Hanna Gracz

... The structure and dynamics of a number of purified tRNA species have been studied in solution using NMR methods (Reid, 1981; Hilbers et al., 1983; Kopper et al., 1983; Hyde & Reid, 1985; Smith et al., 1985; Hare et al., 1985 and references therein; Agris et al., 1986; Schmidt ...

Yeast tRNA/sup Phe/ was C-13 enriched (70 atom %) in vivo in methyl groups. Surprisingly, during the authors studies with the C-13 methyl enriched tRNA/sup Phe/, they observed a downfield carbon NMR signal at 120.9 ppm. Carbon spectra of four chemically synthesized Y-base derivatives and Q Base were compared. The authors can now unambiguously assign the downfield signal to C-13

ABSTRACT In this brief report, we demonstrate that Kerr effect measurements, which determine the excess birefringence contributed by polymer solutes in dilute solutions observed under a strong electric field, are highly sensitive to and capable of determining their microstructures, as well as their locations along the macromolecular backbone. Specifically, using atactic triblock copolymers with the same overall composition of styrene (S) and p-bromostyrene (pBrS) units, but with two different block arrangements, that is, pBrS90-b-S120-b-pBrS90 (I) and S60-b-pBrS180-b-S60 (II), which are indistinguishable by NMR, we detected a dramatic difference in their molar Kerr constants (mK), in agreement with those previously estimated. Although similar in magnitude, their Kerr constants differ in sign, with mK(II) positive and mK(I) negative. In addition, S/pBrS random and gradient copolymers synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization exhibit a heretofore unexpected enhanced enchainment of racemic (r) pBrS-pBrS diads. Comparison of their observed and calculated mKs suggests that the gradient S/pBrS copolymers possess an unanticipated additional gradient in stereosequence that parallels their comonomer gradient, that is, as the concentration of pBrs units decreases from one end of the copolymer chain to the other, so does the content of r diads. This conclusion could only be reached by comparison of observed and calculated Kerr effects, which access the global properties of macromolecules, and not NMR, which is only sensitive to local polymer structural environments, but not to their locations on the copolymer chains. Molar Kerr constants are characteristic of entire polymer chains and are highly sensitive to their constituent microstructures and their distribution along the chain. They may be used to both identify constituent microstructures and locate them along the polymer chain, thereby enabling, for the first time, characterization of their complete macrostructures. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

ABSTRACT The macrostructures of synthetic polymers are essentially the complete molecular chain architectures, including the types and amounts of constituent short-range microstructures, such as the regio- and stereosequences of the inserted monomers, the amounts and sequences of monomers found in co-, ter-, and tetra-polymers, branching, inadvertent, and otherwise, etc. Currently, the best method for characterizing polymer microstructures uses high field, high resolution 13C-nuclear magnetic resonance (NMR) spectroscopy observed in solution. However, even 13C-NMR is incapable of determining the locations or positions of resident polymer microstructures, which are required to elucidate their complete macrostructures. The sequences of amino acid residues in proteins, or their primary structures, cannot be characterized by NMR or other short-range spectroscopic methods, but only by decoding the DNA used in their syntheses or, if available, X-ray analysis of their single crystals. Similarly, there are currently no experimental means to determine the sequences or locations of constituent microstructures along the chains of synthetic macromolecules. Thus, we are presently unable to determine their macrostructures. As protein tertiary and quaternary structures and their resulting ultimate functions are determined by their primary sequence of amino acids, so too are the behaviors and properties of synthetic polymers critically dependent on their macrostructures. We seek to raise the consciousness of both synthetic and physical polymer scientists and engineers to the importance of characterizing polymer macrostructures when attempting to develop structure–property relations. To help achieve this task, we suggest using the electrical birefringence or Kerr effects observed in their dilute solutions. The molar Kerr constants of polymer solutes contributing to the birefringence of their solutions, under the application of a strong electric field, are highly sensitive to both the types and locations of their constituent microstructures. As a consequence, we may begin to characterize the macrostructures of synthetic polymers by means of the Kerr effect. To simplify implementation of the Kerr effect to characterize polymer macrostructures, we suggest that NMR first be used to determine the types and amounts of constituent microstructures present. Subsequent comparison of observed Kerr effects with those predicted for different microstructural locations along the polymer chains can then be used to identify the most likely macrostructures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014

