Change In Electrostatic Energy Research Articles

Local Electric Field Controls Fluorescence Quantum Yield of Red and Far-Red Fluorescent Proteins.

Genetically encoded probes with red-shifted absorption and fluorescence are highly desirable for imaging applications because they can report from deeper tissue layers with lower background and because they provide additional colors for multicolor imaging. Unfortunately, red and especially far-red fluorescent proteins have very low quantum yields, which undermines their other advantages. Elucidating the mechanism of nonradiative relaxation in red fluorescent proteins (RFPs) could help developing ones with higher quantum yields. Here we consider two possible mechanisms of fast nonradiative relaxation of electronic excitation in RFPs. The first, known as the energy gap law, predicts a steep exponential drop of fluorescence quantum yield with a systematic red shift of fluorescence frequency. In this case the relaxation of excitation occurs in the chromophore without any significant changes of its geometry. The second mechanism is related to a twisted intramolecular charge transfer in the excited state, followed by an ultrafast internal conversion. The chromophore twisting can strongly depend on the local electric field because the field can affect the activation energy. We present a spectroscopic method of evaluating local electric fields experienced by the chromophore in the protein environment. The method is based on linear and two-photon absorption spectroscopy, as well as on quantum-mechanically calculated parameters of the isolated chromophore. Using this method, which is substantiated by our molecular dynamics simulations, we obtain the components of electric field in the chromophore plane for seven different RFPs with the same chromophore structure. We find that in five of these RFPs, the nonradiative relaxation rate increases with the strength of the field along the chromophore axis directed from the center of imidazolinone ring to the center of phenolate ring. Furthermore, this rate depends on the corresponding electrostatic energy change (calculated from the known fields and charge displacements), in quantitative agreement with the Marcus theory of charge transfer. This result supports the dominant role of the twisted intramolecular charge transfer mechanism over the energy gap law for most of the studied RFPs. It provides important guidelines of how to shift the absorption wavelength of an RFP to the red, while keeping its brightness reasonably high.

Study of the effect of temperature on light-induced degradation in methylammonium lead iodine perovskite solar cells

In this paper, the effect of temperature on the process of optical degradation of CH3NH3PbI3 perovskite solar cells is studied. First, we promote the theory of the degradation process we have already presented, which leads to more accurate results. Previously, in the proposed model, we had considered the density of the vacancies distributed in the perovskite layer to be constant, while in reality, their density changes over time. In order to obtain the electric field resulting from the migration of vacancies, which is, in fact, the cause of the optical degradation of the perovskite solar cells, we need the initial density of the vacancies and its temperature dependence as well as migration energy (potential barrier against the migration) of the vacancies. Using the Gibbs free energy resulting from the formation of the vacancies, the density of the vacancies is obtained. Also, by calculating the changes in electrostatic energy between the lattice ions during displacement, the migration energy is obtained. When the required parameters are presented, the temperature dependence of the optical degradation process is discussed using the proposed theory, and the results show the appropriate accuracy of this theory.

Universal effects of solvent species on the stabilized structure of a protein.

We investigate the effects of solvent specificities on the stability of the native structure (NS) of a protein on the basis of our free-energy function (FEF). We use CPB-bromodomain (CBP-BD) and apoplastocyanin (apoPC) as representatives of the protein universe and water, methanol, ethanol, and cyclohexane as solvents. The NSs of CBP-BD and apoPC consist of 66% α-helices and of 35% β-sheets and 4% α-helices, respectively. In order to assess the structural stability of a given protein immersed in each solvent, we contrast the FEF of its NS against that of a number of artificially created, misfolded decoys possessing the same amino-acid sequence but significantly different topology and α-helix and β-sheet contents. In the FEF, we compute the solvation entropy using the morphometric approach combined with the integral equation theories, and the change in electrostatic (ES) energy upon the folding is obtained by an explicit atomistic but simplified calculation. The ES energy change is represented by the break of protein-solvent hydrogen bonds (HBs), formation of protein intramolecular HBs, and recovery of solvent-solvent HBs. Protein-solvent and solvent-solvent HBs are absent in cyclohexane. We are thus able to separately evaluate the contributions to the structural stability from the entropic and energetic components. We find that for both CBP-BD and apoPC, the energetic component dominates in methanol, ethanol, and cyclohexane, with the most stable structures in these solvents sharing the same characteristics described as an association of α-helices. In particular, those in the two alcohols are identical. In water, the entropic component is as strong as or even stronger than the energetic one, with a large gain of translational, configurational entropy of water becoming crucially important so that the relative contents of α-helix and β-sheet and the content of total secondary structures are carefully selected to achieve sufficiently close packing of side chains. If the energetic component is excluded for a protein in water, the priority is given to closest side-chain packing, giving rise to the formation of a structure with very low α-helix and β-sheet contents. Our analysis, which requires minimal computational effort, can be applied to any protein immersed in any solvent and provides robust predictions that are quite consistent with the experimental observations for proteins in different solvent environments, thus paving the way toward a more detailed understanding of the folding process.

