Solid‐state 13C and 129Xe NMR study of poly(vinyl alcohol) and poly(vinyl alcohol)/lactosilated chitosan gels

Dry and hydrated poly(vinyl alcohol) (PVA) gels with 55% (a‐PVA) and 61% (s‐PVA) syndiotacticity and related PVA/lactyl chitosan (LC) blends have been investigated with ¹²⁹Xe and cross‐polarization/magic‐angle‐spinning ¹³C NMR techniques. Although the dry gels exhibit two broad ¹²⁹Xe resonances in the slow‐to‐intermediate exchange limit, both hydrated gels show three resonances. The corresponding dry blends exhibit two signals, the chemical shifts and line widths of which change with respect to those of pure PVA, whereas one (a‐PVA/LC) or two (s‐PVA/LC) signals appear in the spectra of the hydrated blends. A comparative analysis of the data demonstrates that LC rearranges the domains of the polymeric matrix in both the dry and hydrated blends according to the syndiotacticity of the PVA chains. Information on the molecular motions of the amorphous and swollen polymeric domains in the kilohertz range has been obtained from an analysis of the spin‐lattice relaxation times. These data indicate that the dynamics and arrangement of the PVA chains in the gels are strongly affected by their tacticity and the addition of the copolymer LC. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3123–3131, 2003     

Molecular recognition: A thermodynamic study of non bonded interactions in aqueous solutions of iminoacids. Effects of steric and geometric isomerism

Enthalpies of dilution of binary and ternary aqueous solutions containing L-and D-proline, cis-L- and cis-D-4-hydroxy proline, trans-L-4-hydroxy proline, have been determined by flow calorimetry at 25°C. Pairwise self-and cross-interaction coefficients of the virial expansion of the excess enthalpies were evaluated. They are positive for solutions of unsubstituted prolines. On the contrary, upon the introduction of a hydroxy group, they become negative, reaching the largest value for the cis-L-lcis-D-4-hydroxy proline system. The results are interpreted in terms of preferential configurations, stabilized by interactions between the hydrophilic groups.     

Photooxidation of 1,3-butadiene containing systems: Rate constant determination for the reaction of acrolein with ⋅OH radicals

The photooxidation of the 1,3-butadiene–NO–air system at 298 ± 2 K was investigated in an environmental chamber under simulated atmospheric conditions. The irradiation gave rise to the formation of acrolein in a 55% yield, based on 1,3-butadiene initial concentration for all the experimental runs. The rate of formation of acrolein was the same as that of 1,3-butadiene consumption, indicating that acrolein is the major product of the 1,3-butadiene oxidation in air. The dependence of acrolein concentration on irradiation time showed thata secondary process, identified as an oxidation of acrolein by ^?OH radicals, was occurring during the photochemical runs. The rate constant of this secondary process was determined by measuring the relative rates of disappearance of acrolein and n-butane during the irradiation of acrolein-n-butane-NO-air mixtures. The so obtained relative rate constant value was placed on an absolute basis using a reported rate constant for the n-butane + ^?OH reaction; a value of (1.6 ± 0.2) × 10¹⁰ M^?1 sec^?1 was obtained.     

Linear kinematical groups

We prove a theorem which states that in an (n+1)-dimensional space-time (n3) the only linear kinematical groups which are compatible with the isotropy of space are the Lorentz and Galilei groups. The special casesn=1 andn=2 are also briefly discussed.On leave of absence from Instituto di Fisica dell' Universitá, Milano, Italy. A. v. Humboldt fellow.     

Extreme affine transformations

We classify the extreme points of the compact convex set of affine maps of IR ⁿ which map into itself the closed unit ball. This work is a preliminary step towards solving the problem of finding the extreme points of the compact convex set of affine maps of theN×N density matrices (dynamical maps of anN-level system) and forn=3 furnishes the solution of the problem in the simplest case of a two-level system.The bulk of this work was performed while the first author was visiting the Center for Particle Theory of the University of Texas at Austin under the partial support of the U.S.A.E.C. under contract ORO-(40-1) 3992. A travel grant under the Fulbright-Hays program is acknowledged.     

113Cd NMR and fluorescence studies of multiple binding mechanisms of Cd(II) by the Suwannee River fulvic acid

The binding of Cd (II) by the fulvic acid from a Suwannee River (FA) was investigated at various pH values and reactants ratios by 113Cd NMR and fluorescence spectroscopy. The NMR results provided evidences that the FA-Cd interactions occur through a variety of binding modes and mechanisms. Different kinds of organically bound Cd-species were detected in the 1.8-10.8 pH range depending on the FA/Cd ratios. Labile complexes (amenable to Cd-aminoacidic and Cd-hydroxy interactions or outer-sphere complexes) were observed at low pH and FA/Cd levels while stronger interactions (of carboxylate-type or inner-sphere complexation) took place as the pH and/or the FA concentration were increased. At pH ca. 6 insoluble FA-Cd adducts were primarily produced but, at relatively large FA concentration, only soluble complexes, stable in the whole pH 1.8-10.8 range, were formed. A complementary analysis, by fluorescence spectroscopy, provided clear evidences of FA-Cd association/aggregation phenomena. While no noticeable effects occurred with soluble samples, the formation of insoluble adducts led to significant enhancements of the emission fluorescence spectra. Although other explanations could not be excluded, this result was accounted for by modifications of the optical properties of the ligand itself due to sedimentation of the heavier components. Fluorescence enhancement was also observed on samples before the effective precipitation and interpreted as spectroscopic evidence of the onset of aggregation phenomena.     

Quantum chemical study of the geometries and stabilities of the two valence-tautomers of C2H2F+

Non-empirical SCF-MO molecular wavefunctions were computed for the two limiting structures of C₂H₂F⁺ with full geometry optimization using double-zeta quality atomic orbital basis sets. The bridged structure (fluorenium ion) was found to be an energy maximum (transition state) about 31 kcal/mole higher than the open structure (fluoro-vinyl cation). The latter, contrary to the unsubstituted vinyl cation, is slightly (4.5 °) bent away from fluorine at the electron deficient centre.     

Investigating the interactions of hyaluronan derivatives with biomolecules. The use of diffusional NMR techniques

[Chemical structure: see text] The interactions between a biomaterial and biomolecules present in body fluids often determine the fate of the biomaterial. This paper presents a study on hyaluronan (HA)-containing materials (in soluble or colloidal form) that focuses on their interactions with lipids and proteins and for the first time uses PFG NMR as an analytical technique for probing these events. The interactions of HA-based polymers with phospholipids (DPPC and DPPG liposomes) are shown to depend both on charge and hydrophobicity factors. Despite the difference in behavior between albumin (substantially non-adhesive) and fibrinogen (adhesive), the interactions of the polymers with proteins do not seem to be based on hydrophobic effects but on surface polar interactions.