Acid-Base Behavior of Amino Acids

Amino acids are amphoteric compounds i.e., they contain both acidic and basic groups. Because of this, they arc capable of bearing a net electrical charge, which depends on the nature of the solution. 

The charge carried by a molecule influences the way in which it interacts with other molecules use can be made of this property in the isolation and purification of amino acids and proteins. Therefore, it is important to have a clear understanding of the factors that influence the charge carried on amino acids. 

The major biological solvent is water, and the acid-base behavior of dissolved molecules is intimately linked with the dissociation of water. Water is a weak electrolyte capable of dissociating to a proton and a hydroxyl ion. In this process, the proton binds to an adjacent water molecule to which it is hydrogen-bonded (Chap. 4) to form a hydronium ion (H30+)  

In pure water at 25°C, at any instant there are 1.0 x 10-7mol L 1 of H30+ and an equivalent concentration of OH- ions. It must be stressed that the proton is hardly ever bare in water because it has such a high affinity for water molecules. Hydrated complexes other than H30+ have been suggested, but since water is so extensively hydrogen-bonded, it is difficult to identify these species experimentally, and as a simplification the hydrated proton is often written as H +.  

The dissociation of water is a rapid equilibrium process for which we can write an equilibrium constant  

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The acid-base behavior of amino acids may also be illustrated via titration curves. If one started with aspartic acid hydrochloride, that is, aspartic acid crystallized from solution in hydrochloric acid, one would require 3 mol base to remove completely the protons from 1 mol aspartic acid. The titration curve obtained with structures at each step of the reaction series is shown in Figure 4.1. Note that the isoelectric point is attained after one proton equivalent has been removed from the molecule. At this point, aspartic acid contains one positive and one negative charge it is zwitterionic. 

Explain the acid-base behavior of amino acids. 

Glycine, with a piCj of 2.34 and pJ 2 of 9.60, exemplifies the acid-base behavior of amino acids whose R groups have no ionizable constituents. Glycine is sometimes used in vitro to prepare buffers at pH 1.0 to 3.8 or 8.1 to 11.0. The ionization constants in amino acids are given in Table 20-2. 

Acid-Base Behavior of Amino Acids Section 16.1B... 

The influence of micelles on the acid-base properties of analytes can be significant and is reflected in changes in log Kh. A gocd example is the acid-base behavior of amino acids and peptides, which present the following equilibria [44,45] ... 

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