Dumitru Popescu

Hydrodynamic effects on the solute transport across endothelial pores and hepatocyte membranes.

In this short note we propose a simple and rapid procedure to calculate the net quantity of metabolites absorbed by hepatocytes from blood plasma. The blood movement through sinusoids determines an opposed circulation of plasma through the space of Disse. Hydrodynamic considerations lead to the conclusion that hepatocytes absorb for their own synthesis processes a quantity of metabolites in a volume flow of the order of 10(-12) nl s(-1) through a sieve plate surface with an area of 1 microm2. At pathological temperature (40 degrees C), the excess of the net absorbed volume flow for the entire sinusoidal surface of the mammalian liver may be as high as 1.9 nl s(-1). Some observations on the effect of red and white blood cells on the chylomicron traffic through endothelial pores are made.

Selective association of phospholipids as a clue for the passive flip-flop diffusion through bilayer lipid membranes☆

We showed that the investigation of the selective association of phospholipids might contribute to the insight of the flip-flop diffusion processes. The process of selective association was studied quantitatively by testing the association probabilities for both parallel and anti-parallel orientations of the polar headgroups. The model of double chain binary mixture confirms a high capacity of phospholipids for self-association in parallel configuration of the electric dipole moments whether the cross-sectional area of the polar headgroups are in an usual range of 25-55 A2. It is demonstrated that the aggregation of a class of phospholipids from a binary mixture is strongly dependent on the dipole-dipole interaction between the same phospholipids and is modulated by the magnitude of the electric dipole moment of the other phospholipids from that binary mixture. There are a great number of mechanisms involved in the transbilayer movement of phospholipids. We referred here only to the passive transport of lipids from one monolayer to the other. The flip-flop mechanisms raised in this paper are the breakdown of bilayer due to the increase of the packing density and the inversion of the coupled phospholipids from the opposite monolayers of the same bilayer. Thus, the pair formation promoting a drop in occupied volume decreases the packing pressure in the respective monolayer and consequently triggers a flip-flop into the other direction since the packing pressure in the other monolayer has not dropped. According to the present model for the binary mixtures of double-chain lipids, the rate of the flip-flop diffusion decreased by increasing the number of the methylene groups added to the acyl chain. This dependence may be perturbed whether the phospholipids possesses a very high cross-section area of the polar headgroups (a > 55 A2). We think that the selective association of phospholipids is neither exclusively, nor only involved in promoting the transbilayer diffusion of phospholipids. Most probably, the selective association determines some phospholipid domains that attract certain particular proteins so that it can modulate the protein activity.

Differential length effects in a binary mixture of single chain amphiphiles in planar monolayers. A three-dimensional approach

The purpose of this work was to investigate the selective association into a binary lipid mixture having the acyl chain lengths of the two components as variables. A more adequate parameter that can describe the selectivity of the association processes into a binary lipid mixture is the mean association probability. In the present paper, we used a three-dimensional approach by evaluating the mean probabilities of association of single chain amphiphile molecules for a group of 36 binary mixtures. The possible correlations between the hydrophobic chain lengths and the association processes have been carried out by using the differential mean association probabilities. These results allowed us to uncover some significant rules that govern the selective association between the two components from a single chain binary mixture. We have found that in a binary mixture of single chain amphiphiles, the distance where the dispersion forces become effective during association processes is about 8.89 A.

Stability and instability properties of aggregation of single chain amphiphiles into binary mixtures

In a lysophospholipid binary mixture, there are three ways of association between the mixture components of single-chain amphiphiles: (a) between two identical molecules each of the first and second component (self-association process) and (b) between two different molecules (cross-association process). Association probabilities for three binary mixtures were analysed as functions depending on the electric dipole moments of the polar head groups. A 3-D view representation is most suitable for this analysis. The most important finding is that for certain values of the electric dipole moments there are molecular couples which have a maximum stability to the changes in the external electrolytic medium. This fact confirms the formation of clusters and their stability, which is equivalent to the existence of micro-heterogeneities within the lipid bilayers. On the other hand, there are unstable molecular associations, and this fact influences the appearance of some phase transitions. Generally, the increase of the electric dipole moment or the increase of the acyl-chain length of one component from a binary lipid mixture decreases the self-association probability between its own molecules, but it increases the self-association probability of the other mixture components. Furthermore, the cross-association probability has high values for any binary lipid mixture of single-chain amphiphiles.

