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Biomedical subjects

V A Parsegian

Publications and source records attributed to V A Parsegian.

At least 19 recordsLinked to original sources
PubMed1992

Protein solvation in allosteric regulation: a water effect on hemoglobin.

The oxygen affinity of hemoglobin varies linearly with the chemical potential of water in the bathing medium, as seen from the osmotic effect of several neutral solutes, namely sucrose, stachyose, and two polyethyleneglycols (molecular weights of 150 and 400). The data, analyzed either by Wyman linkage equations or by Gibbs-Duhem relations, show that approximately 60 extra water molecules bind to hemoglobin during the transition from the fully deoxygenated tense (T) state to the fully oxygenated relaxed (R) state. This number, independent of the nature of the solute, agrees with the difference in water-accessible surface areas previously computed for the two conformations. The work of solvation in allosteric regulation can no longer go unrecognized.

Allosteric Regulation↗
PubMed1992

Energetics of a hexagonal-lamellar-hexagonal-phase transition sequence in dioleoylphosphatidylethanolamine membranes.

The phase diagram of DOPE/water dispersions was investigated by NMR and X-ray diffraction in the water concentration range from 2 to 20 water molecules per lipid and in the temperature range from -5 to +50 degrees C. At temperatures above 22 degrees C, the dispersions form an inverse (HII) phase at all water concentrations. Below 25 degrees C, an HII phase occurs at high water concentrations, an L alpha phase is formed at intermediate water concentrations, and finally the system switches back to an HII phase at low water concentrations. The enthalpy of the L alpha-HII-phase transition is +0.3 kcal/mol as measured by differential scanning calorimetry. Using 31P and 2H NMR and X-ray diffraction, we measured the trapped water volumes in HII and L alpha phases as a function of osmotic pressure. The change of the HII-phase free energy as a function of hydration was calculated by integrating the osmotic pressure vs trapped water volume curve. The phase diagram calculated on the basis of the known enthalpy of transition and the osmotic pressure vs water volume curves is in good agreement with the measured one. The HII-L alpha-HII double-phase transition at temperatures below 22 degrees C can be shown to be a consequence of (i) the greater degree of hydration of the HII phase in excess water and (ii) the relative sensitivities with which the lamellar and hexagonal phases dehydrate with increasing osmotic pressure. These results demonstrate the usefulness of osmotic stress measurements to understand lipid-phase diagrams.

Calorimetry, Differential Scanning↗
PubMed1992

Direct measurement of the intermolecular forces between counterion-condensed DNA double helices. Evidence for long range attractive hydration forces.

Rather than acting by modifying van der Waals or electrostatic double layer interactions or by directly bridging neighboring molecules, polyvalent ligands bound to DNA double helices appear to act by reconfiguring the water between macromolecular surfaces to create attractive long range hydration forces. We have reached this conclusion by directly measuring the repulsive forces between parallel B-form DNA double helices pushed together from the separations at which they have self organized into hexagonal arrays of parallel rods. For all of the wide variety of "condensing agents" from divalent Mn to polymeric protamines, the resulting intermolecular force varies exponentially with a decay rate of 1.4-1.5 A, exactly one-half that seen previously for hydration repulsion. Such behavior qualitatively contradicts the predictions of all electrostatic double layer and van der Waals force potentials previously suggested. It fits remarkably well with the idea, developed and tested here, that multivalent counterion adsorption reorganizes the water at discrete sites complementary to unadsorbed sites on the apposing surface. The measured strength and range of these attractive forces together with their apparent specificity suggest the presence of a previously unexpected force in molecular organization.

Animals↗
PubMed1992

Direct measurement of temperature-dependent solvation forces between DNA double helices.

The assembly of double stranded DNA helices with divalent manganese ion is favored by increasing temperature. Direct force measurements, obtained from the osmotic stress technique coupled with x-ray diffraction, show that the force characteristics of spontaneously precipitated Mn(2+)-DNA closely resemble those observed previously by us for other counterion condensed DNA assemblies. At temperatures below the critical one for spontaneous assembly, we have quantitated the changes in entropy and manganese ion binding associated with the transition from repulsive to attractive interactions between helices mediated by osmotic stress. The release of structured water surrounding the DNA helix to the bulk solution is the most probable source of increased entropy after assembly. Increasing the water entropy of the bulk solution by changing the manganese salt anion from CI- to ClO4- predictably and quantitatively increases the transition entropy. This is further evidence for the dominating role of water in the close interaction of polar surfaces.

