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| In [[chemistry]], [[biochemistry]], and [[pharmacology]], a '''dissociation constant (<math>K_{d}</math> )''' 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 (chemistry)|complex]] falls apart into its component [[molecules]], or when a [[salt (chemistry)|salt]] splits up into its component [[ions]]. The dissociation constant is the [[Multiplicative inverse|inverse]] of the [[association constant]]. In the special case of salts, the dissociation constant can also be called an [[ionization constant]].
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| :<math>
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| \mathrm{A}_{x}\mathrm{B}_{y} \rightleftharpoons x\mathrm{A} + y\mathrm{B}
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| </math>
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| in which a complex <math>\mathrm{A}_{x}\mathrm{B}_{y}</math> breaks down into ''x'' A
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| subunits and ''y'' B subunits, the dissociation constant is defined
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| :<math>
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| K_{d} = \frac{[A]^x \times [B]^y}{[A_x B_y]}
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| </math>
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| where [A], [B], and [A<sub>x</sub>B<sub>y</sub>] are the concentrations of A, B, and the
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| complex A<sub>x</sub>B<sub>y</sub>, respectively.
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| One reason for the popularity of the dissociation constant in biochemistry and pharmacology is that in the frequently encountered case where x=y=1, K<sub>d</sub> has a simple physical interpretation: when [A]=K<sub>d</sub>, [B]=[AB] or equivalently [AB]/([B]+[AB])=1/2. That is, K<sub>d</sub>, which has the dimensions of concentration, equals the concentration of free A at which half of the total molecules of B are associated with A. This simple interpretation presumes the absence of competing reactions and does not apply for higher values of x or y.
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| ==Protein-ligand binding==<!-- This section is linked from [[Antibody]] and [[MDMA]] -->
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| The dissociation constant is commonly used to describe the [[Chemical affinity|affinity]] between a [[Ligand (biochemistry)|ligand]] ('''<math>\mathrm{L}</math>''') (such as a [[drug]]) and a [[protein]] ('''<math>\mathrm{P}</math>''') i.e. how tightly a ligand binds to a particular protein. Ligand-protein affinities are influenced by [[non-covalent| non-covalent intermolecular interactions]] between the two molecules such as [[hydrogen bond]]ing, [[electrostatic| electrostatic interactions]], [[hydrophobic]] and [[Van der Waals force]]s. They can also be affected by high concentrations of other macromolecules, which causes [[macromolecular crowding]].<ref>{{Cite pmid|18573087}}</ref><ref>{{Cite pmid|11279227}}</ref>
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| The formation of a ligand-protein complex ('''<math>\mathrm{C}</math>''') can be described by a two-state process
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| :<math>
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| \mathrm{C} \rightleftharpoons \mathrm{P} + \mathrm{L}
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| </math>
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| the corresponding dissociation constant is defined
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| :<math>
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| K_{d} = \frac{\left[ \mathrm{P} \right] \left[ \mathrm{L} \right]}{\left[ \mathrm{C} \right]}
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| </math>
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| where ['''<math>\mathrm{P}</math>'''], ['''<math>\mathrm{L}</math>'''] and ['''<math>\mathrm{C}</math>'''] represent [[molar concentration]]s of the protein, ligand and complex, respectively.
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| The dissociation constant has [[Concentration#Molarity|molar]] units (M), which correspond to the concentration of ligand ['''<math>\mathrm{L}</math>'''] 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 ['''<math>\mathrm{C}</math>'''], equals the concentration of protein with no ligand bound ['''<math>\mathrm{P}</math>''']. 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 (<math>\mu</math>M) dissociation constant. | |
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| Sub-picomolar 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 <math>10^{-15}</math> M = 1 fM = 0.000001 nM.<ref>{{Cite pmid|8506353}}</ref>
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| [[Ribonuclease inhibitor]] proteins may also bind to [[ribonuclease]] with a similar <math>10^{-15}</math> M affinity.<ref>{{Cite pmid|17350650}}</ref>
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| 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 [[non-covalent|intermolecular interactions]] holding a particular ligand-protein complex together.
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| 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 (Negative Design) with high affinity (typically 0.1-10 nM) or at improving the affinity between a particular drug and its ''[[in-vivo]]'' protein target (Positive Design).
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| ===Antibodies===
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| In the specific case of antibodies (Ab) binding to antigen (Ag), usually the affinity constant is used. It is the inverted dissociation constant.
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| :<math>
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| \text{Ab} + \text{Ag} \rightleftharpoons \text{AbAg}
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| </math>
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| :<math>
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| K_{a} = \frac{\left[ \text{AbAg} \right]}{\left[ \text{Ab} \right] \left[ \text{Ag} \right]} = \frac{1}{K_{d}}
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| </math> | |
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| This [[chemical equilibrium]] is also the ratio of the on-rate (k<sub>forward</sub>) and off-rate (k<sub>back</sub>) constants. Two antibodies can have the same affinity, but one may have both a high on- and off-rate constant, while the other may have both a low on- and off-rate constant.
