Ionizable groups, amino acid structure

At neutral pH (pH =7) both the acid and amine groups will be ionized to give the so-called zwitterion form. There is no pH at which tire amino acid structure will have no ionized groups. 

The Ionization of Amino Acids, The Zwitterion or Dipole Ion. As is well known amino adds are of importance in that they are the structural units from which proteins are formed, and into which food proteins break down during digestion. The simplest compound of the series is glycine, NHaCH2COOH, which like all other amino adds contains an amino and a carboxyl group. The type formula for the series may thus be represented by NH2RCOOH. Amino acids are able to combine with both adds and bases, i.e., they are amphoteric. 

Figure 1.2 Individual amino acids consist of a primary (a) amine, a carboxylic acid group, and a unique side-chain structure (R). At physiological pH, the amine is protonated and bears a positive charge, while the carboxylate is ionized and possesses a negative charge.

Each type of ionizable group in proteins will have a unique pKa based upon the theoretical value for the amino acid and modulated from that value by its own surrounding microenvironment. Minute environmental changes will cause amine containing residues at different structural locations to have different ionization potentials, even if the groups are otherwise chemically identical. 

Successive pK a values of the parent acid H4(EDTA) are 2.0, 2.7, 6.2, and 10.3 at room temperature. The first two represent removal of protons from two of the four —COOH groups. It turns out that the other two ioniz-able protons reside on the nitrogens, so that in Na2H2(EDTA) (the form usually supplied by manufacturers) all the carboxyl groups are ionized to — COO- and the two nitrogens are protonated (giving a zwitterion structure, common among amino acids). When H2(EDTA)2- complexes a metal ion, however, all the protons are displaced, and the flexible molecule wraps... 

Electrostatic charges due to ionized acidic or basic amino acids influence protein solubility. At extremes of pH, many poorly soluble proteins are dissolved and their molecular structures unfolded due to surplus of similar repelling charges. Gluten proteins have few charged groups and so are poorly soluble in neutral solution (15). Dispersions of other proteins must be adjusted to their isoelectric point or have salt added to optimize cohesion and adhesion. 

FIGURE 3-5 The 20 common amino acids of proteins. The structural formulas show the state of ionization that would predominate at pH 7.0. The unshaded portions are those common to all the amino acids the portions shaded in red are the R groups. Although the R group of... 

Amino acids and the structure of the polypeptide chain. Polypeptides are composed of L-amino acids covalently linked together in a sequential manner to form linear chains, (a) The generalized structure of the amino acid. The zwitterion form, in which the amino group and the carboxyl group are ionized, is strongly favored. (b) Structures of some of the R groups found for different amino acids, (c) Two amino acids become covalently linked by a peptide bond, and water is lost, (d) Repeated peptide bond formation generates a polypeptide chain, which is the major component of all proteins. 

Tyrosine contains a phenolic side chain with a pKa of about 9.7—10.1. Due to its aromatic character, tyrosine is second only to tryptophan in contributing to a protein s overall absorptivity at 275—280 nm. Although the amino acid is only sparingly soluble in water, the ionizable nature of the phenolic group makes it often appear in hydrophilic regions of a protein—usually at or near the surface. Thus tyrosine derivatization proceeds without much need for deforming agents to further open protein structure. 

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