A highly effective method has been developed for the simple extraction of lignin from lignocellulosic biomass using a potentially inexpensive protic ionic liquid (PIL). After the lignin-extraction step, the PIL is easily recovered using distillation leaving the separated lignin and cellulose-rich residues available for further processing. Biopolymer solubility tests indicate that increasing the xylan (i.e., hemicellulose) solubility in the PIL results in greater fiber disruption/penetration, which significantly enhances the effectiveness of the lignin extraction.

ABSTRACT The macrostructures of synthetic polymers are essentially the complete molecular chain architectures, including the types and amounts of constituent short-range microstructures, such as the regio- and stereosequences of the inserted monomers, the amounts and sequences of monomers found in co-, ter-, and tetra-polymers, branching, inadvertent, and otherwise, etc. Currently, the best method for characterizing polymer microstructures uses high field, high resolution 13C-nuclear magnetic resonance (NMR) spectroscopy observed in solution. However, even 13C-NMR is incapable of determining the locations or positions of resident polymer microstructures, which are required to elucidate their complete macrostructures. The sequences of amino acid residues in proteins, or their primary structures, cannot be characterized by NMR or other short-range spectroscopic methods, but only by decoding the DNA used in their syntheses or, if available, X-ray analysis of their single crystals. Similarly, there are currently no experimental means to determine the sequences or locations of constituent microstructures along the chains of synthetic macromolecules. Thus, we are presently unable to determine their macrostructures. As protein tertiary and quaternary structures and their resulting ultimate functions are determined by their primary sequence of amino acids, so too are the behaviors and properties of synthetic polymers critically dependent on their macrostructures. We seek to raise the consciousness of both synthetic and physical polymer scientists and engineers to the importance of characterizing polymer macrostructures when attempting to develop structure–property relations. To help achieve this task, we suggest using the electrical birefringence or Kerr effects observed in their dilute solutions. The molar Kerr constants of polymer solutes contributing to the birefringence of their solutions, under the application of a strong electric field, are highly sensitive to both the types and locations of their constituent microstructures. As a consequence, we may begin to characterize the macrostructures of synthetic polymers by means of the Kerr effect. To simplify implementation of the Kerr effect to characterize polymer macrostructures, we suggest that NMR first be used to determine the types and amounts of constituent microstructures present. Subsequent comparison of observed Kerr effects with those predicted for different microstructural locations along the polymer chains can then be used to identify the most likely macrostructures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014

ABSTRACT The use of chitosan, a cationic, biodegradable polysaccharide derived from sea-shells, in nanofibrous form offers a powerful platform to exploit its inherent benefits. However, chitosan nanofiber formation is difficult, requiring corrosive solvents or a carrier polymer blend to successfully electrospin. Our approach entails blending chitosan with a functional small molecule, cyclodextrin, to facilitate nanofiber formation of chitosan in acetic acid and trifluoroacetic acid. In this case the cyclodextrin, with its complexation properties, could serve to improve chitosan fiber formation, thus serving as a multi-functional blend. In this study, we examine the role of each component and the possibility of synergistic effects in nanofiber formation. Significant improvements in chitosan fiber formation were observed in concert with cyclodextrin at solvent concentrations not possible with just the individual components. Multiple fiber morphologies including three-dimensional fiber mats were also achieved. We examine the improved nanofiber formation in relation to solution viscosity, polymer entanglement, and chitosan–cyclodextrin associations. Rheological studies provide evidence of interactions between cyclodextrin and chitosan. NMR and FTIR studies further validate complexation between these two components.

