Influence of salts in the Hofmeister series on the physical gelation behavior of gelatin in aqueous solutions

On the modification of plant proteins: Traditional methods and the Hofmeister effect

With increasing consumer health awareness and demand from some vegans, plant proteins have received a lot of attention. Plant proteins have many advantages over animal proteins. However, the application of plant proteins is limited by a number of factors and there is a need to improve their functional properties to enable a wider range of applications. This paper describes the advantages and disadvantages of traditional methods of modifying plant proteins and the appropriate timing for their use, and collates and describes a method with fewer applications in the food industry: the Hofmeister effect. It is extremely simple but efficient in some respects compared to traditional methods. The paper provides theoretical guidance for the further development of plant protein-based food products and a reference value basis for improving the functional properties of proteins to enhance their applications in the food industry, pharmaceuticals and other fields.

Evolution of the structure and interaction in the surfactant-dependent heat-induced gelation of protein

The addition of a surfactant and/or an increase in temperature disrupt the native structure of proteins, where high temperature further results in protein gelation. However, in a mixed protein-surfactant system, surfactant concentration and temperature have been observed to exhibit both mutually associative and counter-balancing effects towards heat-induced gelation of protein-surfactant dispersion. This study is conducted on globular bovine serum albumin (BSA) protein and cationic surfactant dodecyl trimethyl ammonium bromide (DTAB), which interact strongly owing to their oppositely charged nature. The findings reveal that the BSA-DTAB suspension undergoes gelation with increasing temperature but only at lower concentrations of DTAB, where the presence of the surfactant facilitates gelation (associative effect). Conversely, as the surfactant concentration increases beyond a critical value, temperature-driven gelation of the BSA-DTAB system is completely inhibited, despite surfactant-induced protein denaturation (counter-balancing effect). To conceptualize these results, we compared them with observations made in a system comprising protein and a similarly charged surfactant, sodium dodecyl sulfate (SDS). It has been further demonstrated that the anionic surfactant (SDS) can restrict protein gelation at much lower concentration compared to the cationic surfactant (DTAB). The evolution of the structure and interaction during gel formation/inhibition has been examined to understand the underlying mechanism guiding these sol-gel transitions. We present a comprehensive phase diagram, encompassing the solution/gel states of the protein-surfactant dispersion, with respect to the dispersion temperature, surfactant concentration, and ionic behavior (anionic or cationic) of the surfactants.

Anionic Hofmeister Effect Regulated Conductivity in Polyelectrolyte Hydrogels for High-Performance Supercapacitor

The Hofmeister effect not only affects the stability and solubility of protein colloids but also has specific effects on the polymer molecules. Here, the impact of the Hofmeister effect on the electrochemical properties of polyelectrolyte hydrogels at room temperature and subzero temperature studied for the first time. Polyelectrolyte hydrogels exhibit an anti-polyelectrolyte effect in low concentrations of ammonium salt, while they exhibit an obvious Hofmeister effect in high concentrations of ammonium salt. Kosmotropic ions demonstrate strong interaction with water molecules or polymer chains, resulting in the reduction of conductivity of polyelectrolyte hydrogels. However, chaotropic ions exhibit weak interactions with water molecules or molecular chains, leading to an increase in conductivity. The Hofmeister effect has a more significant effect on the polyzwitterion electrolyte. The conductivity of polyzwitterion hydrogel soaked in chaotropic ion is up to 6.2 mS cm-1 at -40 °C. The supercapacitor assembled by polyzwitterion electrolytes maintains a capacitance retention rate of 85% and ≈100% coulomb efficiency after 15 000 cycles at -40 °C. This study elucidates the influence of the Hofmeister effect on conductivity in polyelectrolytes and expands the regulatory approach for improving the performance of energy storage devices.

Development of New Edible Biodegradable Films Containing Camu-Camu and Agro-Industry Residue

The use of edible films has garnered significant interest in the food and environmental sectors due to their potential to prevent food deterioration and their biodegradability. This study aimed to develop and characterize edible films based on camu-camu residue, gelatin, and glycerol, evaluating their solubility, thermal, degradability, antioxidant, and water vapor permeability properties of the gelatin matrix. This is the first study incorporating camu-camu into a gelatin and glycerol matrix. The films produced with camu-camu residue were manageable and soluble, with some non-soluble residues, providing a shiny and well-presented appearance. In the biodegradation results, samples 3 and 4 appeared to degrade the most, being two of the three most affected samples in the triplicate. The films showed degradation modifications from the third day of the experiment. In the germination and plant growth analysis, sample 4 exhibited satisfactory development compared to the other samples, emerging as the sample with the best overall result in the analyses, attributed to a 13.84 cm increase in the growth of the upper part of the seedling. These results indicate that the produced materials have potential for food packaging applications.

