Cooperative Interactions of Unlike Macromolecules 2: NMR and Theoretical Study of Electrostatic Binding of Sodium Poly(styrenesulfonate)s to Copolymers with Variably Distributed Cationic Groups

Addressing Specific (Poly)ion Effects for Layer-by-Layer Membranes

Layer-by-layer (LbL) assembly of the alternating adsorption of oppositely charged polyions is an extensively studied method to produce nanofiltration membranes. In this work, the concept of chaotropicity of the polycation and its counterion is introduced in the LbL field. In general, the more chaotropic a polyion, the lower its effective charge, charge availability, and hydrophilicity. Here, this is researched for the well-known PDADMAC (polydiallyldimethylammonium chloride) and PAH (poly(allylamine) hydrochloride), and the synthesized PAMA (polyallylmultimethylammonium), with two different counterions (I^- and Cl^-). Higher chaotropicity (PDADMAC > PAMA-I > PAMA-Cl > PAH) translates into a reduced charge availability and a more pronounced extrinsic charge compensation, resulting in more mass adsorption and a higher pure water permeability. PAMA-containing membranes show the most interesting results in the series. Due to its molecular structure, the chaotropicity of this polycation perfectly lies between PDADMAC and PAH. Overall, the chaotropicity of PAMA membranes allows for the formation of the right balance between extrinsic and intrinsic charge compensation with PSS. Moreover, modifying the nature of the counterions of PAMA (I^- or Cl^-) allows to tune the density of the multilayer and results in lower size exclusion abilities with PAMA-I compared to PAMA-Cl (higher MWCO and lower MgSO₄ retention). In general, the contextualization of the polyion interaction within the specific (poly)ion effects expands the understanding of the influence of the charge density of polycations without ignoring the chemical nature of the functional groups in their monomer units.

On the benefits of rubbing salt in the cut: self-healing of saloplastic PAA/PAH compact polyelectrolyte complexes

The inherent room temperature mending and self-healing properties of saloplastic PAA/PAH CoPECs are studied. After ultracentrifugation of PAA/PAH polyelectrolyte complexes, tough, elastic materials are obtained that undergo self-healing facilitated by salt. At intermediate salt concentrations the CoPECs remain elastic enough to recover their original shape while the chains are mobile enough to repair the cut, thus leading to actual self-healing behavior.

Isothermal microcalorimetry to investigate non specific interactions in biophysical chemistry

Isothermal titration microcalorimetry (ITC) is mostly used to investigate the thermodynamics of “specific” host-guest interactions in biology as well as in supramolecular chemistry. The aim of this review is to demonstrate that ITC can also provide useful information about non-specific interactions, like electrostatic or hydrophobic interactions. More attention will be given in the use of ITC to investigate polyelectrolyte-polyelectrolyte (in particular DNA-polycation), polyelectrolyte-protein as well as protein-lipid interactions. We will emphasize that in most cases these “non specific” interactions, as their definition will indicate, are favoured or even driven by an increase in the entropy of the system. The origin of this entropy increase will be discussed for some particular systems. We will also show that in many cases entropy-enthalpy compensation phenomena occur.

Cooperative Hydrogen Bonds of Macromolecules. 3. A Model Study of the Proximity Effect

Experimental and theoretical evidence for the proximity effect as a basic mechanism of the hydrogen bond cooperativity was obtained in a model system. Hydrogen bond (HB) interaction between poly(4-vinylpyridine) (PVP) and selected acids as HB donors was studied using PFG NMR self-diffusion measurements, 1H NMR longitudinal relaxation and quantum-mechanical DFT calculations. Bivalent HB donors, such as glutaric (GA) and adipic acid (AA), were compared to univalent donors 4-chlorobutyric acid, 4-acetylbutyric acid and 5-chlorovaleric acid. PFG NMR established substantially larger HB equilibrium constants for AA and, in particular, GA than for univalent donors, thus indicating cooperativity of COOH groups in bivalent donors. According to the values of these constants, the fraction of the transiently bound GA and AA molecules, which are bound by two hydrogen bonds, is 0.70 and 0.63, respectively. This result, which means substantial cooperativity in particular in GA, was then independently verified by a relaxation study comparing longitudinal relaxation rates of univalent and bivalent donors. Analysis of relaxation led to the same probabilities that HB of one COOH group of a bivalent donor will be accompanied by HB of another COOH group of the same molecule, namely 0.70 for GA and 0.63 for AA. Such cooperativity must be due to the proximity effect, i.e., the lowering of the entropy demand of the next binding by the motional restriction imposed by the already existing bonds. This conclusion is in excellent agreement with DFT calculations on the interaction of GA with a model vinylpyridine dimer, which predict preference of double binding of the same GA molecule over that of two GA molecules and show that this preference is due to a substantially lower entropy demand.

