Biomedical subjects

G Cevc

Publications and source records attributed to G Cevc.

11 recordsLinked to original sources

PubMed1992

The mechanism of the solute-induced chain interdigitation in phosphatidylcholine vesicles and characterization of the isothermal phase transitions by means of dynamic light scattering.

A new method is introduced for the detection of chain interdigitation in phospholipid bilayers. The same method is used to measure the hydrocarbon tilt in the dipalmitoylphosphatidylcholine membranes as a function of the bulk concentration of the interdigitation-inducing solutes, such as ethanol. The hydrocarbon tilt in the phosphatidylcholine bilayers is demonstrated to be limited to angles below approx. 51 degrees. The need for higher tilt values leads to bilayer interdigitation. Solute-induced chain interdigitation is shown to be a cooperative process provoked by the excessively large lateral repulsion in the interfacial region and the concomitant excessive chain tilt. Ethanol-induced phosphatidylcholine interdigitation, for example, proceeds via interdigitated domains formation and finally gives rise to the bilayers with fully intercalated chains tilted by at least 30 degrees (and sometimes as much as 50 degrees) with respect to the membrane normal.

Ethanol↗

PubMed1992

Lipid vesicles penetrate into intact skin owing to the transdermal osmotic gradients and hydration force.

Gradients across the outer skin layers may result in fields enforcing a lipid flow into or through the intact skin surface provided that lipids are applied in the form of special vesicles. The osmotic gradient, for example, which is created by the difference in the total water concentrations between the skin surface and the skin interior, provides one possible source of such driving force. It is sufficiently strong to push at least 0.5 mg of lipids per hour and cm2 through the skin permeability barrier in the region of stratum corneum. The lipid concentration gradient, on the contrary, does not contribute much to the lipid penetration into dermis. Occlusion, therefore, is detrimental for the vesicle penetration into intact skin.

Animals↗

PubMed1992

Noninvasive percutaneous induction of topical analgesia by a new type of drug carrier, and prolongation of local pain insensitivity by anesthetic liposomes.

We studied the duration of action and permeability of common analgesics and local anesthetics applied dermally via new carriers--transfersomes--in rats and humans. The therapeutic potential of analgesic transfersomes was evaluated in Sprague-Dawley rats subjected to heat and pressure stimuli. Results were compared with those obtained from administration of lidocaine-containing standard liposomes. In rats, subcutaneous injections of 2% lidocaine solution and of liposomal or transfersomal suspension resulted in a strong initial analgesic effect that decayed within 6-7 min. Characteristic withdrawal time is approximately 30 s. Dermally applied analgesic transfersomes, by contrast, increased heat stimulus reaction to greater than 70 s, 130% longer than in controls that received a placebo or a standard aqueous lidocaine solution. In humans, we tested two groups of nine male and female volunteers, aged between 25 and 60 yr, for pain-suppressing activity assessed by the pinprick method. Each subject received a total of 0.5 mL of a transfersomal preparation containing 7% lidocaine or 4% tetracaine over a forearm area of 9 cm. We conclude that the effectiveness of dermally applied anesthetic transfersomes is similar to that of the corresponding subcutaneous injections of similar drug quantities and that optimally designed transfersomes offer a suitable and promising means for the noninvasive treatment of local pain with direct, topical drug application.

Administration, Cutaneous↗

PubMed1991

How membrane chain-melting phase-transition temperature is affected by the lipid chain asymmetry and degree of unsaturation: an effective chain-length model.

Hydrocarbon effects on the lipid chain-melting phase-transition temperature are analyzed. The membrane fluidization temperature is shown to increase with the effective chain length, which is proportional to the thickness of the well-packed hydrocarbon region. The latter, as a rule, increases with the length of the longest ordered and aligned segment on each chain. This conclusion is independent of the cause for the reduced chain packing in membrane interior: chain unsaturation (which effectively decouples the two hydrocarbon segments disjoined by a double bond) or chain asymmetry (which causes the terminal hydrocarbon segments to lose close contact) both affect the bilayer chain-melting phase-transition temperature comparably on the effective chain-length scale. Thermodynamic consequences of the trans unsaturation are approximately 50% smaller than the effects of the double bonds in the cis conformation, owing to the smaller membrane perturbation by the former double bonds. A simple quantitative model is introduced for the analysis of the phospholipid chain-melting phase behavior. This new model permits quantitative predictions of the chain-melting transition temperature solely on the basis of the known lipid chemical composition. It also explains lipid sensitivity to the hydrocarbon type and attachment. The model agreement with the experimental data is usually better than to within 99% and thus comparable to experimental scatter, even when only a few or no adjustable parameters are used. The membrane fluidization temperature is calculated for a number of potentially interesting, also as yet unexplored, phospholipids, and the biological significance of the effective chain-length concept is discussed.

