Single Positive Charge Research Articles

Human Interleukin-4 and Variant R88Q: Phasing X-ray Diffraction Data by Molecular Replacement Using X-ray and Nuclear Magnetic Resonance Models

The structure of recombinant human interleukin-4 (hIL-4) as been determined by both NMR and X-ray diffraction methods in several laboratories, including ours. The X-ray and NMR structures were successfully applied for solving the X-ray crystal structure by molecular replacement. Due to the small size of the hIL-4 molecule (129 residues) and its lack of structural diversity (4-helix bundle), this task was especially difficult and required special care with rotation function applications. The crucial point was that proper removal of the Patterson origin peaks was indispensable in all cases.All available structures of hIL-4 were checked, in a standardized procedure, for their suitability as templates for molecular replacement. The models derived from the various structures are close to, but not in all loop details identical with, the genuine X-ray structures. The deviations of the X-ray structure-derived models are of the same magnitude as the differences between the original X-ray structures, while the deviations of the NMR structure-derived models are two to three times as large.The hIL-4 variant R88Q is a binding mutant, its affinity to the receptor is decreased by a factor of about 200. Its X-ray structure was determined by molecular replacement using the wild-type X-ray structure determined in our laboratory as a model.The structure of R88Q is virtually identical with that of the wild-type protein. All differences besides the shortened side-chain of residue 88 occur at surface residues with high temperature factors, i.e. at spots where the structure is not well defined. Since the structure is not perturbed, the biological effect of decreased receptor affinity has to be attributed to the loss of a single positive charge in the surface area of the main receptor contact.

Na+/amino acid coupling stoichiometry of rheogenic system B0,+ transport in Xenopus oocytes is variable.

Using electrophysiological and radiotracer studies in parallel, we have investigated the characteristics of the endogenous Na(+)-dependent amino acid transporter (system B0,+) in Xenopus oocytes with regard to ion dependence, voltage dependence and transport stoichiometry. In voltage-clamped oocytes (-60 mV) superfusion with saturating concentrations of amino acids (1 mM) in 100 mM NaCl resulted in reversible, inward currents (mean +/- SEM): alanine, 1.83 +/- 0.09 nA (n = 21); arginine, 2.54 +/- 0.18 nA (n = 17); glutamine, 1.73 +/- 0.10 nA (n = 19). Only arginine evoked a current in choline medium (0.50 +/- 0.13 nA, n = 10), whereas Cl- replacement had no effect on evoked currents. The glutamine-evoked current was saturable (Imax = 1.73 nA, glutamine Km = 0.12 mM) and linearly dependent upon voltage between -90 and -30 mV. Using direct and indirect (activation) methods, we found that transport can proceed with Na+/amino acid coupling stoichiometry of either 1:1 or 2:1, but coupling was the same for each amino acid tested (alanine, arginine and glutamine) within a batch of oocytes (i.e. from a single toad). Despite the net single positive charge on arginine, the magnitude of the net transmembrane charge movement during Na(+)-coupled arginine transport was identical to that for the zwitterionic neutral amino acids glutamine and alanine; this may be explained by a concomitant stimulation of K+ efflux during arginine transport with a putative coupling of 1 K+:1 arginine.

Isolation of carboxyl-termini and blocked amino-termini of viral proteins by high-performance cation-exchange chromatography

The strong cation-exchanger, PolySulfoethyl Aspartamide, has been assessed as a medium for isolation of carboxyl-terminal and blocked amino-terminal peptides from tryptic digests of small quantities of viral proteins. Peptides with a single positive charge, the blocked amino-terminal peptides of ovalbumin and the Newcastle disease virus (NDV) matrix protein and carboxyl-terminal peptides of ovalbumin and the NDV nucleocapsid protein, eluted in early ion-exchange fractions and were readily isolated in homogeneous form by subsequent reversed-phase HPLC. Some early ion-exchange fractions also contained singly charged peptides derived by “chymotryptic-like” cleavage, whilst other peptides eluted in these fractions due to their highly acidic character. Terminal sequences with additional basic residues were isolated from later eluting ion-exchange fractions. Peptides with this property included the blocked amino-terminus of the NDV nucleocapsid protein and a portion of the carboxyl-terminus of the NDV matrix protein. Hitherto undescribed polymorphism in the amino-terminal region of ovalbumin was revealed in this study. Truncated peptides from the carboxyl-terminus of the NDV matrix protein were also detected. The presence of these peptides could be a reflection of carboxyl-terminal processing of the matrix protein. The strategy described herein should be of general utility for selective microisolation of carboxyl-terminal peptides and blocked amino-terminal peptides from tryptic digests of proteins.