Animals↗
PubMed1992

Membrane dipole potentials, hydration forces, and the ordering of water at membrane surfaces.

We have compared hydration forces, electrical dipole potentials, and structural parameters of dispersions of dipalmitoylphosphatidylcholine (DPPC) and dihexadecylphosphatidylcholine (DHPC) to evaluate the influence of fatty acid carbonyl groups on phospholipid bilayers. NMR and x-ray investigations performed over a wide range of water concentrations in the samples show, that in the liquid crystalline lamellar phase, the presence of carbonyl groups is not essential for lipid structure and hydration. Within experimental error, the two lipids have identical repulsive hydration forces between their bilayers. The higher transport rate of the negatively charged tetraphenylboron over the positively charged tetraphenylarsonium indicates that the dipole potential is positive inside the membranes of both lipids. However, the lack of fatty acid carbonyl groups in the ether lipid DHPC decreased the potential by (118 +/- 15) mV. By considering the sign of the potential and the orientation of carbonyl groups and headgroups, we conclude that the first layer of water molecules at the lipid water interface makes a major contribution to the dipole potential.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1990

Direct measurement of forces between linear polysaccharides xanthan and schizophyllan.

Direct osmotic stress measurements have been made of forces between helices of xanthan, an industrially important charged polysaccharide. Exponentially decaying hydration forces, much like those already measured between lipid bilayer membranes or DNA double helices, dominate the interactions at close separation. Interactions between uncharged schizophyllans also show the same kind of hydration force seen between xanthans. In addition to the practical possibilities for modifying solution and suspension properties through recognition and control of molecular forces, there is now finally the opportunity for theorists to relate macroscopic properties of a polymer solution to the microscopic properties that underlie them.

Carbohydrate Sequence↗
PubMed1990

Membrane curvature, lipid segregation, and structural transitions for phospholipids under dual-solvent stress.

Amphiphiles respond both to polar and to nonpolar solvents. In this paper X-ray diffraction and osmotic stress have been used to examine the phase behavior, the structural dimensions, and the work of deforming the monolayer-lined aqueous cavities formed by mixtures of dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC) as a function of the concentration of two solvents, water and tetradecane (td). In the absence of td, most PE/PC mixtures show only lamellar phases in excess water; all of these become single reverse hexagonal (HII) phases with addition of excess td. The spontaneous radius of curvature R0 of lipid monolayers, as expressed in these HII phases, is allowed by the relief of hydrocarbon chain stress by td; R0 increases with the ratio DOPC/DOPE. Mixtures with very large R0's can have water contents higher than the L alpha phases that form in the absence of td. The drive for hydration is understood in terms of the curvature energy to create large water cavities in addition to direct hydration of the polar groups. Much of the work of removing water to create hexagonal phases of radius R less than R0 goes into changing monolayer curvature rather than dehydrating polar groups. Single HII phases stressed by limited water or td show several responses. (a) The molecular area is compressed at the polar end of the molecule and expanded at the hydrocarbon ends. (b) For circularly symmetrical water cylinders, the degrees of hydrocarbon chain splaying and polar group compression are different for molecules aligned in different directions around the water cylinder. (c) A pivotal position exists along the length of the phospholipid molecule where little area change occurs as the monolayer is bent to increasing curvatures. (d) By defining R0 at the pivotal position, we find that measured energies are well fit by a quadratic bending energy, K0/2 (1/R-1/R0); the fit yields bilayer bending moduli of Kc = (1.2-1.7) X 10(-12) ergs, in good agreement with measurements from bilayer mechanics. (e) For lipid mixtures, enforced deviation of the HII monolayer from R0 is sufficiently powerful to cause demixing of the phospholipids in a way suggesting that the DOPE/DOPC ratio self-adjusts so that its R0 matches the amount of td or water available, i.e., that curvature energy is minimized.

Chemical Phenomena↗
PubMed1990

Solute inaccessible aqueous volume changes during opening of the potassium channel of the squid giant axon.

