Helix-specific interactions induce condensation of guanosine four-stranded helices in concentrated salt solutions.

Deoxyguanosine-5'-monophosphate in water self-associates into stable structures, which include liquid-crystalline hexagonal and cholesteric phases. The structural unit is a four-stranded helix, composed of stacked Hoogsteen-bonded guanosine quartets. By using the osmotic stress method, we recently measured the force between helices in KCl solutions up to 2 M. In addition to the long-range electrostatic force, a short-range hydration repulsive contribution was recognized. The hydration repulsion is exponential, and shows a decay length independent from the ionic strength of the solution. Here, we report that more concentrated KCl solutions cause condensation of the guanosine helix in a hexagonal phase with constant equilibrium separation of approximately 7 A between helix surfaces. Long-range attraction, which induces the self-assembly, and short-range repulsion, which prevents the contact between the helices, are implied. By using osmotic stress, the force needed to push helices closer from the spontaneously assumed position has been measured. The attractive force was then estimated as a difference between the net force and the repulsive contribution, revealing an exponential decay length about two times larger than that of the short-range repulsion. The agreement with the helix interaction theory introduced recently by Kornyshev and Leikin (Kornyshev, A. A., and S. Leikin, 1997. Theory of interaction between helical molecules. J. Phys. Chem. 107:3656-3674) suggests that the repulsive and attractive forces originate from helix-specific interactions.     

Parametrization of direct and soft steric-undulatory forces between DNA double helical polyelectrolytes in solutions of several different anions and cations.

Directly measured forces between DNA helices in ordered arrays have been reduced to simple force coefficients and mathematical expressions for the interactions between pairs of molecules. The tabulated force parameters and mathematical expressions can be applied to parallel molecules or, by transformation, to skewed molecules of variable separation and mutual angle. This "toolbox" of intermolecular forces is intended for use in modelling molecular interactions, assembly, and conformation. The coefficients characterizing both the exponential hydration and the electrostatic interactions depend strongly on the univalent counterion species in solution, but are only weakly sensitive to anion type and temperature (from 5 to 50 degrees C). Interaction coefficients for the exponentially varying hydration force seen at spacings less than 10 to 15 A between surfaces are extracted directly from pressure versus interaxial distance curves. Electrostatic interactions are only observed at larger spacings and are always coupled with configurational fluctuation forces that result in observed exponential decay lengths that are twice the expected Debye-Huckel length. The extraction of electrostatic force parameters relies on a theoretical expression describing steric forces of molecules "colliding" through soft exponentially varying direct interactions.     

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.     

Measuring electrostatic, van der Waals, and hydration forces in electrolyte solutions with an atomic force microscope

In atomic force microscopy, the tip experiences electrostatic, van der Waals, and hydration forces when imaging in electrolyte solution above a charged surface. To study the electrostatic interaction force vs distance, curves were recorded at different salt concentrations and pH values. This was done with tips bearing surface charges of different sign and magnitude (silicon nitride, Al₂O₃, glass, and diamond) on negatively charged surfaces (mica and glass). In addition to the van der Waals attraction, neutral and negatively charged tips experienced a repulsive force. This repulsive force depended on the salt concentration. It decayed exponentially with distance having a decay length similar to the Debye length. Typical forces were about 0.1 nN strong. With positively charged tips, purely attractive forces were observed. Comparing these results with calculations showed the electrostatic origin of this force.

In the presence of high concentrations (> 3 M) of divalent cations, where the electrostatic force can be completely ignored, another repulsive force was observed with silicon nitride tips on mica. This force decayed roughly exponentially with a decay length of 3 nm and was ~0.07-nN strong. This repulsion is attributed to the hydration force.

