Showing papers by "Bulgarian Academy of Sciences published in 1989"

Bending elasticity and thermal fluctuations of lipid membranes. Theoretical and experimental requirements

Abstract: Thermal fluctuations of giant lipid vesicles have been investigated both theoretically and experimentally. At the theoretical level, the model developed here takes explicitly into account the conservation of vesicle volume and membrane area. Under these conditions, the amplitude of thermal fluctuations depends critically not only on the bending elasticity of the bilayer, but also on the membrane tension and/or hydrostatic pressure difference between the interior and exterior of the vesicle. At the experimental level, the determination of the bending modulus kc first requires the analysis of a large number (several hundred) of vesicle contours to obtain a significant statistics. Secondly, the contribution of the experimental error on the contour coordinates, which results in a white noise on the Fourier amplitudes, must be eliminated, and this can be done by using the angular autocorrelation function of the fluctuations. Finally, the amplitudes of harmonics having short correlation times must be corrected from the effect of the integration time (40 ms) of the video camera, which otherwise leads to an overestimation of kc. All these theoretical and experimental requirements have been considered in the analysis of the thermal fluctuations of 42 giant vesicles composed of egg phosphatidylcholine. The behaviour of this population of vesicles can be accounted for with a bending modulus kc equal to 0.4 - 0.5 x 10-19 J, and extremely low membrane tensions, ranging below 15 × 10-5 mN/m.

Paleobotanical evidences of the Tertiary history and origin of the Mediterranean sclerophyll dendroflora

Abstract: The evolution of important woody plant groups, ancestral to the modern Mediterranean dendroflora, is surveyed. Altogether, the history and phytogeography of 86 fossil species or species-groups is considered. The major part of the Paleomediterranean woody plants appears in Miocene mixed mesophyllous and mesoxerophyllous, evergreen and deciduous forests. The initial formation of basic Mediterranean sclerophyllous woody vegetation types is referred to periods from the Late Sarmatian to the Late Pontian, in geographic areas between 37° and 45° N latitude.

Tocopherol stabilizes membrane against phospholipase A, free fatty acids, and lysophospholipids.

Abstract: Tocopherols (vitamin E) are indispensable components of the lipid bilayer of biological membranes; a decrease in their content brings about structural and functional damage of the It is generally known that vitamin E is a lipid-soluble antioxidant in cell membranes, functioning as a free radical scavenger to prevent lipid per~xidation.~.~ In addition to its antioxidant function, tocopherols are capable of quenching singlet molecular oxygen, thus protecting membranes against light-induced oxidative Besides this antioxidative role, vitamin E may become incorporated into biological membranes through a physicochemical association of the tocopherol side chain with polyenoic fatty acid residues in membrane phospholipids, that is, by way of van der Waals interaction^.'^' These molecular mechanisms underlying the biological effects of tocopherols have been studied in great detail and are generally accepted (FIG. 1 ). Recently it has been demonstrated that in addition to its antioxidant and physicochemical stabilizing effects in biological membranes, vitamin E can protect biological membranes against the damaging action of phospholipases, especially phospholipase A, as well as against the phospholipid hydrolysis products by phospholipase A, namely, free fatty acids and lysophospholipids?2‘o The aim of the present paper is to analyze the molecular mechanisms underlying protective effects of vitamin E in lipid bilayers as well as in natural membranes in which modification of physical properties or damage was due to the presence of free fatty acids, lysophospholipids, or both. It is noteworthy that sharp elevation of concentrations of the phospholipid hydrolysis products is a characteristic feature of many pathological processes, such as ischemia, stress damages, and hypoxia.”.”

Error estimates for discretized differential inclusions

Abstract: We present an estimate for the Hausdorff distance between the set of solutions of a differential inclusion and the set of solutions of its Euler discrete approximation, using an averaged modulus of continuity for multifunctions. A computational procedure to obtain a certain solution of the discretized inclusion is proposed.

Peripheral effects on the amplitude of monopolar and bipolar H-reflex potentials from the soleus muscle

Abstract: Variations in the amplitude of mono- and bipolarly measured H-reflex potentials can be influenced by muscle architecture and changes in muscle length. In the passive soleus muscle with the ankle joint fixed at 90 degree, the maximal-amplitude bipolar H-potentials were obtained along the midline of soleus at a distance of 2.0-4.0 cm below the insertion of the gastrocnemii on the Achilles tendon. In contrast, the optimal location of monopolar H-potentials was 5.0-8.0 cm below the gastrocnemii insertion. Stepwise passive shortening of soleus resulted in an increase in the amplitude of both H- and motor-unit potentials. This correspondence implicates peripheral factors, such as changes in muscle fibre diameter and inclination to the skin surface, as mechanisms mediating the changes in the amplitude of the potentials. Such effects necessitate caution in interpretation of the association between H-potential amplitude and monosynaptic reflex excitability.