Electrostatic balance between global repulsion and local attraction in reentrant polymerization of actin

Actin polymerization depends on the salt concentration, exhibiting a reentrant behavior: the polymerization is promoted by increasing KCl concentration up to 100 mM, and then depressed by further increase above 100 mM. We here investigated the physical mechanism of this reentrant behavior by calculating the polymerization energy, defined by the electrostatic energy change upon binding of an actin subunit to a filament, using an implicit solvent model based on the Poisson-Boltzmann (PB) equation. We found that the polymerization energy as a function of the salt concentration shows a non-monotonic reentrant-like behavior, with the minimum at about 100 mM (1:1 salt). By separately examining the salt concentration effect on the global electrostatic repulsion between the like-charged subunits and that on the local electrostatic attraction between the inter-subunit ionic-bond-forming residues in the filament, we clarified that the reentrant behavior is caused by the change in the balance between the two opposing electrostatic interactions. Our study showed that the non-specific nature of counterions, as described in the mean-field theory, plays an important role in the actin polymerization. We also discussed the endothermic nature of the actin polymerization and mentioned the effect of ATP hydrolysis on the G-F transformation, indicating that the electrostatic interaction is widely and intricately involved in the actin dynamics.

A Comprehensive Docking and MM/GBSA Rescoring Study of Ligand Recognition upon Binding Antithrombin.

Background: A high-throughput virtual screening pipeline has been extended from single energetically minimized structure Molecular Mechanics/Generalized Born Surface Area (MM/GBSA) rescoring to ensemble-average MM/GBSA rescoring. The correlation coefficient (R2) of calculated and experimental binding free energies for a series of antithrombin ligands has been improved from 0.36 to 0.69 when switching from the single-structure MM/GBSA rescoring to ensemble-average one. The electrostatic interactions in both solute and solvent are identified to play an important role in determining the binding free energy after the decomposition of the calculated binding free energy. The increasing negative charge of the compounds provides a more favorable electrostatic energy change but creates a higher penalty for the solvation free energy. Such a penalty is compensated by the electrostatic energy change, which results in a better binding affinity. A highly hydrophobic ligand is determined by the docking program to be a non-specific binder.Results: Our results have demonstrated that it is very important to keep a few top poses for rescoring, if the binding is non-specific or the binding mode is not well determined by the docking calculation.

Improving the treatment of coarse-grain electrostatics: CVCEL.

We propose an analytic approach for calculating the electrostatic energy of proteins or protein complexes in aqueous solution. This method, termed CVCEL (Circular Variance Continuum ELectrostatics), is fitted to Poisson calculations and is able to reproduce the corresponding energies for different choices of solute dielectric constant. CVCEL thus treats both solute charge interactions and charge self-energies, and it can also deal with salt solutions. Electrostatic damping notably depends on the degree of solvent exposure of the charges, quantified here in terms of circular variance, a measure that reflects the vectorial distribution of the neighbors around a given center. CVCEL energies can be calculated rapidly and have simple analytical derivatives. This approach avoids the need for calculating effective atomic volumes or Born radii. After describing how the method was developed, we present test results for coarse-grain proteins of different shapes and sizes, using different internal dielectric constants and different salt concentrations and also compare the results with those from simple distance-dependent models. We also show that the CVCEL approach can be used successfully to calculate the changes in electrostatic energy associated with changes in protein conformation or with protein-protein binding.

Energy Conversion in Polyelectrolyte Hydrogels.