Tuning the thermostability of GHG gels by salts at different positions on the Hofmeister scale

The influence of Hofmeister cations (NH4+, Na+, Mg2+) and anions (H2PO4-, CH3COO-, Cl-, NO3-) on the thermostability of a GHG hydrogel was investigated. The combined results of UV circular dichroism (UVCD) and Small Amplitude Oscillatory Shear Rheology experiments reveal that the addition of salt reduces the stability of the gel phase and the underlying fibrils. In line with the cationic Hofmeister hierarchy, the chaotropic Mg2+ ions caused the greatest thermal destabilization of the gel phase with the gel → sol transition temperature Tgs value lowered by 10 °C. In the absence of salt, the gel → sol transition probed by the storage modulus and microscopy is biphasic. In the presence of salt, it becomes monophasic. Contrary to expectations the presence of Hofmeister anions leads to a nearly identical reduction of the gel → sol transition temperatures. However, UVCD spectra suggest that they affect the ππ-stacking between imidazole groups to a different extent. We relate the absence of ion specificity regarding the solubility of fibrils (probed by UVCD) to the observed enthalpy-entropy compensation of the dissolution process. Our results combined show how CD spectroscopy and rheology combined yields a more nuanced picture of the processes underlying the gel → sol transition.

A Biomimetic "Salting Out-Alignment-Locking" Tactic to Design Strong and Tough Hydrogel

Recently, hydrogel-based soft materials have demonstrated huge potential in soft robotics, flexible electronics as well as artificial skins. Although various methods are developed to prepare tough and strong hydrogels, it is still challenging to simultaneously enhance the strength and toughness of hydrogels, especially for protein-based hydrogels. Herein, a biomimetic "salting out-alignment-locking" tactic (SALT) is introduced for enhancing mechanical properties through the synergy of alignment and the salting out effect. As a typical example, tensile strength and modulus of initially brittle gelatin hydrogels increase 940 folds to 10.12 ± 0.50 MPa and 2830 folds to 34.26 ± 3.94 MPa, respectively, and the toughness increases up to 1785 folds to 14.28 ± 3.13 MJ m-3. The obtained strength and toughness hold records for the previously reported gelatin-based hydrogel and are close to the tendons. It is further elucidated that the salting out effect engenders hydrophobic domains, while prestretching facilitates chain alignment, both synergistically contributing to the outstanding mechanical properties. It is noteworthy that the SALT demonstrates remarkable versatility across different salt types and polymer systems, thus opening up new avenues for engineering strong, tough, and stiff hydrogels.

Elucidating the influence of electrostatic force on the re-arrangement of H-bonds of protein polymers in the presence of salts

Polyampholytes/proteins have an intriguing network of hydrogen bonds (H-bonds), especially their secondary structure, which plays a crucial role in determining the conformational stability of the polymer. The changes in protein secondary structure in the protein-salt system have been extensively deciphered by researchers, yet their pathways for breakage and recreation are unknown. Understanding the mechanism of protein conformational changes towards their biological activities, like protein folding, remains one of the main challenges and requires multiscale analysis of this strongly correlated system. Herein, salts have been used to reveal the re-arrangement behavior in the H-bond network of proteins under the influence of electrostatic interactions, as the strength of electrostatic forces is much stronger than that of H-bonds. At lower salt concentrations, there are negligible changes in the secondary structures as the electrostatic forces induced by the salt ions are less. Later, the existing H-bonds break and reconstruct new H-bonds at higher salt concentrations due to the influence of the stronger electrostatic interaction induced by the large number of salt ions. Molecular dynamics (MD) simulations and FTIR studies have been used rigorously to decipher the reason behind the re-arrangement of the H-bonds within gelatin (protein). The re-arrangement in the H-bond has also been observed with time from simulations and experiments. Thus, this study could provide a fresh perspective on the conformational changes of polyampholytes/proteins and will also influence the studies of protein folding-unfolding interaction in the presence of salt ions.

