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Information about Dissociation Constant

“Kd” redirects here. For other uses, see KD.


Concepts in
Chemical Equilibria
Acid dissociation constant
Binding constant
Chemical equilibrium
Dissociation constant
Distribution coefficient
Equilibrium constant
Equilibrium unfolding
Liquid-liquid extraction
Phase diagram
Phase rule
Reaction quotient
Solubility equilibrium
Stability constant
Thermodynamic equilibrium
Vapor-Liquid Equilibrium
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In chemistry and biochemistry, a dissociation constant is a specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex falls apart into its component molecules, or when a salt splits up into its component ions. The dissociation constant is usually denoted and is the inverse of the affinity constant. In the special case of salts, the dissociation constant can also be called an ionization constant.

For a general reaction



in which a complex breaks down into x A subunits and y B subunits, the dissociation constant is defined



where [A], [B], and [AxBy] are the concentrations of A, B, and the complex AxBy, respectively.

Protein-Ligand binding

The dissociation constant is commonly used to describe the affinity between a ligand () (such as a drug) and a protein () i.e. how tightly a ligand binds to a particular protein. Ligand-protein affinities are influenced by non-covalent intermolecular interactions between the two molecules such as hydrogen bonding, electrostatic interactions , hydrophobic and Van der Waals forces.

The formation of a ligand-protein complex () can be described by a two-state process



the corresponding dissociation constant is defined



where [], [] and [] represent the concentrations of the protein, ligand and complex, respectively.

The dissociation constant has molar units (M), which correspond to the concentration of ligand [] at which the binding site on a particular protein is half occupied, i.e. the concentration of ligand, at which the concentration of protein with ligand bound [], equals the concentration of protein with no ligand bound []. The smaller the dissociation constant, the more tightly bound the ligand is, or the higher the affinity between ligand and protein. For example, a ligand with a nanomolar (nM) dissociation constant binds more tightly to a particular protein than a ligand with a micromolar (M) dissociation constant.

Sub-nanomolar dissociation constants as a result of non-covalent binding interactions between two molecules are rare. Nevertheless, there are some important exceptions. Biotin and avidin bind with a dissociation constant of roughly M = 1 fM = 0.000001 nM.[1] While ribonuclease inhibitor proteins may also bind to ribonuclease with a similar M affinity.[2] The dissociation constant for a particular ligand-protein interaction can change significantly with solution conditions (e.g. temperature, pH and salt concentration). The effect of different solution conditions is to effectively modify the strength of any intermolecular interactions holding a particular ligand-protein complex together.

Drugs can produce harmful side effects through interactions with proteins for which they were not meant to or designed to interact. Therefore much pharmaceutical research is aimed at designing drugs that bind to only their target proteins with high affinity (typically 0.1-10 nM) or at improving the affinity between a particular drug and its in-vivo protein target.

Another notation

A dissociation constant is sometimes expressed by its p, which is defined as:



These p's are mainly used for covalent dissociations (i.e., reactions in which chemical bonds are made or broken) since such dissociation constants can vary greatly.

Dissociation constant of water

As a frequently used special case, the dissociation constant of water is often expressed as Kw:



(The concentration of water is not included in the definition of , for reasons described in the article equilibrium constant.

The value of Kw varies with temperature, as shown in the table below. This variation must be taken into account when making precise measurements of quantities such as pH.

Water temperature Kw*10-14pKw
0°C0.114.92
10°C0.314.52
18°C0.714.16
25°C1.213.92
30°C1.813.75
50°C8.013.10
60°C12.612.90
70°C21.212.67
80°C3512.46
90°C5312.28
100°C7312.14

Acid base reactions

For the deprotonation of acids, K is known as Ka, the acid dissociation constant. Stronger acids, for example sulfuric or phosphoric acid, have larger dissociation constants; weaker acids, like acetic acid, have smaller dissociation constants. A molecule can have several acid dissociation constants. In this regard, that is depending on the number of the protons they can give up, we define monoprotic, diprotic and triprotic acids. The first (e.g. acetic acid or ammonium) have only one dissociable group, the second (carbonic acid, bicarbonate, glycine) have two dissociable groups and the third (e.g. phosphoric acid) have three dissociable groups. In the case of multiple pK values they are designated by indices: pK1, pK2, pK3 and so on. For amino acids, the pK1 constant refers to its carboxyl (-COOH) group, pK2 refers to its amino (-NH3) group and the pK3 is the pK value of its side chain.





References

1. ^ Livnah O, Bayer EA. et al (1993). "Three-dimensional structures of avidin and the avidin-biotin complex". Proc Natl Acad Sci USA. 90 (11): 5076-5080. PMID 8506353. 
2. ^ Johnson RJ, McCoy JG. et al (2007). "Inhibition of Human Pancreatic Ribonuclease by the Human Ribonuclease Inhibitor Protein". J. Mol. Biol retrieved ahead of print. PMID 17350650. 

