Biomedical subjects

S Marcelja

Publications and source records attributed to S Marcelja.

6 recordsLinked to original sources

PubMed1992

Electrostatics of membrane adhesion.

We consider electrical double layer interaction under the conditions typically encountered during membrane fusion. Within the physiological concentration range of monovalent electrolytes the interaction is repulsive and the Poisson-Boltzmann calculation may be used to evaluate the force. When divalent counterions are added, strong ion-ion correlations make the Poisson-Boltzmann approximation inapplicable. We use the anisotropic hypernetted chain method to show that in the presence of small amounts of divalent counterions in adsorption equilibrium with the surfaces, the double layer interaction turns into attraction. This attractive electrostatic force may be the balancing contribution controlling membrane adhesion.

Biophysical Phenomena↗

PubMed1990

Electrostatics of phosphoinositide bilayer membranes. Theoretical and experimental results.

We made fluorescence, electron paramagnetic resonance (EPR), electrophoretic mobility, and ionizing electrode measurements to study the effect of the monovalent lipid phosphatidylinositol (PI) and the trivalent lipid phosphatidylinositol 4,5-bisphosphate (PIP2) on the electrostatic potential adjacent to bilayer membranes. When the membranes were formed from mixtures of PI and the zwitterionic lipid phosphatidylcholine (PC), the Gouy-Chapman-Stern (GCS) theory described adequately the dependence of potential on distance (0, 1, 2 nm) from the membrane, mole % negative lipid, and [KCI]. Furthermore, all EPR and fluorescence probes reported identical surface potentials with a PC/PI membrane. With PC/PIP2 membranes, however, the anionic (coion) probes reported less negative potentials than the cationic (counterion) probes; the deviations from the GCS theory were greater for the coions than the counterions. Discreteness-of-charge theories based on the Poisson-Boltzmann equation incorrectly predict that deviations from the GCS theory should be greater for counterions than for coions. We discuss a consistent statistical mechanical theory that takes into account three effects ignored in the GCS theory: the finite size of the ions in the double layer, the electrical interaction between pairs of ions (correlation effects), and the mobile discrete nature of the surface charges. This theory correctly predicts that deviations from GCS theory should be negligible for monovalent lipids, significant for trivalent lipids, and greater for coions than for counterions.

Electrochemistry↗

PubMed1979

Properties of bilayer membranes in the phase transition or phase separation region.

The increase in passive permeability of bilayer membranes near the phase transition temperature is usually explained as caused by either the increase in the amount of 'boundary lipid' present in the membrane, or by the increase in lateral compressibility of the membrane. Since both the amount of 'boundary lipid' and the lateral compressibility show a similar anomaly near the transition temperature, it is difficult to distinguish experimentally between the two proposed mechanisms. We have examined some details of both of the proposed pictures. The fluid-solid boundary energy, neglected in previous work, has been computed as a function of the domain size. For a single component uncharged lipid bilayer, the results rule out the existence of even loosely defined solid domains in a fluid phase, or vice versa. Thermodynamic fluctuations, which are responsible for anomalous behaviour near the phase transition temperature, are not intense enough to approximate the formation of a domain of the opposite phase. Turning next to lateral compressibility of bilayer membranes we have considered two-component mixtures in the phase separation region. We present the first calculation of lateral compressibility for such systems. The behaviour shows interesting anomalies, which should correlate with existing and future data on transport across membranes.

Lipid Bilayers↗

PubMed1979

Initial processing of visual information within the retina and the LGN.

The initial stage of information processing by the visual system reduces the information contained in the continuous image on the retina into a discrete set of responses which are carried from the lateral geniculate nucleus (LGN) to the visual cortex. -- 1. The optimal sampling of the light intensity distribution in the visual environment is achieved only if each channel in the visual pathways carries undistorted information corresponding to an image element. The visual system approaches as closely as possible the scheme of optimal spatial sampling, retaining the full information on the low spatial frequency content of the object light intensity. The ideal receptive field of a sustained LGN cell is then of the form J1(Kr)/Kr. -- 2. The experimentally determined receptive fields of sustained LGN cells (and to some extent retinal ganglion cells as well) in cat closely resemble the functional form J1(Kr)/Kr. The centre-surround organization of the receptive fields is therefore understood as a scheme which leads to a maximal information flow through the visual pathways. -- 3. The optimal sampling scheme cannot be realized by the retina alone, because of restrictions on the size of neural networks. It is therefore constructed in two stages, ending at the LGN level. A recombination of ganglion cell signals into optimal receptive fields is a major role of the LGN.

Animals↗

PubMed1977

Phase transition in charged lipid membranes.

Experimental results on the effect of electrostatics on bilayer phase transitions are compared with corresponding data for monolayers and the predictions of electrical double layer theory. The two substantial conclusions which emerge are that: (i) double layer theory based on a continuous surface charge distribution cannot explain all the relevant data, a situation which may be improved by taking into account the discrete nature of the surface charge distribution; (ii) the crystal - liquid crystal phase transition of charged bilayer membranes is always a continuous one which takes place through an intermediate state consisting of both fluid and frozen domains.

Electricity↗

PubMed1976

Lipid-mediated protein interaction in membranes.

This study describes the effects ensuing from a non-specific interaction between membrane integral proteins and the surrounding lipids. The results are obtained using an appropriate molecular field theory to describe the ordering of membrane lipids. The modification of the lipid structure near a protein molecule, while most pronounced within the annulus of the first neighbour molecules, extends two or three layers beyond the annulus. The ordering of lipids within the annulus has a modified temperature dependence, and becomes a continuous function of temperature for low lipid/protein ratios. The change in order of lipid molecules surrounding a protein leads to an indirect, lipid-mediated interaction between membrane integral proteins. This interaction depends sensitively on the bulk lipid order. Under favourable circumstances, it gives rise to protein aggregation.

Binding Sites↗