The Interaction of Dipole Modifiers with Polyene-Sterol Complexes

Recently, we showed that the effect of dipole modifiers (flavonoids and styrylpyridinium dyes) on the conductance of single amphotericin B (AmB) channels in sterol-containing lipid bilayers primarily resulted from changes in the membrane dipole potential. The present study examines the effect of dipole modifiers on the AmB multi-channel activity. The addition of phloretin to cholesterol-containing membranes leads to a significant increase in the steady-state AmB-induced transmembrane current. Quercetin significantly decreases and RH 421 increases the current through ergosterol-containing bilayers. Other tested flavonoids and styrylpyridinium dyes do not affect the channel-forming activity of AmB independently on the sterol composition of the bilayers. The effects obtained in these trials may instead be attributed to the direct interaction of dipole modifiers with AmB/sterol complexes and not to the effect of dipole potential changes. The presence of double bonds in the Δ7 and Δ22 positions of sterol molecules, the number of conjugated double bonds and amino sugar residues in polyene molecules, and the conformation and adsorption plane of dipole modifiers are important factors impacting this interaction.     

The alpha helix dipole: screened out?

Aligned alpha helix peptide dipoles sum to a "macroscopic" dipole parallel to the helix axis that has been implicated in protein folding and function. However, in aqueous solution the dipole is counteracted by an electrostatic reaction field generated by the solvent, and the strength of the helix dipole may reduce drastically from its value in vacuum. Here, using atomic-detail helix models and Poisson-Boltzmann continuum electrostatics calculations, the net effective dipole moment, mu(eff), is calculated. Some initially surprising results are found. Whereas in vacuum mu(eff) increases with helix length, the opposite is found to be the case for transmembrane helices. In soluble proteins, mu(eff) is found to vary strongly with the orientation and position of the helix relative to the aqueous medium. A set of rules is established to estimate of the strength of mu(eff) from graphical inspection of protein structures.     

Computation of the dipole moments of proteins.

A simple and computationally feasible procedure for the calculation of net charges and dipole moments of proteins at arbitrary pH and salt conditions is described. The method is intended to provide data that may be compared to the results of transient electric dichroism experiments on protein solutions. The procedure consists of three major steps: (i) calculation of self energies and interaction energies for ionizable groups in the protein by using the finite-difference Poisson-Boltzmann method, (ii) determination of the position of the center of diffusion (to which the calculated dipole moment refers) and the extinction coefficient tensor for the protein, and (iii) generation of the equilibrium distribution of protonation states of the protein by a Monte Carlo procedure, from which mean and root-mean-square dipole moments and optical anisotropies are calculated. The procedure is applied to 12 proteins. It is shown that it gives hydrodynamic and electrical parameters for proteins in good agreement with experimental data.     

Genistein derivatives decrease liposome membrane integrity--calcein release and molecular modeling study

The ability of newly synthesized genistein benzyl and glycosylated derivatives to permeabilize the liposome membrane was studied by calcein-leakage method. All studied derivatives appeared to be more effective than their parent compound--genistein. Comparing the experimental results with theoretical calculations we found that in the case of benzyl derivatives the dipole moment of added benzene ring (with its substitutions) might be important for the strength of flavonoids-lipid interactions. This conclusion may have some implications for QSAR studies in which mostly the dipole moments of entire molecules are considered.     

Revealing instances of coordination among multiple cortical areas

Cognitive functions must involve interactions between several (perhaps many) cortical regions. The instances of such interactions may not be tightly time locked to any external cue. Thus averaging over repeated trials of brain activity or its spectrograms may miss these instances. Here, coordinated activity among multiple cortical locations is revealed in ongoing activity with millisecond accuracy without the need for averaging over time or frequencies. This is based on reconstructions of the cortical current dipole amplitudes at multiple points from MEG recordings. In these current dipole traces, instances of brief activity undulations (BAUs) are automatically detected and used to reveal where and when cortical points interact. The article shows that these BAUs truly represent the reorganization of activity at the cortex and are strongly connected to behavior.     

Theory of initial current density in membrane voltage-clamp experiments

Herein we show that the voltage-clamp current density at zero time calculated from electrodiffusion equations is linear in the clamping voltage for a simple membrane (no charge structure) and for a membrae with fixed charges. Such membranes are nonexcitable. Excitable membranes can be represented by a homogeneous membrane with dipole layers at the surface. In this case the initial current density will be linear in the clamping voltage if a critical field for a dipole layer reorientation is not passed through in changing from holding to clamping potential. Otherwise, deviation from nonlinearity may occur. This is in agreement with experimental data for the squid giant axon.     

