Spin-spin relaxation in the presence of dissimilar Fe3+ complexes

It is shown by use of Mössbauer spectroscopy that the spin-spin relaxation time of Fe³⁺ ions in frozen aqueous solutions is increased when two different iron complexes are present.     

Dynamics of coaxial torsional resonators consisting or segments with different radii,material properties and surrounding media

High-Q torsional resonators constitute the most sensitive transducers for high frequency dynamic viscoelastic measurements of dilute polymer solutions. Most such resonators described in the literature are segmented. Because of the need for torque and torsional displacement transducers the Q-value of the individual segments most often differ, but normally all segments have the same radius.A detailed analysis of the dynamics of such resonators when both the radii, material properties and surrounding media may be different for each segment, is presented. For resonators where all segment lengths equal an integer multiple of a quarter of the torsional wavelength, we find that the Q-value of the resonator as a whole is mainly determined by the Q-value of the segment with the smallest radius. We further find that reduction of the radius of the segment surrounded by polymer solution results in a stronger mechanical coupling between the resonator as a whole and the polymer solution. These findings suggest that the segment radii are important optimization parameters of segmented torsional resonators used to measure the high frequency dynamic viscoelastic properties of e.g. polymer solutions.     

Bifurcations in a vocal fold model

An autonomous fourth order model of vocal fold vibrations is proposed. Each fold is represented by a lower and upper mass, and the aerodynamic forces are derived from a modified Bernoulli equation. The model exhibits many features of normal phonation in a wide parameter region. At the borderlines of this region coexistence of limit cycles, period-doubling and chaos are observed. Implications for an understanding of pathological voices are discussed.     

Mucoadhesive Sustained Drug Delivery System Based on Cationic Polymer and Anionic Cyclodextrin/Triclosan Complex

In this study the interactions between a cationic polymer and an anionic cyclodextrin were investigated. The system has the potential for use in a sustained release dosage forms for use on mucous membranes. As mucous membranes are negatively charged the objective of this study was to investigate whether a drug delivery system based on a cationic polymer and an anionic cyclodextrin would be more mucoadhesive than a system containing a cationic polymer and a neutral cyclodextrin. For this purpose the cationic polymer hexadimethrine bromide (HDMBr) and anionic sulfobutylether -cyclodextrin (SBECD) were utilized as well as the neutral hydroxypropyl-cyclodextrin (HPCD). Triclosan was used as a model drug. The drug delivery system was formulated as a solution or semi-solid and its adhesion to porcine buccal mucosa and cation exchange media was measured. In addition the release of triclosan from the system was quantified. No difference was observed between the two systems when they were applied to the mucosal surface. However, the formulations showed improved adhesion, compared to the neutral cyclodextrin/drug delivery system, when they could also reach the underlying surface of the excised tissue. The drug delivery system was much better retained on the cation exchange media than the uncharged system. Significant interactions were observed between the negatively charged cyclodextrin and the positively charged polymer. The results indicate that the interactions could be used to obtain a mucoadhesive sustained drug delivery system under certain circumstances. The positive charge of HDMBr did not have the expected effect on the buccal mucosa and it can be concluded that although a positive charge is likely to promote mucoadhesion, other attributes of polymers, such as molecular weight and viscosity, may have equally beneficial effect.     

Mimicking enzymatic transaminations: attempts to understand and develop a catalytic asymmetric approach to chiral alpha-amino acids

Attempts are made to build a bridge between asymmetric catalysis and enzymatic reactions by mechanistic investigations and the development of a catalytic and enantioselective approach to amination of alpha-keto esters by primary amines catalyzed by chiral Lewis acids as a model for transamination enzymes. Different Lewis acids can catalyze the half-transamination of alpha-keto esters using primary amine nitrogen sources such as pyridoxamine and 4-picolylamine. The mechanistic studies of the Lewis-acid catalyzed half-transamination using deuterium-labelled compounds show the incorporation of deuterium atoms in several positions of the alpha-amino acid derivative, indicating that the enol of the alpha-keto ester plays an important role along the reaction path. The catalytic enantioselective reactions are dependent on the pKa-value of the solvent since enantioselectivities were only obtained in solvents with high pKa-values relative to methanol. However, stronger acidic conditions generally gave better yields, but poor enantioselectivities. A series of chiral Lewis acids were screened as catalysts for the enantioselective half-transamination reactions and moderate yields and enantioselectivities of up to 46% ee were obtained.     

Detection and relative quantification of proteins by surface enhanced Raman using isotopic labels

Accurate quantification of protein content and composition has been achieved using isotope-edited surface enhanced resonance Raman spectroscopy. Synthesis of isotopomeric Rhodamine dye-linked bioconjugation reagents enabled direct labeling of surface lysines on a variety of proteins. When separated in polyacrylamide gels and stained with silver nanoparticles. The spectral signatures reflect the expected statistical distribution of isotopomeric labels on the labeled proteins in the gel matrix format without interference from protein features.     

Differences in self‐assembly features of thermoresponsive anionic triblock copolymers synthesized via one‐pot or two‐pot by atom transfer radical polymerization

The self‐assembly process in aqueous solutions of the methoxyl‐poly(ethylene glycol)‐block‐poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic sodium)‐block‐poly(N‐isopropyl acrylamide) (PNIPAAM) triblock copolymer, synthesized via two different atomic transfer radical polymerization methods, namely “one‐pot” (P3‐sample) and “two‐pot” (P2‐sample), was studied by various experimental techniques. The “one‐pot” procedure leads to a copolymer (P3) where the PNIPAAM block is contaminated with a minor quantity of 2‐acrylamido‐2‐methyl‐1‐propane sulfonate (AMPS) residuals and this sample does not form micelles over the considered temperature region, but unimers and temperature‐induced aggregates coexist in the presence of a small amount of salt. The P2 polymer forms micelles and intermicellar structures, but the former moieties disappear at high temperatures, whereas the latter species contract with increasing temperature. Small‐angle neutron scattering results revealed correlation peaks, both for P3 and P2, and no micelle formation for P3, but a pronounced upturn of the scattered intensity at low wavevector values at elevated temperatures for the P2 copolymer. The findings from this study clearly show that the spurious AMPS residuals have a drastic influence on the self‐assembly and micelle formation of the triblock copolymer. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 524–534     

Effects of addition of anionic and cationic surfactants to poly(N-isopropylacrylamide) microgels with and without acrylic acid groups

The interaction of the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic surfactant hexadecyl trimethyl ammonium bromide with poly(N-isopropylacrylamide) (PNIPAAM) microgels with and without poly(acrylic acid) (PAA) was investigated by means of dynamic light scattering (DLS), zeta potential, and turbidimetry measurements. The DLS results show that the PNIPAAM microgels with PAA will contract when an anionic or cationic surfactant is added to the suspension, while the PNIPAAM microgels without PAA expand in the presence of an ionic surfactant. A collapse of the PNIPAAM microgels is observed when the temperature is increased. From the zeta potential measurements, it is observed that the charge density of PNIPAAM microgels in the presence of an ionic surfactant is significantly affected by temperature and the attachment of the negatively charged PAA groups. The turbidity measurements clearly indicate that the interaction between PNIPAAM and SDS is more pronounced than that of the cationic surfactant.     

Asymmetric organocatalytic conjugate addition of malonates to enones using a proline tetrazole catalyst

5-Pyrrolidin-2-yltetrazole performs as a useful organocatalyst for the asymmetric addition of malonates to a range of enones, with good to excellent enantioselectivities.