Using extensive molecular dynamics simulations of polyelectrolyte hydrogels we demonstrate that, on deformation, these hydrogels adjust their deformed state predominantly by altering electrostatic interactions between their charged groups rather than excluded-volume and bond energies. On deformation, due to the hydrogel's inherent tendency to preserve electroneutrality in its interior, the translational entropy of counterions decreases and the total electrostatic energy becomes more attractive. This result is valid for a wide range of compression ratios and Bjerrum lengths. The change in the electrostatic energy is more marked in highly swollen gels at low ionic strengths. At high Bjerrum lengths, where most of the counterions are condensed on hydrogel chains and the gel resembles a neutral system, the electrostatic-energy change with deformation is weaker.

Computational antigenic epitope prediction by calculating electrostatic desolvation penalties of protein surfaces.

The prediction of antigenic epitopes on the surface of proteins is of great importance for vaccine development and to specifically design recombinant antibodies. Computational methods based on the three-dimensional structure of the protein allow for the detection of noncontinuous epitopes in contrast to methods based on the primary amino-acid sequence only. A method recently developed to predict protein-protein binding sites is presented, and the application to predict putative antigenic epitopes is described in detail. The prediction approach is based on the local perturbation of the electrostatic field at the surface of a protein due to a neutral probe of low dielectric constant that represents an approaching binding partner. The calculated change in electrostatic energy corresponds to an energy penalty of desolvating a protein surface region, and antigenic epitope surface regions tend to be associated with a lower penalty compared to the average protein surface. The protocol to perform the calculations is described and illustrated on an example antigen, the outer surface protein A of Borrelia burgdorferi, a pathogenic organism causing lyme disease.

Fundamentals of charged devices: Energy changes in a capacitor medium

Capacitors and derivative devices continue to be as relevant as ever, both at the frontiers of scientific and technological research and in the development of commercial products. Central to such pursuits is the essential requirement that all aspects of the work done in charging or discharging capacitors are clearly understood. Surprisingly, there is a very significant disagreement among standard texts and journal publications regarding this topic. The present work elaborates a thorough analysis of energy changes in a capacitor medium in terms of thermodynamic work and its extension to the more general, electrostatic energy change. Our conclusions agree, for example, with those of Volker Heine, but not with texts by Pippard or Callen. We also note that electric and magnetic analogues are not equivalent.

Variation of Weak Polyelectrolyte Persistence Length through an Electrostatic Contour Length

There is currently no consensus that the classical perspective of electrostatic persistence length (the so-called Odijk–Skolnick–Fixman, or OSF, formulation), derived by equating changes in electrostatic energy with bending rigidity, is applicable to weak polyelectrolytes. Here, with a focus on polyelectrolyte chains of finite length at a single scale, we formulate a simple and general theoretical model featuring the electrostatic persistence length, Pe, through the introduction of an electrostatic contour length, Le, such that the electrostatic energy balance is applicable to highly flexible charged polyelectrolytes (where the intrinsic stiffness, P0 is on the order of Pe). To isolate the effect of ionization, only the salt-free regime is considered. At the upper limit (relatively rigid molecules), the new formulation converges to the classical OSF form, while the lower bound (highly flexible molecules) approaches proportionality to the Debye screening length, κ–1. In general, the electrostatic persistence can be described retroactively by the Debye screening, κ–1, and the ratio of electrostatic to intrinsic contour lengths, A/a, complementary to predictive theoretical formulations. The theory is validated via full atomistic molecular dynamics simulations of single, isolated model weak polyelectrolyte chains in explicit solvent—specifically poly(acrylic acid), PAA, and poly(allylamine hydrochloride), PAH, both of which undergo significant increases in persistence length under ionization. An ensemble of equilibrated polymer states is obtained via temperature assisted sampling, implementing molecular dynamics to drive the polymers into physically accessible conformations via cyclical temperature fluctuations and equilibration.

Predicting RNA-binding sites from the protein structure based on electrostatics, evolution and geometry

An RNA-binding protein places a surface helix, β-ribbon, or loop in an RNA helix groove and/or uses a cavity to accommodate unstacked bases. Hence, our strategy for predicting RNA-binding residues is based on detecting a surface patch and a disparate cleft. These were generated and scored according to the gas-phase electrostatic energy change upon mutating each residue to Asp−/Glu− and each residue's relative conservation. The method requires as input the protein structure and sufficient homologous sequences to define each residue's relative conservation. It yields as output a priority list of surface patch residues followed by a backup list of surface cleft residues distant from the patch residues for experimental testing of RNA binding. Among the 69 structurally non-homologous proteins tested, 81% possess a RNA-binding site with at least 70% of the maximum number of true positives in randomly generated patches of the same size as the predicted site; only two proteins did not contain any true RNA-binding residues in both predicted regions. Regardless of the protein conformational changes upon RNA-binding, the prediction accuracies based on the RNA-free/bound protein structures were found to be comparable and their binding sites overlapped as long as there are no disordered RNA-binding regions in the free structure that are ordered in the corresponding RNA-bound protein structure.

