Alberto Ciferri

The Supramolecular Association of Polyelectrolytes to Complementary Charged Surfactants and Protein Assemblies

Cohesion matters! The correlation between the conformational rigidity of the polyelectrolyte and the size and stability of the globular assembly is discussed in this review article. Some examples of models for the association of polyelectrolytes to globular assemblies are shown here.Supramolecular complexes of strong polyelectrolytes and oppositely charged ionic micelles or protein assemblies derive their main stabilization from electrostatic interactions that include the counterion condensation/release mechanism and from hydrophobic interactions distributed along apolar sections of the components. The predicted and the experimental behavior of selected complexes differing in the flexibility of the polyelectrolyte and in the cohesion of the complementary assembly is reviewed and analyzed. Depending upon the rigidity of the polyelectrolyte, globular surfactant clusters persist in the final structure or are transformed into elongated assemblies. On the other hand, even the rather rigid DNA molecule is forced to bend by strongly associated cationic protein complexes such as the histone octamers. A general framework should allow the prediction of these structures in terms of the interplay between the persistence length of the polyelectrolyte and the association constant of the protein or the surfactant assembly.

A Polyester Forming a Thermotropic Cholesteric Phase

Abstract Mesomorphic polyesters were synthesized from 4, 4′-dihydroxy-α-methylstilbene and adipic acid (P-6) or (+)-3-methyl adipic acid (P-6M). P-6 forms a thermotropic nematic phase and P6-M a thermotropic cholesteric phase. The nematic phase of P-6 could easily be identified by optical microscopy. For both polymers we observed a biphasic region in which the isotropic and liquid crystalline phases coexist. Bright colors were obtained by increasing the pitch of P6-M by admixture with either a low molecular weight nematogen or with polymer P-6, and also by synthesizing a copolymer containing the two dibasic acids. The copolymeric cholesteric phase, which is stable between 199 and 282[ddot]C, had a predominately planar texture, and these features could be retained in the solid state by quenching to produce a film having a deep blue color at room temperature. The role of the degree of polymerization upon the development of organization, and parameters, of the mesophase is discussed.

Ionic Mixed Interactions in Macromolecules

Purely ionic interactions in natural and synthetic macromolecules involve the mutual interaction of fixed charges and their interaction with mobile ions. Such charge-dependent interactions lead to well-documented effects, including chain expansion of polyelectrolytes, globularization of polyampholytes, distributions of mobile ions according to charge screening, or ion condensation models. A variety of structural features, functions, and applications of these systems is amplified by the superimposition of charge-independent effects associated with the occurrence of less polar or hydrophobic groups, special salts, surfactants, or complementary protein assemblies. For instance, ionic and hydrophobic attractive interactions stabilize pearls (or rings)-on-a-string conformations, possibly a model for the formation of the chromatin assembly. The attractive interactions due to hydrophobic fatty acid groups attached to polysaccharides promote the formation of vesicles that entrap and slowly release water-soluble drugs. Intra- and intermolecular associations based on ion-pairing mixed interactions also control the formation of host-guest compounds, protein conformation, and the assembly of layered polyelectrolytes. Metallo-supramolecular polymers and networks are formed due to the coordination of multivalent cations with bi- and trifunctional organic ligands. The association of lithium salts to polymers in the absence of water allows the formation of highly efficient energy sources. It also allows the identification of the ionic species that control charge-independent contributions to Hofmeister effects. This critical review presents a synthetic classification of systems displaying ionic mixed interactions, and a discussion of underlying molecular mechanisms.

Bond Scrambling and Network Elasticity

Dynamic networks (DNs) recently reported in the literature are based on cross-linked supramolecular chains or on covalent chains with reversible bonds. As originally pointed out by Lehn, these networks should be regarded as dynamic materials exhibiting adaptive features due to continuous scrambling of their bonds and sequences. Results in the recent literature reveal that these networks undergo reversible long-range deformation resembling that of rubber networks. The present analysis of this process in terms of the theory of composite networks is based on the expectation that the scrambling process should allow rupture of bonds in the undeformed state and their reformation in the stretched state. Accordingly, a permanent set of the resting length of DNs should generally be expected and set materials should retain long-range elasticity relative to the set state. However, only a limited set is shown by DNs, implying that a strong memory of the initial network topology assists elastic recovery of the original dimensions. The analysis of reported experimental data further reveals that the stress-strain dependence of dynamic networks accurately follows the classical rubber elasticity theory. In this respect, DNs show better rubber behavior than typical covalent networks. Consistently with theoretical predictions, this surprising finding suggests that bond scrambling relieves local strain constraints on the fluctuations of networks junctions and favors the recovery of the initial network topology. Scrambling therefore allows compliance under stress and enhanced recovery when stress is released. Unprecedented applications of these advanced materials thus become foreseeable.

Charge-dependent and charge-independent contributions to ion-protein interaction.

The thermodynamic framework and recent molecular descriptions of protein-salt interactions are critically reviewed. A reanalysis of earlier and recent data describing the role of salts on the thermal stability of collagen I over a wide range of concentration evidences the complex encroachment of charge-dependent and charge-independent interactions. Charge-independent interactions, operationally quantified by dilution parameters and association constants, are found to be the overriding factor for the large stabilization observed with nonbinding salts and for the large destabilization observed when strong salt binding occurs. Charge-dependent interactions, namely screening and selective ion adsorption, are instead the prevailing components at low ionic strength and nonisoelectric pH.

The vitreous gel: a composite structured network engineered by Nature

The vitreous body of the eye is a gel composed primarily of water and an ordered distribution of collagen II fibrils reinforcing a hyaluroran network. Its age‐induced alteration leads to serious eye pathologies. Notwithstanding extensive biochemical investigation, its assembling mechanism is poorly understood. In this paper we analyse the vitreous components in terms of general principles governing their supramolecular interaction, mesophase behaviour and collagen fibrillogenesis. This basic approach allows the selection of assembling patterns consistent with experimental data in the literature. The conclusions may provide new guidelines for advanced therapy and nanotechnological replacement engineering.

On the molecular mechanism of amyloid fibrillogenesis

Amyloid fibrils are known to form out of partially unfolded human lysozyme through self‐assembly of its disordered sequences into a cross‐β structure. A new molecular mechanism for this process is proposed based on analogies with the mesophase and microsegregation behaviour of copolymers composed of rigid and flexible blocks. According to the model, α‐helical sequences play an essential role on the stabilization of fibrils. The model ought to apply to globular–fibrillar transformations occurring for other proteins and conformational incompatibility might play a role on the stability of native tertiary structures.