Real-Time Localization of Moving Dipole Sources for Tracking Multiple Free-Swimming Weakly Electric Fish

In order to survive, animals must quickly and accurately locate prey, predators, and conspecifics using the signals they generate. The signal source location can be estimated using multiple detectors and the inverse relationship between the received signal intensity (RSI) and the distance, but difficulty of the source localization increases if there is an additional dependence on the orientation of a signal source. In such cases, the signal source could be approximated as an ideal dipole for simplification. Based on a theoretical model, the RSI can be directly predicted from a known dipole location; but estimating a dipole location from RSIs has no direct analytical solution. Here, we propose an efficient solution to the dipole localization problem by using a lookup table (LUT) to store RSIs predicted by our theoretically derived dipole model at many possible dipole positions and orientations. For a given set of RSIs measured at multiple detectors, our algorithm found a dipole location having the closest matching normalized RSIs from the LUT, and further refined the location at higher resolution. Studying the natural behavior of weakly electric fish (WEF) requires efficiently computing their location and the temporal pattern of their electric signals over extended periods. Our dipole localization method was successfully applied to track single or multiple freely swimming WEF in shallow water in real-time, as each fish could be closely approximated by an ideal current dipole in two dimensions. Our optimized search algorithm found the animal’s positions, orientations, and tail-bending angles quickly and accurately under various conditions, without the need for calibrating individual-specific parameters. Our dipole localization method is directly applicable to studying the role of active sensing during spatial navigation, or social interactions between multiple WEF. Furthermore, our method could be extended to other application areas involving dipole source localization.     

Electrostatic potentials of the alpha helix dipole and of elastase

Molecular graphics has been used to display the electrostatic potentials of the α-helix dipole and that of elastase calculated using atomic charges obtained by a new, simple method^1–3. Calculations on the α-helix dipole support the simple dipole model in which the helix is represented by single, half integral charges at the helix termini. The potentials of elastase show some interesting features which may be related to the binding processes.     

Fröhlich electromagnetic field generated by living cells: Computer results

The Fröhlich coherent polar vibrations may be a source of the cell-generated electromagnetic field. The cellular membrane is assumed to be the locus of the dipole vibrations. We adopt a model of the spherical dipole layer with both uniform and domain distributions of the dipole densities and of the dipole moments. Computer results indicate that the electromagnetic field of cellular origin may be significant within a distance of about 1–10 m from the cell surface. The Schottky barrier might be used to determine the frequency and the intensity of the cell-generated field in the frequency range below 1 THz.     

The dipole moments of membrane proteins: potassium channel proteins. II. T1 assembly. Search for the voltage sensor

The dipole moments of potassium channel protein (Kcsa) and beta-subunits were discussed in the previous paper of this series [Takashima, Biophys. Chem. 94 (2001) 209-218]. While the dipole moment of beta-subunits was found to be very large, the dipole moment of Kcsa turned out to be somewhat smaller than beta-subunits. As the continuation of this work, the discussion of the present paper is focussed on the dipole moment of T1 assembly, another component of the K-channel. As discussed later, the calculation using the X-ray crystallographic data by MacKinnon et al., revealed an astoundingly large dipole moment for T1 assembly. The dipole moment of T1 assembly combined with the likewise large dipole moment of beta-subunits amounts to a sufficient value to play an essential role as a voltage sensor of potassium channel.     

A conformational study of N-phenyl-1-naphthylamine and 1-anilino-8-naphthalene sulfonate by the empirical method

Conformational analysis of N-phenyl-1-naphthylamine and 1-anilinonaphthalene-8-sulfonate (ANS) was carried out using the empirical method. Properties such as conformational energies and dipole moments were considered. Furthermore, the effect of solvent medium was examined through the effective dielectic constant. The N-phenyl-1-naphthylamine molecule showed two energy minima which were independent of dielectic constant. The ANS molecule also showed two energy minima but the minima changed positions when the dielectic constant increased from 1.0 (vacuum) to 80.0 (highly polar medium). Hydrogen bonding appeared to play an important role in stabilizing these conformations. The minimum energy conformations may have relevance to the binding of ANS to lipid bilayers and bimembranes. The dipole moment, in contrast to the energy minimum, was found to depend on orientation of the sulfonate group rather than of the benzene ring with respect to the naphthalene ring. Thus binding and fluorescence enhancement of ANS may be attributed to the orientation of the sulfonate group, which to a large extent may determine the magnitude of the dipole moment and the degree of electrostatic interactions between the probe and binding domains. Various dimensions like intra-atomic distances, volume and area of the ANS molecule were calculated.     

