Acid dissociation constant amino acid

The pronounced electron-withdrawing nature of the 1,2,5-thiadiazole system is also evidenced by strong carbonyl electrophilic activation and by enhancement of carboxy acidity. The acid dissociation constants of thiadiazole acids, discussed in Section 4.09.4.1, fall in the range 1.5-2.5. The 1,2,3-thiadiazole carboxylic acids are easily decarboxylated at 160-200 °C. This reaction has been used for the synthesis of monosubstituted derivatives as well as the parent ring and deuterated derivatives <68AHC(9)107>. An efficient bromo-decarboxylation of 3-amino-1,2,5-thiadiazole-carboxylic acid has also been reported <70BRP1190359>. 

At low pH, both the ammonium group and the carboxyl group are protonated. At high pH, neither is protonated. Acid dissociation constants of amino acids are listed in Table 10-1, where each compound is drawn in its fully protonated form. 

Table 10-1 Acid dissociation constants of amino acids... 

Fig. 2 Theoretical influence of pH on the ratio of equilibrium constants FCapp/- pep and Kapp/Kest for peptide (solid line) and ester (dotted line) bond formation, respectively. pKi and pK2 are the acid dissociation constants of the carboxyl and amino groups, respectively, the values used for the calculation (pi i = 3.75, pK2 = 7.75) are representative of an internal peptide bond. Theoretical values calculated using the following equations Kapp/ffpep = 1/(1 + [H+]/fCi)(l + K2/[H+D with Kpep = [peptide]/[RC02-][R NH3+], Kapp/ffest = 1/(1 + Ki/[H+]) with Kest = [ester]/[RC02H][R OH]...

The perturbation of the acid dissociation constant of an amino acid residue as a consequence of its environment within a protein represents another mechanism for enhancing its reactivity relative to a free amino acid in solution. Most amino acid residues react with their respective modification reagents in their unprotonated form instead of in their conjugate acid form. Eq. (4.10) describes the pH dependence of the simple bimolecular reaction (eq. 4.9) of the free base form of nucleophilic amino acid side chain with a non-ionizable modification reagent where is the acid dissociation constant and Aj is the total concentration of amino acid. 

The true acidic dissociation constant of the amino acid is evidently identical with the hydrolysis constant of the basic group with an apparent dissociation constant Kb," the true basic dissociation constant corresponds to the value of the hydrolysis constant for the acid group on the older basis. 

In a recent study the acid dissociation constants of substituted l-amino-4-aryl-2(li/)-pyrimidinethiones of the type 1 and stability constants of their silver complexes have been measured pH-potentiometrically in a 75% (v/v) mixture of dioxane/water11. The influence of the substituents R1 and R2 on pKa and lgf n values has been discussed11. 

Available from Pharmacia Biotech as Immobiline, these are "derivatives of acrylamide with different dissociation constants. For acidic Immobiline (with pK 3.6 or 4.6), carboxylic acid is chosen as a functional group for the bases, tertiary amino groups are selected. When different pK values of Immobiline are mixed in appropriate proportions, together with acrylamide, bisacrylamide, TEMED, and ammonium persulfate, the buffering groups responsible for ereating the pH gradient become covalently attached to the polyacrylamide base. 

If the acid dissociation constants of the amino acid serine CHoOH CHNH2 COOH) are 6.2 x 10 and 7.1 x 10" state which ionic form will predominate at the following pH values 2.0, 5.0, 8.0, and 11.0. What is the isoionic point ... 

Acid Dissociation Constants of Selected n-Fructose-Amino Acids at F = 298 K, 7 = 0.2 M... 

K i and K 2 refer to the concentration acid dissociation constants of carboxyl and amino groups, respectively. Respective values for parent amino acids are given in parentheses. For the imidazole... 

This table lists the and pi (pH at the isoelectric point) values of a-amino acids commonly found in proteins along with their abbreviations. The dissociation constants refer to aqueous solutions at 25°C. 

A more challenging problem is to find the pH of a solution prepared from a polyprotic acid or one of its conjugate species. As an example, we will use the amino acid alanine whose structure and acid dissociation constants are shown in Figure 6.11. 

These are the definitions of the two characteristic dissociation constants normally expressed in terms of p K. When three dissociating groups are present in a molecule there are three piC values, ie, pfC, P 3- knowledge of these piC values is important in the separation or isolation of each amino acid by ion-exchange chromatography. 

Amino-2-hydroxybenZOiC acid. This derivative (18) more commonly known as 4-aminosa1icy1ic acid, forms white crystals from ethanol, melts with effervescence and darkens on exposure to light and air. A reddish-brown crystalline powder is obtained on recrystallization from ethanol —diethyl ether. The compound is soluble ia dilute solutioas of nitric acid and sodium hydroxide, ethanol, and acetone slightly soluble in water and diethyl ether and virtually insoluble in benzene, chloroform or carbon tetrachloride. It is unstable in aqueous solution and decarboxylates to form 3-amiaophenol. Because of the instabihty of the free acid, it is usually prepared as the hydrochloride salt, mp 224 °C (dec), dissociation constant p 3.25. 

Himdin [8001-27-2] is a polypeptide of 66 amino acids found ia the saUvary gland secretions of the leech Himdo medicinalis (45). It is a potent inhibitor of thrombin and biads to y-thrombia with a dissociation constant of 0.8 x 10 ° M to 2.0 x lO " M. Himdin forms a stable noncovalent complex with free and bound thrombin completely iadependent of AT-III. This material has now been cloned and expressed ia yeast cells (46,47). Its antigenic poteatial ia humans remains to be estabUshed. 

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