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J R Grigera

Publications and source records attributed to J R Grigera.

4 recordsLinked to original sources
PubMed1992

Water and electrolyte solutions. Concentration and activity.

In this work we briefly describe the concept of activity of a solution. We treat the case of electrolyte solutions and the existing experimental difficulties to determine single ion activities. Then, we review the theoretical methods available to compute ionic activities. The mean spherical approximation (MSA) provides a simple and reliable method for computation of single ion activities. We present the case of an arbitrary mixture of ions. Particular attention is given to biological-like solutions, including magnesium and calcium ions.

Electrolytes↗
PubMed1991

Humidity-dependent structural changes in native collagen studied by X-ray diffractometry.

1. X-ray diffractometry was used in this work to study structural modifications of powdered native collagen submitted to repeated cycles of gradual drying and hydration. 2. Hysteresis effects known to exist in water sorption isotherms of this fibrous protein were detected in the plots of relative humidity vs integrated intensity of the wide angle X-ray reflections which constitute the main features of the diffraction pattern. 3. A gradual loss of structured material was observed after each drying and rehydration process. An increase in the amorphous regions of the fibrils could also be inferred from the diffraction data. 4. Drying the samples up to a critical degree of hydration (0.12 g H2O per g protein) did not produce a hysteresis loop in the plots of the parameters studied. 5. One-step drying-rehydration cycles did not seem to affect the order of the samples since they repeatedly recovered their original structure. The difference between these results and those of the gradual hydration processes may be attributed to the kinetic properties of biopolymer hydration. The rate of water removal seems to be an important factor in the structural modifications produced by the hydration (dehydration) process.

Collagen↗
PubMed1986

Effect of Triton X-100 on the physical properties of liposomes.

The interaction of the nonionic detergent Triton X-100 with phospholipid bilayers of liposomes made of egg yolk phosphatidylcholine was studied through the behavior of several physical properties. The dielectric permittivity spectra between 30 kHz and 13 MHz, the viscosity, the density, and the d.c. conductivity (1 kHz) of aqueous liposomes suspensions at various mole ratios were measured at 22 degrees C. For detergent-to-phospholipid ratios lower than 3, a dielectric relaxation process of characteristic frequency of about 50 kHz was recorded. This process does not appear for the liposomes in water, and becomes smaller and smaller for detergent-to-phospholipid ratios higher than 3. The viscosity of these suspensions showed a biphasic behavior, being remarkably increased by the detergent for concentration ratios lower than 3. The measured d.c. conductivity of these samples showed no relation with this process, being slightly increased when the detergent content is increased. As a conclusion of these results a well defined concentration range appears where the phospholipid organization changes forming highly asymmetrical structures.

Electric Conductivity↗