Electrolyte-induced aggregation of zein protein nanoparticles in aqueous dispersions

Ion specific effects on the charging and aggregation features of zein nanoparticles (ZNP) were studied in aqueous suspensions by electrophoretic and time-resolved dynamic light scattering techniques. The influence of mono- and multivalent counterions on the colloidal stability was investigated for positively and negatively charged particles at pH values below and above the isoelectric point, respectively. The sequence of the destabilization power of monovalent salts followed the prediction of the indirect Hofmeister series for positively charged particles, while the direct Hofmeister series for negatively charged ones assumed a hydrophobic character for their surface. The multivalent ions destabilized the oppositely charged ZNPs more effectively and the aggregation process followed the Schulze-Hardy rule. For some multivalent ions, strong adsorption led to charge reversal resulting in restabilization of the suspensions. The experimental critical coagulation concentrations (CCCs) could be well-predicted with the theory developed by Derjaguin, Landau, Verwey and Overbeek indicating that the aggregation processes were mainly driven by electrical double layer repulsion and van der Waals attraction. The ion specific dependence of the CCCs is owing to the modification of the surface charge through ion adsorption at different extents. These results are crucial for drug delivery applications, where inorganic electrolytes are present in ZNP samples.

Molecular interactions of acids and salts with polyampholytes

The Hofmeister series characterizes the ability of salt anions to precipitate polyampholytes/proteins. However, the variation of protein size in the bulk solution of acids and the effect of salts on the same have not been studied well. In this article, the four acids (CH3COOH, HNO3, H2SO4, and HCl) and their effects on the hydrodynamic radius (RH) of gelatin in the bulk solution are investigated. The effects of Na salt with the same anions are also considered to draw a comparison between the interactions of acids and salts with polyampholytes. It is suggested that the interactions of polyampholytes with acids are different from those of salts. The interaction series of polyampholytes with acids with respect to the RH of the polyampholyte is CH3COO->NO3->Cl->SO42- whereas the interaction series with salts is SO42->CH3COO->Cl->NO3-. These different interactions are due to equilibration between acid dissociation and protonation of polyampholytes. Another important factor contributing to the interactions in weak acids is the fact that undissociated acid hinders the movement of dissociated acid. Experiments and simulations were performed to understand these interactions, and the results were identical in terms of the trend in RH (from the experiments) and the radius of gyration (Rg) (from the simulations). It is concluded that the valence of ions and dissociation affect the interaction in the case of acids. However, the interactions are influenced by the kosmotropic and chaotropic effect, hydration, and mobility in the case of salts.

Electrostatic-Dominated Conformational Fluctuations and Transition States of Phase Separation in Charge-Balanced Protein Polymer

Hydration of the protein/polymer is the most important aspect of stability. It is well-known that salts alter the charged polymer's electrostatic forces, ultimately impacting its conformations in solution. The solvent effects lead to certain conformational fluctuations. Previous studies have shown the screening of electrostatic repulsion within the charge-imbalanced protein following charge inversion owing to counterion condensation and phase separation. This article studies conformation stability and phase separation of charge-balanced gelatin (a protein polymer at the isoelectric point) with the addition of different salts. A phenomenon has been reported where the electrostatic effect of salts results in conformational fluctuations in gelatin due to its insufficient hydrations (termed as starvation), which scales with salt concentration. This article also presents different transition states for charge-balanced proteins prior to phase separation. It is concluded that phase separation of a charge-balanced protein passes through a stable state followed by an unstable transition state, where certain unique interactions with salts occur.

Collagen and gelatin: Structure, properties, and applications in food industry

Food-producing animals have the highest concentration of collagen in their extracellular matrix. Collagen and gelatin are widely used in food industry due to their specific structural, physicochemical, and biochemical properties, which enable them to improve health and nutritional value as well as to increase the stability, consistency, and elasticity of food products. This paper reviews the structural and functional properties including inherent self-assembly, gel forming, water-retaining, emulsifying, foaming, and thickening properties of collagen and gelatin. Then the colloid structures formed by collagen such as emulsions, films or coatings, and fibers are summarized. Finally, the potential applications of collagen and gelatin in muscle foods, dairy products, confectionary and dessert, and beverage products are also reviewed. The objective of this review is to provide the current market value, progress as well as applications of collagen and its derivatives in food industry.