We have applied solutions with varying osmotic pressures symmetrically to the inside and outside of perfused, TTX-treated, giant axons. The potassium conductance G decreased with increasing osmotic stress, but there was no effect on either the shape or the position of the voltage-current curve. One must distinguish three possible actions of the osmotic agent: osmotic stress, channel blocking, and lowered solution conductivity. To do so, we compared results obtained working with pairs of internal and external solutions of either (a) equal osmotic stress, (b) equal conductivity, or (c) the same blocking agent. There was the same change in G irrespective of the type of stressing species (sorbitol or sucrose); this provides some evidence against a blocking mechanism. The conductivity of the external solution had a small effect on K currents; internal solution conductivity had none. A change in series resistance of the Schwann cell layer could account for the small effect of external solution conductivity. The primary cause of G depression appears, then, to be the applied osmotic stress. Using this result, we have developed models in which the channel has a transition between closed states under voltage control but osmotically insensitive and a closed/open step that is voltage-independent but osmotically sensitive. We have assumed that the conductance of this open state does not change with osmotic stress. In this way, we estimate that an additional 1,350 +/- 200 A3 or 40-50 molecules of solute-inaccessible water appear to associate with the average delayed rectifier potassium channel of the squid axon when it opens.

Animals↗
PubMed1988

Variation in hydration forces between neutral phospholipid bilayers: evidence for hydration attraction.

It is now generally recognized that hydration forces dominate close interactions of lipid hydrophilic surfaces. The commonality of their characteristics has been reasonably established. However, differences in measured net repulsion, particularly evident when phosphatidylethanolamine (PE) and phosphatidylcholine (PC) bilayers are compared, suggest there exists a variety of behavior wider than expected from earlier models of hydration and fluctuation repulsion balanced by van der Waals attraction. To find a basis for this diverse behavior, we have looked more closely at measured structural parameters, degrees of hydration, and interbilayer repulsive forces for the lamellar phases of the following lipids: 1-palmitoyl-2-oleoyl-PE (POPE), egg PE, transphosphatidylated egg PE (egg PE-T), mono- and dimethylated egg PE-T (MMPE and DMPE), 1-stearoyl-2-oleoyl-PC (SOPC), and mixtures of POPE and SOPC. POPE and SOPC bilayers differ not only in their maximum degrees of hydration but also in the empirical hydration force coefficients and decay lengths that characterize their interaction. When mixed with POPE, SOPC effects sudden and disproportionate increases in hydration. POPE, egg PE, and egg PE-T differ in their degree of hydration, molecular area, and hydration repulsion. A single methylation of egg PE-T almost completely converts its hydration and bilayer repulsive properties to those of egg PC; little progression of hydration is seen with successive methylations. In order to reconcile these observations with the conventional scheme of balancing interbilayer hydration and fluctuation-enhanced repulsion with van der Waals attraction, it is necessary to relinquish the fundamental idea that the decay of hydration forces is a constant determined by the properties of the aqueous medium. Alternatively, one can retain that fundamental idea if one recognizes the possibility that polar group hydration has an attractive component to it. In the latter view, that attractive component originates from interbilayer hydrogen-bonded water bridges between apposing bilayer surfaces, arising from correlation of zwitterionic or other complementary polar groups or from factors that affect polar group solubility. The same Marcelja and Radic formalism that accounts so well for the repulsive component also leads to an estimate of the attractive one. We suggest that the full range of degrees of hydration and of interbilayer spacings observed for different neutral bilayers results in part from variable contributions of the attractive and repulsive hydration components.(ABSTRACT TRUNCATED AT 400 WORDS)

Hydrogen Bonding↗
PubMed1987

Water movement during channel opening and closing.

By applying the osmotic stress of a nonpenetrating polymer, we have measured the change delta nu in polymer-inaccessible internal water volume of a voltage-gated ionic channel. The voltage-dependent anion channel (VDAC) from mitochondrial outer membranes shows a delta nu comparable in magnitude to the full channel volume estimated from solute penetrability, single-channel conductance, or image reconstruction. It thus appears that channel "gating" involves significant structure reorganization and water movement rather than the minimal changes caused by a local constriction or blockade. Hydration of the inner channel surface may be an important factor in channel gating as is the hydration of molecular surfaces in controlling macromolecular interaction in solution.

Body Water↗
PubMed1986

Thermal-mechanical fluctuations enhance repulsion between bimolecular layers.

Beginning with the free energy potential for long-range interactions between parallel flat sheets and the mechanical energy of bending those sheets, we have derived an upper bound for repulsion enhanced by thermal undulations of sheets in a multilamellar array. Through a self-consistent (mean-field) potential, we cover the full range of layer separations from close-in to far-apart, where the enhanced repulsion approaches the weak steric interaction derived previously by Helfrich [Helfrich, W. (1978) Z. Naturforsch. 33a, 305-315]. We have examined the effect of the fluctuation-enhanced repulsion in experimental studies of osmotic (and mechanical) compression of multilamellar lipid arrays, mechanical compression of bilayers immobilized on mica substrates, and controlled adhesion of giant bilayer membrane vesicles.