Selective Adsorption of Poly(ethylene oxide) onto a Charged Surface Mediated by Alkali Metal Ions

Using a surface force balance, we have measured normal and shear interactions between mica surfaces across pure water and across 0.1 M aqueous solutions of LiNO3, NaNO3, KNO3, and CsNO3, both prior to adding polymer and following addition of 1.5 x 10(-4) w/w poly(ethylene oxide) (PEO, Mw = 170 kD) and overnight incubation. Our results reveal that while the PEO adsorbs strongly from the KNO3 and CsNO3 solutions, unexpectedly it does not adsorb at all from the LiNO3 and NaNO3 salt solutions. We attribute this to the different nature of the hydration layers about the alkali metal ions: these favor liganding to the negatively charged mica surface of the etheric -O- group on the ethylene oxide monomer for the case of the more weakly hydrated K+ and Cs+, but not for the case of Na+ or Li+ with their more strongly bound water. A simple model relating the electrostatic energy changes occurring upon such liganding to the experimentally measured hydration energies of the different alkali metal ions supports this attribution.

Electrostatic Energy Calculation on the pH-Induced Conformational Change of Influenza Virus Hemagglutinin

The pH-induced conformational change of influenza virus hemagglutinin (HA) has been investigated by calculating the change of electrostatic energy of the fragment of HA2 upon pH change. The average charge and electrostatic free energy are calculated as a function of pH for the fusion peptide (residues 1–20 of HA2) and the polypeptide of residues 54–77 of HA2 by using the finite difference Poisson-Boltzmann method. It is found that as pH decreases from 8 to 5, the electrostatic free energy of the fusogenic state is lowered by ∼2 kcal/\\ and the fusogenic state is less ionized compared to that of the native state for both polypeptides. For the fusion peptide at the fusogenic state, most of ionizable residues are neutral at acidic pH except Glu-11. For the polypeptide of residues 54–77 at the fusogenic state, most of residues except Glu-74 and His-64 are fully charged between pH 5 and pH 8.

In situ measurements of simultaneous electronic behavior of Cu and Al induced by mechanical deformation

Studies of mechanical force acting on electrons inside a solid are of both fundamental and practical importance. This is especially emphasized by the recent applications of the Kelvin probing technique to investigate mechanical behavior of surfaces and interfaces. However, the effect of deformation on the electron work function (EWF) and its physical mechanism have not been well understood. In this study, simultaneous responses of the EWF of copper and aluminum to deformation during bending tests were measured in situ. It was demonstrated that in the elastic range, the tensile strain decreased the EWF, whereas compressive strain increased the EWF. However, in the plastic range, the EWF always decreased with strain for both tension and compression deformations. Considering changes in electrostatic energy of a one-dimensional lattice caused by dislocation density, we proposed a simple model and quantitatively established a correlation between the EWF and strain, which is consistent with the experimental observations.

Is Silica Really an Anomalous Oxide? Surface Acidity and Aqueous Hydrolysis Revisited

The single-site Solvation, Bond Strength, and Electrostatic (SBE) model accounts for the anomalous position of silica onthe surface acidity versus aqueous acidity correlation developed for metal oxides, by considering the solvation energy change in the protonation reaction implemented through the dielectric constant (1/epsilon(k)) and the electrostatic energy change through the Pauling bond strength to bond length ratio (s/r) of the oxide. I address here why inclusion of the solid's dielectric constant brings silica into the same correlation as other oxides like TiO2, Al2O3, and Fe2O3. The solvation and electrostatic contributions are interpreted in terms of classical concepts such as chemical hardness, polarizability, ionicity, electronegativity, and local charge densities. Silica is acidic (PZC < 7), not because of its small dielectric constant, its tetrahedral coordination, or its high bond strength alone. Surface acidity depends largely on high values of the s/r ratio. The dielectric constant of the solid affects acidity mainly by reflecting the nature of water-surface interactions. Solids with large values of epsilon(k) are interpreted as being less polarizable and more ionic so that water, a hard polar solvent, interacts favorably with such surfaces and scales similar to water-water interactions regardless of whether the metal-oxide bond is in the solid or in the aqueous state. For these oxides, pKa(s) = pKa(aq) +/- 1. Silica, with a small dielectric constant, is interpreted as being more polarizable and more covalent so that water-SiO2 interactions scale differently than for the more ionic oxides. Such an interpretation when combined with the Partial Charge Model for metal hydrolysis suggests that the surfaces of RuO2, W03, Sb2O5, and Ta2O5 should be acidic similar to silica. But, unlike silica, they would lie on the pKa correlation defined by the other oxides because of their larger dielectric constants. The mixed oxide, AlPO4, is predicted to behave like silica.

