Ionization of amino acids

Zwitterion self-ionization of amino acid to produce COO" and -NH3+... 

Enzymes are active over a limited pH range the pH value of maximum activity is known as the pH optimum, and this is characteristic of the enzyme. It typically reflects the pH necessary to achieve the appropriate ionization of amino acid side-chains at the active site. 

Each enzyme has an optimum pH at which the rate of the reaction that it catalyzes is at its maximum. Slight deviations in the pH from the optimum lead to a decrease in the reaction rate. Larger deviations in pH lead to denaturation of the enzyme due to changes in the ionization of amino acid residues and the disruption of noncovalent interactions. 

B. A. Mansoori, D. A. Volmer, and R. K. Boyd, Wrong-way-round electrospray ionization of amino acids. Rapid Commun. Mass Spectrom. 11 (1997), 1120-1130. 

Ionization of Amino Acids Section 16.1 See problems 16.4 and 16.5 for ionization information. 

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. 

Electron Donation Ionization, OH-Adduct Formation, and Hydrogen Abstraction. Ionization of amino acids or peptides either through photolysis or radiolysis leads to cation radicals, the fate of which will be influenced by the nature of the compound and the medium. 

ANIMATED FIGURE 3.5 The ionization of amino acids. Sign in at www.thomsonedu.com/login to see an animated versien ef this fignre. 

HEAT OF IONIZATION OF AMINO ACID SIDE CHAINS IN ENZYMES... 

Table 1 Heats of ionization of amino acid side chains in enzymes...

Chen Y, Chen H, Aleksandrov A, Orlando TMJ (2008) Role of water, acidity, and surface morphology in surface-assisted laser desorption/ionization of amino acids. Phys Chem C 112 6953-6960... 

Can we utilize this pH-dependent ionization of amino acids for their separation A small example will answer the question. We are given a mixture of three amino acids, alanine, aspartic acid, and arginine. It is required that we separate these three amino acids from each other and obtain them in a more or less pure state. From Table 1.8 we can find out that the pi of alanine. 

The factor 0.5 relates to identically shaped reactant and product potential energy curves. Thus, through perturbation of A /, ionization of amino acids D and E will slow down the forward electron transfer and accelerate the reverse electron transfer. Because of the large distance between Qa and (3b (== 18 A), we neglect the electrostatic interactions between D and E and Qa, but some effect can be expected. 

Figure 1 Ionization of amino acids in solutions. pH dependencies of ionic fractions a - amino acids with aliphatic radicals (alanine, glycine, isoleucine, leucine, and valine) b - amino acids with aromatic radical (phenylalanine), sulfur-containing radical (methionine), and nitrogen in the cycle (proline) c -amino di-carboxylic acids (aspartic acid and glutamic acid) d - amino acid with phenol group (tyrosine) e - di-amino acid (lysine) f - dihydroxyphenylalanine (DOPA). One can see that there is no significant difference in ionization of amino acids that have aliphatic radicals while ionization of amino acids with more complicated radicals differs significantly...

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