     

Exclusion of alcohols from spermidine-DNA assemblies: probing the physical basis of preferential hydration

The interaction of the alcohols 2-methyl-2,4-pentanediol (MPD) and 2-propanol and of glycerol with condensed spermidine(3+)-DNA arrays are investigated with direct force measurements using osmotic stress coupled with X-ray scattering. Thermodynamic forces between DNA helices are measured from the dependence of helical interaxial spacings on the osmotic pressure applied by poly(ethylene glycol) solutions in equilibrium with the DNA phase. The sensitivity of these forces to solute concentration can be transformed into a change in the number of excess or deficit solutes or waters in the DNA phase by applying the Gibbs-Duhem equation. The alcohols examined are excluded from the condensed DNA array and strongly affect the osmotic stress force curves. DNA is preferentially hydrated. MPD is significantly more excluded than 2-propanol. The exclusion of these alcohols, however, is not due to a steric repulsion since glycerol that is intermediate in size between MPD and 2-propanol does not observably affect DNA force curves. As the distance between DNA helices varies, the change in the number of excess waters is independent of alcohol concentration for each alcohol. These solutes are acting osmotically on the condensed array. The distance dependence of exclusion indicates that repulsive water structuring forces dominate the interaction of alcohols with the DNA surface. The exclusion measured for these condensed arrays can quantitatively account for the effect of these alcohols on the precipitation of DNA from dilute solution by spermidine(3+).     

Solvent hydrogen-bond network in protein self-assembly: solvation of collagen triple helices in nonaqueous solvents.

Forces between type I collagen triple helices are studied in solvents of varying hydrogen-bonding ability. The swelling of collagen fibers in reconstituted films is controlled by the concentration of soluble polymers that are excluded from the fibers and that compete osmotically with collagen for available solvent. The interaxial spacing between the triple helices as a function of the polymer concentration is measured by x-ray diffraction. Exponential-like changes in the spacing with increasing osmotic stress, qualitatively similar to the forces previously found in aqueous solution, are also seen in formamide and ethylene glycol. These are solvents that, like water, are capable of forming three-dimensional hydrogen-bond networks. In solvents that either cannot form a network or have a greatly impaired ability to form a hydrogen-bonded network, strikingly different behavior is observed. A hard-wall repulsion is seen with collagen solvated by ethanol, 2-propanol, and N,N-dimethylformamide. The spacing between helices hardly changes with increasing polymer concentration until the stress exceeds some threshold where removal of the solvent becomes energetically favorable. No solvation of collagen is observed in dimethoxyethane. In solvents with an intermediate ability to form hydrogen-bonded networks, methanol, 2-methoxyethanol, or N-methylformamide, the change in spacing with polymer concentration is intermediate between exponential-like and hard-wall. These results provide direct evidence that the exponential repulsion observed between collagen helices at 0-8-A surface separations in water is due to the energetic cost associated with perturbing the hydrogen-bonded network of solvent molecules between the collagen surfaces.     

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)     

Species-specific long range interactions between receptor/ligand pairs.

Total internal reflection microscopy (TIRM) monitors Brownian fluctuations in elevation as small as 1 nm by measuring the scattering of a single sphere illuminated by an evanescent wave when the sphere is levitated by colloidal forces such as electrostatic double-layer repulsion. From the Boltzmann distribution of elevations sampled by the sphere over time, the potential energy profile can be determined with a resolution of approximately 0.1 of the thermal energy kT. Thus, the interaction between a receptor-coated (goat, horse, or rabbit immunoglobulin G (IgG)) latex sphere and a protein A (SpA)-coated glass microscope slide was studied. A typical TIRM potential energy profile measured between a bare sphere and a bare glass plate, where the sphere fluctuates around the secondary potential energy minimum formed between double-layer repulsion and gravitational attraction, agrees well with DLVO theory. The interactions measured between IgG-coated spheres and SpA-coated slides, on the other hand, displayed a weaker repulsion compared with that observed between bare surfaces under the same conditions. Analysis of the results obtained between the coated surfaces suggests an additional attractive force. The decay length of this attraction correlates with the known dissociation constants for the binding of IgG with SpA in free solution.     

Membrane interactions in nerve myelin. I. Determination of surface charge from effects of pH and ionic strength on period.

