PubMed · 14268845
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D H NORTHCOTE. 1964. POLYSACCHARIDES.. https://doi.org/10.1146/annurev.bi.33.070164.000411
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Lipidic polylysine dendrimers, synthesized using Fmoc solid phase peptide techniques, have been formulated as nanoparticles by precipitation from solution in dichloromethane. The effect of concentration on the diameter and stability of nanoparticles formed from two short homologous series of dendrimers--one fifth generation and one sixth generation series and with surface C4, C10 or C12 groups--was investigated using photon correlation spectroscopy. The increase in generation from fifth to sixth resulted in increased diameter for each chain length. An increase in the surface lipidic chain length from C4 to C12 had no effect on the particle diameter of aggregates derived from fifth generation dendrimers, and a small and variable effect on the sixth generation derived nanoparticles. Using pyrene (excitation 340 nm) as a hydrophobic fluorescent probe, a decrease in intensity peak I1 (374 nm)/I3 (385 nm) in the emission spectra (340-600 nm) was observed in the two dendrimers studied, fifth generation dendrimers with C10 or C12 surface lipidic chains, as the dendrimer concentration increased, reaching a plateau at higher concentrations, indicating that a more compact form of the aggregates with a more hydrophobic interior was obtained. Apart from the hydrophobicity of the dendrimers and dendrimer concentration, the flexibility of the dendrimers might have a significant effect in determining nanoparticle size. The aggregates derived from the fifth generation dendrimers with C10 or C12 surface lipidic chains are stable in purified intestinal fluid but not in purified stomach fluid, in which further aggregation of the nanoparticulate dendrimer aggregates occurs as an effect of pH, salts, proteins and enzymes in these fluids. This study demonstrates, inter alia, the importance of testing nanoparticulate delivery systems in relevant physiologically based fluids prior to their use in vivo.
The phase behavior and some physicochemical properties of homopolymers (HP) and hydrophobically modified (HMP) polymers, as well as of polyelectrolytes (PE) and proteins (PR), in the presence of aqueous surfactants, or their mixtures, are discussed. Mixing the above components gives rise to the formation of organized phases, whose properties are controlled by polymer and/or surfactant content, temperature, pH, and ionic strength. Depending on the nature, concentration, and net charge of both solutes, molecular solutions, polymer-surfactant complexes, adsorption onto micelles and vesicles, gels, liquid crystalline phases, and precipitates are observed. Such rich polymorphic behavior is the result of a complex balance between electrostatic, excluded volume, van der Waals, and other contributions to overall system stability. It is also modulated by the molecular details and architecture of both the polymer and the surfactant. Different experimental methods allow investigation of the above systems and getting information on the nature of polymer-surfactant interactions (PSI). Surface adsorption and thermodynamic methods, together with investigation of the phase diagrams, give information on the forces controlling PSI and on the existence of different phases. Conductivity, QELS and viscosity allow estimating the size and shape of polymer-surfactant (protein-surfactant) complexes. Optical microscopy, cryo-TEM, AFM, NMR, fluorescence, and relaxation methods give more information on the above systems. Use of the above mixtures in controlling gelation, surface covering, preparing dielectric layers, and drug release is suggested.
This study is the second of two publications that investigate the interactions between volatile and nonvolatile components in coffee brew. The purpose here was to shed some light into the chemical mechanisms responsible for the decrease of volatile thiols when in contact with coffee nonvolatiles. A mixture of volatile thiols covering a large range of physicochemical properties was monitored over time in the presence of a coffee brew model. The binding potential was estimated by SPME-GC-MS. Additives inhibiting specific reaction pathways were preincubated with the coffee brew 1 h prior to addition of the volatile compounds. Degradation kinetics of the volatile thiols were characterized by their rate constants k(obs). The effect of individual additives was shown by calculating k(rel), the relative rate constant as compared to the reference without additive. The conclusion was that thiols, mainly responsible for the "roasty" and "burnt" notes, disappear via two main chemical mechanisms. The results suggest that nucleophilic addition is the major pathway for thiol degradation. Addition occurs on oxidized species generated in the matrix in the presence of air. This mechanism prevails for aliphatic thiols (e.g., ethanethiol, methanethiol). Benzylic thiols (such as 2-furfurylthiol) can react in parallel via another pathway that is slowed in the absence of oxygen and in the presence of a radical scavenger. This points to a radical mechanism, but further work is needed to support this hypothesis. A direct correlation between thiol hydrophobicity and the magnitude of the interactions was shown as well. Therefore, weak physical interactions or hydrophobic assistance accelerating chemical reactions cannot be excluded at this point of the study.