Amino dissociation

The above method of preparing a neutral solution of the ammonium salt cannot be applied to extremely weak acids (e.g., some amino-acids), the ammonium salts of which dissociate in boiling aqueous solution. 

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. 

Dissociation. In aqueous solution, amino acids undergo a pH-dependent dissociation (37) ... 

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. 

The hterature suggests that more than one mechanism may be operative for a given antiozonant, and that different mechanisms may be appHcable to different types of antiozonants. All of the evidence, however, indicates that the scavenger mechanism is the most important. All antiozonants react with ozone at a much higher rate than does the mbber which they protect. The extremely high reactivity with ozone of/)-phenylenediamines, compared to other amines, is best explained by their unique abiUty to react ftee-tadicaHy. The chemistry of ozone—/)-PDA reactions is known in some detail (30,31). The first step is beheved to be the formation of an ozone—/)-PDA adduct (32), or in some cases a radical ion. Pour competing fates for dissociation of the initial adduct have been described amine oxide formation, side-chain oxidation, nitroxide radical formation, and amino radical formation. 

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. 

Deprotonation of enols of P-diketones, not considered unusual at moderate pH because of their acidity, is faciUtated at lower pH by chelate formation. Chelation can lead to the dissociation of a proton from as weak an acid as an aUphatic amino alcohol in aqueous alkaU. Coordination of the O atom of triethanolamine to Fe(III) is an example of this effect and results in the sequestration of iron in 1 to 18% sodium hydroxide solution (Fig. 7). Even more striking is the loss of a proton from the amino group of a gold chelate of ethylenediamine in aqueous solution (17). 

In base the tetrahedral intennediate is fonned in a manner analogous to that proposed for ester saponification. Steps 1 and 2 in Figure 20.8 show the fonnation of the tetrahedral intennediate in the basic hydrolysis of amides. In step 3 the basic amino group of the tetrahedral intennediate abstracts a proton from water, and in step 4 the derived ammonium ion dissociates. Conversion of the carboxylic acid to its conesponding carboxylate anion in step 5 completes the process and renders the overall reaction ineversible. 

Histidine is one of the 20 naturally occurring amino acids commonly found in proteins (see Chapter 4). It possesses as part of its structure an imidazole group, a five-membered heterocyclic ring possessing two nitrogen atoms. The pAl for dissociation of the imidazole hydrogen of histidine is 6.04. 

From a chemical point of view, the common amino acids are all weak polyprotic acids. The ionizable groups are not strongly dissociating ones, and the degree of dissociation thus depends on the pH of the medium. All the amino acids contain at least two dissociable hydrogens. 

Values for for the common amino acids are typically 0.4 to 1.0 X 10 M, so that typical values of pAl2 center on values of 2.0 to 2.4 (see Table 4.1). In a similar manner, we can write the second dissociation reaction as... 

Typical values for pAlg are in the range of 9.0 to 9.8. At physiological pH, the a-carboxyl group of a simple amino acid (with no ionizable side chains) is completely dissociated, whereas the a-amino group has not really begun its dissociation. The titration curve for such an amino acid is shown in Figure 4.7. 

If the a-amino group is one-third dissociated, there is one part Gly for every two parts Gly°. The important is the for the amino group. The glycine a-amino group has a of 9.6. The result is... 

Note that the dissociation constants of both the a-carboxyl and a-amino groups are affected by the presence of the other group. The adjacent a-amino group makes the a-COOH group more acidic (that is, it lowers the pAl, ) so... 

Calculate the pH at which the e-amino group of lysine is 20% dissociated. 

I) derived from this by dissociation, or in a mobile equilibrium mixture of both these forms. Dobbie et reproduced the spectra of cotarnine solutions containing varying amounts of potassium hydroxide by using cotarnine chloride and hydrocotamine and by dissolving mixtures of the latter two compounds or by placing the separate solutions of these compounds in the apparatus in series. Thus no evidence could be obtained for the occurrence of the amino-aldehyde (3) postulated by Roser. Steiner, Kitasato, and Skinner came to similar conclusions. The band at 285 m/x in alkaline solutions is not due to an aromatic aldehyde. This band also occurs in the spectrum of hydrocotamine (10a) and in the carbinolamine... 

The determination of the degree of dissociation of cotarnine ° and the good agreement with the values derived from measurements of electrical conductivity with those from the spectrophotometric methods is indirect evidence that no significant part of the undissociated cotarnine is in the amino-aldehyde form. In the conductance calculation, the undissociated part was neglected. If this included a significant amount of amino-aldehyde (i.e., a secondary base), there would be a noticeable discrepancy in the degree of dissociation obtained by the two methods. 

On the basis of the dissociation constant values, it seems sensible to conclude that, in these moderately basic carbinolamines, the hydrogen atom of the hydroxyl group is suflQciently acid to be eliminated under the influence of an alkali and by its transfer to the nitrogen atom of the mesomeric anion, the formation of the amino-aldehyde form may result. Instead of the amino-aldehyde, however, the corresponding bimolecular ether (15a-c) can be obtained. " It can be concluded that the formation of the bimolecular ether (S l or 8 2 mechanism) and the formation of the amino-aldehyde (B-SeI or B-Se2 mechanism) are competitive reactions. It seems probable that where the first reaction can occur the latter one is pushed into the background. The triple tautomeric system postulated by Gadamer... 

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