Study on an Estimation of the Contributed Energy to the Transfer Free Energies of Complex Ions. Part I. [Fe(CN)2n(phen)(3-n)](2-2n)+ from Water to a Water-ethanol Mixed Solvent

Abstract The solubilities of [Fe(phen)3](ClO4)2·H2O, [Fe(CN)2(phen)2]·0.5H2O, [Fe(phen)3][Fe(CN)4phen]·8H2O, and [Fe(phen)3]2[Fe(CN)6]·3.5H2O in ethanol-water mixed solvents were measured at 25.0 ± 0.1 °C, and the transfer free energy of each complex ion from water to water-ethanol mixed solvent was calculated. From the obtained data, the transfer free energies were split into contributions from 1,10-phenanthroline-solvent, cyano ligand-solvent interaction energies, and electrostatic energies. The ratio of the contributed energy from 1,10-phenanthroline vs. transfer free energy of free (non-coordinated) 1,10-phenanthroline is almost constant at about 0.6—0.7 over the whole mole-fraction range. There is thus a possibility to estimate the contributed energy from the transfer free energy of the free ligand in the future. On the other hand, the contributed energy from the cyano ligand can not be obtained by multiplying a certain coefficient to the transfer free energy of the free cyanide ion. The contributed energy from electrostatic energy changes in a somewhat different manner, as expected by the Born equation. However, the estimated transfer free energy as the sum of each contributed energy obtained by a least-squares method is in fair agreement with the observed transfer free energy of each complex ion. It has been confirmed that the proposed conception of the contributed energy to the transfer free energy is a useful means to predict the unknown transfer free energy of complex ions.

A compound derived from prodocarpic acid inhibits influenza a virus replication in vitro: K.A. Staschke, S.D. Hatch, J.C. Tang, W.J. Hornback, S. Mauldin, J.E. Munroe, J.M. Colacino∗, and M.A. Muesing∗. Lilly Research Laboratories, Indianapolis, IN USA

The pH-induced conformational change of influenza virus hemagglutinin (HA) has been investigated by calculating the change of electrostatic energy of the fragment of HA2 upon pH change. The average charge and electrostatic free energy are calculated as a function of pH for the fusion peptide (residues 1–20 of HA2) and the polypeptide of residues 54–77 of HA2 by using the finite difference Poisson-Boltzmann method. It is found that as pH decreases from 8 to 5, the electrostatic free energy of the fusogenic state is lowered by ∼2 kcal/\ and the fusogenic state is less ionized compared to that of the native state for both polypeptides. For the fusion peptide at the fusogenic state, most of ionizable residues are neutral at acidic pH except Glu-11. For the polypeptide of residues 54–77 at the fusogenic state, most of residues except Glu-74 and His-64 are fully charged between pH 5 and pH 8.

Characterization of a new class of respiratory syncytial virus agents inhibiting the fusion process: B. O'Hara 1, G. Ellestad 1, J. Morin 1, A. Gazumyan 1, J. LaRocque 1, B. Mitsner 1, U. Raifeld 1, A. Nikitenko 1, W.-D. Ding 1, P. Wyde 2, B. Gilbert 2, and Y. Gluzman 1 1 Wyeth-Ayerst Research, Pearl River NY 10965, 2 Baylor

The pH-induced conformational change of influenza virus hemagglutinin (HA) has been investigated by calculating the change of electrostatic energy of the fragment of HA2 upon pH change. The average charge and electrostatic free energy are calculated as a function of pH for the fusion peptide (residues 1–20 of HA2) and the polypeptide of residues 54–77 of HA2 by using the finite difference Poisson-Boltzmann method. It is found that as pH decreases from 8 to 5, the electrostatic free energy of the fusogenic state is lowered by ∼2 kcal/\ and the fusogenic state is less ionized compared to that of the native state for both polypeptides. For the fusion peptide at the fusogenic state, most of ionizable residues are neutral at acidic pH except Glu-11. For the polypeptide of residues 54–77 at the fusogenic state, most of residues except Glu-74 and His-64 are fully charged between pH 5 and pH 8.