Chemical Phenomena↗
PubMed1986

Mimicry and mechanism in phospholipid models of membrane fusion.

In spite of heroic efforts, phospholipid bilayer models of fusion do not mimic the cellular process closely enough to be confident that the cellular mechanism is being probed. Probably the experimental factors that are most difficult to control are the uniformity of the vesicles and the constraints that determine their behavior. In most conditions where fusion is triggered, the probability of bilayer mechanical rupture and leakage is increased. Confining that rupture to the contact area remains the most elusive characteristic of cell fusion to mimic. It may depend on fusogenic proteins. Some membrane characteristics, such as lipid asymmetry, may be crucial and may be determined biochemically, but they are difficult to duplicate in model systems. The contact-induced formation of nonbilayer lipid structures triggered by phosphatidylinositol turnover may offer some guidance in how the biochemical and required physical changes may be coupled.

Cations, Divalent↗
PubMed1985

Chemical potential measurements of deoxyhemoglobin S polymerization. Determination of the phase diagram of an assembling protein.

We have used the "osmotic stress" method to determine the phase diagram of deoxyhemoglobin S polymerization. This method involves equilibration, through a semipermeable membrane, of the protein with solutions of inert polymers of known osmotic pressure. With deoxyhemoglobin A and S solutions, in which we have demonstrated achievement of equilibrium, plots of osmotic pressure versus concentration initially agree closely with the results of other methods of measurement of colligative properties. However, once the known solubility value is exceeded for the deoxyhemoglobin S solutions at various temperatures, there is a rapid rise in hemoglobin concentration over a narrow osmotic pressure range and then a more gradual increase in concentration. We believe that these two regions correspond, respectively, to the onset of the polymerization process, and of subsequent continuing growth and compression or alignment of polymer. We derive the thermodynamic values for these processes and show that the behavior of the deoxyhemoglobin S system is analogous to the phase transition for a simple chemical system. These results are relevant to understanding the intracellular polymerization of deoxyhemoglobin S in sickle cell disease, and these concepts are applicable to other protein assembly systems.

Gels↗
PubMed1984

Lessons for the study of membrane fusion from membrane interactions in phospholipid systems.

'Fusion' in model systems usually refers to the decay of membrane configurations that are inherently unstable because of the method of preparation. Natural fusion is a controlled event during which the underlying forces and instabilities are subject to the additional effects of biochemical reactions. To understand biological fusion one must be able first to assess the interplay among these physical and chemical factors. This paper reviews traditional measurements of electrostatic double layer and electrodynamic van der Waals forces acting between bilayer membranes. It also describes the much stronger hydration forces that have now been systematically studied. An essential part of any fusion event is the ability of membrane surfaces to overcome or circumvent the hydration barrier in order to make contact. This may be accomplished through applied force, through bridging substances that displace water from the membrane surface, or through biochemical modification of surfaces. In model systems, destruction of the hydration layer can cause violent adhesion, membrane deformation, and rupture. Natural fusion proceeds by more subtle processes whereby interfacial forces are harnessed in ways not yet understood.

Animals↗
PubMed1984

Measurement of the repulsive force between polyelectrolyte molecules in ionic solution: hydration forces between parallel DNA double helices.

We have measured the repulsive force between B-form double helices in parallel packed arrays of polymer-condensed DNA in the presence of 0.005-1.0 M ionic solutions. Molecular repulsion is consistently exponential with a 2.5-3.5 A decay distance, when the separation between DNA surfaces is 5-15 A. Only weakly dependent on ionic strength and independent of molecular size, this intermolecular repulsion does not obey the predictions of electrostatic double-layer theory. Rather, it strongly resembles the "hydration force" first recognized and quantified between phospholipid bilayers. Only beyond 15 A separation between molecules is there evidence of electrostatic double-layer forces. The quantitative failure of electrostatic double-layer theory seen here must gravely affect accepted analyses of other polyelectrolyte systems. Because the packing of condensed DNA resembles the spacings of DNA in many bacteriophages, our results permit estimation of the "DNA pressure" in phage heads.

Anions↗