Hydrocarbons↗

PubMed1991

Polymorphism of the bilayer membranes in the ordered phase and the molecular origin of the lipid pretransition and rippled lamellae.

The lamellar-to-undulated-lamellar phase transition (L beta'----P beta', pretransition) in lipid bilayers is shown to be a phenomenon which exists only if lipid polar headgroups are sufficiently hydrated and if the interchain packing is sufficiently weak. The minimal lipid hydrophilicity and the critical amount of the lipid-bound water can be related to the lipid chain-melting transition temperature; the latter must not exceed some maximal, chainlength-dependent value if the pretransition is to exist. The minimally required amount of the lipid-bound water itself is essentially chainlength independent, however, and unaffected by the method of hydration variation: physical dehydration, hydrational competition between the lipid molecules and the substances dissolved in the aqueous subphase, or decreasing lipid headgroup polarity all affect the pretransition temperature similarly on the appropriate scale. Simple, phenomenological expressions for the evaluation of the bilayer subtransition, pretransition and chain-melting phase transition temperature as a function of the lipid chainlength are presented. They show that, even in excess water, bilayers will tend to undulate only as long as each of the two identical lipid chains will contain between 12 +/- 1 and 22 +/- 1 carbon atoms, the P beta-phase region for the less polar lipids being as a rule narrower. To get a theoretical means for quantitatively studying the effects of the lipid hydration on the bilayer pretransition, an interaction-balance method is proposed for describing undulated membranes at the molecular level. This is based on comparing the free-energy gain from the increased headgroup hydration with the free-energy loss caused by the reduced chain-chain attraction upon ripple formation. A rationale is thus found for scaling the pretransition temperature in terms of the hydration-induced chain-melting phase transition shift or of the lipid surface hydrophilicity. Within the framework of such a model the recently reported (de)hydration dependence of the bilayer-undulation period is reproduced with reasonable accuracy. Furthermore, it is estimated that at least 12 +/- 2 water molecules must be associated with each lipid head for the bilayer undulation to be feasible. The closer the system is to this boundary condition the longer is the repeat-distance for the surface undulations and the less stable is the undulated bilayer phase.

Gels↗

PubMed1991

Isothermal lipid phase transitions.

In liotropic lipid systems phase transitions can be induced isothermally by changing the solvent concentration or composition; alternatively, lipid composition can be modified by (bio)chemical means. The probability for isothermal phase transitions increases with the decreasing transition entropy; it is proportional to the magnitude of the transition temperature shift caused by transformation-inducing system variation. Manipulations causing large thermodynamic effects, such as lipid (de)hydration, binding of protons or divalent ions and macromolecular adsorption, but also close bilayer approach are, therefore, likely to cause structural lipid change(s) at a constant temperature. Net lipid charges enhance the membrane susceptibility to salt-induced isothermal phase transitions; a large proportion of this effect is due to the bilayer dehydration, however, rather than being a consequence of the decreased Coulombic electrostatic interactions. Membrane propensity for isothermal phase transitions, consequently, always increases with the hydrophilicity of the lipid heads, as well as with the desaturation and shortening of the lipid chains. Upon a phase change at a constant temperature, some of the interfacially bound solutes (e.g. protons or calcium) are released in the solution. Membrane permeability and fusogenicity simultaneously increase. In mixed systems, isothermal phase transitions, moreover, may result in lateral phase separation. All this opens up ways for the involvement of isothermal phase transitions in the regulation of biological processes.

Lipid Bilayers↗

PubMed1990

Liposomes for the sustained drug release in vivo.