Structural characteristics, component interactions and functional properties of gelatins from three fish skins extracted by five methods

Fish skin gelatin is an important functional product used in food, medicine and other industries. However, the structure and function of gelatins extracted with different methods differ significantly, thus limiting its production and application. This study used dry-salting, wet-salting, pepsin, acid and heat methods to extract gelatins from the skins of tilapia, grass carp and sea perch. Then, their structural characteristics (micro- and ultra-structure, amyloid-like fibril, etc.) and functional properties (viscosity, emulsifying performance, antioxidant abilities, etc.) were analyzed, and interaction between gelatin components were also explored. According to the results, the gelatins extracted with dry-salting and wet-salting methods had better reticular structure, larger fiber length/height, and higher viscosity properties, emulsifying and antioxidant capacity. The gelatin extracted by applying heat has the highest gel strength, and the gelatin extracted using pepsin had better thermal stability, water absorption capacity, and fat absorption capacity. Further analysis of component interaction showed that 11 types of collagens detected in the gelatins might promote the conversion of collagen to gelatin through self-assembly ability. The co-assembly of different types of collagens enhanced the properties of gelatin. Decorin had a positive effect on gelatin network structure, but Metallopeptidase inhibited the formation of network structure. Different methods can produce personalized gelatin products according to specific needs. The mining of component interaction would reveal the mechanism of gelatin formation and promote the development of gelatin synthetic biology.

Mechanism behind the deterioration in gel properties of collagen gel induced by high-temperature treatments: A molecular perspective

This study aims to elucidate the mechanism behind the deterioration in the gel properties of collagen gel resulting from high-temperature treatment. The results show that the high level of triple-helix junction zones and related lateral stacking contribute to the dense and orderly collagen gel network with high gel strength and storage modulus. The analysis of the molecular properties of heated collagen shows that high-temperature treatment leads to serious denaturation and degradation of collagen, resulting in the formation of gel precursor solutions composed of low-molecular-weight peptides. The short chains in the precursor solution are not easy to nucleation and can limit the growth of triple-helix cores. To conclude, the decrease in triple-helix renaturation and crystallization abilities of peptide components is the reason for the deterioration in the gel properties of collagen gel induced by high temperature. The findings presented in this study add the understanding of texture deterioration in high-temperature processed collagen-based meat products and related products, and provide a theoretical basis for establishing methods to overcome the production dilemma faced by these products.

Influences of Trypsin Pretreatment on the Structures, Composition, and Functional Characteristics of Skin Gelatin of Tilapia, Grass Carp, and Sea Perch

Fish skin gelatin is an important functional product in the food, cosmetics, and biomedicine industries, and establishing a green and effective fish skin gelatin extraction method is an effective way to obtain high-quality gelatin and improve its production efficiency. In this study, a trypsin method was used to extract the skin gelatin of sea perch, tilapia, and grass carp, and the microstructures of skin gelatin of these three fish species were analyzed, with such functional characteristics as thermal stability, gel strength, and emulsifying properties measured. The study results show that the skin gelatin of sea perch and tilapia obtained through the trypsin method has a relatively big molecular mass, a dense network structure, and a stable trihelix conformation. In addition, the skin gelatin of these three fish species has a relatively high β-turn content in the secondary structure, good gel strength, and water absorption properties. The compositions of the collagen-associated proteins in the skin gelatins of these three fish species extracted with the trypsin method are significantly different from each other, with positive effects of decorin and biglycan on the stability of the network structure of gelatin and a certain damaging effect of metalloendopeptidase on the network structure of gelatin. The skin gelatin of tilapia has high thermal stability and good emulsifying performance. Therefore, this gelatin type has bright application prospects in such fields as food processing, cosmetics, and drug development. In contrast, the skin gelatin of grass carp has poor functional properties. Therefore, there are significant differences among the structures and functions of skin gelatin extracted from different kinds of fish through the trypsin method. This finding has provided a useful reference for the production of customized fish gelatin according to demand.

