A nitroxide spin label has been covalently linked to the weak polyacid poly­(ethylene-alt-maleic acid) (P­(E-alt-MA)) to study the rotational mobility of the polyacid backbone in polyelectrolyte complexes (PEC) formed with the oppositely charged strong polycation poly­(diallyldi­methylammonium chloride) (PDADMAC) in dependence on the pH of the dispersion and the temperature. The rotational mobility of the polyacid chain segments has been determined by simulation of the line shape of the continuous wave (CW) electron paramagnetic resonance (EPR) spectra using the microscopic order/macroscopic disorder (MOMD) model of restricted rotational diffusion. The study has shown that the diffusion coefficient characterizing the rotational motions of the polyacid backbone is significantly smaller at low degree of dissociation at pH 4 than at high degree of dissociation at pH 7 and pH 10.

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The influence of maltose-modified poly(propylene imine) (PPI) dendrimers on dimyristoylphosphatidylcholine (DMPC) or dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol (DMPC/DMPG) (3%) liposomes was studied. Fourth generation (G4) PPI dendrimers with primary amino surface groups were partially (open shell glycodendrimers - OS) or completely (dense shell glycodendrimers - DS) modified with maltose residues. As a model membrane, two types of 100nm diameter liposomes were used to observe differences in the interactions between neutral DMPC and negatively charged DMPC/DMPG bilayers. Interactions were studied using fluorescence spectroscopy to evaluate the membrane fluidity of both the hydrophobic and hydrophilic parts of the lipid bilayer and using differential scanning calorimetry to investigate thermodynamic parameter changes. Pulsed-filed gradient NMR experiments were carried out to evaluate common diffusion coefficient of DMPG and DS PPI in D2O when using below critical micelle concentration of DMPG. Both OS and DS PPI G4 dendrimers show interactions with liposomes. Neutral DS dendrimers exhibit stronger changes in membrane fluidity compared to OS dendrimers. The bilayer structure seems more rigid in the case of anionic DMPC/DMPG liposomes in comparison to pure and neutral DMPC liposomes. Generally, interactions of dendrimers with anionic DMPC/DMPG and neutral DMPC liposomes were at the same level. Higher concentrations of positively charged OS dendrimers induced the aggregation process with negatively charged liposomes. For all types of experiments, the presence of NaCl decreased the strength of the interactions between glycodendrimers and liposomes. Based on NMR diffusion experiments we suggest that apart from electrostatic interactions for OS PPI hydrogen bonds play a major role in maltose-modified PPI dendrimer interactions with anionic and neutral model membranes where a contact surface is needed for undergoing multiple H-bond interactions between maltose shell of glycodendrimers and surface membrane of liposome.

The dynamics of spin-labeled poly­(acrylic acid) (PAA) in polyelectrolyte complexes (PECs) has been studied by EPR. It has been found that the segmental mobility of the PAA in the PECs is nearly constant in the pH range 10 to 5 but decreases dramatically for pH < 5. Recently, we have studied the dynamics of spin-labeled poly­(ethylene-alt-maleic acid) (P­(E-alt-MA)) in PECs. The rotational mobility of the P­(E-alt-MA) has been observed to depend on pH, which has been related to the formation of hydrogen bonds of the maleic acid units at low pH [Macromolecules2015483577]. To test this hypothesis, in the present study, P­(E-alt-MA) has been replaced with PAA, which is a structural isomer of P­(E-alt-MA). Now with the results for PAA at hand, we can conclude that the low mobility of the P­(E-alt-MA) at low pH cannot be exclusively attributed to the presence of maleic acid units. Nevertheless, it has been confirmed that the dynamics in the PECs is strongly influenced by the degree of dissociation of the weak polyacid.

Spin‐labeled poly(ethylene‐alt‐maleic acid) (SL‐P(E‐alt‐MA)) with 2.5 mol% spin‐labeled repeat units is prepared to study the interaction of this polyanion with an oppositely charged polyelectrolyte by electron paramagnetic resonance (EPR) spectroscopy. The internal rotation of the spin label and the segmental rotational mobility of the polyanion in aqueous solution are determined by simulating the line shapes of the experimental EPR spectra as a function of temperature. The complex formation of SL‐P(E‐alt‐MA), a weak polyanion, with the strong polycation poly(diallyldimethylammonium chloride) (PDADMAC) is studied, in dependence of the mixing ratio. If the spin‐labeled polyanion is the excess component, the spectrum of a slow‐moving component is superimposed by the spectrum of a fast‐moving component. In the opposite case, the spectra are dominated by a slow‐moving component.