We have used x-ray diffraction to study the interactions between myelin membranes in the sciatic nerve (PNS) and optic nerve (CNS) as a function of pH (2-10) and ionic strength (0-0.18). The period of myelin was found to change in a systematic manner with pH and ionic strength. PNS periods ranged from 165 to 250 A or more, while CNS periods ranged from 150 to 230 A. The native periods were observed only near physiological ionic strength at neutral or alkaline pH. The smallest periods were observed in the pH range 2.5-4 for PNS myelin and pH 2.5-5 for CNS myelin. The minimum period was also observed for PNS myelin after prolonged incubation in distilled water. At pH 4, within these acidic pH ranges, myelin period increased slightly with ionic strength; however, above these ranges, the period increased with pH and decreased with ionic strength. Electron density profiles calculated at different pH and ionic strength showed that the major structural alteration underlying the changes in period was in the width of the aqueous space at the extracellular apposition of membranes; the width of the cytoplasmic space was virtually constant. Assuming that the equilibrium myelin periods are determined by a balance of nonspecific forces/i.e., the electrostatic repulsion force and the van der Walls attractive force, as well as the short-range repulsion force (hydration force, or steric stabilization), then values in the period-dependency curve can be used to define the isoelectric pH and exclusion length of the membrane. The exclusion length, which is related to the minimum period at isoelectric pH, was used to calculate the electrostatic repulsion force given the other forces. The electrostatic repulsion was then used to calculate the surface potential, which in turn was used to calculate the surface charge density (at different pH and ionic strength). We found the negative surface charge increases with pH at constant ionic strength and with ionic strength at constant pH. We suggest that the former is due to deprotonation of the ionizable groups on the surface while the latter is due to ion binding. Interpretation of our data in terms of the chemical composition of myelin is given in the accompanying paper (Inouye and Kirschner, 1988). We also calculated the total potential energy functions for the different equilibrium periods and found that the energy minima became shallower and broader with increasing membrane separation. Finally, it was difficult to account directly for certain structural transitions from a balance of nonspecific forces.(ABSTRACT TRUNCATED AT 400 WORDS)     

Binding of divalent cations of dipalmitoylphosphatidylcholine bilayers and its effect on bilayer interaction

We have confirmed that CaCl2 swells the multilayer lattice formed by dipalmitolyphosphatidylcholine (DPPC) in an aqueous solution. Specifically, at room temperature 1 mM CaCl2 causes these lipid bilayers to increase their separation, dw, from 19 A in pure water to greater than 90 A. CaCl2 concentrations greater than 4 mM cause less swelling. We have measured the net repulsive force between the bilayers in 30 mM CaCl2 at T = 25 degrees C (below the acyl chain freezing temperature). For interbilayer separations between 30 and 90 A, the dominant repulsion between bilayers is probably electrostatic; Ca2+ binds to DPPc lecithin bilayers, imparting a charge to them. The addition of NaCl to CaCl2 solutions decreases this repulsion. For dw less than 20 A, the bilayer repulsion appears to be dominated by the "hydration forces" observed previously between both neutral and charged phospholipids. From the electrostatic repulsive force, we estimate the extent of Ca2+ binding to the bilayer surface. The desorption and bound Ca2+, apparent when bilayers are pushed together, is more rapid than one would expect if an association constant governed Ca2+ binding. The association affinity does not appear to be a fixed quantity but rather a sensitive function of ionic strength and bilayer separation.     

Interactions between charged, uncharged, and zwitterionic bilayers containing phosphatidylglycerol.