Microprobe analysis of technetium-mibi accumulation in cultured heart cell mitochondria: Correlation of structure and membrane potential

Hexakis (methoxyisobutylisonitrile)99mTechnetium (Tc-MTBI) is a gamma emitting radiopharmaceutical currently in clinical use for evaluating myocardial perfusion. Because Tc-MIBI exists as a lipophilic cation with a single positive charge, it has been shown to concentrate in mitochondria in response to membrane potential in a Nernstian fashion similar to rhodamine 123. Use of electron probe x-ray microanalysis (EPXMA) for localization and quantitation of the element technetium in mitochondria and calculation of membrane potential allows correlation for the first time between mitochondrial structure and membrane potential in intact cells.Heart cells from 11-day-old chick embryos were cultured into spherical aggregates 50 -100 μm in diameter after Ebihara et al. Aggregates were then incubated for 60 minutes in HEPES balanced buffer salt solution containing either 0, 36,72, or 144 μMTc99-MIBI as well as 20 mCi metastable Tc99m-MIBI. Some aggregates were prepared for conventional fixation and subsequent electron microscopic evaluation, while other aggregates were plunge frozen in liquid propane and cryosectioned for quantitative EPXMA.

Ion gradient-induced membrane translocation of model peptides

The K+ diffusion potential-induced association of synthetic model peptides carrying a single positive charge originating from the NH2-terminal amino function with large unilamellar vesicles (LUV) consisting of phosphatidylcholine (PC) has been reported previously (de Kroon, A. I. P. M., J. de Gier, and B. de Kruijff. 1989. Biochim. Biophys. Acta. 981:371-373). To determine the vesicle localization of the associated peptides, fluorescence measurements utilizing the peptides' tryptophan residue as intrinsic fluorescent probe were performed. The application in these measurements, of vesicles that exhibit an asymmetric transbilayer distribution of brominated PC which is a quencher of tryptophan fluorescence, unequivocally demonstrated that the peptide H3N(+)-AIMLWA-Ome (AIXme+) is accumulated in the interface of the inner leaflet of the vesicle membrane in response to the valinomycin-induced K+ diffusion potential (negative inside). The relative contributions of the membrane potential (delta psi) and the pH gradient (delta pH, acidic inside) induced by the K+ diffusion potential, to the process have been assessed. An analysis of the pH and delta pH dependencies of the process demonstrated that the K+ diffusion potential-induced peptide accumulation is largely determined by a redistribution of peptide according to the transbilayer pH gradient, in agreement with a translocation across the vesicle membrane of the neutral, deprotonated form of the peptide. The general validity of the mechanism proposed for the vesicle-uptake of AIXme+ has been examined by extending the experiments to peptide analogues with a single negative charge and to peptides with two positive charges, and by investigating the effect of incorporating the acidic phospholipid cardiolipin (CL) into the LUV. The incorporation of CL appeared not to affect the K+ diffusion, potential-induced vesicle uptake of AIXme+. The peptide H3N(+)-RMLWA-Ome (RXme2+) showed a small delta pH independent fluorescence response to the delta psi upon raising the CL content of the vesicles to 25%.

Optical measurements of Na-Ca-K exchange currents in intact outer segments isolated from bovine retinal rods.

