José A. Fornés

pH fluctuations in unilamellar vesicles

pH fluctuations in small unilamellar vesicles (SUV) are theoretically estimated. We determine that these fluctuations are dependent on macroscopic variables such as pH, pKa, buffer group concentration and surface electrical potential. Based on a previously reported definition of buffer electrical capacitance (Procopio and Forne′s, Phys. Rev. E, 1995, 51, 829) an equation is derived which relates the pH fluctuation, the buffering power and the SUV size. We also derived an equation for the mean square charge thermal fluctuation using the fluctuation–dissipation theorem which coincides with that independently obtained by Kirkwood and Shumaker (Proc. Natl. Acad. Sci., 1952, 38, 863) using mechanical statistical methods. From our results it is inferred that measurement of pH in small systems has to be performed near the pK of the buffer groups in order that the fluctuational errors be minimized. We show that pH fluctuations diminish with increasing the size of the SUV and the predicted pH fluctuations decrease as the surface potential becomes less negative as a consequence of decreasing density of charged groups in the inner vesicular surface. It is predicted that measurable effects will appear on the fluorescence detection due to protonic fluctuations close to the pH sensing region of the probes.

Electrical Fluctuations on the Surfaces of Proteins from Hydrodynamic Data

We calculate the electrical capacitance on the surfaces of protein molecules from hydrodynamic data of the proteins. Then we estimate the electrical fluctuations (charge, voltage) through the fluctuation-dissipation theorem, which links the electrical capacitance of the system with these fluctuations. From the intrinsic viscosity of the proteins, we estimate the polarizability, which leads to knowledge of the field and dipole fluctuations. From the fitting of the capacitance, polarizability, and electrical fluctuations as a function of the molecular weight of the proteins, we report numerical equations that make it possible to estimate these physical magnitudes for a given protein, knowing the molecular weight. Charge fluctuations are in fractions of unit charge range, voltage fluctuations are in the three-mV-digit range, field fluctuations are in the two-digit mV/nm (10(6) V/m) range, and the dipole moment fluctuations range from two to three digits, times the dipole moment of the water molecule. These surface properties of proteins have not been reported before.

Electrical Fluctuations in Colloid and Ionic Solutions

A method is developed in order to determine the natural electrical thermal fluctuations and its spectral distribution across two points of a solution of ions or spherical charged particles immersed in an ionic solution. The electrical equivalent between two points of a solution is considered as a capacitor and a resistor in parallel. The method is applied within the Debye-Huckel approximation (linearized Poisson-Boltzmann equation), although it is valid in general. Among the results is the diminution of electrical fluctuations as particle sizes increase; as a consequence, large particles produce electrical stabilization in their neighborhood. It can also be observed that fluctuations are not quite sensitive to ionic concentrations for large particles. When the size of the particles become negligible we obtain similar results with the already obtained using the method of the mode expansion.

Hydrodynamic interactions induce movement against an external load in a ratchet dimer Brownian motor

We use the Brownian dynamics with hydrodynamic interactions simulation in order to describe the movement of a elastically coupled dimer Brownian motor in a ratchet potential. The only external forces considered in our system were the load, the random thermal noise and an unbiased thermal fluctuation. For a given set of parameters we observe direct movement against the load force if hydrodynamic interactions were considered.

Thermal length fluctuations of a linear chain of masses connected by springs in a viscous medium

By applying the Fluctuation Dissipation Theorem the root mean-square amplitude (RMSA) of the longitudinal vibrations of a linear chain of masses connected by springs in a viscous medium is estimated as a function of the mass of the particles, the force constant of the springs, the number of particles, the damping parameter and temperature. At room temperature the RMS end-to-end length is found to vary from a few tenths of an Angstrom for hydrocarbon chains of the length found in phospholipid membrane bilayers to several Angstroms in α-helices of peptide residues.

H-Bond vibrations of the α-helix

The collective low frequency longitudinal vibrational modes of the α-helix are isolated using Deans method, (P. Dean, Proc. Phys. Soc., London, 1959, 73, 413; P. Dean, Proc. R. Soc. London, Ser. A, 1960, 254, 507), which considers the different masses of the amino acids involved. The spectrum range extends up to f = 100 cm−1 for the α-portion of several proteins within the range of experimental results (S. Cusack, J. Smith, J. Finney, B. Tidor and M. Karplus, J. Mol. Biol., 1988, 202, 903).

Incorporation of arachidonic and palmitic acids in large unilamellar vesicles. A comparison of electrical surface parameters

We determine the influence caused by the incorporation, of arachidonic and palmitic acids on the electrical surface parameters (potential, charge density, pK) of large, electrically charged and uncharged, unilamellar vesicles, utilizing fluorescence spectroscopy. The molecular relation of incorporated acid varies in the range of 8–24 lipid pairs per acid molecule. The surface potential and charge density vary in the range 12–19 mV and 1200–7000 esu cm−2 (1 esu cm−2 = 3.335 × 10−9 C m−2). The ΔpK is one-tenth of the pK unit. We found in vesicles without charge (DMPC) that the saturation ratio for the incorporation of fatty acids is arachidonic: palmitic = 0.6; this relation is inverted in vesicles with charge (soy lecithin), arachidonic: palmitic = 8, showing an electrical influence on the arachidonic acid incorporation.

EVIDENCE FOR MULTIPOLAR CHARGE DISTRIBUTION IN FALLING WATER DROPS

A method for producing water drops with very small net charge (few fC) was employed in conjunction with a sensing device, in order to identify the electric signals produced by falling water drops. The sensing device consists of a wire loop hooked to a patch‐clamp amplifier set into voltage‐clamp mode. Water drops with very small net charge produce a negligible signal at the sensing device but when falling through a horizontal charged capacitor they produce well‐defined and reproducible signals which are proportional to the capacitor voltage. The experimental curves can be fitted to the theoretical ones considering the drop as possessing a multipolar charge distribution. There is evidence that this distribution is due to the contribution of the field‐oriented water molecules multipoles.

Local transient fluctuational density as producing ionic flow through cell membranes

Abstract The possibility of a coupling between a flow of information and a net flow of a given ionic species through cell membranes is considered. Conditions meeting the requirements imposed are discussed.