Monocotyledonous leaves subjected to osmotica used for protoplast isolation accumulate a massive amount of putrescine (Put), lose chlorophyll and senesce rapidly. Treatment with spermidine (Spd) or spermine (Spm) prevents the loss of chlorophyll, indicating preservation of the thylakoid membranes at the site of the chlorophyll-protein complexes. Using several recently produced antibody probes, the effects on the stabilization of thylakoid membranes of applying either difluoromethylarginine (DFMA), a specific inhibitor of putrescine synthesis via arginine decarboxylase, or the polyamines Spd, Spm, or diaminopropane (Dap) to osmotically shocked oat leaves (Arena sativa L.) have been investigated. High protein levels were maintained in thylakoid membranes of leaf tissue incubated in the dark in the presence of 0.6 M sorbitol when pretreated with DFMA. After 48 h incubation, the level of the thylakoid protein D 1, at the core of photosystem II, was higher in the DFMA-pretreated leaves as was the stromal protein ribulose-l,5-bisphosphate carboxylase-oxygenase (Rubisco; as indicated by the level of large subunits). Applications of Spd, Spm or Dap were effective in retarding the loss of D1, D2 and cytochrome f from the thylakoid membranes as well as Rubisco large subunits and chlorophyll from the leaf tissue. The effects of polyamine applications may be mediated through Dap since most of the added Spd or Spm was converted to Dap within 6 h. The possible mechanisms of action of polyamine application s and DFMA-pretreatment on stabilizing the composition of the thylakoid membrane are also discussed.* To whom correspondence should be addressed; FAX: 44 (903) 72 67 80 Abbreviations : Cyt = cytochrorne; Dap = diaminopropane; DFMA = DL-cc-difluoromethylarginine ; LSU = large subunit (of Rubisco); Put=putrescine; Rubisco=ribulose-l,5-bisphosphate carboxylase-oxygenase; Spd = spermidine; Spin = spermine; SDS-PAGE = sodium dodecyl sulphate-polyacrylamide gel electrophoresis Key words: Polyamine -Ribulose-l,5-bisphosphate carboxylase/oxygenase (large subunit) -Thylakoid membrane (stabilization) -Thylakoid protein (D1, D2, cytochrome J)

The structure of the meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS 4 ) J-aggregates could be determined by X-ray and electron diffraction methods. A sheet-like architecture reveals the relationship between structure and chirality, optics and shapes of the J-aggregate of the meso 4-sulfonatophenyl-and phenyl-10 substituted porphyrins. The structure of the J-aggregates of H 4 TPPS 4 belongs to the chiral space group P2 1 and includes four porphyrin molecules in its unit cell. The intermolecular stabilization of the zwitterionic units by hydrogen bonding and electrostatic interactions between the positively charged central NH groups and the periphery anionic sulfonato groups results in a structure of porphyrins sheets along the [

ABSTRACT:The cure reaction kinetics of epoxy resin, with organically modified montmorillonite loadings of up to 20 wt % and with stoichiometric conditions, has been studied by differential scanning calorimetry with a view to understanding further the fabrication of epoxy-based polymer layered silicate nanocomposites. The kinetic analysis of isothermal and nonisothermal cure shows that the autocatalytic model is the more appropriate to describe the kinetics of these reactions, and it is observed that a dominant effect of the montmorillonite is to catalyze the curing reaction. However, it was not possible to model the reactions over the whole range of degrees of conversion, in particular for nonisothermal cure. This attributed to the complexity of the reactions, and especially to the occurrence of etherification by cationic homopolymerization catalyzed by the onium ion of the organically modified montmorillonite. The homopolymerization reaction results in an excess of diamine in the system, and hence in practice the reaction is off stoichiometric, which leads to a reduction in both the heat of cure and the glass transition temperature as the montmorillonite content increases. Small angle X-ray scattering of the cured nanocomposites shows that an exfoliated nanostructure is obtained in nonisothermal cure at slow heating rates, whereas for nonisothermal cure at faster heating rates, as well as for isothermal cure at 708C and 1008C, a certain amount of exfoliation is accompanied by the growth of dspacings of 1.4 nm and 1.8 nm for dynamic and isothermal cure, respectively, smaller than the d-spacings of the modified clay before intercalation of the resin. A similar nanostructure, consisting of extensive exfoliation accompanied by a strong scattering at distances less than the d-spacing of the modified clay, is also found for resin/clay mixtures, before the addition of any crosslinking agent, which have been preconditioned by storage for long times at room temperature. The development of these nanostructures is attributed to the presence of clay agglomerations in the original resin/clay mixtures and highlights the importance of the quality of the dispersion of the clay in the resin in respect of achieving a homogeneous exfoliated nanostructure in the cured nanocomposite.

