Dipolar ions amino acids

The zwitterion contains carboxlate and ammonium ionic centers but has a net charge of zero. a-Amino acids exist exclusively as zwitterions in the solid state. Even when drawn as the uncharged molecule, we must understand that the actual structure is that of the dipolar ion. Amino acids are referred to as amphoteric compounds because both acidic and basic groups are contained in the same molecule. 

The dipolar ion has both an acidic group, the ammonium cation, and a basic group, the carboxylate anion, so it can act as either an acid or a base. Such compounds are termed amphoteric. The species that is present in aqueous solution depends on the pH. In the pH range near neutral the amino acid is present in the form of the dipolar ion. In acidic solution the carboxylate group becomes protonated and the amino acid is present as a cation, whereas in basic solution the ammonium group gives up a proton and the molecule exists as an anion ... 

Edsall JT. Dipolar ions and acid-base equilibria. In Proteins, Amino Acids and Peptides as Dipolar Ions. Cohn EJ, Edsall JT, eds. 1965. Hafner Publishing, New York. pp. 75-115. 

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"). 

Cohn, E. J., and Edsall, J. T., Proteins, Amino Acids, and Peptides as Ions and Dipolar Ions, ... 

For glycine, it is known that the dielectric constants of water increases rapidly and linearly with the concentration of the amino acid, reaching a value of about 135 at a concentration of 2.5 mol liter" at 25 C. D is given by D = 78.54 + 22.58C, where 78.54 is the measured value of D for pure water, 22.58 is the numerical value of the dielectric increment, and C is the concentration in mol/liter. This great increase of D reflects the extremely large moment of glycine as a dipolar ion, and the linearity of the relationship represents the proportionality between D and polarizability that is characteristic of strongly polar media. 

Equation (24) renders intelligible the behavior of the dielectric constant of dipolar ions in polar solutions. It explains the linear increase of D with concentration, since changes in partial molar volumes, only slightly dependent on concentration, can only affect the DyVi term. It also explains the nearly identical values of D of the amino acids of the same moment, and the fact that D of a given amino acid is insensitive to changes in the dielectric constant of the solvent, for the change of solvent can directly affect 8 only through the term D V2. 

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). 

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). 

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. 

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) ... 

Like amino acids, this tripeptide is a dipolar ion. The same structure can be abbreviated Ala-Val-Met or, using one-letter abbreviations, AVM. It is customary in describing amino acid sequences to place the amino-terminal (N-terminal) residue at the left end and the carboxyl-terminal (C-terminal) residue at the right end. Residues are numbered sequentially with the N-terminal residue as 1. An example is shown in Fig. 2-6. 

The dissociation constants fC — Kn for a multi-protic acid HrA are defined as stepwise or macroscopic constants (also called molecular constants). For some compounds, e.g. alanine, the pfCa values are far apart (pfC and pK2 are 2.4 and 9.8, respectively). The macroscopic constants can be assigned specifically, fCj to the carboxyl group and K2 to the protonated amino group. At the isoelectric pH of 6.1 the alanine exists almost entirely as the dipolar ion. However, for compounds in which the macroscopic pfCa values are closer together, they cannot be assigned to specific groups. We will consider some specific examples in the next section. 

Normally, amino acids exist as dipolar ions. RCH(NH,+ iCOO-. in a neutral state, where both amino and carboxyl groups are ionized. The dipolar form, RCH(NH2)COOH may be considered, but the dipolar form predominates for the usual monoamino monocarboxylic acid and it is estimated that these forms occur 10s to 106 times more frequently than the non-polar forms. Amino acids decompose thermally at what might be considered a relatively high temperature (200-300°C). The compounds are practically insoluble in organic solvents, have low vapor pressure, and do not exhibit a precisely defined melting point. 

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. 

Exercise 25-3 How would the general features of the plot of concentration of dipolar ion and charged species versus pH for glycine (Figure 25-1) change for 6-amino-hexano ic acid, which has pKa values of 4.43 and 10.75 Give special attention to the position of the isoelectric point and the width of the pH range over which the dipolar ion is expected to be the most stable species present. 

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. 

Solubility of horse carbon monoxide hemoglobin in different salt solutions. The addition of a moderate amount of salt (salting in) is required to solubilize this protein. At high concentrations, certain salts compete more favorably for solvent, decreasing the solubility of the protein and thus leading to its precipitation (salting out). (Source-. E. J. Cohn and J. T. Edsall, Proteins, Amino Acids, and Peptides as Ions and Dipolar Ions. Copyright 1942, Reinhold, New York, N.Y.)... 

Certain substituents (e.g. the amino group) may markedly affect the solubility and other properties of a sulphonic acid or a carboxylic acid. Thus such sulphonic acids as the aminobenzenesulphonic acids and the pyridine- and quinoline-sulphonic acids exist in the form of inner salts or dipolar ions that result from the interaction of the basic amino group and the acidic sulphonic acid group. Sulphanilic acid, for example, is more accurately represented by formula (1) than by formula (2)-. 

Know the meaning of a-amino acid, essential amino acid, dipolar ion, amphoteric, L configuration, isoelectric point (p/), electrophoresis, pKa. 

Review Sec. 17.7 if necessary, and use Table 17.1 for the structures of the amino acids. Write the structures in neutral form, recognizing that dipolar ion structures are possible and that the exact form and degree of ionization depend on the pH of the solution. 

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