The properties of Na-Ca-K exchange current through the plasma membrane of intact rod outer segments (ROS) isolated from bovine retinas were studied with the optical probe neutral red. Small cellular organelles such as bovine ROS do not offer an adequate collecting area to measure Na-Ca-K exchange currents with electrophysiological techniques. This study demonstrates that Na-Ca-K exchange current in bovine ROS can be measured with the dye neutral red and dual-wavelength spectrophotometry. The binding of neutral red is sensitive to transport of cations across the plasma membrane of ROS by the effect of the translocated cations on the surface potential of the intracellular disk membranes (1985. J. Membr. Biol. 88: 249-262). Electrogenic Na+ fluxes through the ROS plasma membrane were measured with a resolution of 10(5) Na+ ions/ROS per s, equivalent to a current of approximately 0.01 pA; maximal electrogenic Na-Ca-K exchange flux in bovine ROS was equivalent to a maximal exchange current of 1-2 pA. Electrogenic Na+ fluxes were identified as Na-Ca-K exchange current based on a comparison between electrogenic Na+ flux and Na(+)-stimulated Ca2+ release with respect to flux rate, Na+ dependence, and ion selectivity. Neutral red monitored the net entry of a single positive charge carried by Na+ for each Ca2+ ion released (i.e., monitored the Na-Ca-K exchange current). Na-Ca-K exchange in the plasma membrane of bovine ROS had the following properties: (a) Inward Na-Ca-K exchange current required internal Ca2+ (half-maximal stimulation at a free Ca2+ concentration of 0.9 microM), whereas outward Na-Ca-K exchange current required both external Ca2+ (half-maximal stimulation at a free Ca2+ concentration of 1.1 microM) and external K+. (b) Inward Na-Ca-K exchange current depended in a sigmoidal manner on the external Na+ concentration, identical to Na(+)-stimulated Ca2+ release measured with Ca(2+)-indicating dyes. (c) The neutral red method was modified to measure Ca(2+)-activated K+ fluxes (half-maximal stimulation at 2.7 microM free Ca2+) via the Na-Ca-K exchanger in support of the notion that the rod Na-Ca exchanger is in effect a Na-Ca-K exchanger. (d) Competitive interactions between Ca2+ and Na+ ions on the exchanger protein are described.

A molecular dynamics study of radiation induced electrostatic disruption of insulator surfaces

We apply a molecular dynamics simulation to study radiation induced electrostatic disruption of insulator surfaces. With a scaled Lennard-Jones potential, we construct a model for 600 atoms in a 2D surface surrounded by the bulk of the solid. Electrostatic disruption is initiated by placing single positive charges on two to five neutral atoms in the surface region. We identify three stages of the disruption process. First, rapid ejection of several atoms (5 to 15) occurs at ~ 0.5–2.0 ps, compared to the Debye period 0.1 ps. Next, after 1 to 5 ps many atoms in the vicinity of the charged region are greatly disturbed. Crater-like defects and voids are generated by the Coulomb explosion, and atoms are piled up at the surface. Third, the temperature of the region near the explosion becomes higher than the background temperature and enhances sublimation and annealing of atoms in this region even at much longer times. It is shown that fluxes and density of ionization for high-energy protons in the space environment are such that electrostatic disruption may produce considerable damage of exposed surfaces of good insulators.

The role of charge and hydrophobicity in peptide-lipid interaction: a comparative study based on tryptophan fluorescence measurements combined with the use of aqueous and hydrophobic quenchers