The structure and dipole moment of globular proteins in solution and crystalline states: use of NMR and X-ray databases for the numerical calculation of dipole moment

The large dipole moment of globular proteins has been well known because of the detailed studies using dielectric relaxation and electro-optical methods. The search for the origin of these dipolemoments, however, must be based on the detailed knowledge on protein structure with atomic resolutions. At present, we have two sources of information on the structure of protein molecules: (1) x-ray databases obtained in crystalline state; (2) NMR databases obtained in solution state. While x-ray databases consist of only one model, NMR databases, because of the fluctuation of the protein folding in solution, consist of a number of models, thus enabling the computation of dipole moment repeated for all these models. The aim of this work, using these databases, is the detailed investigation on the interdependence between the structure and dipole moment of protein molecules. The dipole moment of protein molecules has roughly two components: one dipole moment is due to surface charges and the other, core dipole moment, is due to polar groups such as N--H and C==O bonds. The computation of surface charge dipole moment consists of two steps: (A) calculation of the pK shifts of charged groups for electrostatic interactions and (B) calculation of the dipole moment using the pK corrected for electrostatic shifts. The dipole moments of several proteins were computed using both NMR and x-ray databases. The dipole moments of these two sets of calculations are, with a few exceptions, in good agreement with one another and also with measured dipole moments.     

Contribution of asymmetric ligand binding to the apparent permanent dipole moment of DNA

Despite its antiparallel symmetry, DNA often appears to possess a permanent electric dipole moment in transient electro-optical experiments. We propose that this may be due to the asymmetric binding of charged ligands to the DNA. We have used the fluctuating dipole theory of Kirkwood and Shumaker to calculate the contribution of asymmetric ligand binding to the electro-optic orientation function, and Monte Carlo computer simulation to calculate the reversing pulse behavior, as a function of ligand binding density. The results indicate that the effect should be observable even against the background of the sizable induced dipole moment produced by polarization of the counterion atmosphere.     

The effects of heart-cavity blood resistivity and volume on endocardial potentials: theoretical predictions

Theoretical concepts regarding the potential variations within the bloodfilled cavities of the heart caused by excitation of the myocardium are scarce. In an attempt to understand these potential variations, calculations were made of the potentials due to a dipole adjacent to a disk of finite resistivity. The dipole was assumed to be in an infinite two-dimensional medium of higher resistivity. Equations for the potential inside a sphere due to an external source are also given.On the basis of the results, predictions were made on the effects of the intracardiac blood on the endocardial potentials: (1) the potential at the center of the cavity is independent of blood resistivity and equivalent to the free space potential; (2) potentials due to a tangential dipole are uniformly reduced; (3) potentials due to a radial dipole are smaller near the dipole, but greater at the far end of the dipole axis; (4) for a dipole radial to or making an angle with the cavity, potentials are reduced at some points but increased at others; (5) the potential along the transverse axis of the dipole has a constant value depending on conductivity; (6) intracardiac blood tends to smooth out potential variations within the cavity; (7) the potential depends both on dipole distance and direction; and (8) if the cavity volume increases, potentials decrease with distance from the dipole; the potential at the adjacent endocardial wall changes little.     

Electric field strength of membrane lipids from vertebrate species: membrane lipid composition and Na+-K+-ATPase molecular activity

Intramembrane electric field strength is a very likely determinant of the activity of ion-transporting membrane proteins in living cells. In the absence of any transmembrane electrical potential or surface potential, its magnitude is determined by the dipole potential of the membrane's lipid components and their associated water of hydration. Here we have used a fluorometric method to quantify the dipole potential of vesicles formed from lipids extracted from kidney and brain of 11 different animal species from four different vertebrate classes. The dipole potential was compared with the fatty acid composition and with the Na(+)-K(+)-ATPase molecular activity of each preparation. The magnitude of the dipole potential was found to be relatively constant across all animal species, i.e., 236-334 mV for vesicles prepared from the total membrane lipids and 223-256 mV for phospholipids alone. The significantly lower value for phospholipids alone is potentially related to the removal of cholesterol and/or other common soluble lipid molecules from the membrane. Surprisingly, no significant dependence of the dipole potential on fatty acid composition was found. This may, however, be due to concomitant compensatory variations in lipid head group composition. The molecular activity of the Na(+)-K(+)-ATPase was found to increase with increasing dipole potential. The fact that the dipole potential is maintained at a relatively constant value over a wide range of animal species suggests that it may play a fundamental role in ensuring correct ion pump conformation and function within the membrane.     