Hofmeister anion effects synergize with microbial transglutaminase to enhance the techno-functional properties of pea protein

Pea protein are emerging as the most potential alternative for meat products, but its application was hindered by their weaker gelling properties. Here, the feasibility of combining the Hofmeister anion (CO₃^2-, Citrate^3-, and SO₄^2-) effect with microbial transglutaminase (MTG) cross-linking strategy to improve the techno-functional properties of pea protein was studied. Hofmeister anions or/and MTG treatment of pea protein caused a clear shift in far-UV CD spectra towards β-turn and random coil structures. Furthermore, Hofmeister anion and MTG-induced crosslinking caused a reduction of surface hydrophobicity in contrast with anions-treated. Compared to CO₃^2- and SO₄^2-, Citrate^3- treatment can better improve the efficiency of MTG-crosslinking, as demonstrated by a reduction in free amino group contents and an increase in mean diameter size. Using MTG in combination with Hofmeister anions showed significantly improved foam property and gel hardness as well as decrease gelation temperature of pea protein, specifically Citrate^3- treatment. Thus, this research provides a novel and effective method to improve the effect of MTG-cross-linked pea protein, which will play an essential role in future food production.

Hofmeister effect-based soaking strategy for gelatin hydrogels with adjustable gelation temperature, mechanical properties, and ionic conductivity

As a natural polymer with good biocompatibility, gelatin hydrogel has been widely used in the field of biomedical science for a long time. However, the lack of suitable gelation temperature and mechanical properties often limit the clinical applicability in diverse and complex environments. Here, we proposed a strategy based on the Hofmeister effect that gelatin hydrogels were soaked in the appropriate concentration of sodium sulfate solution, and the change in molecular chain interactions mainly guided by kosmotropic ions resulted in a comprehensive adjustment of multiple properties. A series of gelatin hydrogels treated with different concentrations of the salt solution gave rise to microstructural changes, which brought a decrease in the number and size of pores, a wide range of gelation temperature from 32 °C to 46 °C, a stress enhancement of about 40 times stronger to 0.8345 MPa, a strain increase of about 7 times higher to 238.05 %, and a certain degree of electrical conductivity to be utilized for versatile applications. In this regard, for example, we prepared microneedles and obtained a remarkable compression (punctuation) strength of 0.661 N/needle, which was 55 times greater than those of untreated ones. Overall, by integrating various characterizations and suggesting the corresponding mechanism behind the phenomenon, this method provides a simpler and more convenient performance control procedure. This allowed us to easily modulate the properties of the hydrogel as per the intended purpose, revealing its vast potential applications such as smart sensors, electronic skin, and drug delivery.

Chinese Tofu-Inspired Biomimetic Conductive and Transparent Fibers for Biomedical Applications

Conductive fibers are vital for next-generation wearable and implantable electronics. However, the mismatch of mechanical, electrical, and biological properties between existing conductive fibers and human tissues significantly retards their further development. Here, the concept of neuro-like fibers to meet these aforementioned requirements is proposed. A new wet spinning process is established to continuously produce pure gelatin hydrogel fibers. The key is the controllable and rapid gelation of spinning solutions based on the salting-out effect, which is inspired by the Chinese food tofu. The resultant fibers exhibit neuro-like features of soft-while-strong mechanical properties, high ionic conductivity, and superior biological properties including biodegradability, biocompatibility, and edibility, which are crucial for implanted applications but seldom reported. Furthermore, all-weather suitable neuro-like fibers with excellent anti-freezing and water retention properties are developed by introducing glycerol for wearable applications. The optical fiber, transient electronics, and electronic data glove made of neuro-like fibers profoundly demonstrate their potential in biomedical applications.

Secondary structure and stability of a gel-forming tardigrade desiccation-tolerance protein

Protein-based pharmaceuticals are increasingly important, but their inherent instability necessitates a "cold chain" requiring costly refrigeration during production, shipment, and storage. Drying can overcome this problem, but most proteins need the addition of stabilizers, and some cannot be successfully formulated. Thus, there is a need for new, more effective protective molecules. Cytosolically, abundant heat-soluble proteins from tardigrades are both fundamentally interesting and a promising source of inspiration; these disordered, monodisperse polymers form hydrogels whose structure may protect client proteins during drying. We used attenuated total reflectance Fourier transform infrared spectroscopy, differential scanning calorimetry, and small-amplitude oscillatory shear rheometry to characterize gelation. A 5% (wt/vol) gel has a strength comparable with human skin, and melts cooperatively and reversibly near body temperature with an enthalpy comparable with globular proteins. We suggest that the dilute protein forms α-helical coiled coils and increasing their concentration drives gelation via intermolecular β-sheet formation.