The interaction of interrelated model peptides with model membranes has been studied by techniques based on tryptophan fluorescence. The peptides used are derivatives of the sequence H-Ala-Met-Leu-Trp-Ala-OH, which was designed for this purpose. Several modifications yielded a set of 13 penta- and hexapeptides varying in net charge, hydrophobicity, charge distribution, and the intramolecular position of the tryptophan residue with respect to the charge(s). The affinity of these peptides for small unilamellar vesicles (SUV) consisting of zwitterionic egg phosphatidylcholine (eggPC) and negatively charged beef heart cardiolipin (bhCL) has been investigated in a comparative way. The criteria for affinity comprise (1) intrinsic fluorescence changes upon titration of the peptides with the lipid vesicles, (2) reduced accessibility of the peptides to aqueous quenchers of tryptophan fluorescence (I- and acrylamide) in the presence of lipid, and (3) exposure to membrane-incorporated fluorescence quenchers, brominated phosphatidylcholines (BrPC). Application of BrPC brominated at different positions along the acyl chains provided information on the membrane topology of the peptides. With respect to the extent of affinity for zwitterionic membranes, the overall hydrophobicity of the peptides is the main determinant. A comparison of the affinity for PC of equally hydrophobic peptides carrying either a single positive or negative charge reveals preferential interaction of the cationic peptide. Both hydrophobic and electrostatic interactions determine the affinity of positively charged mono- and divalent peptides for CL vesicles. The distribution of the charged moieties in divalent positively charged peptides, either both at one end of the molecule or one at each end, has little influence on the affinity of these peptides for CL but does affect the extent of exposure to BrPC. Upon decreasing the surface charge density of the vesicles by diluting CL with increasing amounts of PC, both types of peptides show different behavior. The position of the tryptophan relative to the charged moiety in the peptide molecule is shown to affect the fluorescent properties upon interaction with vesicles. Concerning the membrane topology, all peptides adopt a localization near the membrane surface, with the neutral peptides inserting slightly deeper into the bilayer than the charged peptides. The results allow a comparative analysis of the factors determining the extents and modes of lipid-model peptide interaction; in addition, the validity of the methods applied is discussed.

Misplacement of the amino-terminal positive charge in the prepro-α-factor signal peptide disrupts membrane translocation in vivo

Abstract We have identified a series of mutations in the signal peptide of yeast prepro-alpha-factor which specifically attenuate translocation across the endoplasmic reticulum membrane in vivo. In prepro-alpha-factor-somatostatin hybrids, transposition of the amino-terminal tripeptide from wild-type NH2-Met-Arg-Phe to NH2-Met-Phe-Lys or NH2-Met-Phe-Arg causes a 45-75% reduction in the efficiency of membrane translocation. This is evidenced by the intracellular accumulation of unglycosylated, signal-containing precursors which are membrane-associated and are exposed to the cytosol. Surprisingly, abolition of the single positive charge by replacing arginine with phenylalanine has little effect on translocation into the endoplasmic reticulum. We conclude that the presence of an amino-terminal positive charge is not necessary for efficient targeting or translocation; however, misplacement by one position markedly disrupts translocation without affecting targeting. These mutations thus define an early stage of membrane interaction that is sensitive to local charge effects. Furthermore, our data suggest that post-translational translocation, signal cleavage, and core glycosylation of these polypeptides may occur to a significant extent in vivo.

Experimental identification of the energy level of substitutional manganese in silicon

A combination of deep level transient spectroscopy (DLTS) and electron spin resonance (ESR) measurements was used to determine the energy level of substitutional manganese in silicon. Samples of p-type silicon were subjected to a copper-manganese codiffusion. Successfully prepared samples show the typical ESR signal of substitutional manganese with a single positive charge. The Hamiltonian parameters g=2.029 and A=−62.7×10−4 cm−1 are different from those for negatively charged interstitial manganese. The DLTS measurements reveal an energy level of M1=0.39 eV above the valence-band edge for the substitutional manganese. Because of the codiffusion of Cu also the previously reported levels C1=0.098 eV, C2=0.22 eV, and C3=0.41 eV were found. The combination of ESR and DLTS results allowed a conclusive identification of the defect level M1 and provided no evidence for ordinary amphoteric or negative U behavior in the lower half of the band gap. Furthermore, isothermal and isochronal annealing experiments were performed which support the conclusion that the defect level M1 orginates from substitutional manganese in p-type silicon.

Voltage dependence of Na translocation by the Na/K pump

During each complete reaction cycle, the Na/K pump transports three Na ions out across the cell membrane and two K ions in. The resulting net extrusion of positive charge generates outward membrane current but, until now, it was unclear how that net charge movement occurs. Reasonable possibilities included a single positive charge moving outwards during Na translocation; or a single negative charge moving inwards during K translocation; or either positive or negative charges moving during both translocation steps, but in unequal quantities. Any step that involves net charge movement through the membrane must have voltage-dependent transition rates. Here we report measurements of transient, voltage-dependent, displacement currents generated by the pump when its normal Na/K transport cycle has been interrupted by removal of external K and it is thus constrained to carry out Na/Na exchange. The quantity and voltage sensitivity of the charge moved during these transient currents suggests that Na translocation includes a voltage-dependent transition involving movement of one positive charge across the membrane. This single step can thus fully account for the electrogenic nature of Na/K exchange. The result provides important new insight into the molecular mechanism of active cation transport.

