 | This article may be too technical for most readers to understand.(March 2018) |
Lyotropy (a portmanteau of lyo- "dissolve" and -tropic "change") refers to concentration-dependent physical effects in solutions and often more specifically to ion-specific behavior in aqueous solutions.
Ions in an aqueous solutions display ion specific behavior that has been commonly exemplified by the Hofmeister series. Stemming from observations by Franz Hofmeister in the 1870s with egg white lysozyme, lyotropic effects led to a classification of ions on their abilities to salt in or salt out proteins. [1]
Because of the positive charge of lysozyme, the original series turned out to be different than the series for most proteins. Thus, the series can change depending on the protein in solution and the concentrations of the ions in solution. Lyotropy- like the Hofmeister series- classifies ions and their abilities to salt in/ salt out proteins.
In 1936, Voet investigated lyotropic behavior to quantify the effects of salt action on molecules and predict the behavior using mathematical models. Using agar and gelatin, he formulated an equation to predict the salting-out action of different ions for other colloids. Lyotropic numbers, Nlyo, based on this work appear to be related to the charge density of the ions. [2]
Lyotropic activity also influences swelling of gels, surface tension, rate of saponification processes, viscosity of salt solutions, and heats of hydration. [3] [4]
The current understanding of ion-pairing equilibria in an aqueous environment can also be traced to the Eigen-Tamm model that introduced the use of two equilibria states for ion pairs: the contact ion pair (CIP, ion pairs in close contact) and the separated ion pair (SIP, ion pairs separated from each other). [5] As an early application of ion-pairing equilibria, Kester and Pytkowicz studied the role of sulfate and divalent cation ion-pairing in seawater. [6]
This ion-specific behavior was also elucidated through Collin's Law of Matching Water Affinities that describes the strength of ion-pairing in terms of ion size and counterion, while also incorporating coordination state and entropy. [7] [8] Modern computational approaches to simulation of ion-pairing involve molecular dynamic simulations and ab initio calculations that often incorporate polarizable continuum solvent models. [9]
Following the law of matching water affinities, chaotrope-chaotrope and kosmotrope-kosmotrope ion pairs prefer the CIP state; whereas, chaotrope-kosmotrope ion pairs prefer SIP or unpaired states. [5] Another important lyotropic effect is the pairing of ions to charged headgroups of biological molecules. Vlachy et al. proposed that from chaotropic to kosmotropic headgroups, the ordering follows carboxylate, sulfate and sulfonate groups. In this context, a sodium ion (Na+) will prefer a carboxylate, and a potassium ion (K+) will prefer a sulfonate, which has important partitioning effects in biological systems. [10] Protein solubility depends on pH and salt concentration, where small changes in the local environment can lead to Hofmeister series reversals. [9]
In aqueous solutions of glycans, lyotropic ion-pairing effects often dominate molecular interactions by controlling salt-bridge binding. [11] Modern computational approaches in salt-bridge formation in proteins demonstrate mechanisms underlying the favorable arginine-arginine pairing that is due to reduction in electrostatic repulsion due to pi-stacking interactions. [12]
In carbohydrates, electrostatic and ion pairing are the dominant mechanisms for molecular interactions. Modern computational approaches in salt bridge formation in protein demonstrate that the favorable arginine-arginine pairing (i.e. conserved arginine) is due to reduction in electrostatic repulsion. [12]
Electrolyotropy incorporates Donnan-potential spatial gradients and ion-specific pairing, and is used to determine the distribution of the ions and electric potential by modeling charges as being either fixed or free. [13] A canonical example is a surface-tethered polyelectrolyte brush with a variety of different fixed charged groups interacting with free ions and ion-pairs to minimize Gibbs free energy.
Using streaming current measurements in a microslit electrokinetic system (MES), electrolyotropic theory can be used to determine stoichiometric dissociation constants of ion pairing. [14] This approach proves useful in characterizing complex polyelectrolytes and mixtures of ions in solutions like those found in biological systems. In fact, pH and salt concentrations directly affect stoichiometric dissociation constants. Application of electrolyotropic theory has been proposed as a model of mucosal tissue. [13] [15]
Solvation describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the solute, including solubility, reactivity, and color, as well as influencing the properties of the solvent such as its viscosity and density. If the attractive forces between the solvent and solute particles are greater than the attractive forces holding the solute particles together, the solvent particles pull the solute particles apart and surround them. The surrounded solute particles then move away from the solid solute and out into the solution. Ions are surrounded by a concentric shell of solvent. Solvation is the process of reorganizing solvent and solute molecules into solvation complexes and involves bond formation, hydrogen bonding, and van der Waals forces. Solvation of a solute by water is called hydration.
Salting out is a purification technique that utilizes the reduced solubility of certain molecules in a solution of very high ionic strength. Salting out is typically used to precipitate large biomolecules, such as proteins or DNA. Because the salt concentration needed for a given protein to precipitate out of the solution differs from protein to protein, a specific salt concentration can be used to precipitate a target protein. This process is also used to concentrate dilute solutions of proteins. Dialysis can be used to remove the salt if needed.
A chaotropic agent is a molecule in water solution that can disrupt the hydrogen bonding network between water molecules. This has an effect on the stability of the native state of other molecules in the solution, mainly macromolecules by weakening the hydrophobic effect. For example, a chaotropic agent reduces the amount of order in the structure of a protein formed by water molecules, both in the bulk and the hydration shells around hydrophobic amino acids, and may cause its denaturation.
Ammonium sulfate precipitation is one of the most commonly used methods for large and laboratory scale protein purification and fractionation that can be used to separate proteins by altering their solubility in the presence of a high salt concentration.
Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes (salts) and polymers and are sometimes called polysalts. Like salts, their solutions are electrically conductive. Like polymers, their solutions are often viscous. Charged molecular chains, commonly present in soft matter systems, play a fundamental role in determining structure, stability and the interactions of various molecular assemblies. Theoretical approaches to describe their statistical properties differ profoundly from those of their electrically neutral counterparts, while technological and industrial fields exploit their unique properties. Many biological molecules are polyelectrolytes. For instance, polypeptides, glycosaminoglycans, and DNA are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.
Aqueous biphasic systems (ABS) or aqueous two-phase systems (ATPS) are clean alternatives for traditional organic-water solvent extraction systems.
Co-solvents are defined as kosmotropic (order-making) if they contribute to the stability and structure of water-water interactions. In contrast, chaotropic (disorder-making) agents have the opposite effect, disrupting water structure, increasing the solubility of nonpolar solvent particles, and destabilizing solute aggregates. Kosmotropes cause water molecules to favorably interact, which in effect stabilizes intramolecular interactions in macromolecules such as proteins.
In chemistry, a salt bridge is a combination of two non-covalent interactions: hydrogen bonding and ionic bonding. Ion pairing is one of the most important noncovalent forces in chemistry, in biological systems, in different materials and in many applications such as ion pair chromatography. It is a most commonly observed contribution to the stability to the entropically unfavorable folded conformation of proteins. Although non-covalent interactions are known to be relatively weak interactions, small stabilizing interactions can add up to make an important contribution to the overall stability of a conformer. Not only are salt bridges found in proteins, but they can also be found in supramolecular chemistry. The thermodynamics of each are explored through experimental procedures to access the free energy contribution of the salt bridge to the overall free energy of the state.
Cation–π interaction is a noncovalent molecular interaction between the face of an electron-rich π system (e.g. benzene, ethylene, acetylene) and an adjacent cation (e.g. Li+, Na+). This interaction is an example of noncovalent bonding between a monopole (cation) and a quadrupole (π system). Bonding energies are significant, with solution-phase values falling within the same order of magnitude as hydrogen bonds and salt bridges. Similar to these other non-covalent bonds, cation–π interactions play an important role in nature, particularly in protein structure, molecular recognition and enzyme catalysis. The effect has also been observed and put to use in synthetic systems.
Franz Hofmeister was an early protein scientist, and is famous for his studies of salts that influence the solubility and conformational stability of proteins. In 1902, Hofmeister became the first to propose that polypeptides were amino acids linked by peptide bonds, although this model of protein primary structure was independently and simultaneously conceived by Emil Fischer.
The Hofmeister series or lyotropic series is a classification of ions in order of their lyotrophic properties, which is the ability to salt out or salt in proteins. The effects of these changes were first worked out by Franz Hofmeister, who studied the effects of cations and anions on the solubility of proteins.
Enzyme catalysis is the increase in the rate of a process by a biological molecule, an "enzyme". Most enzymes are proteins, and most such processes are chemical reactions. Within the enzyme, generally catalysis occurs at a localized site, called the active site.
Implicit solvation is a method to represent solvent as a continuous medium instead of individual “explicit” solvent molecules, most often used in molecular dynamics simulations and in other applications of molecular mechanics. The method is often applied to estimate free energy of solute-solvent interactions in structural and chemical processes, such as folding or conformational transitions of proteins, DNA, RNA, and polysaccharides, association of biological macromolecules with ligands, or transport of drugs across biological membranes.
Protein precipitation is widely used in downstream processing of biological products in order to concentrate proteins and purify them from various contaminants. For example, in the biotechnology industry protein precipitation is used to eliminate contaminants commonly contained in blood. The underlying mechanism of precipitation is to alter the solvation potential of the solvent, more specifically, by lowering the solubility of the solute by addition of a reagent.
Lyotropic liquid crystals result when fat-loving and water-loving chemical compounds known as amphiphiles dissolve into a solution that behaves both like a liquid and a solid crystal. This liquid crystalline mesophase includes everyday mixtures like soap and water.
In surface science, a double layer is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge, consists of ions which are adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the "diffuse layer".
The Bromley equation was developed in 1973 by Leroy A. Bromley with the objective of calculating activity coefficients for aqueous electrolyte solutions whose concentrations are above the range of validity of the Debye–Hückel equation. This equation, together with Specific ion interaction theory (SIT) and Pitzer equations is important for the understanding of the behaviour of ions dissolved in natural waters such as rivers, lakes and sea-water.
Polyelectrolytes are charged polymers capable of stabilizing colloidal emulsions through electrostatic interactions. Their effectiveness can be dependent on molecular weight, pH, solvent polarity, ionic strength, and the hydrophilic-lipophilic balance (HLB). Stabilized emulsions are useful in many industrial processes, including deflocculation, drug delivery, petroleum waste treatment, and food technology.
Polysulfobetaines are zwitterionic polymers that contain a positively charged quaternary ammonium and a negatively charged sulfonate group within one constitutional repeat unit. In recent years, polysulfobetaines have received increasing attention owing to their good biotolerance and ultralow-fouling behavior towards surfaces. These properties are mainly referred to a tightly bound hydration layer around each zwitterionic group, which effectively suppresses protein adsorption and thus, improves anti-fouling behavior. Therefore, polysulfobetaines have been typically employed as ultrafiltration membranes, blood-contacting devices, and drug delivery materials.
An ion network is an interconnected network or structure composed of ions in a solution. The term "ion network" was coined by Cho and coworkers in 2014. The notion of extended ion aggregates in electrolyte solutions, however, can be found in an earlier report. The ion network is particularly relevant in high-salt solutions where ions can aggregate and interact strongly and it has been investigated in an increasing number of research and review articles.