Synergy of Hofmeister effect and ligand crosslinking enabled the facile fabrication of super-strong, pre-stretching-enhanced gelatin-based hydrogels

Hydrogels are becoming increasingly popular in biomedical and soft machine manufacturing, but their practical application is limited by poor mechanical properties. In recent years, Hofmeister effect-enhanced gelatin hydrogels have become popular. However, the synergy of the Hofmeister effect using other toughening methods is still less investigated. We have fabricated an ultra-high strength gelatin-based hydrogel by introducing ligand cross-linking and hydrogen bonds. Unlike conventional double-network hydrogels, the dense physical cross-linking involving sacrificial bonds gives the hydrogel excellent fatigue resistance and self-recovery properties. The enhancement of mechanical properties by the Hofmeister effect is attributed to the disruption of the hydration shell of the gelatin molecular chains, which leads to stronger interactions between the molecular chains. The mechanical properties of the hydrogels are adjustable over a wide range by varying the concentration of the soaked (NH₄)₂SO₄ solution. The fixation of the gelatin molecular chain orientation by the Hofmeister effect and the reorganization of the coordination bonds allow the hydrogels to be self-reinforced by pre-stretching. At the same time, the modulus contraction of hydrogels in high-concentration salt solutions, and relaxation and swelling in dilute solutions exhibit ionic stimulation responses and shape recovery capability, and hybrid hydrogels have great potential as bio-actuators.

Ionogels Derived from Fluorinated Ionic Liquids to Enhance Aqueous Drug Solubility for Local Drug Administration

Gelatin is a popular biopolymer for biomedical applications due to its harmless impact with a negligible inflammatory response in the host organism. Gelatin interacts with soluble molecules in aqueous media as ionic counterparts such as ionic liquids (ILs) to be used as cosolvents to generate the so-called Ionogels. The perfluorinated IL (FIL), 1-ethyl-3-methylpyridinium perfluorobutanesulfonate, has been selected as co-hydrosolvent for fish gelatin due to its low cytotoxicity and hydrophobicity aprotic polar structure to improve the drug aqueous solubility. A series of FIL/water emulsions with different FIL content and their corresponding shark gelatin/FIL Ionogel has been designed to enhance the drug solubility whilst retaining the mechanical structure and their nanostructure was probed by simultaneous SAXS/WAXS, FTIR and Raman spectroscopy, DSC and rheological experiments. Likewise, the FIL assisted the solubility of the antitumoural Doxorubicin whilst retaining the performing mechanical properties of the drug delivery system network for the drug storage as well as the local administration by a syringe. In addition, the different controlled release mechanisms of two different antitumoral such as Doxorubicin and Mithramycin from two different Ionogels formulations were compared to previous gelatin hydrogels which proved the key structure correlation required to attain specific therapeutic dosages.

Hofmeister effect in gelatin-based hydrogels with shape memory properties

The soaking strategy with the Hofmeister effect has been proposed to fabricate gelatin- based hydrogels with excellent properties. However, the modulation mechanism of hydrogels lacks in-depth study. In this work, we studied in detail the effects of Hofmeister ions on the structural, thermal, viscoelastic and mechanical properties of gelatin hydrogels. The results showed that kosmotropic anions (Cit^3-, SO₄^2-, H₂PO₄^- and S₂O₃^2-) enhanced hydrogen bonds and hydrophobic interactions between gelatin molecules, resulting in increases in the length and content of triple helices and thus improving the properties of gelatin hydrogels. In contrast, chaotropic anions (I^- and SCN^-) weakened the interactions between gelatin molecules, and thus attenuated the properties. Based on the Hofmeister effect, we successfully fabricated gelatin poly N-methylolacrylamide (PNMA) double network hydrogels with shape memory properties. The Hofmeister effect provides an excellent route for the rational design and fabrication of functional gelatin-based hydrogels.

