Ordered collagen membranes: production and characterization.

A collagen membrane with microscopic order is presented. The membranes were produced with acid-soluble collagen, using two different methods to obtain orientation. The product was characterized by mean of UV and IR spectra, scanning electronic microscopy, optical microscopy and laser diffractometry. The results clearly show a high level of order in the membranes obtained by both techniques. Permeability for rifamycin, ascorbic acid and NaCl was also measured. Due to the characteristics of the membranes, they have a potential application for treatment of surface injuries.

[Electrodynamics of interactions in electrolyte solutions and their biological significance].

Attention is drawn to the fact that the interaction of charges in aqueous solutions of electrolytes, such as media having physiological characteristics, depends not only on the distance between interacting charges but also on the frequency that determines their dynamics. This fact has significant consequences for some biological processes and their kinetics. The analysis of reasons for charge shielding, including the dynamic effects, shows that, even at distances exceeding the Debye length, electric interactions in systems similar to physiological are effective provided that charges move with frequencies higher than 250 MHz. For each electrolyte solution, the threshold frequency (Maxwell frequency) can be found, which determines the transition from the conducting to the dielectric mode of interactions of charges in physiological solutions.

Electrostatic recognition between enzyme and inhibitor: interaction between papain and leupeptin.

Electrostatic forces are involved in a wide variety of molecular interactions that are of biological interest, including, among others, DNA-Protein interactions, protein folding, and the interactions between enzymes and their substrates and inhibitors. In this work, the interaction between papain and an inhibitor, leupeptin, is analyzed from the point of view of their electrostatic interaction. The computations enable one to suggest that negatively charged amino acids located in the region of the active site are responsible for creating an environment that enables efficient binding of the inhibitor. This binding occurs despite the fact that the net global charge of both molecules is positive; an explanation for this apparent contradiction is proposed.

Molecular dynamics simulation of double proton transfer: adenine-thymine base pair.

Molecular dynamics of proton transfer was simulated from normal (N) to tautomeric (T) form of the base pair (bp) adenine-thymine (A-T). The results for the model system, consisting of one A-T base pair in SPC/E water, was analysed in terms of its hydration properties, difference on free energy, conformational changes, and magnetic properties. It is suggested that during proton transfer the base pair remains on the same conformational state. The free energy of the protons transfer appears to be almost insensitive to the presence of water. The overall hydrophobicity of the A-T pairs is slightly modified by the proton transfer the N form being a little more hydrophobic. The behavior of the magnetic shielding suggests that a current is produced in the ring of the hydrogen bonds due to strong pi -electron delocalization in the amide-amidine complex.

Electrostatics of the phospholipase-membrane interaction.

The electrostatic interaction of the Phospholipase A2 (PLA2)-membrane complex in the presence and absence of calcium is analysed by the computation of the electrostatic profiles of the components and the complex. The electrostatic potential was computed by using of the program MOLPOT that implement the boundary element method to solve the electrostatic problem. It considers a closed surface in three dimensions that contains the macromolecule that follows as close as possible the macromolecule shape. The results show that the presence of calcium ions contributes to the stability of the complex and at the same time creates a favourable electrostatic potential pattern that may be favourable for the lipolysis of the membrane components.

Hydration properties of xylitol: computer simulation.

We present a molecular dynamics simulation of xylitol in SPC/E water using classical Gibbs ensemble molecular dynamics simulation. The simulation is done both with and without periodic charge update, and no qualitative difference in the results obtained by both methods is found. The analysis of the radial and angular distribution functions, the water-water hydrogen bond distributions, and water residence times allow the conclusion that there is a relatively strong hydration of xylitol. This polyol adopts a single linear conformation and, from the point of view of the hydration dynamics, it should be classified as positively hydrated.

A multicopy modeling of the water distribution in macromolecular crystals.