We present here for the first time the crystal structure of an AT-hook domain. We show the structure of an AT-hook of the ubiquitous nuclear protein HMGA1, combined with the oligonucleotide d(CGAATTAATTCG)2, which has two potential AATT interacting groups. Interaction with only one of them is found. The structure presents analogies and significant differences with previous NMR studies: the AT-hook forms hydrogen bonds between main-chain NH groups and thymines in the minor groove, DNA is bent and the minor groove is widened.

The J aggregates of 4-sulfonatophenyl meso-substituted porphyrins are non-covalent polymers obtained by self-assembly that form nanoparticles of different morphologies. In the case of high aspect-ratio nanoparticles (bilayered ribbons and monolayered nanotubes), shear hydrodynamic forces may modify their shape and size, as observed by peak force microscopy, transmission electron microscopy of frozen solutions, small-angle X-ray scattering measurements in a disk-plate rotational cell, and cone-plate rotational viscometry. These nanoparticles either show elastic or plastic behaviour: there is plasticity in the ribbons obtained upon nanotube collapse on solid/air interfaces and in viscous concentrated nanotube solutions, whereas elasticity occurs in the case of dilute nanotube solutions. Sonication and strong shear hydrodynamic forces lead to the breaking of the monolayered nanotubes into small particles, which then associate into large colloidal particles.

Suggested roles for polyamine function, and the evidence for these functions, is reviewed. These include membrane stabilization, free radical scavenging, effects on DNA, RNA and protein synthesis, effects on the activities of RNase, protease and other enzymes, the interaction with ethylene biosynthesis, and effects on second messengers. It is concluded that in addition to interacting with plant hormones, polyamines are able to modulate plant development through a fundamental mechanism(s) common to all living organisms.Abbreviations: ACC = 1 -aminocyclopropane-1 -carboxylic acid; ADC = arginine decarboxylase; Chl = chlorophyll; DAP = diaminopropane; DFMA = DL-a-difluoromethylarginine; DFMO = DL-oldifluoromethylornithine; PAS = polyamines; Put = putrescine; SAM = S-adenosylmethionine; Spd = spermidine; Spm = spermine

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Various methods of preparation of epoxy resin/clay mixtures, before the addition of the crosslinking agent and curing to form epoxy-based polymer layered silicate (PLS) nanocomposites, have been investigated to determine their effect on the nanostructure. Organically modified montmorillonite clay was used, and the mixtures were prepared by both simple mixing and solvent-based methods. X-ray diffraction shows that intercalation of the resin into the clay galleries occurs for all clay loadings up to 25 wt % and for both preparation methods, but the dispersion of the clay in the resin, observed by optical microscopy, is significantly better for the solvent preparation method. Differential scanning calorimetry (DSC) shows that the intercalated resin has the same molecular mobility as the extra-gallery resin, but suggests that the intercalated resin does not penetrate completely into the galleries. Prolonged storage of the resin/clay mixtures at room temperature leads to changes in the DSC response, as well as in the response to thermogravimetry, which are interpreted as resulting from homopolymerization of the epoxy resin, catalyzed by the onium ion in the modified clay. This confirms and explains the earlier observation of Benson Tolle and Anderson (J Appl Polym Sci 2004, 91, 89) that ''conditioning'' of the resin/clay mixtures at ambient temperature has a significant effect when the crosslinking agent is subsequently added, and indicates that the preparation method has important consequences for the nanostructure development in the PLS nanocomposites.