Dipolar ion, Zwitterion

This chapter deals with the very important a-amino acids, the building blocks of the proteins that are necessary for the function and structure of living ceils. Enzymes, the highly specific biochemical catalysts are proteins. or-Amino acids are dipolar ions (zwitterions), RCH(N" H,)COO , as is indicated by their crystallinity, high melting point, and solubility in water rather than in nonpolar solvents. The standard (naturally occurring) amino acids are listed in Table 21-1 those marked with an asterisk are essential amino acids that cannot be synthesized in the body and so must be in the diet. They have 1° NHj s except for proline and hydroxyproline (2°). They have different R groups. 

The complete hydrolysis of proteins produces 20 a-amino acids that also occur as free metabolic intermediates. As free acids, they exist mostly as dipolar ions (zwitterions). Except for glycine, they contain chiral a-carbons and therefore exist in the d- and L-enantiomeric pair, of which the L-isomers are the monomeric units of proteins. They are differentiated structurally by their side-chain groups with varying chemical reactivities that determine many of the chemical and physical properties of proteins. These side-chain groups include ... 

In weak acid solution (about pH 5 6.0) the carboxyl group of a neutral amino acid (one amino group and one carboxyl group) is dissociated, and the amino group binds a proton to give a dipolar ion (zwitterion). The pH at which the concentration of the dipolar ion is a maximum is called the isoelectric point (pi) of that amino add. The isoelectric point of an amino add is calculated from the relationship... 

The R group is what gives each AA its identity and determines its specific potential fimctions in the body. At the pH of body fluids, free AAs exist as dipolar ions (zwitterions) in the following basic form ... 

Draw the structures for the dipolar ions (zwitterions) of serine and valine. 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

The zwitterion is also often referred to as a dipolar ion. Note, however, that it is not an ion, but a neutral molecule. 

We saw in Sections 20.3 and 24.5 that a carboxyl group is deprotonated and exists as the carboxylate anion at a physiological pH of 7.3, while an amino group is protonated and exists as the ammonium cation. Thus, amino acids exist in aqueous solution primarily in the form of a dipolar ion, or zwitterion (German zwitter, meaning "hybrid"). 

Zaigmondygold number chem The number of milligrams of protective colloid necessary to prevent 10 milliliters of gold sol from coagulating when 0.5 milliliter of 10% sodium chloride solution is added. zig-mon-de gold, n3m-b3r zwitterion See dipolar ion. tsfid-3r,T,an ... 

Therefore, we can deduce that a-amino acids in solution at pH 7.0 exist as dipolar ions these are called zwitterions (German zwitter = hybrid) (see Section 13.1). 

A zwitterion must not be confused with an ylide, which has opposite electric charges residing on adjacent atoms. The term dipolar ion, while often used in place of zwitterion, is not considered by lUPAC to be an acceptable synonym, and should be avoided. 

When proteins undergo hydrolysis, you wind up with 22 a amino acids, 20 of which are regulcir amino acids and 2 of which are derived amino acids. Amino acids are amphoteric (they possess the characteristics of both acids and bases and can react as either) because both acidic and basic groups are present. An internal acid-base reaction produces a dipolar ion known as a zwitterion (you can see the general structure of one in Figure 16-33). 

When an amino acid is dissolved in water, it exists in solution as the dipolar ion, or zwitterion (German for hybrid ion ), shown in Figure 3-9. A zwitterion can act as either an acid (proton donor) ... 

ZWITTERION. An ion carrying charges of opposite sign, which thus constitutes an electrically neutral molecule with a dipole moment looking like a posilive ion at one end and a negative ion at the other. Most aliphatic amino acids form such dipolar ions, hence react with both strong acids and strong bases. 

An amino-acid therefore can have a number of charged forms, it is an anion at high pH and a cation at low pH at neutral pH it normally has no net charge, but exists as a dipolar ion or zwitterion with both a positive and negative charge. 

All solid amino acids exist as dipolar ions known as zwitterions (Figure 1.2(a)). In aqueous solution the structure of amino acids are dependent on the pH of the solution (Figure 1.2(b)). The pH at which an aqueous solution of an amino acid is electrically neutral is known as the isoelectric point (pi) of the amino acid (Table 1.1). Isoelectric point values vary with temperature. They are used in the design of electrophoretic and chromatographic analytical methods for amino acids. 

Amino acids have both an acidic group, the carboxylic acid group, and a basic group, the amino group. These two functional groups undergo an intramolecular acid base reaction to form a dipolar ion, also known as a zwitterion ... 

Dipolar ion or zwitterion (Section 26.2) A neutral compound containing a covalently linked cation and anion. 

Examples of these molecules are amino acids, which exist in the zwitterionic form in acid solution, and in which the molecule contains an equal quantity of positively and negatively charged groups. Thus, zwitterionic molecules are dipolar ions that can have a total positive or negative charge or be neutral, according to solution pH. 

Although we commonly write amino acids with an intact carboxyl (—COOH) group and amino (—NH2) group, their actual structure is ionic and depends on the pH. The carboxyl group loses a proton, giving a carboxylate ion, and the amino group is proto-nated to an ammonium ion. This structure is called a dipolar ion or a zwitterion (German for dipolar ion ). 

Complexes of monomers with acceptors can initiate polymerizations of the respective monomers even without photoexcitation. The properties of DA complexes and the conditions of the thermal activation are determined by the type of donor (monomer), acceptor, and solvent [300,303]. Ion radicals from the monomer, dipolar intermediates (zwitterions), as well as the relatively... 

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