Pressure vs. distance relationships have been obtained for phosphatidylglycerol bilayers, in both charged and uncharged states. Water was removed from the lipid multilayers by the application of osmotic pressures in the range of 0-2.7 x 10(9) dyn/cm2, and the distance between adjacent bilayers was obtained from Fourier analysis of lamellar x-ray diffraction data. For phosphatidylglycerol bilayers made electrically neutral either by lowering the pH or by adding equimolar concentrations of the positively charged lipid stearylamine, the pressure-distance data could be fit with a single exponential. The measured decay lengths were 1.1 A at low pH and 1.5 A with stearylamine, which are similar to decay lengths of the hydration pressure found for gel phases of other neutral bilayers. In addition, the magnitude of this repulsive pressure was proportional to the square of the Volta potential (equivalent to the dipole potential for electrically neutral bilayers) measured in monolayers in equilibrium with bilayers, in agreement with results previously found for the hydration pressure between phosphatidylcholine bilayers. For charged phosphatidylglycerol bilayers, the pressure-distance relation had two distinct regions. For bilayer separations greater than 10 A, the pressure-distance data had an exponential decay length (11 A) and a magnitude consistent with that expected for electrostatic repulsion from double-layer theory. For bilayer separations of 2-10 A, the pressure decayed much more rapidly with increasing bilayer separation (decay length less than 1 A). We interpret these data at low bilayer separations in terms of a combination of hydration repulsion and steric hindrance between the lipid head groups and the sodium ions trapped between apposing bilayers.     

Structural dimensions and their changes in a reentrant hexagonal-lamellar transition of phospholipids.

A hexagonal-lamellar-hexagonal (HII-L-HII) reentrant phase transition sequence on dehydration of dioleoylphosphatidylethanolamine occurs below 22 degrees C. This provides an unusual opportunity to measure how several structural dimensions change during this transition. Using x-ray diffraction, we have measured these dimensions with a hope of gaining some clue about the accompanying internal stresses. The principal dimensions described are molecular areas and molecular lengths projected onto the hexagonal lattice. In contrast with large changes in average area at the polar and hydrocarbon ends of the molecule, a position near the polar group/hydrocarbon interface is one of constant molecular area. It remains constant both as the monolayers curl from changing water content and in the transition from one structure to the other. In the L-to-HII transition, the most obvious change in molecular length is a 25% decrease in the distance between aqueous cylinders, the interaxial direction. There is little change in the interstitial direction, the direction toward the interstice equidistant from three aqueous cylinders. As the hexagonal phase is dehydrated, a number of internal changes in molecular lengths are described. Increases in the interaxial direction are much larger than in the interstitial. Simultaneously however, hydrocarbon chain lengths decrease, and polar group lengths increase. It is likely that molecules move axially and the cylinders become longer with dehydration. These dimensions and their changes might be used in the search for a better understanding of the energetics of molecular packing, of the interpretation of spectroscopic measurements of these phases, and of the mechanics of lipid layers.     

Investigation of DNA orientation on gold by EC-STM.

The immobilization of thiol-derivatized DNA on a Au (111) single crystal surface by self-assembly has been investigated by electrochemical scanning tunneling microscopy (EC-STM). Continuous potential-dependent orientation changes of double-stranded oligodeoxynucleotides (ODN) have been observed in a certain potential range from 200 to 600 mV (versus SCE). It is suggested that the DNA duplexes stand straight on the gold surface at potentials negative of the potential of zero charge (pzc) and then lay down on the surface when the potential shifts positively. These results are in agreement with the expectation based on the Coulombic interaction consideration between negatively charged DNA helices and gold surface. As the applied potential shifts positively, the surface charge changes from negative to positive, that is, the Coulombic force between negatively charged DNA helices and gold surfaces changes from repulsion to attraction. However, for the single-stranded oligodeoxynucleotides, no distinct changes in the surface structure were observed with the applied potential.     

Charge structure and counterion distribution in hexagonal DNA liquid crystal

A hexagonal liquid crystal of DNA fragments (double-stranded, 150 basepairs) with tetramethylammonium (TMA) counterions was investigated with small angle neutron scattering (SANS). We obtained the structure factors pertaining to the DNA and counterion density correlations with contrast matching in the water. Molecular dynamics (MD) computer simulation of a hexagonal assembly of nine DNA molecules showed that the inter-DNA distance fluctuates with a correlation time around 2 ns and a standard deviation of 8.5% of the interaxial spacing. The MD simulation also showed a minimal effect of the fluctuations in inter-DNA distance on the radial counterion density profile and significant penetration of the grooves by TMA. The radial density profile of the counterions was also obtained from a Monte Carlo (MC) computer simulation of a hexagonal array of charged rods with fixed interaxial spacing. Strong ordering of the counterions between the DNA molecules and the absence of charge fluctuations at longer wavelengths was shown by the SANS number and charge structure factors. The DNA-counterion and counterion structure factors are interpreted with the correlation functions derived from the Poisson-Boltzmann equation, MD, and MC simulation. Best agreement is observed between the experimental structure factors and the prediction based on the Poisson-Boltzmann equation and/or MC simulation. The SANS results show that TMA is too large to penetrate the grooves to a significant extent, in contrast to what is shown by MD simulation.     

