Eckhard Nordmeier

Synthesis and characterisation of dextran and pullulan sulphate

Dextrans and pullulans of different molar masses in the range of 10(4)-10(5) g/mol were sulphated via a SO3-pyridine complex. The degree of substitution achieved was DS = 2.4 and DS = 1.4 for dextran sulphate and DS = 2.0 and DS = 1.4 for pullulan sulphate, respectively. Confirmation of sulphation was given by FTIR spectroscopy. Asymmetrical S=O and symmetrical C-O-S stretching vibrations were detected at 1260 and 820 cm(-1). Reactivity of the polysaccharide C-atoms was determined by 13C NMR spectroscopy: For dextran this was C-3 > C-2 > C-4, while for pullulan it was C-6 > C-3 > C-2 > C-4.

Poly(ethylene glycol)-induced DNA condensation in aqueous/methanol containing low-molecular-weight electrolyte solutions.: Part II. Comparison between experiment and theory

Abstract Viscometry, UV/Vis–centrifugation and dynamic light scattering (DLS) were used to monitor the coil–globule transition of calf-thymus DNA by poly(ethylene glycol) (PEG) in aqueous/methanolic NaCl solutions. All three methods confirm that methanol and PEG promote the transition synergistically. The PEG concentration at which the DNA collapses decreases as the methanol fraction of the solvent is increased. The values found for the critical PEG and methanol concentrations agree quite well with those predicted by the modified Flory–Huggins theory. This is rather surprising, because effects such as selective solvent adsorption or intramolecular charge repulsion are neglected. The most informative experimental technique for the present investigations is DLS. The DNA molecules are not affected by outside forces and DLS allows the measurement of molecular size as well as the distribution of the conformational states. It was observed that there are no intermediate conformational states. A DNA molecule is either in the expanded coil state or in the collapsed state.

Poly(ethylene glycol)-induced DNA condensation in aqueous/methanol containing low-molecular-weight electrolyte solutionsI. Theoretical considerations

Abstract In a certain unfavourable environment, a single DNA molecule undergoes a conformational transition from an expanded coil state to a collapsed form. Here, this transition is induced by poly(ethylene glycol) (PEG) in a solvent mixture composed of an aqueous salt buffer and methanol. A theoretical description is presented in terms of the free energy of mixing DNA, PEG, and solvent, the elastic part of the free energy for DNA chains, and the translational entropy of the low molecular ions. Further effects taken into account are DNA–counterion binding and solvent quality. The theoretical predictions are: (1) The transition between the coil and the collapsed state is discontinuous. There exists a coexistence region where both states coexist side by side, but its width is very small. (2) The collapse takes place at a certain critical PEG concentration cPEG,c. The value of this PEG-concentration depends on the degree of PEG polymerisation, Pw,PEG, the molar fraction, xmethanol, of methanol, and on the concentration, csalt, of the added salt. For given values of xmethanol and csalt, cPEG,c decreases with increasing Pw,PEG. That is, it is easier to induce the DNA collapse with PEG of high molar mass than with PEG of low molar mass. If both Pw,PEG and csalt are constant, cPEG,c increases as the methanol concentration decreases. This means that addition of methanol promotes DNA condensation. If finally Pw,PEG and xmethanol are chosen constant, cPEG,c decreases as csalt increases. Thus we can say, the collapsed DNA state is the more stabile the higher are Pw,PEG, xmethanol as well as csalt. That is, these three parameters act synergistically. (3) Theory gives some information about the DNA-molecule size. While in the coil state the expansion factor, α, is of the order of 1.8–2.4, it is of the order of 0.3 in the compact state. Results of measurements presented in the companion paper affirm these results.

STUDY OF AQUEOUS DEXTRAN SOLUTIONS UNDER HIGH PRESSURES AND DIFFERENT TEMPERATURES BY DYNAMIC LIGHT SCATTERING

Abstract The behaviour of dextrans of different molar masses in aqueous solutions was investigated by dynamic light scattering at different temperatures, ranging from 30 to 100°C and different pressures, in the range 1–3000 bar. It has been shown that concentration dependence of the apparent diffusion coefficient, D app , of all samples changes the behaviour about the overlap concentration C∗ . It has also been shown, that the diffusion coefficient increases with temperature (as a consequence of faster fluctuations of the molecules) and decreases with pressure. This could be accounted for by increase of solvent (water) viscosity with pressure. The activation energy of diffusion is independent of molar mass of dextran for the samples of molar masses from 7.4–21.1 × 10 6 g mol −1 , but it is considerably lower for the sample of molar mass of 1.0 × 10 6 g mol −1 . Pressure has a very small effect on activation energy of diffusion. The characteristic exponent for diffusion coefficient D 0 dependence on molar mass was found to be 0.32.

