PubMed · 7754360
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C Colaco, J Kampinga, B Roser. 1995-05-12. Amorphous stability and trehalose.. https://doi.org/10.1126/science.7754360
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The Zn(2+)-glutathione system is studied as a model for metal-peptide systems where some critical factors must be considered when using voltammetric techniques for the determination of stability constants. These factors are the presence of side reactions (in this case, both the protonation of glutathione and the hydrolysis of Zn(2+)), the association-dissociation rates of the complexes compared with the time scales of the measurements (which makes the complexes electrochemically labile or inert), and the electron transfer kinetics on the electrode surface (which makes the metal ion reduction reversible or irreversible). For the study of these factors, three data treatment approaches have been applied: (i) the electrochemical hard-modeling approach (modelization of both chemical equilibrium and electrochemical processes), (ii) a chemical hard-modeling approach (modelization of chemical equilibria only, based on the least-squares curve-fitting program SQUAD), and (iii) a previously developed model-free soft-modeling approach based on multivariate curve resolution with a constrained alternating least-squares optimization. By analyzing differential pulse polarographic data obtained under different experimental conditions, the influence of the mentioned factors on every approach is discussed and, if possible, the corresponding stability constants are computed. The results of this study showed the potential usefulness of voltammetry in combination with hard- and soft-modeling data analysis for the study of peptide complexation equilibria of metal ions such as Zn which have neither relevant spectroscopic properties nor proper isotopes for NMR measurements.
A study of microemulsions with retinol and its esters. Physical properties of w/o and o/w microemulsions containing Tween 60, Tween 80, Epicurone 135 (soy bean lecithin) as surfactants, n-butanol, triacetin, propylene glycol as cosurfactants were examined. The drug-containing systems were characterised in regard to their ophthalmic parameters. Physiologically well-tolerated and physically stable multiple-components were developed. The concentrations of surfactants and cosurfactants which are necessary to form stable systems were evaluated. The values of the following parameters--refractive index, viscosity, pH value, osmotic tension, obtained in the study, proved suitable for the purpose and the preparations were physiologically tolerated, the use of microemulsions as potential drug delivery systems for ocular administration has been discussed. The influence of retinol and its esters on the physical parameters the preparation was investigated. Microemulsion stored at 20 degrees C up to 6 months showed no significant physical changes.
The effect of electrostatic interactions on the conformation and thermal stability of plastocyanin (Pc) was studied by infrared spectroscopy. Association of any of the two redox states of the protein with positively charged membranes at neutral pH does not significantly change the secondary structure of Pc. However, upon membrane binding, the denaturation temperature decreases, regardless of the protein redox state. The extent of destabilization depends on the proportion of positively charged lipid headgroups in the membrane, becoming greater as the surface density of basic phospholipids increases. In contrast, at pH 4.8 the membrane binding-dependent conformational change becomes redox-sensitive. While the secondary structures and thermal stabilities of free and membrane-bound oxidized Pc are similar under acidic conditions, the conformation of the reduced form of the protein drastically rearranges upon membrane association. This rearrangement does not depend on electrostatic interactions to occur, since it is also observed in the presence of uncharged lipid bilayers. The conformational transition, only observed for reduced Pc, involves the exposure of hydrophobic regions that leads to intermolecular interactions at the membrane surface. Membrane-mediated partial unfolding of reduced Pc can be reversed by readjusting the pH to neutrality, in the absence of electrostatic interactions. This redox-dependent behavior might reflect specific structural requirements for the interaction of Pc with its redox partners.