unilamellar vesicles (ULVs). Here, we report a kinetic study on lipid mixtures composed of DMPC, DHPC, and DMPG. Two membrane charge densities (i.e., [DMPG]/[DMPC] = 0.01 and 0.001) and two solution salinities (i.e., [NaCl] = 0 and 0.2 M) are investigated. Upon dilution of the high-concentration samples at 50 °C, thermodynamically stable MLVs are formed, in the case of both weakly charged and high salinity solution mixtures, implying that the electrostatic interactions between bilayers are insufficient to cause MLVs to unbind. Importantly, in the case of these samples small angle neutron scattering (SANS) data show that, initially, nanodiscs (also known as bicelles) or bilayered ribbons form at low temperatures (i.e., 10 °C), but transform into uniform size, nanoscopic ULVs after incubation at 10 °C for 20 h, indicating that the nanodisc is a metastable structure. The instability of nanodiscs may be attributed to low membrane rigidity due to a reduced charge density and high salinity. Moreover, the uniform-sized ULVs persist even after being heated to 50 °C, where thermodynamically stable MLVs are observed. This result clearly demonstrates that these ULVs are kinetically trapped, and that the mechanical properties (e.g., bending rigidity) of 10 °C nanodiscs favor the formation of nanoscopic ULVs over that of MLVs. From a practical point of view, this method of forming uniform-sized ULVs may lend itself to their mass production, thus making them economically feasible for medical applications that depend on monodisperse lipid-based systems for therapeutic and diagnostic purposes.

Solution structure of copper-seamed C-alkylpyrogallol[4]arene nanocapsules with varying chain lengths

The stability of copper-seamed C-alkylpyrogallol[4]arene hexamers with varying chain lengths in solution has been studied using small-angle neutron scattering (SANS). The progression in diameter of spherical capsules with increasing alkyl chain lengths of copper-seamed hexamers in solution suggests both robustness as well as a close correlation between the solid phase and solution phase structures.

Thermally Switchable One- and Two-Dimensional Arrays of Single-Walled Carbon Nanotubes in a Polymeric System

Fabrication of highly ordered arrays of single-walled carbon nanotubes (SWNTs) has been of great interest for a wide range of potential applications. Here, we report thermally switchable one- and two-dimensional arrays of individually isolated SWNTs formed by cooperative self-assembly of functionalized SWNTs and a block copolymer/water system. Small-angle X-ray scattering measurements reveal that when the block copolymer/water system is in an isotropic phase, two-dimensional hexagonal arrays of SWNTs are formed by depletion attraction, and when the block copolymer/water system is in a lamellar phase, one-dimensional lattices of SWNTs intercalated in the polar regions of the polymeric lamellar structure are formed by entropically driven segregation and two-dimensional depletion attraction. These two SWNT arrays are thermally interchangeable, following the temperature-dependent phase behavior of the block copolymer/water system.

Exploring the Ellipsoidal and Core–Shell Geometries of Copper-Seamed C-Alkylpyrogallol[4]arene Nanocapsules in Solution

Small-angle neutron scattering (SANS) studies were used to probe the stability and geometry of copper-seamed C-alkylpyrogallol[4]arene (PgC(n)Cu; n = 11, 13, 17) hexamers in solution. Novel structural features are observed at chain lengths greater than 10 in both solid and solution phase. Scattering data for the PgC(11)Cu and PgC(13)Cu in chloroform fitted as core-shell spheres with a total spherical radius of about 22.7 and 22.9 Å respectively. On the other hand, the scattering curve for the PgC(17)Cu hexamer at both 1% and 5% mass fractions in o-xylene did not fit as a discrete sphere but rather as a uniform ellipsoid. The geometric dimensions of the ellipsoid radii are 24 Å along the minor axis and 115 Å along the major axis. It is expected that an individual hexamer with heptadecyl chains would exhibit a uniform radius of ca. 24 Å. However, an approximate ratio of 1:5 between radii lengths for the minor axis and major axis is consistent with interpenetration of the heptadceyl chains of adjacent hexamers to form a single ellipsoidal assembly.