See also

KD is an abbreviation for:
  • Kharbanda Don
  • Kraft Dinner
  • Kawasaki disease
  • Knocked Down car kits for assembly
  • Kapal DiRaja, ship prefix used in the Royal Malaysian Navy
  • Kaliningradavia, as its IATA airline designator

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chemical equilibrium is the state in which the chemical activities or concentrations of the reactants and products have no net change over time. Usually, this state results when the forward chemical process proceeds at the same rate as their reverse reaction.
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An acid dissociation constant, denoted by Ka, is an equilibrium constant for the dissociation of a weak acid. According to the Brønsted-Lowry theory of acids and bases an acid is only recognised by its reaction with a base.
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The binding constant is a special case of the equilibrium constant K. The equilibrium state of molecular binding, i.e. the balance between the binding and dissociation processes after infinite reaction time, may be formalized as the unbound compounds (reactants) transforming into
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chemical equilibrium is the state in which the chemical activities or concentrations of the reactants and products have no net change over time. Usually, this state results when the forward chemical process proceeds at the same rate as their reverse reaction.
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In the fields of organic and medicinal chemistry, a partition or distribution coefficient (KD) is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium.
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equilibrium constant. See also Determination of equilibrium constants for experimental and computational methods.

Types of equilibrium constants

Association and dissociation constants

In organic chemistry and biochemistry it is customary to use pKa
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In biochemistry, equilibrium unfolding is the process of unfolding a protein or RNA molecule by gradually changing its solution conditions, i.e., its environment. Since equilibrium is maintained at all steps, the process is reversible (equilibrium folding).
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Separation processes

Processes
Acid-base extraction • Chromatography • Crystallization • Dissolved air flotation • Distillation • Drying • Electrochromatography • Filtration • Flocculation • Froth flotation
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In physical chemistry, mineralogy, and materials science, a phase diagram is a type of graph used to show the equilibrium conditions between the thermodynamically-distinct phases.
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The phase rule of Willard Josiah Gibbs in the 1870 is the fundamental rule which phase diagrams are based on.

P + F = C + 2

P is the number of phases present in equilibrium (Types of solid, liquid, gas phases etc).
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In chemistry, reaction quotient is a quantitative measure of the extent of reaction, the relative proportion of products and reactants present in the reaction mixture at some instant of time.
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Solubility equilibrium is any chemical equilibrium between solid and dissolved states of a compound at saturation.

Solubility equilibria involve application of chemical principles and constants to predict solubility of substances under specific conditions (because
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equilibrium constant. See also Determination of equilibrium constants for experimental and computational methods.

Types of equilibrium constants

Association and dissociation constants

In organic chemistry and biochemistry it is customary to use pKa
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thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, and chemical equilibrium. The local state of a system at thermodynamic equilibrium is determined by the values of its intensive parameters, as pressure, temperature, etc.
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Vapor-liquid equilibrium, abbreviated as VLE by some, is a condition where a liquid and its vapor (gas phase) are in equilibrium with each other, a condition or state where the rate of evaporation (liquid changing to vapor) equals the rate of condensation (vapor changing to
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Biochemistry is the study of the chemical processes in living organisms.[1] The word "biochemistry" comes from the Greek word βιοχημεία biochēmeia, which means "the chemistry of life.
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equilibrium constant. See also Determination of equilibrium constants for experimental and computational methods.

Types of equilibrium constants

Association and dissociation constants

In organic chemistry and biochemistry it is customary to use pKa
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complex in chemistry usually is used to describe molecules or ensembles formed by the combination of ligands and metal ions. Originally, a complex implied a reversible association of molecules, atoms, or ions through weak chemical bonds.
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molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.[1][2] In organic chemistry and biochemistry, the term molecule
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Salt is a mineral essential for animal life, composed primarily of sodium chloride. Salt for human consumption is produced in different forms: unrefined salt (such as sea salt), refined salt (table salt), and iodized salt.
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ion is an atom or molecule which has lost or gained one or more electrons, making it positively or negatively charged. A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion
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Inverse or Inversion may refer to:
  • Homosexuality, an older term for homosexuality
  • Inverse (program), a program for solving Inverse and Optimization problems
  • Inversion (music)
  • Inversion (prosody), the reversal of the order of a foot's elements

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Salt is a mineral essential for animal life, composed primarily of sodium chloride. Salt for human consumption is produced in different forms: unrefined salt (such as sea salt), refined salt (table salt), and iodized salt.
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An acid dissociation constant, denoted by Ka, is an equilibrium constant for the dissociation of a weak acid. According to the Brønsted-Lowry theory of acids and bases an acid is only recognised by its reaction with a base.
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complex in chemistry usually is used to describe molecules or ensembles formed by the combination of ligands and metal ions. Originally, a complex implied a reversible association of molecules, atoms, or ions through weak chemical bonds.
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Affinity may refer to:
  • Chemical affinity - the force of attraction between chemical species.
  • Biochemistry, protein-ligand binding affinity
  • Biology, cell affinity refers to how close a cell group tightly to the surrounding cells.

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In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions (these ligands act as
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drug, broadly speaking, is a substance used as a medicine or narcotic.[1] There is no single, precise definition, as there are different meanings in medicine, government regulations, and colloquial usage.[2]

In pharmacology, Dictionary.
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