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20-September-2008 09:55:59 - Dissociation constant Kd redirects here. For other uses, see KD. Concepts in Chemical Equilibria Acid dissociation constant Binding constant Buffer solution Chemical equilibrium Chemical stability Dissociation constant Distribution coefficient Distribution ratio Equilibrium constant Equilibrium unfolding Equilibrium stage Liquid-liquid extraction Phase diagram Phase rule Reaction quotient Relative volatility Solubility equilibrium Stability constant Thermodynamic equilibrium Theoretical plate Vapor-liquid equilibrium 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 Kd 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 \mathrmA_x\mathrmB_y \rightleftharpoons x\mathrmA + y\mathrmB in which a complex AxBy breaks down into x A subunits and y B subunits, the dissociation constant is defined K_d = \fracA^x \times B^yA_x B_y where A, B, and AxBy are the concentrations of A, B, and the complex AxBy, respectively. Contents 1 Protein-Ligand binding 2 Another notation 3 Dissociation constant of water 4 Acid base reactions 5 References 6 See also Protein-Ligand binding The dissociation constant is commonly used to describe the affinity between a ligand L such as a drug and a protein P 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 C can be described by a two-state process \mathrmC \rightleftharpoons \mathrmP + \mathrmL the corresponding dissociation constant is defined K_d = \frac\left \mathrmP \right \left \mathrmL \right\left \mathrmC \right where P, L and C represent the concentrations of the protein, ligand and complex, respectively. The dissociation constant has molar units M, which correspond to the concentration of ligand L 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 C, equals the concentration of protein with no ligand bound P. 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 μM 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 10 - 15 M = 1 fM = 0.000001 nM.1 Ribonuclease inhibitor proteins may also bind to ribonuclease with a similar 10 - 15 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 Ka is sometimes expressed by its pKa, which is defined as: pKa = - log10Ka These pKa'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: Kw = H + OH - The concentration of water \left \mboxH_2\mboxO \right is not included in the definition of kw, 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 Kw10-14 pKw 0°C 0.1 14.92 10°C 0.3 14.52 18°C 0.7 14.16 25°C 1.2 13.92 30°C 1.8 13.75 50°C 8.0 13.10 60°C 12.6 12.90 70°C 21.2 12.67 80°C 35 12.46 90°C 53 12.28 100°C 73 12.14 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. 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 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 H B ^-2 \rightleftharpoons\ H ^ + + B ^-3 \qquad K_3 = H ^ + \cdot B ^ -3 \over H B ^ -2 \qquad pK_3 = - log K_3 References ^ 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. doi:10.1073/pnas.90.11.5076. PMID 8506353. ^ Johnson RJ, McCoy JG. et al April 2007. Inhibition of Human Pancreatic Ribonuclease by the Human Ribonuclease Inhibitor Protein. Journal of Molecular Biology 368 2: 434-449. doi:10.1016/j.jmb.2007.02.005. PMID 17350650. See also Ki Database Acid pH Scatchard plot Retrieved from http://en..org/wiki/Dissociation_constant Categories: Enzyme kinetics Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages العربية ÄŒesky Deutsch Eesti Español Français í•œêµì–´ Italiano עברית Lietuvių Polski РуÑ?Ñ?кий Svenska УкраїнÑ?ька This page was last modified on 8 August 2008, at 15:28
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