Influence of Hofmeister anions on structural and thermal properties of a starch-protein-lipid nanoparticle

A self-assembled soluble nanoparticle, composed of common food biopolymers (carbohydrate, protein) and lipid, was previously reported by our laboratory. Although carrying capacity of valuable small molecules was demonstrated, physical functional properties are also important. Given the stabilization or destabilization characteristics of Hofmeister anion on macromolecular structures, mainly on proteins, here, we investigated the effects of different sodium salts composed of different Hofmeister anions on the structural and thermal properties of these self-assembled nanoparticles for improved functionalities. The salts were added into the mixture that was prepared in a diluted system during nanoparticle formation. Increased concentration of kosmotropic anions, in contrast to the chaotropic anion tested, resulted in nanoparticles with higher molar mass, hydrodynamic radius, and molecular density with more compact arrangement. The nanoparticles produced in presence of kosmotropic anions dissociated at higher temperatures and required higher enthalpies compared to the control sample. Spherical nanoparticles were formed for the kosmotropes with shear thinning behavior, contrary to rod-like nanoparticles for the chaotrope with near-Newtonian behavior. These findings help to gain an understanding of the effect of altering environmental conditions on the nanoparticles with an aim of producing desired structures for applications.

Tannic acid modified antifreezing gelatin organohydrogel for low modulus, high toughness, and sensitive flexible strain sensor

Current hydrogel strain sensors have met assorted essential requirements of wearing comfort, mechanical toughness, and strain sensitivity. However, an increment in the toughness of a hydrogel usually leads to an increase in elastic moduli that could be unfavorable for wearing comfort. In addition, traits of biofriendly and sustainability require synthesis of the hydrogels from natural polymer-based networks. We propose a novel strategy to fabricate an ionic conductive organohydrogel from natural biological macromolecule "gelatin" and polyacid "tannic acid" to resolve these challenges. Tannic acid modified the structure of the gelatin network in the ionic conductive organohydrogels, that not only led to an increase in toughness accompanying a decrease in elastic moduli but also headed to higher strain sensitivity and tunability. The proposed methodology exhibited tunable tensile modulus from 27 to 13 kPa, tensile strength from 287 to 325 kPa, elongation at fracture from 510 to 620%, toughness from 500 to 550 kJ/m³_, conductivity from 0.29 to 0.8 S/m, and strain sensitivity (GF = 1.4-6.5). Moreover, the proposed organohydrogel exhibited excellent freezing tolerance. This study provides a facile yet powerful strategy to tune the mechanical and electrical properties of organohydrogels which can be adapted to various wearable sensors.

3D Bioprinting of Living Materials for Structure-Dependent Production of Hyaluronic Acid

3D bioprinting of living materials represents an interesting paradigm toward the efficacy enhancement for the biosynthesis of various functional compounds in microorganisms. Previous studies have shown the success of 3D-printed bioactive systems in the production of small molecular compounds. However, the feasibility of such a strategy in producing macromolecules and how the geometry of the 3D scaffold influences the productivity are still unknown. In this study, we printed a series of 3D gelatin-based hydrogels immobilized with fermentation bacteria that can secrete hyaluronic acid (HA), a very useful natural polysaccharide in the fields of biomedicine and tissue engineering. The 3D-printed bioreactor was capable of producing HA, and an elevated yield was obtained with the system bearing a grid structure compared to that either with a solid structure or in a scaffold-free fermentation condition. As for the grid structure, bioreactors with a 90° strut angel and a median interfilament distance displayed the highest HA yield. Our findings highlighted the significant role of 3D printing in the spatial control of microorganism-laden hydrogel structures for the enhancement of biosynthesis efficiency.

Impact of Magnesium Salt on the Mechanical and Thermal Properties of Poly(vinyl alcohol)

The effects of magnesium salts with various anion species on the structure and properties of a poly(vinyl alcohol) (PVA) film were studied. The glass transition temperature of the PVA film increased following the addition of a magnesium salt. Furthermore, the salt greatly enhanced the modulus and yield stress and reduced the crystallinity of the film. These effects were attributed to the strong ion–dipole interactions between the magnesium salts and the PVA chains. The strength of interaction, i.e., the reduction of segmental motions, depended on the anion species in the following order: Mg(ClO₄)₂, MgBr₂, MgCl₂, Mg(CH₃COO)₂, and MgSO₄. The order corresponded to the Hofmeister series, which predicts the ability to break the structure of water.

