Amino acids dissociation

It must be pointed out that in general it is not possible to obtain an isoelectric point for a pure ampholyte, since equation (13) for the hydrogen ion concentration in the latter will be equal to that given by (14) for the isoelectric point in special cases only. It is usual, therefore, to add a small amount of alkali or acid in order to obtain an isoelectric solution. At the isoelectric point an amino-acid dissociates equally to yield RHt and R " ions, but if the hydrogen ion concentration exceeds the theoretical value for this point, there will be a tendency for the hydrogen ions to react with R- or RH= = to yield RH= = or RHj, respectively. In a solution of smaller hydrogen ion concentration, however, the RHt or RH" = ions will tend to ionize further. These conclusions have been utilized to determine the isoelectric points of amino-acids a small amount of ampholyte is added to each of a series of solutions of known pH these solutions should not be too strongly buffered. If there... 

Yang, J.Z. (1992). Thermodynamics of amino acid dissociation in mixed solvents. 3 Glycine in aqueous glucose solutions from 5 to 45°C, /. Solution Chem, Vol. 11, No. 9, pp. 1131-1143, ISSN 0095-9782... 

Table 1.2. Amino acids dissociation constants and isoelectric points at 25 °C...

In the next step, Glu 270 is a general-acid catalyst, increasing the leaving tendency of the amino group. When the reaction is over, the amino acid (phenylalanine in this example) and the peptide with one less amino acid dissociate from the enzyme, and another molecule of substrate binds to the active site. 

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. 

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. 

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. 

Different Types of Proton Transfers. Molecular Ions. The Electrostatic Energy. The ZwiUertons of Amino Acids. Aviopro-tolysis of the Solvent. The Dissociation Constant of a Weak Acid. Variation of the Equilibrium Constant with Temperature. Proton Transfers of Class I. Proton Transfers of Classes II, III, and IV. The Temperature at Which In Kx Passes through Its Maximum. Comparison between Theory and Experiment. A Chart of Occupied and Vacant Proton Levels. 

Table 9 includes data on the first dissociation constants of seven weak acids it will be recalled that we expect these to fall into class III. The table includes the second dissociation constants of five acids, phosphoric, sulfuric, oxalic, malonic, and carbonic, which fall into class IV, while the amino acids glycine and alanine provide four examples that should fall into class II. 

More specifically, the pi of any amino acid is the average of the two acid-dissociation constants that involve the neutral zwitterion. For the 13 amino acids with a neutral side chain, pi is the average of pKal and p/amino acids with either a strongly or weakly acidic side chain, pi is the average of the two lowest pKa values. For the three amino acids with a basic side chain, pi is the average of the two highest pKz values. 

The isoelectric species is the form of a molecule that has an equal number of positive and negative charges and thus is electrically neutral. The isoelectric pH, also called the pi, is the pH midway between pA values on either side of the isoelectric species. For an amino acid such as alanine that has only two dissociating groups, there is no ambiguity. The first pK (R—COOH) is 2.35 and the second pK (R—NH3+) is 9.69. The isoelectric pH (pi) of alanine thus is... 

Peptides inside the mass specttometet ate broken down into smaller units by coUisions with neuttal helium atoms (collision-induced dissociation), and the masses of the individual fragments are determined. Since peptide bonds are much more labile than carbon-carbon bonds, the most abundant fragments will differ from one another by units equivalent to one or two amino acids. Since—with the exception of leucine and isoleucine—the molecular mass of each amino acid is unique, the sequence of the peptide can be reconstructed from the masses of its fragments. 

Initiation of protein synthesis requires that an mRNA molecule be selected for translation by a ribosome. Once the mRNA binds to the ribosome, the latter finds the correct reading frame on the mRNA, and translation begins. This process involves tRNA, rRNA, mRNA, and at least ten eukaryotic initiation factors (elFs), some of which have multiple (three to eight) subunits. Also involved are GTP, ATP, and amino acids. Initiation can be divided into four steps (1) dissociation of the ribosome into its 40S and 60S subunits (2) binding of a ternary complex consisting of met-tRNAf GTP, and eIF-2 to the 40S ribosome to form a preinitiation complex (3) binding of mRNA to the 40S preinitiation complex to form a 43S initiation complex and (4) combination of the 43S initiation complex with the 60S ribosomal subunit to form the SOS initiation complex. 

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