Range of the solvation pressure between lipid membranes: dependence on the packing density of solvent molecules

Well-ordered multilamellar arrays of liquid-crystalline phosphatidylcholine and equimolar phosphatidylcholine-cholesterol bilayers have been formed in the nonaqueous solvents formamide and 1,3-propanediol. The organization of these bilayers and the interactions between apposing bilayer surfaces have been investigated by X-ray diffraction analysis of liposomes compressed by applied osmotic pressures up to 6 X 10(7) dyn/cm2 (60 atm). The structure of egg phosphatidylcholine (EPC) bilayers in these solvents is quite different than in water, with the bilayer thickness being largest in water, 3 A narrower in formamide, and 6 A narrower in 1,3-propanediol. The incorporation of equimolar cholesterol increases the thickness of EPC bilayers immersed in each solvent, by over 10 A in the case of 1,3-propanediol. The osmotic pressures of various concentrations of the neutral polymer poly(vinylpyrrolidone) dissolved in formamide or 1,3-propanediol have been measured with a custom-built membrane osmometer. These measurements are used to obtain the distance dependence of the repulsive solvation pressure between apposing bilayer surfaces. For each solvent, the solvation pressure decreases exponentially with distance between bilayer surfaces. However, for both EPC and EPC-cholesterol bilayers, the decay length and magnitude of this repulsive pressure strongly depend on the solvent. The decay length for EPC bilayers in water, formamide, and 1,3-propanediol is found to be 1.7, 2.4, and 2.6 A, respectively, whereas the decay length for equimolar EPC-cholesterol bilayers in water, formamide, and 1,3-propanediol is found to be 2.1, 2.9, and 3.1 A, respectively. These data indicate that the decay length is inversely proportional to the cube root of the number of solvent molecules per unit volume.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ion-mediated nucleic acid helix-helix interactions

Salt ions are essential for the folding of nucleic acids. We use the tightly bound ion (TBI) model, which can account for the correlations and fluctuations for the ions bound to the nucleic acids, to investigate the electrostatic free-energy landscape for two parallel nucleic acid helices in the solution of added salt. The theory is based on realistic atomic structures of the helices. In monovalent salt, the helices are predicted to repel each other. For divalent salt, while the mean-field Poisson-Boltzmann theory predicts only the repulsion, the TBI theory predicts an effective attraction between the helices. The helices are predicted to be stabilized at an interhelix distance approximately 26-36 A, and the strength of the attractive force can reach -0.37 k(B)T/bp for helix length in the range of 9-12 bp. Both the stable helix-helix distance and the strength of the attraction are strongly dependent on the salt concentration and ion size. With the increase of the salt concentration, the helix-helix attraction becomes stronger and the most stable helix-helix separation distance becomes smaller. For divalent ions, at very high ion concentration, further addition of ions leads to the weakening of the attraction. Smaller ion size causes stronger helix-helix attraction and stabilizes the helices at a shorter distance. In addition, the TBI model shows that a decrease in the solvent dielectric constant would enhance the ion-mediated attraction. The theoretical findings from the TBI theory agree with the experimental measurements on the osmotic pressure of DNA array as well as the results from the computer simulations.     