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| :<math>
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| K_{a} = \frac{k_\text{forward}}{k_\text{back}} = \frac{\mbox{on-rate}}{\mbox{off-rate}}
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| </math>
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| == Acid–base reactions ==
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| {{Acids and bases}}
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| {{Main|Acid dissociation constant}}
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| For the [[deprotonation]] of [[acid]]s, ''K'' is known as ''K''<sub>a</sub>, the [[acid dissociation constant]]. Stronger acids, for example [[sulfuric acid|sulfuric]] or [[phosphoric acid]], have larger dissociation constants; weaker acids, like [[acetic acid]], have smaller dissociation constants.
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| (The symbol <math>K_{a}</math>, used for the acid dissociation constant, can lead to confusion with the [[association constant]] and it may be necessary to see the reaction or the equilibrium expression to know which is meant.)
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| Acid dissociation constants are sometimes expressed by p<math>K_{a}</math>, which is defined as:
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| :<math>
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| \mathrm{p}K_{a} = -\log_{10}{K_{a}}
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| </math> | |
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| This <math>\mathrm{p}K</math> notation is seen in other contexts as well; it is mainly used for [[covalent]] dissociations (i.e., reactions in which chemical bonds are made or broken) since such dissociation constants can vary greatly.
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| 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'' [[acid#Polyprotic acids|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 p''K'' values they are designated by indices: p''K''<sub>1</sub>, p''K''<sub>2</sub>, p''K''<sub>3</sub> and so on. For amino acids, the p''K''<sub>1</sub> constant refers to its [[carboxyl]] (-COOH) group, p''K''<sub>2</sub> refers to its [[amino]] (-NH<sub>3</sub>) group and the p''K''<sub>3</sub> is the p''K'' value of its [[side chain]].
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| <math>H_3 B \rightleftharpoons\ H ^ + + H_2 B ^ - \qquad K_1 = {[H ^ +] \cdot [H_2 B ^ -] \over [H_3 B]} \qquad pK_1 = - \log K_1 </math>
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| <math>H_2 B ^ - \rightleftharpoons\ H ^ + + H B ^ {-2} \qquad K_2 = {[H ^ +] \cdot [H B ^{-2}] \over [H_2 B^ -]} \qquad pK_2 = - \log K_2 </math>
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| <math>H B ^{-2} \rightleftharpoons\ H ^ + + B ^{-3} \qquad K_3 = {[H ^ +] \cdot [ B ^ {-3}] \over [H B ^ {-2}]} \qquad pK_3 = - \log K_3 </math>
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| ==Dissociation constant of water==
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| {{Main|Self-ionization of water}}
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| The dissociation constant of water is denoted ''K''<sub>w</sub>:
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| <math>K_w = [\mbox{H}^+] [\mbox{OH}^-]</math>
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| The concentration of water <math>\left[ \mbox{H}_2\mbox{O} \right]</math> is omitted by convention, which means that the value of ''K''<sub>w</sub> differs from the value of ''K''<sub>eq</sub> that would be computed using that concentration.
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| The value of ''K''<sub>w</sub> 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.
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| {| class="wikitable" style="text-align: center;"
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| |-
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| ! Water temperature
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| ! K<sub>w</sub> / 10<sup>−14</sup>
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| ! pK<sub>w</sub><ref>{{cite journal | last1 = Bandura | first1 = Andrei V. | last2 = Lvov | first2 = Serguei N. | year = 2006 | title = The Ionization Constant of Water over Wide Ranges of Temperature and Density | journal = Journal of Physical and Chemical Reference Data | volume = 35 | issue = 1 | pages = 15-30 | doi = 10.1063/1.1928231 | url = http://www.nist.gov/data/PDFfiles/jpcrd696.pdf }}</ref>
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| |-
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| |0°C
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| |0.112
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| |14.95
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| |-
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| |25°C
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| |1.023
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| |13.99
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| |-
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| |50°C
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| |5.495
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| |13.26
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| |-
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| |75°C
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| |19.95
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| |12.70
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| |-
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| |100°C
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| |56.23
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| |12.25
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| |}
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| == See also ==
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| * [[Equilibrium constant]]
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| * [[Ki Database|''K''<sub>i</sub> Database]]
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| * [[Acid]]
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| * [[pH]]
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| * [[Scatchard plot]]
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| ==References==
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| {{Reflist}}
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| {{Chemical equilibria}}
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| [[Category:Equilibrium chemistry]]
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| [[Category:Enzyme kinetics]]
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The lowest in the structure of rare metal called credit cards, gold credit cards have now been around since the understanding of credit cards in the mid-1950s. normal or basic credit cards today magic cards have been exceeded in reputation by gold credit cards and platinum credit cards and so have more commonly become known.
Features of gold charge cards
The benefits that gold credit cards have over other types of cards include:
* reduce yearly membership fees (if any)
* lower tolerance income which if you want to use you need certainly to make
* the same credit control as other credit cards when you yourself have a good credit history or handle your card sensibly
* 0% attention for between 6 9 months when moving your balance from one credit company to a different
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