PubMed · 14221191
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D F HAYMAN, V PETROW, O STEPHENSON. 1964. HYPOGLYCAEMIC AGENTS.. https://doi.org/10.1111/j.2042-7158.1964.tb07510.x
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A series of butterfly-type molecular constructs has been prepared in good yield by using a double Stille coupling synthetic protocol. They are composed of a terpyridine (terpy) scaffold and two wings composed of appended porphyrins that are capable of switching from an extended W geometry to a compact U geometry upon cation coordination of the terpy unit. The porphyrin moieties exist in the constructs either as free bases or they can be sequentially metallated, thus giving rise to wings of different "colours". Stationary and time-resolved emission studies of the HZn, ZnAu and Zn2Au constructs show that the electronic properties are strongly dependent on the geometry. In the extended W conformation an energy-transfer process is seen from the free base to the Zn-metallated porphyrin. In the U conformation in Zn2Au the donor luminescence resulting from the singlet excited state of the Zn wing is strongly, quenched not only due to the heavy atom effect but also due to a fast electron-transfer process to the ground state of the Au wing. Furthermore, the binding of (alpha,omega)-diamine substrates to the Zn(II)-porphyrin sites can also influence the conformation of the system. For the Zn2Zn construct, single-crystal diffraction experiments with synchrotron radiation allowed the structure to be solved by direct methods and fully refined; it shows the expected U conformation. The central Zn atom is six-coordinate, whereby the zinc atom is coordinated by the eta3-terpy ligand as well by monodentate and semi-chelating acetate anions. The structure is made rigid by hydrogen bonds involving the aqua ligands on the outer Zn centres and acetate oxygen atoms. The present system thus represents a double-trigger-modulated optomechanical switching device with selective substrate binding for either metal atoms or tailored ligands. Both energy- and electron-transfer processes can be controlled opening a means of improving the on/off ratio in future constructs.
Isothermal dielectric loss spectra of neutralized and nonneutralized chitosan were acquired in successive runs from -130 degrees C up to increasing final temperatures, in a frequency range between 20 Hz and 1 MHz. Essentially, three relaxation processes were detected in the temperature range covered: (i) a beta-wet process, detected when the sample has a higher water content that vanishes after heating to 150 degrees C; (ii) a beta process, which is located at temperatures below 0 degrees C, becoming better defined and maintaining its location after annealing at 150 degrees C independently of the protonation state of the amino side group; and (iii) a sigma process that deviates to higher temperatures with drying, being more mobile in the nonneutralized form. Moreover, in dried neutralized chitosan, a fourth process was detected in the low frequency side of the secondary beta process that diminishes after annealing. Whether this process is a distinct relaxation of the dried polymer or a deviated beta-wet process due to the loss of water residues achieved by annealing is not straightforward. Only beta and sigma processes persist after annealing at 150 degrees C. The changes in molecular mobility upon drying of these two relaxation processes were evaluated.
The kinetics of degradation and surface color changes of beta-carotene encapsulated in a polymeric matrix (PVP-40) and its relationship with physical changes (manifested as structural collapse) of the matrix were studied during storage of samples at several water activities at constant temperature. The degradation rate constants obtained decreased with an increase in the relative humidity (RH) of the storage atmosphere. Beta-carotene losses were observed mainly at RHs below the glass transition temperature (Tg) of the corresponding systems, and the lower degradation constant rates were observed under conditions where the matrices were fully plasticized (i.e., rubbery) and collapsed (RH, 64 and 75%). An inverse correlation was observed between collapse and degradation rate constants. The results presented here indicated that the molecular mobility of the matrix is not rate limiting for the degradation of beta-carotene. Factors such as microstructure and porosity of the polymeric matrix may be more important as modifiers of kinetic reactions. Surface color was not a sensitive indicator of beta-carotene retention, because it was mostly affected by the degree of matrix hydration and collapse phenomena.