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1Author    W. HelfrichRequires cookie*
 Title      
 Abstract    In two previous publications1; 2 we have pro­ posed an elastic theory for simple lipid bilayers which may be viewed as two-dimensional fluids. In particular, we have shown that bilayer spheres in an aqueous medium can be deformed into ellipsoidal bodies if they are submitted to a magnetic field or excess outside pressure. Here we consider a possible deformation by electric fields. It will be seen that the electric effect can be quite strong for large vesicles. As before, we restrict ourselves to small deforma­ tions, assuming the bilayer to be unstretchable and the sphere to become an ellipsoid of revolution. To calculate the ellipticity we minimize the total ener­ gy consisting of curvature-elastic and electric parts. The conductivity of the bilayer will in general be very much smaller than that of the aqueous environ­ ment, so it seems permissible to treat the membrane as a perfect insulator. The electric energy of deformation may be ob­ tained from the Maxwell stresses. Those inside the membrane will be balanced by equal but opposite elastic stresses. The membrane is likely to sustain the latter without undergoing an appreciable defor­ mation as they induce neither curvature nor shear flow in the bilayer. The only unbalanced force is due to the Maxwell stress exerted by the electric field just outside the vesicle, since in the enclosed water the field must be identical to zero. We assume here that the membrane is dielectrically isotropic, which implies that the forces caused by the Maxwell stresses are confined to the interfaces with water. If it is not, electrical torque densities may be ex­ pected within the bilayer. The torques may induce curvature, but this should be negligible as the polarizabilities of the bilayer should be very much smaller than that of water. The standard expression for the electric potential Ue around a sphere in a uniform applied field is Ue= -F eoz + Ae(cos 0/r2). 
  Reference    (Z. Naturforsch. 29c, 182 [1974]; received November 8/December 6 1973) 
  Published    1974 
  Keywords    Lipid Bilayer, Deformation, Electric Field 
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 TEI-XML for    default:Reihe_C/29/ZNC-1974-29c-0182_n.pdf 
 Identifier    ZNC-1974-29c-0182_n 
 Volume    29 
2Author    G. Renger, D. Difiore, B. Luuring, P. GräberRequires cookie*
 Title    The Variation of the Electrochromic Difference Spectrum at Various Stages of the Chloroplast Developm ent+  
 Abstract    The flash-induced difference spectrum in the range of 450 — 550 nm of protochloroplasts isolated from pea-leaves greened under intermittent illumination (2 min light, 98 min dark) was measured and compared with that of fully developed chloroplasts from pea leaves. Because of the sensitivity of the decay o fthe absorption changes to the ionophore valinomycin they were shown to mainly be due to an electrochromic bandshift of the membrane pigments (chlorophylls-a, -b and caro-tenoids). The differences in the shape and the amplitude between both spectra are consistently explained within the framework of a recent hypothesis supposed by Sewe and Reich (Z. Natur-forsch. 33 c, 161 — 171 (1978)) by the lack of chlorophyll-b in the protochloroplasts. It is con­ cluded, that the transformation of the protochloroplasts into chloroplasts which is accompanied by the incorporation of the light harvesting complex and the formation of grana stacks does not seriously change the orientation of the field indicating pigments within the membrane with respect of the polarity of the light induced vectorial electron transport. 
  Reference    Z. Naturforsch. 34c, 120 (1979); received October 23 1978 
  Published    1979 
  Keywords    Chloroplast Development, Electric Field, Electrochromism, Pigment Orientation 
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 TEI-XML for    default:Reihe_C/34/ZNC-1979-34c-0120.pdf 
 Identifier    ZNC-1979-34c-0120 
 Volume    34 
3Author    Julia Yanevaa, Jordanka Zlatanova30Requires cookie*
 Title    The Electrophoretic Separation of Curved Cisplatin-Modifled DNA Fragments on Polyacrylamide Gels Is Dependent on the Voltage Gradient  
 Abstract    Polyacrylamide gel electrophoresis has been widely used to study DNA fragments containing sequence-de-pendent curvature. The anomalous electrophoretic be­ havior of curved DNA fragments on such gels allows their separation from straight fragments of the same length. H ere we dem onstrate that polyacrylamide gels can be successfully used to resolve DNA fragments m od­ ified at a single site by the antitum or drug ds-diam-m inedichloroplatinum (II) (ds-DDP, cisplatin) from their unmodified counterparts. However, the resolution strongly depends on the voltage gradient, being com­ pletely lost when it drops below a certain threshold level. The param eters of the electric field do not affect separation of 'norm al' DNA fragments of comparable length. 
  Reference    Z. Naturforsch. 53c, 921—923 (1998); received April 27/May 12 1998 
  Published    1998 
  Keywords    Cisplatin, Curved DNA, Electric Field, Electrophoresis, Polyacrylamide Gels 
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 TEI-XML for    default:Reihe_C/53/ZNC-1998-53c-0921_n.pdf 
 Identifier    ZNC-1998-53c-0921_n 
 Volume    53 
4Author    Stephan Bamberger, Günter Valet, Friedrich Storch, Gerhard Ruhenstroth-BauerRequires cookie*
 Title    Electromagnetically Induced Fluid Streaming as a Possible Mechanism of the Biomagnetic Orien­ tation of Organisms  
 Abstract    The results show that both the direction and the intensity of the geomagnetic field can be sensed within the chamber of a Zeiss Cytopherometer. This suggests that electro­ magnetically induced fluid streaming might play a role in the perception of the geomagnetic field by organisms, although it is not clear at present in which organ the per­ ception occurs. A basic requirement for such an organ would be an anisotropy of cells or cellular structures. The nervous system with its parallel axons, or specific cells associated with the nervous system, could thus be possible locations of the sensitivity towards magnetic fields. 
  Reference    Z. Naturforsch. 33c, 159—160 (1978); received November 24 1977 
  Published    1978 
  Keywords    Electric Field, Magnetic Earth Field, Suspended Particle, Magnetic, Sensitivity, Organism 
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 TEI-XML for    default:Reihe_C/33/ZNC-1978-33c-0159_n.pdf 
 Identifier    ZNC-1978-33c-0159_n 
 Volume    33 
5Author    U. Zimmermann, J. Vienken, G. PilwatRequires cookie*
 Title    Rotation of Cells in an Alternating Electric Field: the Occurrence of a Resonance Frequency  
 Abstract    Cells suspended in a low-conducting medium were exposed to an alternating electric field whose frequency was altered between 1 kHz and 2 MHz. A resonance frequency was observed at which all suspended cells rotated about an axis normal to the field lines (when the electric field strength was larger than a threshold value of about 400 V/cm). This resonance frequency varied from species to species o f cells (mesophyll protoplasts of Avena sativa = 2 0 -4 0 kHz, human erythrocytes and ghost cells = 8 0 -1 0 0 kHz, yeast cells = 140-180 kHz, Friend cells = 3 0 -4 0 kHz, at room temperature). The resonance frequency o f cell rotation was observed only under specific experimental conditions which excluded interference by reversible electrical breakdown of cell membranes and by gravitational forces. Glutardialdehyde fixed and heated cells exhibited no rotation in the frequency and field range investigated. The phenomenon o f rotation is discussed in terms 
  Reference    Z. Naturforsch. 36c, 173—177 (1981); received October 22 1980 
  Published    1981 
  Keywords    Electric Field, Cell Rotation, Dipole, Cell Membrane, Resonance Frequency 
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 TEI-XML for    default:Reihe_C/36/ZNC-1981-36c-0173.pdf 
 Identifier    ZNC-1981-36c-0173 
 Volume    36