Amino isoelectric point

Isobutyl group (Section 2 13) The group (CH3)2CHCH2— Isoelectric point (Section 27 3) pH at which the concentration of the zwittenonic 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 isoelec tnc point the ammo acid has no net charge Isolated diene (Section 10 5) Diene of the type... 

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 large part of the dissolved amino acid exists as the ampholyte (2witterion). The isoelectric point (pi) is the pH at which the net electric charge of a dissolved amino acid molecule is 2ero. p /is expressed as... 

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... 

Soybean Protein Isolates. Soybean protein isolates, having a protein content of >90 wt%, are the only vegetable proteins that are widely used in imitation dairy products (1). Most isolates are derived from isoelectric precipitation, so that the soybean protein isolates have properties that are similar to those of casein. They are insoluble at thek isoelectric point, have a relatively high proportion of hydrophobic amino acid residues, and are calcium-sensitive. They differ from casein in that they are heat-denaturable and thus heat-labile. The proteins have relatively good nutritional properties and have been increasingly used as a principal source of protein. A main deterrent to use has been the beany flavor associated with the product. Use is expected to increase in part because of lower cost as compared to caseinates. There has been much research to develop improved soybean protein isolates. 

Table 27.2 includes a column labeled pi, which is the isoelectric point of the anino acid. The isoelectric point, also called the isoionic point, is the pH at which the anino acid has no net charge. It is the pH at which the concentration of the zwitterion is a maximum. At a pH lower than pi, the amino acid is positively charged at a pH higher than pi, the amino acid is negatively charged. For the anino acids in Table 27.2, pi is the average of and pK 2 and lies slightly to the acid side of neutrality. 

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. 

Each amino acid is characterized by an isoelectric point , the pH at which it exists in neutral form. Differences in isoelectric points may be exploited to separate amino acids in what is termed an electrophoresis experiment. 

Amino Acids, the Henderson-Hasselbalch Equation, and Isoelectric Points... 

Thomson MOW Click Organic Interactive to learn to estimate isoelectric points for simple amino acids and peptides. 

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. 

Just as individual amino acids have isoelectric points, proteins have an overall p/ because of the acidic or basic amino acids they may contain. The enzyme lysozyme, for instance, has a preponderance of basic amino acids and thus has a high isoelectric point (p/= 11.0). Pepsin, however, has a preponderance of acidic amino acids and a low- isoelectric point pi 1.0). Not surprisingly, the solubilities and properties of proteins with different pi s are strongly affected by the pH of the medium. Solubility- is usually lowest at the isoelectric point, where the protein has no net charge, and is higher both above and below the pi, where the protein is charged. 

Isoelectric point, pi (Section 26.2) The pH at which the number of positive charges and the number of negative charges on a protein or an amino acid are equal. 

Isoelectric point The pH at which an amino acid does not migrate in an electric field, 624-625 Isoleucine, 622t... 

Crystallisation is often used as a method to recover tire amino add. Because of tire amphoteric character (contains both addic and basic groups) of amino adds, their solubility is greatly influenced by the pH of the solution and usually show minima at the isoelectric point (zero net charge). 

The deduced PGE protein consists of 378 amino acid residues. Based on the known composition of N-termini of PGI and PGII proteins it is assumed that the 39 N-terminal amino acids of PGE form the pre-prosequence of the protein. The molecular weight (35 584) and isoelectric point (pi = 3.6) of mature PGE were calculated based on deduced amino acid sequence. 

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). 

True zwitterionic compounds are rare among drugs. The oral absorption of truly zwitterionic compounds is poor unless the compound is a substrate for an absorptive biological transporter as in an a-amino acid which is a substrate for the PepTl nutrient transporter. The aqueous solubility of a true zwitterionic compound will be at a minimum at the isoelectric point which unfortunately for many compounds happens close to the neutral pH at which oral absorphon occurs. Species extrapolation predicting oral absorphon and pk/pD from preclinical animal tests to man are difficult for zwitterions. 

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