New lipidic carriers suitable for the sustained drug release in vivo are presented. They consist of middle sized, compact phospholipid vesicles with one or up to few lipid bilayers which are sterically stabilized with a small amount of large-head phospholipids. As an example, phosphatidylcholine (PC) liposomes casted with up to 10 mol% of phosphatidylethanolamine with a covalently attached polyethyleneglycol 5000 headgroup (PE-PEG) are discussed. Such vesicles exhibit a very long circulation time after an i.v. administration in mice; the improvement over pure phosphatidylcholine liposomes within the first 24 h exceeds 8000%, at this point nearly 25% of the applied PE-PEG liposomes being still in the circulation. This advantage is a consequence of reduced phagocytosis of the lipidic carriers, as shown by an in vitro assay with blood monocyte cells in the flow cytometric experiments. For example, after 6 h incubation with THP-1 monocyte cells in human plasma the difference between the uptake of standard distearoylphosphatidylcholine (DSPC) and novel liposomes containing 10% distearoylphosphatidylethanolamine-PEG is by 1000%. Vesicles with 2.5 mol% DSPE-PEG are also taken-up via phagocytosis relatively slowly. But the latter vesicles, moreover, retain most of the enclosed model-drug carboxyfluorescein after an incubation in plasma. The rate of permeation of the encapsulated substance from such DSPE-PEG liposomes is below 2.4% per h. This is by approximately a factor of two less than for pure DSPC liposomes; vesicles with a higher PE-PEG content are inferior in this respect. Long circulation time and high retention of the newly developed liposomes open up ways for the future systemic use as such stabilized drug carriers for the therapeutic applications in vivo.

Drug Carriers↗

PubMed1990

Membrane electrostatics.

In conclusion, charged membrane together with their adjacent electrolyte solution form a thermodynamic and physico-chemical entity. Their surfaces represent an exceptionally complicated interfacial system owing to intrinsic membrane complexity, as well as to the polarity and often large thickness of the interfacial region. Despite this, charged membranes can be described reasonably accurately within the framework of available theoretical models, provided that the latter are chosen on the basis of suitable criteria, which are briefly discussed in Section A. Interion correlations are likely to be important for the regular and/or rigid, thin membrane-solution interfaces. Lateral distribution of the structural membrane charge is seldom and charge distribution perpendicular to the membranes is nearly always electrostatically important. So is the interfacial hydration, which to a large extent determines the properties of the innermost part of the interfacial region, with a thickness of 2-3 nm. Fine structure of the ion double-layer and the interfacial smearing of the structural membrane charge decrease whilst the surface hydration increases the calculated value of the electrostatic membrane potential relative to the result of common Gouy-Chapman approximation. In some cases these effects partly cancel-out; simple electrostatic models are then fairly accurate. Notwithstanding this, it is at present difficult to draw detailed molecular conclusions from a large part of the published data, mainly owing to the lack of really stringent controls or calibrations. Ion binding to the membrane surface is a complicated process which involves charge-charge as well as charge-solvent interactions. Its efficiency normally increases with the ion valency and with the membrane charge density, but it is also strongly dependent on the physico-chemical and thermodynamic state of the membrane. Except in the case of the stereospecific ion binding to a membrane, the relatively easily accessible phosphate and carboxylic groups on lipids and integral membrane proteins are the main cation binding sites. Anions bind preferentially to the amine groups, even on zwitterionic molecules. Membrane structure is apt to change upon ion binding but not always in the same direction: membranes with bound ions can either expand or become more condensed, depending on the final hydrophilicity (polarity) of the membrane surface. The more polar membranes, as a rule, are less tightly packed and more fluid. Diffusive ion flow across a membrane depends on the transmembrane potential and concentration gradients, but also on the coulombic and hydration potentials at the membrane surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1988

Phosphatidylcholine-fatty acid membranes. I. Effects of protonation, salt concentration, temperature and chain-length on the colloidal and phase properties of mixed vesicles, bilayers and nonlamellar structures.