Preferential hydration of DNA: the magnitude and distance dependence of alcohol and polyol interactions

The physical forces that underlie the exclusion of solutes from macromolecular surfaces can be probed in a similar way as the measurement of forces between macromolecules in condensed arrays using the osmotic stress technique and x-ray scattering. We report here the dependence of alcohol exclusion or, equivalently, the preferential hydration of DNA on the spacing between helices in condensed arrays. The actual forces describing exclusion are quite different from the commonly assumed steric crowding coupled with weak binding. For a set of 12 nonpolar alcohols, exclusion is due to repulsive hydration interactions with the charged DNA surface. Exclusion amplitudes do not depend simply on size, but rather on the balance between alkyl carbons and hydroxyl oxygens. Polyols are included at very close spacings. The distance dependence of polyol inclusion, however, is quite different from nonpolar alcohol exclusion, suggesting the underlying mechanism of interaction is different.     

Differential stability of DNA crossovers in solution mediated by divalent cations

The assembly of DNA duplexes into higher-order structures plays a major role in many vital cellular functions such as recombination, chromatin packaging and gene regulation. However, little is currently known about the molecular structure and stability of direct DNA–DNA interactions that are required for such functions. In nature, DNA helices minimize electrostatic repulsion between double helices in several ways. Within crystals, B-DNA forms either right-handed crossovers by groove–backbone interaction or left-handed crossovers by groove–groove juxtaposition. We evaluated the stability of such crossovers at various ionic concentrations using large-scale atomistic molecular dynamics simulations. Our results show that right-handed DNA crossovers are thermodynamically stable in solution in the presence of divalent cations. Attractive forces at short-range stabilize such crossover structures with inter-axial separation of helices less than 20 Å. Right-handed crossovers, however, dissociate swiftly in the presence of monovalent ions only. Surprisingly, left-handed crossovers, assembled by sequence-independent juxtaposition of the helices, appear unstable even at the highest concentration of Mg²⁺studied here. Our study provides new molecular insights into chiral association of DNA duplexes and highlights the unique role divalent cations play in differential stabilization of crossover structures. These results may serve as a rational basis to understand the role DNA crossovers play in biological processes.     

Force measurements on myelin basic protein adsorbed to mica and lipid bilayer surfaces done with the atomic force microscope.

The mechanical and adhesion properties of myelin basic protein (MBP) are important for its function, namely the compaction of the myelin sheath. To get more information about these properties we used atomic force microscopy to study tip-sample interaction of mica and mixed dioleoylphosphatidylserine (DOPS) (20%)/egg phosphatidylcholine (EPC) (80%) lipid bilayer surfaces in the absence and presence of bovine MBP. On mica or DOPS/EPC bilayers a short-range repulsive force (decay length 1.0-1.3 nm) was observed during the approach. The presence of MBP always led to an attractive force between tip and sample. When retracting the tip again, force curves on mica and on lipid layers were different. While attached to the mica surface, the MBP molecules exhibited elastic stretching behavior that agreed with the worm-like chain model, yielding a persistence length of 0.5 +/- 0.25 nm and an average contour length of 53 +/- 19 nm. MBP attached to a lipid bilayer did not show elastic stretching behavior. This shows that the protein adopts a different conformation when in contact with lipids. The lipid bilayer is strongly modified by MBP attachment, indicating formation of MBP-lipid complexes and possibly disruption of the original bilayer structure.     

DNA cholesteric pitch as a function of density and ionic strength

The nature of chiral interactions among chiral biopolymers, such as DNA, protein alpha-helices, and rodlike virus particles, remains elusive. In particular, a satisfactory model connecting molecular chiral interactions and the pitch of the resulting chiral mesophases is lacking. We report the measurement of short-fragment (146-bp) DNA cholesteric spherulite pitch as a function of osmotic pressure, average DNA interaxial spacing, and salt concentration. We determined cholesteric pitch and interaxial spacing by polarizing optical microscopy and x-ray scattering, respectively, from which the twist-angle between DNA molecules can be calculated. Surprisingly, we found that decreasing ionic strength resulted in weaker chiral interactions between DNA chains, as evidenced by the decrease in the twist-angle, and consequent increase in the cholesteric pitch, for a fixed interaxial spacing. We propose that this behavior can be explained by increased smearing-out of the helical charge pattern along DNA as the Debye screening length is increased.