THE HYDROPHOBIC ACYL-CHAIN EFFECT IN THE LIPID DOMAINS APPEARANCE THROUGH PHOSPHOLIPID BILAYERS

Abstract An intermolecular interaction model for selective association processes of double-chain phospholipids in bilayer lipid membranes has been proposed, analysed and solved numerically. A large variety of binary mixtures of asymmetrical double-chain phospholipids with the cross-sectional areas of the polar headgroups a 1 =40xa0A 2 (the first component) and a 2 =60xa0A 2 (the second component) have been investigated. Changing the hydrophobic acyl-chain lengths of both mixture components, we found in all cases that the self-association probability (the association of like-pairs of phospholipids) of the first component in parallel alignment of the electric dipole moments of the polar headgroups is higher than the cross-association probability (the association of cross-pairs of phospholipids) and the self-association probability of the second component. This result is in good agreement with the experimental evidence that where the cross-sectional area of the polar headgroups matches the hydrocarbon chain-packing cross-sectional area ( a ≅2Σ≅40xa0A 2 ), lipids possess a high tendency to aggregate into well packed bilayer structures with the acyl-chains oriented perpendicularly to the bilayer plane. Our theoretical data confirm that the double-chain phospholipids may associate themselves into anti-parallel alignment of the polar headgroups ( P ′ 22 ) as well. The hydrophobic acyl-chain effect of phospholipids may modulate the distribution of lipid domains within bilayers that have a large variety of functional roles in cellular metabolism.

The working of a pulsatory liposome

Under positive osmotic stress, a greater lipid vesicle swells to a critical diameter, when suddenly a transbilayer pore appears and grows to a maximum radius, then decreases and finally disappears. An amount of liquid was leaked out through the pore and the vesicle returns to the initial state and can start another cycle. This is a pulsatory lipid liposome. In this paper, we have considered the problem of such liposomes. We have obtained the condition that a pulsatory liposome to run an a priori settled number of cycles. The length time of each cycle and its activity life was calculated. Also, we have calculated the quantities of solute leaked out through a pore in each cycle. The pulsatory liposome may be regarded as a biotechnological device to dose drugs at fixed intervals time.

GLOBAL RATIO OF EFFICIENCY IN A SINGLE CHAIN BINARY MIXTURE

This paper is devoted to a newly proposed subject regarding the association processes from a single chain binary mixture. A test system was developed for 36 binary mixtures to evaluate the mean association probabilities. A methylene group added to the short hydrophobic chain is four times more effective on the mean self-association probability h11 than a methylene group added to the long hydrophobic chain. As far as this comparing effect is concerned, this is twice more effective for the mean self-association probability h22.

A theoretical model for the association probabilities of saturated phospholipids from two-component bilayer lipid membranes

The non-random mixing of biomembrane components, especially saturated phospholipids, exhibits important consequences in molecular biology. Particularly, the distribution of lipids within natural and model membranes is strongly determined by the selective association processes. These processes of phospholipids take place due to the cooperative modes in multiparticle systems as well as the specific lipid-lipid interactions both in the hydrophobic core and in the region of the polar headgroups. We demonstrated that the investigation of the selective association processes of saturated phospholipids might contribute to the insight of the lipid domains appearance inside the bilayer membranes. The association probabilities of like-pairs and cross-pairs from a binary mixture of saturated phospholipids were tested for both parallel and anti-parallel alignments of the polar headgroups. The present model confirms the experimental evidence for saturated phospholipids to have a high tendency for association in parallel configuration of the electric dipole moments of the polar headgroups whether the cross-sectional area of the polar headgroup is in an usual range of 25-55 Å2. There are three major lipid domains in a binary mixture of saturated phospholipids: (i) lipid domains in non-mixed phase of the first mixture component, in parallel alignment of the polar headgroups; (ii) lipid domains in non-mixed phase of the second mixture component, in anti-parallel alignment of the polar headgroups; (iii) lipid domains in mixed phase. We think that the selective association processes of phospholipids are neither exclusively, nor only involved in promoting the lipid domains appearance through bilayer phospholipid membranes.

Control and optimization for a petrochemical reactor

The main objective of this work is the design and implementation a numerical control solution for the ethylene reactor, the key-installation in the petrochemical industry. We introduce some technological characteristics of this petrochemical process and the main aspects about the control system solution for the process plant. A robust control strategy for numerical control systems are proposed. Finally, we found an optimal operating point, considering that the process has a nonlinear multivariable structure. The results have been implemented on an assembly of pyrolysis reactors on the petrochemical platform from Pitesti, Romania.

Artificial Neural Network Based Model of Photovoltaic Cell

This work concerns the modeling of a photovoltaic system and the prediction of the sensitivity of electrical parameters (current, power) of the six types of photovoltaic cells based on voltage applied between terminals using one of the best known artificial intelligence technique which is the Artificial Neural Networks. The results of the modeling and prediction have been well shown as a function of number of iterations and using different learning algorithms to obtain the best results.