Uniqueness of the multi-dimensional inverse scattering problem for time dependent potentials

Abstract: te]R, x~N\", n > 3 odd. The potential q(t, x) depends on t and satisfies the following conditions: (i) qe C ~ (~t x ~-~:~), (ii) there exist some positive constants p, C, N such that q(t, x ) = 0 for Ixl >P and Iq(t, x)l < C(1 + Itl) N. The aim of this paper is to prove that the potential q(t, x) is uniquely determined by the scattering data. The inverse scattering problem for stationary (time independent) potentials q(x) has been attacked by many authors. The reader should consult [2], [,10], [18] and the references given there for the history and the recent progress in the analysis of this problem. Most of the works in this direction deal with the Schr6dinger equation. Nevertheless it is clear that the results obtained in these papers are applicable to the wave equation u , A u + q ( x ) u=0. It is known that for stationary potentials the uniqueness holds. The proof is based on the Born approximation of the scattering amplitude A(k, co', og) as k ~ (see [-7], [,16], [-22]). The situation considerably changes when we deal with time dependent potentials. The techniques used in the papers cited above are not available in this case. One of the reasons is that we cannot use the tools from the spectral theory. The local energy decay for (1.1) was examined by Tamura [,28], while the existence of the scattering operator was proved in [,1], [8], [19], [20] for certain classes of potentials q(t, x). In [-8] Ferreira and Perla Menzala proved that if the scattering operators related to qi(t, x), i= 1, 2 coincide and if ql >~-~q2 then ql=q2, provided that q~ are non-negative, \"small\" in a suitable sense and qt=O(Itl -~) as l t l ~ o% 0

Round Robin computer simulation of ejection probability in sputtering

Abstract: We have studied the ejection of a copper atom through a planar copper surface as a function of recoil velocity and depth of origin. Results were obtained from six molecular dynamics codes, four binary collision lattice simulation codes, and eight Monte Carlo codes. Most results were found with a Born-Mayer interaction potential between the atoms with Gibson 2 parameters and a planar surface barrier, but variations on this standard were allowed for, as well as differences in the adopted cutoff radius for the interaction potential, electronic stopping, and target temperature. Large differences were found between the predictions of the various codes, but the cause of these differences could be determined in most cases. A fairly clear picture emerges from all three types of codes for the depth range and the angular range for ejection at energies relevant to sputter ejection, although a quantitative discussion would have to include an analysis of replacement collision events which has been left out here.

Dielectric Spectroscopy: a Rapid Method for the Determination of Solvent Biocompatibility During Biotransformations

Abstract: Dielectric spectroscopy provides a convenient means of determining the degree of intactness of biological cells. 4-terminal dielectric measurements of suspensions of Saccharomyces cerevisiae at 0.4 MHz show that, as with all other biological cells, these organisms possess a substantial β-dispersion. The additional of octanol to such suspensions causes a rapid decrease in the electrical capacitance of the suspension, which parallels the cellular viability as determined by methylene blue staining. The kinetics of cell death are determined in part by the rate of dissolution of the organic solvent in the aqueous phase. The toxicity of several organic solvents to S. cerevisiae is studied using this technique, and is found to be dependent upon the polarity of the solvent. The present method provides a simple and rapid means for assessing the biocompatibility of solvents used in biotransformations.

Precambrian ophiolites, Cambrian island arc, and Variscan suture in the South Carpathian-Balkan region

Abstract: A Late Proterozoic ophiolite in the South Carpathian-Balkan region has a total surface-outcrop area of 400 km 2 . It is a complete ophiolitic section that includes mid-ocean ridge basalts, indicating that it was formed in a large ocean. The oceanic crust is unconformably overlain by an ensimatic island arc of Cambrian age. The ophiolite-island-arc assemblage was obducted over the Moesian platform during Early Ordovician time. Both sequences are now incorporated in a continental block as a result of intracontinental convergence, which took place in Variscan time. The ophiolite is situated along the Thracian suture, which divides the Moesian platform and the Thracian continental fragment. This suture is part of a larger one, referred to as the South European suture, that is situated between the Thracian massif and the Adriatic plate to the southwest and the Moesian platform, Bohemian, and Central French massif to the northeast.

Transient processes in cell proliferation kinetics

Abstract: This is a memorial survey paper on some joint works with Andrei Yu. Yakovlev† in the field of branching processes and their applications in cell proliferation kinetics. The following topics are considered: distributions of discrete and continuous labels, age and residual lifetime distributions, models of leukemia cell kinetics, age-dependent branching populations with randomly chosen paths of evolution as models of progenitor cell populations (in vitro) and estimating of offspring distributions, multitype branching populations with a large number of ancestors and asymptotic likelihood estimation of the basic mitotic parameters. A part of the presented results is not published yet. The paper is focused on some new ideas for branching processes theory arising in cell proliferation modeling.