Carbides of group VA transition metals

AbstractEstimates are made of the cohesive energies of the group VA transition metal carbides for various C contents and crystal structures. In the cubic structure MCx, several factors help to provide optimal cohesion at x < 1, even in the presence of excess C. Notable among these is a drop in the Fermi level when vacancies appear in the C sublattice. A strong C–C repulsion between very close neighbours prevents the hexagonal structure becoming stable when x is large, even though the underlying metal sub lattice is expected to prefer hcp to fcc at these electron concentrations. In M2 C the C–C repulsive contribution is sufficiently reduced to allow the hexagonal structure to appear. The opportunity for afurther reduction in this repulsive contribution, together with a change in electrostatic energy, provides a strong tendency for ordering in the C sub lattice of M2 C, leading to high ordering temperatures.MST/3031

The influence of salt on the structure and energetics of supercoiled DNA

We present a detailed computational study of the influence of salt on the configurations, energies, and dynamics of supercoiled DNA. A potential function that includes both elastic and electrostatic energy components is employed. Specifically, the electrostatic term, with salt-dependent coefficients, is modeled after Stigter's pioneering work on the effective diameter of DNA as a function of salt concentration. Because an effective charge per unit length is used, the electrostatic formulation does not require explicit modeling of phosphates and can be used to study long DNAs at any desired resolution of charge. With explicit consideration of the electrostatic energy, an elastic bending constant corresponding to the nonelectrostatic part of the bending contribution to the persistence length is used. We show, for a series of salt concentrations ranging from 0.005 to 1.0 M sodium, how configurations and energies of supercoiled DNA (1000 and 3000 base pairs) change dramatically with the simulated salt environment. At high salt, the DNA adopts highly compact and bent interwound states, with the bending energy dominating over the other components, and the electrostatic energy playing a minor role in comparison to the bending and twisting terms. At low salt, the DNA supercoils are much more open and loosely interwound, and the electrostatic components are dominant. Over the range of three decades of salt examined, the electrostatic energy changes by a factor of 10. The buckling transition between the circle and figure-8 is highly sensitive to salt concentration: this transition is delayed as salt concentration decreases, with a particularly sharp increase below 0.1 M. For example, for a bending-to-twisting force constant ratio of A/C = 1.5, the linking number difference (delta LK) corresponding to equal energies for the circle and figure-8 increases from 2.1 to 3.25 as salt decreases from 1.0 to 0.005 M. We also present in detail a family of three-lobed supercoiled DNA configurations that are predicted by elasticity theory to be stable at low delta Lk. To our knowledge, such three-dimensional structures have not been previously presented in connection with DNA supercoiling. These branched forms have a higher bending energy than the corresponding interwound configurations at the same delta Lk but, especially at low salt, this bending energy difference is relatively small in comparison with the total energy, which is dominated by the electrostatic contributions. Significantly, the electrostatic energies of the three-lobed and (straight) interwound forms are comparable at each salt environment. We show how the three-lobed configurations change slowly with ALk, resulting in branched interwound forms at higher salt. In longer chains, the branched forms are highly interwound, with bent arms. At low salt, the branched supercoils are asymmetric, with a longer interwound stem and two shorter arms. From molecular dynamics simulations we observe differences in the motions of the DNA as a function of salt. At high salt, the supercoiled chain is quite compact but fairly rigid, whereas at low salt the DNA is loosely coiled but more dynamic. Especially notable at low salt are the large-scale opening and closing of the chain as a whole and the rapid "slithering"of individual residues past one another. Toroidal forms are not detected under these conditions. However, the overall features of the open, loose supercoils found at low salt are more similar to those of toroidal than interwound configurations. Indeed,simulated x-ray scattering profiles reveal the same trends observed experimentally and are consistent with a change from closed to open forms as salt is decreased. Like the minimization studies, the dynamics reveal a critical point near 0.1 M associated with the collapse of loose to tight supercoils. Near this physiological concentration, enhanced flexibility of the DNA is noted. The collective observations suggest a potential regulatory role for salt on supercoiled DNA function, not only for closed circular DNA,but also for linear DNA with small looped regions.