We combined single-molecule force spectroscopy with nuclear magnetic resonance measurements and molecular mechanics simulations to examine overstretching transitions in single-stranded nucleic acids. In single-stranded DNA and single-stranded RNA there is a low-force transition that involves unwinding of the helical structure, along with base unstacking. We determined that the high-force transition that occurs in polydeoxyadenylic acid single-stranded DNA is caused by the cooperative forced flipping of the dihedral angle formed between four atoms, O5'-C5'-C4'-C3' (γ torsion), in the nucleic acid backbone within the canonical B-type helix. The γ torsion also flips under force in A-type helices, where the helix is shorter and wider as compared to the B-type helix, but this transition is less cooperative than in the B type and does not generate a high-force plateau in the force spectrums of A-type helices. We find that a similar high-force transition can be induced in polyadenylic acid single-stranded RNA by urea, presumably due to disrupting the intramolecular hydrogen bonding in the backbone. We hypothesize that a pronounced high-force transition observed for B-type helices of double stranded DNA also involves a cooperative flip of the γ torsion. These observations suggest new fundamental relationships between the canonical structures of single-and double-stranded DNA and the mechanism of their molecular elasticity.

Chain-level mixing in polyolefins is investigated for blends of polyisobutylene (PIB) and polyethylene-co-1-butene (PEB). Previous reports suggest that PIB exhibits unusual mixing behavior in certain saturated blends relative to other polyolefins, even though it is ...

ABSTRACT Direct spectroscopic probes of individual chain conformation and free volume are used to measure the increasing perturbation in the local glass-transition temperature of one polymer chain with decreasing length scale of mixing in binary polyolefin blends. Solid-state 2H and 129Xe NMR experiments reveal a compositional miscibility window in side-chain concentration for polyisobutylene (PIB)/poly(ethylene-co-butene) (PEB) blends. A combination of pulsed-field gradient and chemical shift data for xenon gas absorbed in these polymer blends indicates that the presence of polymer chains within a radius of 35 nm of a different chain structure will perturb the intermolecular packing contribution to the total conformational energy of that chain, thereby changing its Tg.

... JB; Van Tendeloo, G.; Pietrass, T. J. Am. Chem. Soc.2000, 122, 10591. ... (19) van Amerongen, GJ Rubber Chem. Technol.1964, 37, 1065. ... Free Volume Investigation of Polymers of Intrinsic Microporosity (PIMs): PIM-1 and PIM1 Copolymers Incorporating Ethanoanthracene Units. ...

ABSTRACT A series of solid inclusion complexes (ICs) containing the aromatic guests aniline, benzene, ethylbenzene, phenol, p-xylene, styrene, and toluene were formed with host γ-cyclodextrin (γ-CD). IC stoichiometry was observed to depend on the nature of the included aromatic guest. The molar ratio of styrene, aniline, and phenol guests to γ-CD host was 2:1 in their individual IC crystals, whereas ethylbenzene, p-xylene, and toluene guests formed 1:1 inclusion complexes with γ-CD. Thermogravimetric analysis showed that the thermal stabilities of these volatile aromatic guest molecules increased due to guest−host interactions once they were included in their γ-CD-ICs. X-ray diffraction (WAXD) observations performed on the aromatic guest−CD-IC crystals showed that all of them have channel-type crystalline structures. Moreover, it was observed that the presence of guest molecules inside the γ-CD cavities stabilized the channel structure of stacked γ-CDs. However, a solid-phase transition from tetragonal to hexagonal packing of γ-CD stacks was observed upon dehydration of the styrene−, toluene−, p-xylene−, benzene−, and ethylbenzene−γ-CD-IC crystals. We have also discovered that the solid-state transformation between tetragonal and hexagonal channel crystal structures is reversible upon the desorption or sorption of water. In the cases of aniline− and phenol−γ-CD-ICs, no solid-phase transition from tetragonal packing to hexagonal packing was observed upon dehydration. Instead, the tetragonal packing structure was stable even after vacuum-drying at elevated temperatures for an extended period. The stability of the tetragonal packing in aniline− and phenol−γ-CD-ICs was attributed to the presence of hydrogen bonding between the included guests and neighboring host γ-CD molecules.

ABSTRACT The efficient synthesis of eight new macrocyclic amides (lactams) via reaction of diesters with diamines under normal dilution conditions is described. The role of intermolecular H-bond formation and steric hindrance is discussed based on 1H- and 15N-NMR studies of appropriate model compounds. Principles for the optimal choice of esters that can be efficiently transformed into diamides have been developed.