Rheological properties and interactions of fish gelatin-κ-carrageenan polyelectrolyte hydrogels: The effects of salt

This study mainly explored the effects of low-concentration salts (0.1, 0.5 mM NaCl and Na₂ SO₄ ) on the gel, rheological and structural properties of fish gelatin (FG)-κ-carrageenan (κC) polyelectrolyte hydrogels. The results showed that κC could increase the gel strength, hardness, and chewiness of the FG-κC polyelectrolyte hydrogels, while the addition of salts had a negative effect. The rheological behaviors showed that the addition of salts reduced the apparent viscosity, gel, and melting points of the FG-κC polyelectrolyte hydrogels. Compared with NaCl, Na₂ SO₄ -treated FG-κC had lower gel strength, hardness, viscosity, gelation, and melting points, while the addition of salts increased the fluorescence intensity by unfolding FG molecules. The secondary structure analysis results showed that the addition of NaCl and Na₂ SO₄ decreased α-helix and β-sheet contents of FG-κC by destroying the hydrogen bond of FG-κC.

Hofmeister effect on the viscosity properties of gelatin in dilute solutions

The effect of various Hofmeister anions on the molecular conformation of gelatin in dilute solutions was investigated by viscosity, optical rotation and dynamic light scattering (DLS). The results showed that the intrinsic viscosity of gelatin decreased in the presence of the kosmotropic anions such as Citrate^3-, SO₄^2-, H₂PO₄^- and MeCOO^-, whereas it was increased with the addition of chaotropes such as Cl^- and KSCN^-. Furthermore, the intrinsic viscosity of gelatin was directly correlated to the hydration entropy of kosmotropic anions, suggesting that the decrease of the intrinsic viscosity was attributed to the strong hydration effect of kosmotropes. The strong dehydration of gelatin facilitated the folding of the polymer chains into helix bundles, validated by the results of optical rotation. On the contrary, the chaotropic anions could interact directly with polypeptide backbones, and the intrachain hydrogen bonds were destroyed. As a result, the polymer chains expanded, which was confirmed by DLS data, and the intrinsic viscosity was increased. These observations indicate that the molecular conformation of gelatin can be modulated by Hofmeister anions.

Hygroscopicity of Hofmeister Salts and Glycine Aerosols-Salt Specific Interactions

The Hofmeister effect of inorganic ions to precipitate proteins has been used to understand the coagulation phenomenon in colloid and protein science. Herein, for the first time, this effect is studied on the hygroscopicity of aerosols using ATR-FTIR spectroscopy. The representative Hofmeister salts (MgSO₄, KCl, NH₄NO₃) and amino acid (glycine) with different amino acid/salt molar ratios (ASRs) are mixed and atomized into micrometer-sized particles. For mixed kosmotrope (MgSO₄)/glycine and chaotrope (NH₄NO₃)/glycine with an ASR of 1:1, both ERHs (efflorescence relative humidities) and DRHs (deliquescence relative humidities) are absent. However, for the mixtures of glycine and neutral salt (KCl), no DRH is observed while 66.2 and 61.4% ERH of glycine is detected for mixtures with ASRs of 1:1 and 1:3, respectively, which is similar to pure glycine. For the mixture of NH₄NO₃/glycine with an ASR of 1:3, ERH and DRH are found to be 15.4 and 32.2% RH, less than that of pure NH₄NO₃. Further, interactions between glycine-salt and/or water is also studied in the mixtures during hydration and dehydration. Water-mediated ion-glycine interaction is detected based on the two glycine bands merging into one band. Glycine-SO₄^2- interaction is present for glycine/sulfate in all ASRs, while glycine-NO₃^- interaction is only seen for 1:3 glycine/NH₄NO₃ mixtures during hydration. This work opens a window to understand the Hofmeister effect on the hygroscopicity of atmospheric aerosols.

Strengthening gelatin hydrogels using the Hofmeister effect

A simple yet effective soaking treatment has been proposed to fabricate hydrogels with desirable mechanical properties, but the strengthening mechanism of hydrogels lacks an in-depth study. Here, we investigated the influence of kosmotropic citrate anion on the structure and properties of immersed gelatin hydrogels. The obtained hydrogels possessed the properties of high strength, modulus and toughness simultaneously. The dehydration of hydrogels facilitated the interactions among gelatin molecules, resulting in the formation of helix structures. Both the content and length of the triple helices increase with an increase in citrate concentration, which in turn contributes to the strengthening of hydrogels. The excellent mechanical performances of these hydrogels may open up new applications for protein materials.