The role of charge microheterogeneity of basic protein in the formation and maintenance of the multilayered structure of myelin: A possible role in multiple sclerosis

Isolation of several of the charge isomers from both normal and multiple sclerosis (MS) myelin basic protein (MBP) was achieved on CM-52 columns at pH 10.6. In liquid x-ray diffraction and aggregation experiments, corresponding charge isomers were equally effective in the formation of multilayers, demonstrating the dominant role of overall net positive charge. These studies demonstrated that the change in overall charge of MBP of one net positive charge was sufficient to produce large changes in aggregation and in multilayer formation. The x-ray diffraction experiments showed that component 1 was twice as effective as component 2 although they differed in charge by a single positive charge. Component 3 was less effective than component 2 and component "8" was not effective at all. Vesicle aggregation also showed a dependence on net positive charge. In order of decreasing effectiveness, component 1 greater than component 2 greater than component 3 greater than component "8". Since overall charge on MBP is determined by contributions from the various charge isomers, the relative proportions of these charge isomers favoring the less cationic components could explain the observation that MBP from MS victims was less effective than MBP from normal brain in vesicle aggregation and multilayer formation. The isolation of myelin-containing white matter fractions from both normal and MS tissue in which the loss of some of the most cationic charge isomers was correlated with presence of less compact myelin supports this hypothesis.

The syntheses, structures, and electronic and resonance-Raman spectroscopy of halogen-bridged platinum(II,IV) chain complexes involving equatorially co-ordinated terdentate amines: X- ray crystal structure of [Pt II (Me-dien)I][Pt IV (Me-dien)I3]I2, and spectroscopy of [Pt II (dien)(NO2)][Pt IV (dien)(NO2)Br2]Br2

Three chain platinum(II,IV) complexes of the type [PtL3X][PtL3XY2]Z2(L3= terdentate ligand, X = halogen or NO2–, Y = halogen, Z = monoanion) have been synthesised. Two of these have been studied in detail, one by X-ray and the other by spectroscopic methods. The first, [Pt(Me-dien)I]-[Pt(Me-dien)I3]I2(Me-dien =N-methyldiethylenetriamine), has been found to crystallize in the monoclinic space group P21/n, with a= 19.830(8), b= 12.220(5), c= 11.079(6)A, β= 96.49(4)°, and Z= 4. Refinement on 1 543 independent reflections led to a final R value of 0.043. The Pt–I chain distances are 2.679(3) and 2.691(3)A(PtIV) and 3.443(3) and 3.560(3)A(PtII) the chain deviating substantially at iodine and platinum(II) from linearity [PtIV–I–PtII= 150.5(1) and 149.1(1)°; I–PtII-I = 162.8(1)°], but not at platinum(IV)[I–PtIV–I = 179.4(1)°]. The second complex studied in detail, [Pt(dien)(NO2)][Pt(dien)(NO2) Br2] Br2(dien = diethylenetriamine), was found to yield a very intense resonance-Raman spectrum, the principal observed progression being v1ν1, where ν1(181.5 cm–1) is the symmetric stretching mode of the chain, νsym(Br–PtIV–Br). Analysis of these data yields ω1 and x11 values of 181.9 and –0.2 cm–1, respectively. Further, much weaker, progressions of the sorts v1ν1+νsym(NO2)(to v1= 5) and v1ν1+δsym(NO2)(to v1= 4) were also detected. The results add substantially to the few previously known data on halogenbridged mixed-valence chain complexes of platinum in which each unit carries a single positive charge.

A two sodium ion/D-glucose symport mechanism: membrane potential effects on phlorizin binding.

