Isoelectric point , of amino acids

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... 

To calculate the isoelectric point of amino acids having other ionizable functional groups, we must also take into account the pKa of the additional functional group in the side chain. 

Pogliani, L. (1992a). Molecular Connectivity Model for Determination of Isoelectric Point of Amino Acids. J.Pharm.ScL, 81,334-336. 

Pogliani, L. (1992a) Molecular connectivity model for determination of isoelectric point of amino acids. /. Pharm. Sci., 81, 334-336. 

Isoelectric point (Section 27.3) pH at which the concentration of the zwitterionic form of an amino acid is a maximum. At a pH below the isoelectric point the dominant species is a cation. At higher pH, an anion predominates. At the isoelectric point the amino acid has no net charge. 

This article describes the further purification of this endo-xylanase and the measurement of its isoelectric point, partial amino acid sequence, activity and stability at various pHs and tempCTatures, and characteristic products when it hydrolyzes linear xylo-oligosaccharides. 

Comment P2 is called the isoelectric point the amino acid exists there mainly in the zwitterionic form, and only in very small and equal concentrations of the cationic and anionic forms. In an electric field, amino acids do not move at that pH, which is important in electrophoresis for isoelectric focussing. 

At the isoelectric point, ionized amino acids have an overall charge of zero. 

The enzyme contains FAD as a prosthetic group and there are many isozymes with different isoelectric points. 1-Amino acid oxidase is largely responsible for the yellow color of snake venoms and does not cause any major toxic actions. 

Properties and Structure. a -Acid glycoprotein (a -AGP) has a molecular mass of about 41,000 and consists of a peptide chain having 181 amino acid residues and five carbohydrate units (14,15). Two cystine disulfide cross-linkages connect residues 5 and 147 and residues 72 and 164. The carbohydrate units comprise 45% of the molecule and contain siaUc acid, hexosamine, and neutral hexoses. In phosphate buffer the isoelectric point of the... 

The isoelectric points of the amino acids in Table 27.3 are midway between the pK values of the zwitterion and its conjugate acid. Take two exanples aspartic acid and lysine. Aspartic acid has an acidic side chain and a pi of 2.77. Lysine has a basic side chain and a pi of 9.74. 

The isoelectric point of an amino acid depends on its structure, with values for the 20 common amino acids given in Table 26.1. The 15 neutral amino acids have isoelectric points near neutrality, in the pH range 5.0 to 6.5. The two acidic amino acids have isoelectric points at lower pH so that deprotonation of the side-chain -C02H does not occur at their pi, and the three basic amino acids, have isoelectric points at higher pH so that protonation of the side-chain amino group does not occur at their pi. 

Solutions of polyelectrolytes contain polyions and the free (individual) counterions. The dissociation of a polyacid or its salt yields polyanions, and that of a polybase or its salt yields polycations, in addition to the simple counterions. The polyampholytes are amphoteric their dissociation yields polyions that have anionic and cationic functions in the same ion and often are called zwitterions (as in the case of amino acids having HjN and COO groups in the same molecule). Such an amphoter will behave as a base toward a stronger acid and as an acid toward a stronger base its solution properties (particularly its effective charge) will be pH dependent, and an isoelectric point (pH value) exists where anionic and cationic dissociation is balanced so that the polyion s charges add up to zero net charge (and solubility is minimal). 

AMCA-NHS, succinimidyl-7-amino-4-methylcoumarin-3-acetic acid, is an amine-reactive derivative of AMCA containing an NHS ester on its carboxylate group (Thermo Fisher). The result is reactivity directed toward amine-containing molecules, forming amide linkages with the AMCA fluorophore (Figure 9.23). Proteins labeled with AMCA show little-to-no effect on the isoelectric point of the molecule. 

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