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Related Concept Videos

Ionic Strength: Overview01:12

Ionic Strength: Overview

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The ionic strength of a solution is a quantitative way of expressing the total electrolyte concentration of a solution. This concept was first introduced in 1921 by two American physical chemists, Gilbert N. Lewis and Merle Randall, while describing the activity coefficient of strong electrolytes. During the calculation of ionic strength (I or μ), all the cations and anions are considered. However, the concentration (c) of an ion with a greater charge number (z) has a greater contribution...
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The extended Debye-Hückel equation indicates that the activity coefficient of an ion in an aqueous solution at 25°C depends on three partially interdependent properties: the ionic strength of the solution, the charge of the ion, and the ion size. 
The activity coefficient value for an ion is close to one when the solution has almost zero ionic strength, i.e., when the solution shows close to ideal behavior. As the ionic strength of the solution increases from 0 to 0.1 mol/L, a...
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Crystal Field Theory - Octahedral Complexes02:58

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Crystal Field Theory
To explain the observed behavior of transition metal complexes (such as colors), a model involving electrostatic interactions between the electrons from the ligands and the electrons in the unhybridized d orbitals of the central metal atom has been developed. This electrostatic model is crystal field theory (CFT). It helps to understand, interpret, and predict the colors, magnetic behavior, and some structures of coordination compounds of transition metals.
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Formation of Complex Ions03:45

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A type of Lewis acid-base chemistry involves the formation of a complex ion (or a coordination complex) comprising a central atom, typically a transition metal cation, surrounded by ions or molecules called ligands. These ligands can be neutral molecules like H2O or NH3, or ions such as CN− or OH−. Often, the ligands act as Lewis bases, donating a pair of electrons to the central atom. These types of Lewis acid-base reactions are examples of a broad subdiscipline called coordination...
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Ligand Binding and Linkage00:49

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Allosteric proteins have more than one ligand binding site; the binding of a ligand to any of these sites influences the binding of ligands to the other sites. When a protein is allosteric, its binding sites are called coupled or linked.  In the case of enzymes, the site that binds to the substrate is known as the active site and the other site is known as the regulatory site. When a ligand binds to the regulatory site, this leads to conformational changes in the protein that can influence...
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Ionic Strength: Effects on Chemical Equilibria01:19

Ionic Strength: Effects on Chemical Equilibria

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The addition of an inert ionic compound increases the solubility of a sparingly soluble salt. For example, adding potassium nitrate to a saturated solution of calcium sulfate significantly enhances the solubility of calcium sulfate. Le Châtelier's principle cannot predict this shift in the equilibrium. Instead, this could be explained in terms of changes in the effective concentration of the ions in solution in the presence of added inert salt.
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Solution Ionic Strength Can Modulate Functional Loop Conformations in E. coli Dihydrofolate Reductase.

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Ionic strength significantly impacts enzyme structure and function. High salt concentrations stabilize the occluded M20 loop in E. coli dihydrofolate reductase, affecting catalysis and drug design insights.

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Area of Science:

  • Biochemistry
  • Structural Biology
  • Enzyme Kinetics

Background:

  • The M20 loop in E. coli dihydrofolate reductase (ecDHFR) exists in multiple conformations, influencing enzyme catalysis.
  • The transition between closed and occluded M20 loop states was hypothesized to facilitate product release.

Purpose of the Study:

  • To investigate the effect of solution ionic strength on ecDHFR M20 loop conformations, independent of ligand binding.
  • To understand how environmental factors influence enzyme structure relevant to catalysis and drug design.

Main Methods:

  • Molecular dynamics simulations of ecDHFR in model CaCl2 solutions.
  • Analysis of free energy barriers between M20 loop conformations at varying ionic strengths (IM).

Main Results:

  • A significant free energy barrier forms between occluded and closed M20 loop states at ionic strengths above the E. coli physiological threshold (~0.24 M).
  • At high ionic strengths (>0.3 M), the occluded conformation is stabilized, consistent with crystal structures.
  • At lower ionic strengths (≤0.15 M), the M20 loop favors open/partially closed conformations, also aligning with experimental structures.

Conclusions:

  • Solution ionic strength is a critical factor influencing ecDHFR M20 loop conformations.
  • Enzyme structures obtained at non-physiological ionic strengths may not accurately represent catalytic mechanisms or be suitable for structure-based drug design.