ALBERT

Notes: Abstract. Electrophoresis following digestion of Myzus persicae genomic DNA with HindIII showed the presence of a prominent band of approximately 200 bp whereas a faint electrophoretic band corresponding to DNA fragments of about 3000 bp was observed after digestion with ApaI. In situ digestion with restriction enzymes, followed by in situ nick translation, showed that ApaI targets are localized at the nucleolus organizer-bearing X telomeric region, whereas HindIII restriction sites are clustered in intercalary C-positive areas on the same X chromosome. Fluorescent in situ hybridization (FISH) carried out by using digoxygenin-labeled HindIII repeats as probe fully confirmed overlapping between the hybridization sites of this probe and the AT-rich intercalary heterochromatic bands on the X chromosome. These findings, together with published data, allow us to conclude that the M. persicae genome possesses three classes of C-positive heterochromatin: (i) a GC-rich argentophilic band located on one telomere of the X chromosome that contains ApaI targets; (ii) AT-rich intercalary bands located on the X chromosome containing clustered HindIII fragments; (iii) AT-rich telomeric bands, located on autosomes, consisting of HaeIII repeats. Molecular analysis has shown that the length of the HindIII repeat consensus sequence is 189 bp with an AT content of 67%. Southern blotting with HindIII monomers revealed a regular ladder of bands composed of multimers of basic length that are characteristic of satellite DNAs. The HindIII repeat displays other features typical of eukaryotic satellite arrays such as overlapping with heterochromatic bands and a high degree of sequence similarity among monomers (84%–94%). A similarity plot showed that sequences were particularly variable in the 50–100 bp region whereas they proved to be highly conservative in the first 50 bp, thus suggesting that this portion of the repeat might be functionally important.

Notes: Abstract The pressure dependencedT g /dP of glass transition temperatureT g has received considerable interest due to its connection with solid state thermodynamic properties and theories of glass transition. Free volume considerations (1, 2) led to an estimate of the pressure effect onT g , showing thatdT g /dP had to depend on thermal expansion and compressibility changes atT g through the equation: [1] $$\frac{{dT_g }}{{dP}} = \frac{{\Delta \beta }}{{\Delta \alpha }}$$ whereΔβ=β e −β g andΔα=α e −α g Later work (3, 4, 5, 6) has shown that eq. [1] is not verified by experimental facts, the ratioΔβ/Δα being much larger than (dT g /dP) exp. Recent analysis of the properties of glasses obtained under different pressures have complicated the situation, showing that the experimental value ofdT g /dP depends, of course, on the polymer usedbut also on the experimental procedure used in its determination. Since it is obvious that in order to measure anyΔT g -value we need to operate on at leasttwo glasses, these should be identical in all properties which could influenceT g except pressure. Any difference in morphology,which could lead to a change in T g at constant pressure, should therefore be avoided in order to get a sound value for the pure pressure effectdT g /dP. To reveal this effect, we have performed (7)dT g /dP determinations on two polymers, polyvinylacetate (PVAC) and polyvinylchloride (PVC), following three different procedures: A. Measurement of the changeΔT g induced by application of a pressure incrementΔP on the liquid polymer (T〉T g ). This is the procedure normally used; the liquid is cooled down at a fixed rate of temperature change (∼5 °C/day) andT g is dilatometrically recorded at 1 atmosphere. Then the polymer is taken again to the liquid state, pressure ΔP is applied and, at the same rate, the system is cooled down isobarically; the newT g is recorded anddT g /dP calculated. B. Measurement of the change ΔTg induced by application of a pressure increment ΔP on the glassy polymer (T〈T g ). Once determinedT g at 1 atmosphere, pressureΔP is applied on the glass, time is given to the system to equilibrate; then the glass is heated isobarically. Intersection of the glassy line to the liquid line in a volume/temperature plot gives the newT g and therefore allows the calculation ofdT g /dP. C. Measurement ofΔT g during the heating of a glass along an isochor (5, 8). Here the polymer glass is heated at constant volume, by application of an increasing pressure at increasing temperatures given by(∂P/∂T) v . By repeating this procedure two times, starting from two different specific volumes of the glass, two values ofT g at different pressures can be recorded anddT g /dP calculated. Table 1 shows the result of this work Polymer A dT g /dP, °C/atm B C PVAC 0.015 0.037 0.022 PVC 0.013 0.038 0.028 Comparison of values for different procedures shows quite convincingly howdT g /dP, at a given heating or cooling rate, is a property not only of the polymer but also of the experiment. Any information which, like the pressure effect onT g , is based on a comparison between two or more glasses, should be therefore carefully analysed in terms of other contributions, besides pressure, to the total change inT g .

Notes: Summary Shearing at constant gradient of polyisobutylene (PIB) solutions in viscous polybutene oils results in time dependent changes of their viscosities. Two PIB fractions (M=8·106 and 3.8·106) have been studied in a variety of solvents, covering the viscosity range from 5 to 0.5·10−2 poise. The overall phenomenon is similar to that already observed for polymethylmethacrylate, polystyrene, and polyethylene oxide solutions in Aroclor. Results have been discussed in terms of polymer molecular weight and concentration, solvent viscosity and gradient. The systems investigated are free from strong polar interactions and tendency toward molecular aggregation; time dependent changes of viscosity are therefore observable also for true molecularly dispersed solutions.

Notes: Four polystyrene samples, with molecular weight ranging from 43,300 to 1,112,000 have been irradiated in benzene solution by γ-rays, at four different intensities. Data obtained through intrinsic viscosity and light-scattering measurements on irradiated polymers indicate that, at each intensity, there exists a molecular weight below which any polymer chain is no further degraded. An equilibrium between absorbed and dissipated energy by the macromolecule is supposed, and the equation, MlimI = const. arrived at in a previous study, is verified also by the present experimental results.

Notes: The behaviour of the limiting viscosity number [η] and diffusion coefficient D is studied as function of molecular weight, for some polystyrene fractions with molecular weight ranging from 600 to 50,000.The experimental results show that the two equations: \documentclass{article}\pagestyle{empty}\begin{document}$\begin{array}{*{20}c}{{\rm D} = {\rm K} \cdot {\rm M}^{ - 0.5} } & [ \\ \end{array} {\rm \eta }] = {\rm K}' \cdot {\rm M}^{0.5} $\end{document} hold for low polymers irrespective of solvent nature, i.e. good and Θ-solvent.Some considerations are made about the dimensions of the studied fractions.

Notes: With an increasing gradient, the intrinsic viscosity of a high molecular weight polyisobutylene (M̄n = 7 × 106) in polybutene oil L.100 (ηs = 5 poise) first drops to a minimum and then rises again. The minimum occurs at β = M[η]0ηsG/NkT = 240, which is about ten times the value predicted by the dumbbell model. Such a shift to larger gradient is in good agreement with the more realistic necklace model of macromolecules in a good solvent. The increase of intrinsic viscosity after the minimum is nearly linear with the gradient and continues beyond the value at zero gradient. Experiments with capillaries of different length-to-diameter ratios yield identical flow curves so that one may exclude the possibility that the observed upturn is an artifact caused by end effects or time dependence of viscosity.