No abstract

We have measured the self-diffusion coefficients of a series of solute probes, including ethylene glycol and its oligomers and polymers in aqueous solutions and gels of poly(vinyl alcohol) (PVA) using the pulsed-gradient spin−echo NMR techniques. In an effort to link the diffusion properties of small and large molecules in polymer systems, we have selected this group of diffusant probes with various molecular weights, ranging from 62 to 4000. The self-diffusion coefficients of the solute probes decrease with increasing PVA concentrations (from 0 to 0.38 g/mL) and with increasing molecular size of the probes. The temperature dependence of the self-diffusion coefficients has also been studied for ethylene glycol and poly(ethylene glycol)s of molecular weights 600 and 2000. Energy barriers of 30.0, 36.5, and 39.0 kJ/mol have been calculated respectively for the probes, in the temperature range 23−53 °C. The experimental data are used to fit a new physical model of diffusion (Petit et al. Macromolecules, 1996, 29, 6031), which is shown to be successful in describing the effects of polymer concentration, temperature, and molecular size of the diffusants on the self-diffusion coefficients of small and large molecular probes in the polymer system.

To test the effect of the matrix polymer on diffusion, we have measured the self-diffusion coefficients of water and poly(ethylene glycol) of a molecular weight of 600 (PEG-600) in aqueous systems of selected polymers using the pulsed-gradient spin-echo NMR technique. The polymers used in this study include poly(vinyl alcohol) (PVA), hydroxypropyl methyl cellulose (HPMC), poly(N,N-diethylacrylamide) (PNNDEA), and poly(N-isopropylacrylamide) (PNIPA). The polymer concentrations varied from 0 to 0.38 g/mL. The effect of the polymer network on the self-diffusion coefficients of the solvent (water) and a solute (PEG-600) was investigated by analyzing the diffusion data with the use of the free volume model of Yasuda et al. [Yasuda, H.; Lamaze, C. E.; Ikenberry, L. D. Makromol. Chem. 1968, 118, 19], the diffusion model proposed by Phillies [Phillies, G. D. J. Macromolecules 1986, 19, 2367], and the model of Petit et al. [Petit, J.-M.; Roux, B.; Zhu, X. X.; Macdonald, P. M. Macromolecules 1996, 29, 6031]. The results obtained with PVA, HPMC, PNNDEA, and PNIPA are used to evaluate the applicability of these models in polymer−water−solute ternary systems. The physical significance of the parameters used in the models is discussed.

The conformations of three angiotensin II (AII) peptide antagonists ([Sar1]-AII(1-7)-NH(2), [Sar1,Val5,Ala8]-AII and the AII antipeptide, [Glu1,Gly2,Val5,Val8]-AII) were assessed in a lipid medium. A common backbone turn was identified through modeling and spectroscopic studies. The His6 residue acted as a pivoting point beyond which each peptide adopted two distinct conformations. One principle conformer resembled that previously determined for AII while the other was designated as an AII antagonist like conformer. A computational overlay between the nonpeptide antagonist, Losartan, and both the AII and the AII like conformation of [Sar1,Val5,Ala8]-AII revealed common pharmacophoric points with RMS deviations between 1 and 1.5 A. Both the AII conformer and the AII antagonist like conformer of [Sar1,Val5,Ala8]-AII were docked into a model of the AT(1) receptor. Receptor residue Phe289 and Asp281 provided good contact points for both peptides. Some differences were also noted. The terminal carboxyl of AII contacted Lys199 of the receptor while that of [Sar1,Val5,Ala8]-AII bridged Arg23 at the top of helix 1. The Asp1 side chain of AII interacted with His183 of the receptor.