Apical membrane vesicles isolated from a continuous renal cell line, LLC-PK1, catalyze electrogenic Na+-stimulated hexose transport and Na+-dependent binding of 3H-labeled 1-[2-(beta-D-glucopyranosyloxy)-4, 6-dihydroxyphenyl]-3-(4-hydroxyphenyl)-1-propanone [( 3H]phlorizin), a competitive ligand of this transport system. Phlorizin was not itself transported across the membrane and thus can serve as a probe of the binding step. The stoichiometry of Na+-dependent phlorizin binding in vesicles was 1:1, whereas Na+/hexose cotransport in vesicles exhibited a 2:1 stoichiometry. Na+ increased the affinity of phlorizin binding without affecting the total number of binding sites. An increased number of Na+-dependent phlorizin binding sites was observed under conditions of interior-negative membrane potential. These results are consistent with a model of the Na+/glucose cotransport cycle in which the unloaded transporter is negatively charged and its orientation influenced by membrane potential. Glucose and one sodium ion interact with the transporter, resulting in an uncharged complex. Binding of a second sodium ion triggers translocation of glucose and both sodium ions via formation of a loaded carrier complex bearing a single positive charge.

Structural Investigations on a Peptide Regulating Hemopoiesis in vitro and in vivo

Clonal growth in agar of myeloid committed stem cells (CFU-c) is inhibited by a specifically acting "hemoregulatory peptide" which had been isolated from human leukocytes. The inhibitory peptide is composed of aspartic acid, glutamic acid, lysine, cysteine and possibly glycine. It was [3H]carboxymethylated with [3H]iodoacetic acid. From the electrophoretic behaviour of this derivative at pH 1.9 a single positive charge and a molecular mass of about 600-650 Da could be inferred. This positive charge is lost after acetylation and is identified as the epsilon-amino group in the side chain of a lysine residue since the N-terminal group is a pyroglutamic residue. Electrophoresis of the native peptide at pH 6.5 reveals the presence of three carboxyl groups, two of which were localized in the N-terminal part of the molecule, while cysteine and lysine were located in the C-terminal portion. Lysine was identified as the C-terminal residue by a double labeling technique applied to the Cys-[(3H]Cm)-containing fission peptide after partial hydrolysis. Uncertainties in the molecular mass determinations allow for the possible presence of a small residue without reactive side chain (glycine?) in the C-terminal portion of the molecule. The structure of the inhibitory peptide was thus determined as less than Glu-(Asp/Glu)-(Asp/Glu)-Cys-(Gly?)-Lys. This structure is supported by comparison with synthetic peptides. The analogue less than Glu-Glu-Asp-Cys-Lys has properties which are similar to those of the natural inhibitor.

The Determination of Surface Potential Fluctuation Values and Spatial Distribution of Traps in Insulators from the Temperature Dependence of the (Gp/ω)–ω Curves of MOS Structures

The (Gp/ω)–ω curves at 163 and 295 K for MOS-structures on silicon with the different SiO2 oxide thickness and different surface charge values are measured. A technique is suggested for determining the fluctuation value of the surface potential and the distance of traps in the bulk oxide from the interfac? SiSiO2 from the above measurements. This distance turns out to be in the range of 1.5 to 3 A for different samples and contributes greatly to the broadening of the (Gp/ω) vs.ω curves for all the samples investigated. The fluctuation values of surface potential are 13 to 80 mV for different samples. A comparison of potential fluctuation values with the predicted ones obtained in assuming a random position of single positive charges shows a good agreement at the limiting parameter λ = 30 A. [Russian Text Ignored]

Localization of a trifluoperazine binding site on troponin C.