The phase and colloidal properties of phosphatidylcholine/fatty acid (PC/FA) mixed vesicles have been investigated by optical methods, acid-base titration, and theoretically as a function of temperature (5-80 degrees C), molar lipid ratio (0-1), lipid chain length (C14-C18), headgroup ionization (1.5 less than or equal to pH less than or equal to 10), vesicle concentration (0.05-32 mumol vesicle.dm-3, and ionic strength (0.005 less than or equal to J less than or equal to 0.25). Increasing the fatty acid concentration in PC bilayers causes the phase transition temperatures (at 4 less than or equal to pH less than or equal to 5) to rise until, for more than 2 FA molecules per PC molecule, the sample turbidity exhibits only two transitions corresponding to the chain-melting of the 1:2 stoichiometric complexes of PC/FA, and pure fatty acid. The former transition is into a nonlamellar phase and is accompanied by extremely rapid vesicle aggregation (with association rates on the order of Ca approximately 10(7) dm3.mol-1.s-1) and massive lipid precipitation. Fluid-phase vesicles with less than 2 FA per PC associate much more slowly (Ca approximately 10(3) dm3.mol-1.s-1), their aggregation being comparable to that of the ordered-phase liposomes. Under no conditions was the relation between the fatty acid concentration and the vesicle association rate for the fluid-phase vesicles linear. In contrast to the X-ray diffraction data, optical measurements reveal a 'pretransitional region' between the chain-melting temperature of the PC component and the temperature at which the gross transformation into a nonlamellar phase sets in. This is seen for all lipid mixtures investigated. On the relative temperature scale, lipids with different chain lengths behave qualitatively similarly; however, the effective association constants determined for samples of constant lipid concentration seem to decrease somewhat with the number of CH2 groups per chain. Fatty acid protonation, which yields electrically neutral bilayers, invariably increases the rate of vesicle association; we have measured, for example, Ca approximately 10(2) at pH approximately 7 and Ca approximately 10(7) dm3.mol-1.s-1 at pH approximately 4). Protonation of the phosphatidylcholine phosphate groups, which causes a net positive charge to accumulate on the lipid vesicles, initially increases (Ca approximately 10(8) dm3.mol-1.s-1) but ultimately decreases (Ca approximately 10(7) dm3.mol-1.s-1) the rate of association between PC/FA (1:2) mixed vesicles.(ABSTRACT TRUNCATED AT 400 WORDS)

Chemical Phenomena↗

PubMed1987

How membrane chain melting properties are regulated by the polar surface of the lipid bilayer.

The principle of regulation of various membrane properties by the hydrocarbon membrane interior is now well understood. The mechanism by which the interfacial membrane region including aqueous solution affects the state of the lipid bilayer matrix, however, is as yet unclear, despite its great biological and physiological significance. Data and analysis presented in this paper show that apart from the lipid chain type, length, and degree of unsaturation the main factors determining the characteristics of lipid membranes are surface polarity and interfacial hydration. These incorporate the effects of head group dipole and multipole moments as well as the head group ability for hydrogen bonding and can account for most of the changes in the physicochemical membrane state caused by the lipid head group structure, bulk pH value, salt content, solute adsorption, etc. The effects of membrane potential are much less, only 10-30% of the former. Variations in hydration thus not only govern the short- and medium-range intermolecular and intermembrane interactions but also provide a fast and energetically inexpensive regulatory mechanism for lipid membranes to adapt their characteristics, at least locally or transiently, to new requirements.

Lipid Bilayers↗

PubMed1985

Hydration of noncharged lipid bilayer membranes. Theory and experiments with phosphatidylethanolamines.

Calorimetric measurements have been made on the thermodynamics of the chain-melting phase transition of saturated diacylphosphatidylethanolamines, with chains containing 12-20 carbons, as a function of water content. The transition temperature, Tt, and the transition enthalpy, and entropy all decrease with an increase in water content; however, the effect on Tt lessens with an increase in chainlength. These results are compared with a theoretical description of lipid hydration in terms of the interlamellar water polarization (i.e., modified water structure) in the interbilayer region. The measured free energy, enthalpy and entropy of the transition and the transition temperature have an approximate hyperbolic tangent dependence on water content, infinity tanh (dw/2 xi), where dw is the interlamellar water-layer thickness and xi approximately equal to 0.25 nm is the water-order correlation length, in agreement with the theory. Auxiliary x-ray diffraction experiments yield results on the repulsive hydration forces between lipid lamellae consistent with the theory, and allow an estimate of the water orienting potential of the interface. The molecular origin of this potential is discussed in electrostatic terms, and the values of its associated molecular parameters are found to yield the right size of hydrational thermodynamic quantities. The theory thus provides an integrated, clear, and simple approach to the hydration properties of lipid membranes.

Biophysical Phenomena↗