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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Macromolecules 23 (1990), S. 3472-3480 
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Macromolecules 28 (1995), S. 736-744 
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    European biophysics journal 18 (1990), S. 245-253 
    ISSN: 1432-1017
    Keywords: Lipid-protein bilayers ; Permeability ; Computer simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract We have modelled the effect of integral proteins upon the permeability of a lipid bilayer membrane to small ions. Our intention is to predict the temperature-and protein concentration-dependence of the relative ionic diffusion coefficient if it is assumed that the ions diffuse entirely through the lipid regions of the membrane, and do not cross the bilayer either by moving through the protein or along its hydrophobic surface. We used models of lipid-protein bilayers that have been used successfully before, together with a modification of the (protein-free) permeability mechanism of Kanchisa, Tsong, Cruzeiro-Hansson and Mouritsen (KTCM). We used Monte Carlo techniques to simulate the thermodynamics and permeability of the membrane because such a calculation, which depends upon spatial and time correlations, cannot be done using a mean field approximation. We took the protein to be analogous to bacteriorhodopsin and calculated the relative diffusion coefficients, R, of small ions through a DMPC bilayer containing such proteins. We found that although there is only a small change in R when T 〉 T m , R can change by an order of magnitude, depending upon the protein concentration, for T 〈 T m . We showed that regions of largest lipid area fluctuations and, hence, compressibility occur (i) at the interfaces between regions (which may be very small and not macroscopic phases) of extended-chain and excited-chain lipids, thus showing the similarity between this model and the KTCM model, and (ii) in regions where the temperature is close to T m but the lipids are prevented from becoming ordered because of the proximity of proteins which try to keep them in their excited-chain states. We have plotted lipid fluctuation and ionic permeability maps to show which regions of the membrane display the largest permeability, and we show how these regions change as a function of temperature.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1432-1017
    Keywords: Bilayers ; monolayers ; theoretical models ; thermodynamics ; internal pressure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract We have modelled a phospholipid bilayer as two monolayer sheets which interact with each other by a coupling which depends upon the states of the lipid hydrocarbon chains in each sheet. We make use of a model (Georgallas and Pink 1982a) and its parameters, already used to study monolayer phase changes at the LC-LE transition, in order to study the lipid main transition. Although the monolayer coexistence curve can be calculated exactly, we have made use of high-temperature series expansions to calculate the critical point of the bilayer. We also present the results of computer simulations on triangular lattices for the pressure-area isotherms. We find: (i) the interaction between the sheets of a DPPC bilayer is about 1.5–2% of the maximum interaction within the plane of each sheet; (ii) the internal lateral pressure of a DPPC bilayer is about 30.5 dyne/cm; (iii) the bilayer transition enthalpy depends sensitively upon the coupling between the sheets. Should this coupling vary from sample to sample (due, possibly, to its preparation) then very different values of transition enthalpy may be measured. (iv) We present a rough rule-of-thumb for estimating the internal lateral pressure of a bilayer from a knowledge of the corresponding monolayer pressure-area isotherms.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    European biophysics journal 12 (1985), S. 143-151 
    ISSN: 1432-1017
    Keywords: Lipids ; proteins ; electron spin resonance ; computer simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract We analyse recent ESR measurements on Ca2+ ATPase and Myelin proteolipid apoprotein reconstituted in phosphatidylcholine bilayer membranes. Our intention is to discover whether the measurements indicate significant protein-protein repulsive or attractive interactions. In order to do so we have studied a model of a lipid bilayer membrane containing transbilayer proteins. It represents the proteins by hexagons moving on a triangular lattice interacting via an energy E 0 which can be attractive, repulsive or zero. The last-named represents the “random” case studied earlier. We find that all of the Ca2+ ATPase data is best described either by the “random” model or one in which there is always at least one layer of lipid chains between every pair of proteins. We find that all of the Myelin PLA data is best described by a “random” distribution of hexamers and not by an “annulus” model of hexamers. We suggest measurements that can be done in order to unambiguously settle the question of whether these systems are best described by a “random”-type model or an “annulus”-type model.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    European biophysics journal 16 (1988), S. 31-37 
    ISSN: 1432-1017
    Keywords: Lipid bilayers ; protein distribution ; computer simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract We consider four models of the lateral distribution of proteins in lipid bilayer membranes and study the fraction of lipids which are adjacent to at least one protein (adjacent lipids) and how this quantity depends upon protein concentration. The models are (i) hard hexagons free to move from one lattice site to another; (ii) hard disks moving on a continuum; (iii) a mixture of two sizes of “nearly-hard” disks moving on a continuum; (iv) a modification of (ii). The hexagons or disks represent proteins, while unocupied lattice sites or the remainder of the continuum represents lipids. In (iii) large disks represent proteins and small disks represent lipids. In (iv) some of the continuum between pairs of disks, where packing defects might occur, is not occupied by lipids. We find that an analytical expression for the adjacent lipids (Hoffmann et al. 1981), which is in excellent agreement with the results of the Hexagon model (i), breaks down at a packing density of f A≊0.805, and we show by considering the hexagon pair correlation function, that this indicates the onset of random close packing, and that a transition to ordered close packing occurs at f A=0.866. We thus obtain an operational definition for a “random” distribution of hexagons: distributions of packing densities≲0.805. We show that the Disk model (ii) gives results for adjacent lipids that are greater than the Hexagon model and compare these results to the Two Disk model (iii) which gives a result substantially less than the Hexagon model (Mountain et al. 1986). We show that the Modified Disk model (iv) gives results in essential agreement with the Hexagon model except for f A≳0.77. Finally we discuss the general appearance of the “motion restricted” ESR spectrum and conclude that, of these four models, the Modified Disk or the Hexagon models best account for the data. We discuss why this is so with reference to the representation of a 3-dimensional membrane by a 2-dimensional plane.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 41 (1992), S. 0 
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Differential interactions between six crisphead cultivars of lettuce and five isolates of lettuce mosaic virus (LMV) have identified a novel resistance factor in the cultivar Ithaca. The LMV isolates could be grouped into three pathotypes, two of which have not been identified previously. One of these, pathotype III represented by an isolate from Spain, was virulent on all of the cultivars. There were differences in aggressiveness between isolates of the same pathotype.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 40 (1991), S. 0 
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Resistance to beet western yellows virus (BWYV) in the lettuce cultivars Burse 17 and Crystal Heart was controlled by a single recessive gene designated bwy. ELISA tests showed that resistant plants were not immune to infection by BWYV, and may develop some mild symptoms. However, the concentration of virus found in resistant plants was less than in susceptible ones. The cultivar Crystal Heart is being used as a source of resistance for breeding BWYV-resistant crisp lettuce (iceberg) cultivars.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 40 (1991), S. 0 
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 34 (1985), S. 0 
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Aneuploid and intervarietal chromosome substitution lines of wheat ev. Chinese Spring were used to study the effects of homoeologous chromosomes 7A, 7B and 7D upon adult plant resistance to yellow rust (Puccinia striiformis). Chromosomes 7B and 7D carry factors upon their short arms which interact to influence resistance. The results can be explained if there is a single locus determining resistance upon the short arms of chromosomes 7B and 7D. These loci may be homoeo-allelic; however, no evidence was found for a corresponding locus upon the short arm of chromosome 7A. Three homologous variants of the factor on 7DS were found but no variation was found for the factor on 7BS.
    Type of Medium: Electronic Resource
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