Cooperative Hydrogen Bonds of Macromolecules. 2. Two-Dimensional Cooperativity in the Binding of Poly(4-vinylpyridine) to Poly(4-vinylphenol)

The hydrogen bond interaction of poly(4-vinylphenol) (PVF), ligated by a 20 mol/mol excess of pyridine-d(5) (PD) in tetrahydrofuran-d(8), with poly(4-vinylpyridine) (PVP) was studied using liquid and solid-state NMR and quantum mechanical calculations. Because of its cooperative interaction, PVP substitutes PD in its hydrogen bond with PVF, thus forming a PVF-PVP complex, which gradually precipitates from solution. On the basis of the 1H/13C NMR spin-diffusion experiments and density functional theory quantum calculations, the complex is shown to have the fairly regular structure of a polymer sheet with intermittent H-bond links between PVF and PVP chains. The cooperativity of PVP interaction with PVF was studied by measuring the dependence of the binding degree alpha of PVP on its polymerization degree (P(n), being 10, 17, 30, 36, 48, 65, and 84) at various PVP/PVF molar ratios. The value of alpha was established indirectly by measuring the fraction of liberated PD using its 2H quadrupolar relaxation and pulsed field-gradient spin-echo measurement of self-diffusion. The cooperativity is shown to be of a higher order and two-dimensional, that is, dependent on both the polymerization degree of PVP and its ratio to PVF. A mathematical model of such two-dimensional cooperativity based chiefly on a proximity effect is suggested.

Compression-Inhibited Pore Formation of Polyelectrolyte Multilayers Containing Weak Polyanions: A Scanning Force Microscopy Study

Morphological changes of poly(acrylic acid)/poly(diallyldimethylammonium chloride) multilayers induced by low pH were investigated by scanning force microscopy. The weakened interaction between the charged polymer chains in the protonation process is believed to be the reason for this variation. Kinetic studies have shown that during protonation phase separation and dissociation of the multilayers took place successively. The compression of the multilayers, however, caused a transition of the multilayers from a rubbery state to a glassy state. As a result, the closely compacted multilayers lost their sensitivity to pH change. An increase of electrostatic and hydrophobic interactions, can decrease the free energy of the multilayers, and stabilize the films. By compression of the multilayers with a rubber stamp having geometric patterns, films with spatially localized pores were produced.

Cooperative H-Bonds of Macromolecules. 1. Binding of Low-Molecular-Weight Ligands to Polymers

Equilibrium bindings of two low-molecular-weight hydrogen bond acceptor ligands, tetrahydrofuran (THF) and pyridine (PH), and their fully deuterated analogues (TDF, PD) with poly(4-vinylphenol) (PVF) and its low-molecular-weight model, a hydrogen bond donor 4-isopropylphenol (IPP), were studied using spectroscopic (NMR) and theoretical (ab initio SCF-DFT B3LYP/6-31G(d)) methods as a first step of a general study of cooperative hydrogen bonding of H-bond-donating and -accepting macromolecules. Two reliable experimental methods of measuring the fraction of H-bonded or free ligands, which can be used as an indirect tool in further studies of macromolecular cooperative binding, were devised and examined in this study. The methods are complementary and independent. They are based on one hand on (2)H quadrupolar or (1)H longitudinal NMR relaxation and, on the other hand, on (2)H or (1)H PFG NMR self-diffusion measurement, using the pulsed-field-gradient spin-echo (PGSE) and pulsed-field-gradient stimulated echo (PGSTE) methods. It was shown that both the relaxation and PFG methods need viscosity correction using as an internal standard a relatively inert compound of a relaxation/diffusion rate similar to that of the internal standard (CDCl(3) or HMDSO in the present case). After such normalization, the reliability of both methods was checked by calculating the equilibrium constants K of binding under a variety of ligand/donor ratios beta. Excepting the rather impractical region of low beta values, where cooperative self-association of the H-donating polymer takes place, both methods were found to be reasonably reliable. A slight anomaly in the relaxation behavior of pyridine and its stronger-than-expected binding to PVF was plausibly explained (using high-precision quantum mechanical calculations) by additional formation of weak C-H...O bonds to the neighboring units.