A multicopy protocol is proposed for modeling macromolecular hydration using diffraction experimental data (X-ray or neutron) to search for a better description of delocalized water sites than that given by point water models. The model consists of one macro-molecule and several copies of each water molecule, refined simultaneously against diffraction data using molecular dynamics techniques. The protocol was applied to BPTI and an RNA tetradecamer. The sites defined by the different copies range from very ordered ones to continuous channels; they fit the density maps and agree with the diffraction amplitudes with an accuracy comparable with usual crystallographic methods. The delocalization of water in channels agrees with the high mobility observed in NMR experiments.

Osmotic permeability in a molecular dynamics simulation of water transport through a single-occupancy pore.

The aim of this work is to determine plausible values for the rate constants of kinetic models representing water transport through narrow pores. We present here the results of molecular dynamics simulations of the movement of water molecules through a single-site hydrophilic pore. The system consists of a rectangular box of water molecules, some of which are positionally restrained so as to act as a membrane. This membrane separates two compartments where water molecules move freely; one of the positions in the membrane is initially vacant (the 'single-site pore'), but can be occupied by mobile molecules. To analyze the results, we represented the pore by a two-state kinetic diagram in which the vacant and occupied states are linked by transitions corresponding to the binding and release of water molecules. The mean occupancy and vacancy times directly yield the rate constants of binding and release, which in turn yield the osmotic water permeability coefficient per pore pf. We also compute the apparent activation energies delta E* for the rate constants and for pf. The pf value was (1.56 +/- 0.04).10(-11) cm3/s (at 307 K), which is much larger than those determined for CHIP28 and for gramicidin A (of about 10(-13) and 10(-14) cm3/s, respectively). These values were compared with those arising from a model of a symmetric single-file pore through which one-vacancy-mediated water transport takes place. The model yields an expression for pf as a function of the rate constants and of the number of molecular positions (n) in the file. When n = 1, this expression becomes the one corresponding to the single-site pore studied in our current simulation. Using the rate constants of binding and release derived from our simulation, the pf values are consistent with an occupancy value of 5-6 found for gramicidin A, and with occupancies of 4-7 that can be estimated for the single-file pore of a recently proposed model for CHIP28. delta E* for pf is 3.0 kcal/mol, a value similar to that determined for CHIP28. Hence, the system simulated here appears plausible and can be used to mimic some physical properties of water transport through biological pores.

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.

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

X-ray diffractometry was in this work to study structural modifications of powdered native collagen submitted to repeated cycles of gradual drying and hydration. 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. A gradual loss of structural 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. 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. 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 o water removal seems to be an important factor in the structural modifications proceduced by the hydration (dehydration) process_

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.

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.

Some biophysicochemical properties of Bufo arenarum oocytes jelly coats.

The state of water in the oocyte jelly coat in Bufo arenarum has been studied by different methods. The water exchange and adsorption isotherm show a high interaction between water and the matrix of jelly coat. Some biological implications are discussed.

Some biophysicochemical properties of Bufo arenarum oocytes jelly coats.

The state of water in the oocyte jelly coat in Bufo arenarum has been studied by different methods. The water exchange and adsorption isotherm show a high interaction between water and the matrix of jelly coat. Some biological implications are discussed.

Some biophysicochemical properties of Bufo arenarum oocytes jelly coats

The state of water in the oocyte jelly coat in Bufo arenarum has been studied by different methods. The water exchange and adsorption isotherm show a high interaction between water and the matrix of jelly coat. Some biological implications are discussed.

The state of water in the outer barrier of the isolated frog skin.

The flux of water across the outer barrier of the frog skin is generally regarded as the rate-limiting step in the movement of water across the whole membrane. This paper presents some evidence that, at room temperature, the flux of water across the outer barrier occurs through water in a non-liquid state. The organization of water in a non-liquid state lowers the diffusion coefficient of water through water by several orders of magnitude. The study employs a method recently developed in this laboratory which permits measurement of unidirectional fluxes at the outermost part of an epithelial membrane mounted as a flat sheet. Only above 25°C is the activation energy for the flow of tritiated water (4.3 kcal mole(-1)) similar to the one observed in free water (4.6 kcal mole(-1)). At temperatures around 15°C, the energy of activation is 8.5 kcal mole(-1). At temperatures near 0°C, at which the frog lives only part of the year, the energy of activation is 16.7 kcal mole(-1).