Flexible, charged biopolymers in monovalent and mixed-valence salt: Regimes of anomalous electrostatic stiffening and of salt insensitivity

Glycosaminoglycans: What Remains To Be Deciphered?

Glycosaminoglycans (GAGs) are complex polysaccharides exhibiting a vast structural diversity and fulfilling various functions mediated by thousands of interactions in the extracellular matrix, at the cell surface, and within the cells where they have been detected in the nucleus. It is known that the chemical groups attached to GAGs and GAG conformations comprise "glycocodes" that are not yet fully deciphered. The molecular context also matters for GAG structures and functions, and the influence of the structure and functions of the proteoglycan core proteins on sulfated GAGs and vice versa warrants further investigation. The lack of dedicated bioinformatic tools for mining GAG data sets contributes to a partial characterization of the structural and functional landscape and interactions of GAGs. These pending issues will benefit from the development of new approaches reviewed here, namely (i) the synthesis of GAG oligosaccharides to build large and diverse GAG libraries, (ii) GAG analysis and sequencing by mass spectrometry (e.g., ion mobility-mass spectrometry), gas-phase infrared spectroscopy, recognition tunnelling nanopores, and molecular modeling to identify bioactive GAG sequences, biophysical methods to investigate binding interfaces, and to expand our knowledge and understanding of glycocodes governing GAG molecular recognition, and (iii) artificial intelligence for in-depth investigation of GAGomic data sets and their integration with proteomics.

Single-molecule AFM study of hyaluronic acid softening in electrolyte solutions

Investigation of hyaluronic acid (HA) morphology and mechanical properties at a single-molecule level is important for the development of HA based biomaterials. We have developed the atomic force microscopy (AFM) based approach for quantitative characterization of conformation of HA molecules. HA molecules adsorbed on a modified graphitic surface form oriented linear segments. Conformation of HA molecules can be considered as two-dimensional quasi-projection of a three-dimensional conformation locally straightened by a substrate. The persistence length and Young's modulus of biomolecules estimated using wormlike chain model decrease from 15.7 to 9.9 nm, and from ∼21 to ∼13 GPa, respectively, when KCl concentration increases from 0 to 100 mM. The dependence of the persistence length on ionic strength supports the Odijk-Skolnick-Fixman model of polyelectrolyte stiffening in electrolyte solution. The obtained results represent a new insight into the conformation and mechanical characteristics of HA molecules and complement the characterization of this biopolymer by bulk methods.

Ion Specificity of Confined Ion-Water Structuring and Nanoscale Surface Forces in Clays

Ion specificity and related Hofmeister effects, which are ubiquitous in aqueous systems, can have spectacular consequences in hydrated clays, where ion-specific nanoscale surface forces can determine large-scale cohesive swelling and shrinkage behaviors of soil and sediments. We have used a semiatomistic computational approach and examined sodium, calcium, and aluminum counterions confined with water between charged surfaces representative of clay materials to show that ion-water structuring in nanoscale confinement is at the origin of surface forces between clay particles which are intrinsically ion-specific. When charged surfaces strongly confine ions and water, the amplitude and oscillations of the net pressure naturally emerge from the interplay of electrostatics and steric effects, which cannot be captured by existing theories. Increasing confinement and surface charge densities promote ion-water structures that increasingly deviate from the ions' bulk hydration shells, being strongly anisotropic, persistent, and self-organizing into optimized, nearly solid-like assemblies where hardly any free water is left. Under these conditions, strongly attractive interactions can prevail between charged surfaces because of the dramatically reduced dielectric screening of water and the highly organized water-ion structures. By unravelling the ion-specific nature of these nanoscale interactions, we provide evidence that ion-specific solvation structures determined by confinement are at the origin of ion specificity in clays and potentially a broader range of confined aqueous systems.

Effect of mono- and multi-valent ionic environments on the in-lattice nanoparticle-grafted single-stranded DNA

Polyelectrolyte (PE) chains respond in a complex manner to multivalent salt environments, and this behavior depends on pH, temperature, and the presence of specific counter ions. Although much work has been done to understand the behaviour of free PE chains, it is important to reveal their behaviour on a nanoparticle's surface, where surface constraints, particle geometry, and multi-chain environment can affect their behaviour and contribute to particles' assembly states. Our work investigates, using in situ small-angle X-ray scattering (SAXS), the morphology of PE (single-stranded DNA) chains grafted onto the surface of spherical gold nanoparticles assembled in a lattice in the presence of monovalent, divalent and trivalent salts. For divalent salts, the DNA brush length was found to decrease at a faster rate with salt concentration than in the monovalent salt environment, while trivalent salts led to chain collapse. Using a power law analysis and the modified Daoud-Cotton model, we have obtained insight into the mechanism of a nanoparticle-grafted chain's response to ionic environments. Our analysis suggests that the decrease in brush length is due to the conventional electrostatic screening for monovalent systems, whereas for divalent systems both electrostatic screening and divalent ion bridging must be considered.