Fluxionality and stereochemistry of 5-coordinate Cu2+ complexes. The potential energy surface and spectroscopic implications

Abstract: Cu2+ ions in five-coordination (Cl−1, NH3, NCS−, etc.) generally stabilize an elongated square pyramid, which is slightly preferred to a compressed trigonal bipyramid. This stereochemical behaviour can be understood by considering the vibronic interaction between the A′1 ground state and the first excited E′ state via the e′ vibrations in D3h symmetry (pseudo-Jahn-Teller coupling), in combination with an E′ ⊗ e′ interaction (Jahn-Teller coupling in the excited state), leading into the lower C2v (C4v) and Cs symmetries. The adiabatic ground state potential surface is calculated in a semi-empirical model on the basis of available spectroscopic data. The minima at the points, which characterize the elongated C2v (C4v) geometry, are rather flat and can be shifted to any other point of the potential surface by steric ligand and/or geometric packing influences. The CuCl53− square pyramids in [Co (NH3)6] CuCl5 undergo a pseudorotation to (dynamically averaged) trigonal bipyramids at 280 K, with the Cl− ligands along the threefold axis remaining fixed in space. In contrast the nuclear displacements along the e′ coordinates occur in an unrestricted manner above 285 K in case of the Cu(NH3)52+ square pyramids in [Cu(NH3)5]Br2 (“Berry rotation”). The wavefunctions, g-tensor components and d-d transition energies have been derived as functions of the e′ distortion coordinates and were explicitly calculated for the CuCl53− model case.

Phthalocyanines on mineral carriers, 4. Low‐molecular‐weight and polymeric phthalocyanines on SiO2, γ‐Al2O3 and active charcoal as catalysts for the oxidation of 2‐mercaptoethanol

Abstract: Low-molecular-weight phthalocyaninatocobalt compounds 2a–d and polymeric phthalocyaninatocobalt derivatives 4, 6a, b were prepared on the surface of the inorganic carriers SiO2 or γ-Al2O3 and of active charcoal, the range of loading being 11,9 – 0,2 wt.-%. The oxidation of 2-mercaptoethanol in an aqueous solution at pH 9 was used as a model reaction for demercaptizations of petroleum fractions with phthalocyanines on carriers as heterogeneous catalysts. The catalytic activity, related to the phthalocyanine content, was found to increase with increasing dispersion of the complex on the carrier. The influence of the type of support on the activity is given by the following order: SiO2 < γ-Al2O3 < Norit charcoal. Polymeric phthalocyanines exhibit a higher catalytic activity as compared with low-molecular-weight phthalocyanines. The mechanism of the oxidation of the thiol is discussed.

Laser-induced crosslinking of histones to DNA in chromatin and core particles: implications in studying histone-DNA interactions

Abstract: UV laser irradiation has been used to covalently crosslink histones to DNA in nuclei, chromatin and core particles and the presence of the different histone species in the covalently linked material was detected immunochemically. When nuclei were irradiated and then trypsinized to cleave the N- and C- terminal histone tails, no histones have been found covalently linked to DNA. This finding shows that UV laser-induced crosslinking of histones to DNA is accomplished via the non-structured domains only. This unexpected way of crosslinking operated in chromatin, H1-depleted chromatin and core particles, i.e. independently of the chromatin structure. The efficiency of crosslinking, however, showed such a dependence: whilst the yield of crosslinks was similar in total and H1-depleted chromatin, in core particles the efficiency was 3-4 times lower for H2A, H2B and H4 and 10-12 times lower for H3. The decreased crosslinking efficiency, especially dramatic in the case of H3, is attributed to a reduced number of binding sites, and, respectively, is considered as a direct evidence for interaction of nonstructured tails of core histones with linker DNA.

The role of the adjacency between background cues and objects in visual localization during ocular pursuit.

Abstract: Subjects used eye movements to pursue a light target that moved from left to right with a velocity of 15 deg s−1. The stimulus was a sudden five-fold decrease in target intensity during the movement. The subject's task was to localize the stimulus relative to either a single stationary background point or the midpoint between two points (28 deg apart) placed 0.5 deg above the target path. The stimulus was usually mislocated in the direction of eye movement; the mislocation was affected by the spatial adjacency between background and stimulus. When an auditory, rather than a visual, stimulus was presented during tracking, target position at the time of stimulus presentation was visually mislocated in the direction opposite to that of eye movement. The effect of adjacency between background and target remained the same. The involvement of processes of subject-relative and object-relative visual perception is discussed.

An estimate from above of the number of periodic orbits for semi-dispersed billiards

Abstract: For a large class of semi-dispersed billiards an exponential estimate from above is found for the number of periodic points of the billiard ball map.