Trifluoperazine (TFP) was shown to interact with the cyanogen bromide fragment 9 (CB9) (residues 84-135) of rabbit skeletal troponin C and with a synthetic peptide representing the N-terminal region of CB9. The phenothiazine did not affect the calcium binding property of CB9 as observed by proton magnetic resonance and circular dichroism spectroscopies. The calculated calcium binding constants for CB9 in the presence and absence of trifluoperazine were identical (KCa2+ = 1.3 X 10(5) M-1). Localization of the trifluoperazine binding site was achieved by analyzing the 1H NMR spectrum of CB9 and of a synthetic fragment corresponding to residues 90-104 of CB9. Drug-induced shifting and broadening of the ring protons of phenylalanine residues and the methyl resonances of alanine, leucine, and isoleucine residues suggest that the segment 95-102 is in close proximity to the phenothiazine aromatic region. The neighboring negative side chains in the peptide sequence also suggest that the single positive charge present on the piperazine nitrogens of trifluoperazine may interact with them and sterically block a region of interaction of calmodulin (CaM) and troponin C (TnC) with modulated proteins such as phosphodiesterase. Primary sequence analysis of CaM and troponin C reveals that a homologous hydrophobic region to site 3 is also found in the N-terminal region of site 1 of both calcium binding proteins. Binding of TFP to CB9 occurs both in the presence and absence of calcium since the hydrophobic region in these small fragments is completely accessible to TFP whether calcium is present or not. The dissociation constant of the drug to apoCB9 (8 microM) was obtained by ellipticity measurements at 222 nm and was comparable to the 5 microM value obtained by Levin and Weiss [Levin, R. M., & Weiss, B. (1978) Biochim. Biophys. Acta 540, 197-204] for calcium-saturated rabbit skeletal troponin C.

Equilibrium properties of molecular fluids with charge distributions of quadrupolar symmetry

Molecular dynamics simulations are reported for dense fluids of molecules with discrete charge distributions of quadrupolar symmetry. The nonelectrostatic parts of the intermolecular potentials were modeled by Lennard-Jones site–site interactions with the two sites in each molecule separated by 0.547σ, where σ is the distance scaling parameter in the Lennard-Jones function. The charge distributions were comprised of a double negative charge at the molecular center of symmetry plus two single positive charges placed on the symmetry axis at equal distances from the center. The magnitudes and separation distances of the charges were varied so that changes in the thermodynamic, quasithermodynamic (mean square force and torque), and structural properties of the fluid could be determined at a fixed quadrupole moment. At finite charge separation distances, the multipole expansion yields ideal quadrupole–quadrupole plus higher order interactions. It is shown that the observed changes in thermodynamic properties can be accurately calculated by including the next highest multipole interaction terms in the series; namely, the ideal quadrupole–hexadecapole energies (plus a small contribution from the hexadecapole–hexadecapole terms at the largest charge separations). A low order perturbation theory calculation of the changes in structural properties is also presented; it is found not to give satisfactory values for the observed changes in radial distribution function with charge separation distance.

Loop currents in chlorophyll a

Loop currents involve the circulation of electrons (or holes) around closed conjugated-bond loops. From the length of the orbital perimeter the kinetic energy per electron can be calculated. In chlorophyll a, four electronic excitations have been analyzed, including: (1) a single positive charge (hole) circling a 19-bond orbit with a Mobius twist (this is proposed as the effective ground state); (2) nine electrons, unevenly spaced, circling the same 19-bond orbit but without the Mobius twist (proposed as the red excited state); (3) nine electrons in the same orbit but evenly spaced and vibrationless (the hopping exciton); (4) ten electrons, unevenly spaced, in a 16-bond orbit (the blue excited state). The fit of theory to experiment is examined.

Active transport of 5-hydroxytryptamine by plasma membrane vesicles isolated from human blood platelets.

Plasma membrane vesicles isolated from human platelets accumulate 5-hydroxytryptamine when an electrical potential (interior negative) or an Na+ gradient (out greater than in) is imposed across the vesicle membrane. Kinetic studies reveal a Km of 0.5 micronM for the transport process. Uptake is inhibited strongly by tricyclic antidepressants and by ionophores such as gramicidin which catalyze transmembrane exchange of Na+ for K+. Transport is absolutely dependent upon external Na+ and Cl- and is only mildly, if at all, inhibited by reserpine, cinanserin, ouabain, or arsenate. Experiments are presented which suggest that a single positive charge crosses the vesicle membrane with each molecule of 5-hydroxytryptamine. The results provide direct evidence for Na+-coupled active 5-hydroxytryptamine transport by the platelet plasma membrane.