The apparently symmetrical hexagonal bilayer hemoglobin from Lumbricus terrestris has a large dipole moment

The giant approximately 3.6 MDa hexagonal bilayer hemoglobin (HBL Hb) from Lumbricus terrestris consists of 12 213-kDa dodecamers of four globin chains ([b + a + c]3[d]3) tethered to a central scaffold of approximately 36 non-globin, linker subunits L1-L4 (24-32 kDa). Three-dimensional reconstructions obtained by electron cryomicroscopy showed it to have a D6 point-group symmetry, with the two layers rotated approximately 16 degrees relative to each other. Measurement of the dielectric constants of the Hb and the dodecamer over the frequency range 5-100 kHz indicated relaxation frequencies occurring at 20-40 and 300 kHz, respectively, substantially lower than the 700-800 kHz in HbA. The dipole moments calculated using Oncley's equation were 17,300 +/- 2300 D and 1400 D for the Hb and dodecamer, respectively. The approximately threefold higher dipole moment of the dodecamer relative to HbA is consistent with an asymmetric shape in solution suggested by small-angle X-ray scattering. Although a two-term Debye equation and a prolate ellipsoid of revolution model provided a good fit to the experimental dielectric dispersion of the dodecamer, a three-term Debye equation based on an oblate ellipsoid of revolution model was required to fit the asymmetric dielectric dispersion curve of the Hb: the required additional term may represent either an induced dipole moment or a substructure which rotates independently of the main permanent dipole component of the Hb. The D6 point-group symmetry implies that the dipole moments of the dodecamers cancel out. Thus, in addition to a possible contribution from fluctuations of the proton distribution, the large dipole moment of the Hb may be due to an asymmetric distribution of the heterogeneous linker subunits.     

Dipole source analysis of auditory P300 response in depressive and anxiety disorders

This paper is to study auditory event-related potential P300 in patients with anxiety and depressive disorders using dipole source analysis. Auditory P300 using 2-stimulus oddball paradigm was collected from 35 patients with anxiety disorder, 32 patients with depressive disorder, and 30 healthy controls. P300 dipole sources and peak amplitude of dipole activities were analyzed. The source analysis resulted in a 4-dipole configuration, where temporal dipoles displayed greater P300 amplitude than that of frontal dipoles. In addition, a right-greater-than-left hemispheric asymmetry of dipole magnitude was found in patients with anxiety disorder, whereas a left-greater-than-right hemispheric asymmetry of dipole magnitude was observed in depressed patients. Results indicated that the asymmetry was more prominent over the temporal dipole than that of frontal dipoles in patients. Patients with anxiety disorder may increase their efforts to enhance temporal dipole activity to compensate for a deficit in frontal cortex processing, while depressed patients show dominating reduction of right temporal activity. The opposite nature of results observed with hemispheric asymmetry in depressive and anxiety disorders could serve to be valuable information for psychiatric studies.     

Favorable and unfavorable lateral interactions of ceramide, neutral glycosphingolipids and gangliosides in mixed monolayers

Interactions among four natural neutral sphingolipids (ceramide, glucosyl-ceramide, lactosyl-ceramide and asialo-GM1) and six gangliosides (GM3, GM2, GM1, GD3, GD1a and GT1b) were studied in binary Langmuir monolayers at the air-buffer interface in terms of their molecular packing, compressibility, dipole potential and mixing behavior. The changes of surface organization can be grouped into three sets: (a) binary films of neutral GSLs, and of the latter with ceramide, exhibit thermodynamically unfavorable mixing with mean molecular area expansions and dipole moment hyperpolarization; (b) mixed monolayers of ceramide, or of GlcCer, and gangliosides occur with thermodynamically favorable interactions leading to mean molecular area condensation and depolarisation; (c) binary mixtures of LacCer or Gg4Cer with gangliosides, and all ganglioside species among them, revealed molecular immiscibility characterized by additive mean molecular area and dipole potential, with composition-independent constant collapse pressure. These results disclose basic tendencies of GSLs to molecularly mix or demix, leading to their surface segregation, which may underlay vectorial separation of their specific biosynthetic pathways.     

Position-dependent interactions between cysteine residues and the helix dipole

A protein model was developed for studying the interaction between cysteine residues and the helix dipole. Site-directed mutagenesis was used to introduce cysteine residues at the N-terminus of helix H in recombinant sperm whale myoglobin. Based on the difference in thiol pK(a) between folded proteins and an unfolded peptide, the energy of interaction between the thiolate and the helix dipole was determined. Thiolates at the N1 and N2 positions of the helix were stabilized by 0.3 kcal/mole and 0.7 kcal/mole, respectively. A thiolate at the Ncap position was stabilized by 2.8 kcal/mole, and may involve a hydrogen bond. In context with other studies, an experimentally observed helix dipole effect may be defined in terms of two distinct components. A charge-dipole component involves electrostatic interactions with peptide bond dipoles in the first two turns of the helix and affects residues at all positions of the terminus; a hydrogen bond component involves one or more backbone amide groups and is only possible at the capping position due to conformational restraints elsewhere. The nature and magnitude of the helix dipole effect is, therefore, position-dependent. Results from this model system were used to interpret cysteine reactivity in rodent hemoglobins and the thioredoxin family.