Denaturation experiments on calf-thymus DNA/polycation complexes in aqueous/organic solvent mixtures

Abstract Polyelectrolyte complexes of calf-thymus DNA and polycations, such as PDADMAC, IONENE, and P4VP were formed and analysed with respect to their thermodynamical stability. The UV-VIS spectrographic melting curves yield two denaturation temperatures, Tm,1 and Tm,2. This is explained as follows: DNA molecules contain two kinds of repeat unit sequences. There are sequences that are not complexed, they denature at Tm,1; the other DNA repeat units are complexed with polycation repeat units, they denature at Tm,2. Thereby, Tm,2 is much larger than Tm,1. That is, polycation binding stabilises calf-thymus DNA. Surprisingly, the absolute value of Tm,1 depends neither on the degree of complexation nor on the polycation type. Tm,1 agrees quite well with the denaturation temperature of the pure, uncomplexed DNA. Also Tm,2 does depend not on the degree of polycation complexation, but on the polycation type. One observes Tm,2(PDADMAC)>Tm,2(P4VP)>Tm,2(IONENE). The probable reason for this series is the polycation molar mass. Tm,2 increases slightly as Mw is increased. While Tm,1 depends on the concentration of the added NaCl-salt, Tm,2 does not. This is explained using the fact that the complexed DNA repeat units are electrically neutral, so that there are no salt/charge interactions. Measurements in aqueous/organic solvent mixtures show that both Tm,1 as well Tm,2 decrease continually as the content of the organic solvent is increased. Very marked is this effect for water/N-methylformamide. At wNMF≈40%, Tm,1 and Tm,2 are nearly half as large as in pure water.

Dilute solution properties of pullulan by dynamic light scattering

SummaryPullulan, produced by the fungus Aureobasidium, has been studied in dilute solution by viscometry, static low angle light scattering and dynamic light scattering at temperatures of 25, 40, 60 and 80 °C. The samples used were fractions obtained by partial hydrolysis of native pullulan and successive fractionation with ethanol. The mass-average molar mass, MW, of the studied pullulans were in the range of 1.67*105 to 1.0*106 g/mol. The translational diffusion coefficient at infinite dilution, Do, was obtained from the reduced first cumulant of the intensity autocorrelation function of the light scattered by extrapolation to zero scattering angle and zero polymer concentration. The activation energy of diffusion, ED, of pullulan macromolecules was calculated from Do=Doexp(-ED/RT). It was found that in the given temperature rangeED = 16.5 ± 0.5 kJmol−1 for all samples.

Nonstoichiometric polyelectrolyte complexes: A mathematical model and some experimental results

A mathematical model is presented to describe nonstoichiometric watersoluble polyelectrolyte complexes, and the predictions are compared with some experimental results. The theory is a mixture of Madelungs theory for ionic crystals and Mannings counterion condensation theory. The central parameters are the degree of complexation, φ, and the degree of counterion binding, θ. All other quantities are known in principle. It is found that there is a competition between complexation and counterion binding. When φ is large, θ is small, or vice versa. The degree of complexation, φ, depends sensibly on the concentration, c s , of the added low molecular salt, the polyanion chain length, N, and the dielectric constant, E, of the solvent. There exists a critical salt concentration, c s,c , at which the complexes salt out and where for c s > c s,c the complexes dissociate back into their single strands, the polyanions, and polycations. Further, φ is larger the smaller the polyanion length and the smaller the solvent dielectric constant are. To prove these predictions we have formed nonstoichiometric complexes between IONENE and PAA and IONENE and PMAA, respectively. The degree of complexation was determined by ultracentrifugation and checked by viscometry. The accord found between theory and experiment is both qualitatively as well quantitatively quite well.