The Acid-Base Properties of Amino Acids

The carboxylic acid and amine functional groups are simultaneously present in amino acids, and we might ask whether they are mutually compatible since one group is acidic and the other is basic. Although we have represented the amino acids in Table 17.1 as having amino and carboxyl groups, these structures are oversimplified. 

Amino acids with one amino group and one carboxyl group are better represented by a dipolar ion structure.  

The amino group is protonated and present as an ammonium ion, whereas the carboxyl group has lost its proton and is present as a carboxylate anion. This dipolar structure is consistent with the salt-like properties of amino acids, which have rather high melting points (even the simplest, glycine, melts at 233°C) and relatively low solubilities in organic solvents. 

Amino acids are amphoteric (page 212). They can behave as acids and donate a proton to a strong base, or they can behave as bases and accept a proton from a strong acid. These behaviors are expressed in the following equilibria for an amino acid with one amino and one carboxyl group  

Titration curve for alanine, showing how its structure varies with pH. Electrostatic potential maps of alanine at low pH (a), at the isoelectric point (b), and at high pH (c) illustrate the difference in charge distribution as a function of pH. 

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The following two sections illustrate how a knowledge of the acid-base properties of amino acids and peptides can be used to advantage in designing separation procedures, or predicting separation patterns. 

WhatAre the Acid-Base Properties of Amino Acids ... 

WE BEGIN THIS CHAPTER with a study of amino acids, compounds whose chemistry is built on amines (Chapter 10) and carboxylic acids (Chapter 13). We concentrate in particular on the acid-base properties of amino acids because these properties are so important in determining many of the properties of proteins, polymers of amino acids that have many functions in living organisms. With this understanding of the chemistry of amino acids, we then examine the structure of proteins themselves. 

What Are the Acid-Base Properties of Amino Acids A. Acidic and Basic Groups of Amino Acids... 

Amino acids are characterized by the presence of adjacent carboxylic (-C0 H) and amino (-NH) functional groups. The equilibrium constant for protonation or dissociation of these groups is a function of their position in the amino acid molecule. Therefore, widely differing acid-base properties of amino acids occur, depending upon the number of functional groups and their relative position in the molecule. The dissociation constants for various amino acids used in this investigation are given in Table I. 

Stereochemistry of Amino Acids 339 Acid-Base Properties of Amino Acids 339 The Isoelectric Point 342 Electrophoresis 343 The Peptide Bond 343 Primary Structure of Proteins 344 Secondary Structure of Proteins 344 Tertiary Structure of Proteins 345 The Folding Problem 346 Denaturing Proteins 346 Enzymes 347... 

A compelling case can be made for replacing empirically-derived scales of amino acid properties with parameters either measured directly (e.g., chemical shift data or infrared spectra) or calculated from basic principles. The goal would be to develop all-electronic expressions for the physico-chemical properties of amino acids based on computational methods that include QM calculations. A start in this direction was provided by the successful description of steric effects in terms of polarizability and hydrophUicity as a function of electron density (32). Application of more sophisticated computational approaches wiU speed progress toward this objective. 

Another very important factor influencing the reactivity of both reaction partners is pH. Protonisation of the carbonyl group in acidic solutions increases the reactivity of nucleophOic reagents, while protonised amino groups are less reactive as the nitrogen atom does not have a free electron pair (Figure 4.74). Acid base properties of amino acids, peptides and proteins (formation of cations, amphions and anions) are described in Section 2.2.3.1. 

In many cases only the racemic mixtures of a-amino acids can be obtained through chemical synthesis. Therefore, optical resolution (42) is indispensable to get the optically active L- or D-forms in the production of expensive or uncommon amino acids. The optical resolution of amino acids can be done in two general ways physical or chemical methods which apply the stereospecific properties of amino acids, and biological or enzymatic methods which are based on the characteristic behavior of amino acids in living cells in the presence of enzymes. 

More recently, Stepanov et al. (1989) investigated the acid-base properties of the zwitterion 3.22 which is obtained in the diazotization of 5-amino-3-nitro-l,2,4-triazole. Under alkaline conditions the (Z)-diazoate dianion 3.23 is formed. It can be isomerized thermally to give the (E)-diazoate dianion 3.24. If the solution of this compound is acidified, the primary addition of a proton takes place at the anionic ring nitrogen yielding 3.25, and subsequently the hydrogen-bond-stabilized (Z)-iso-mer (3.26). Further acidification gives the nitrosoamine (3.27). 

The structures of the amino acids reveal any functional groups that result In acidic or basic properties. To determine the acid-base properties of these compounds, look for carboxyl groups and amine groups. 

The oldest method of resolving enantiomers by TLC takes advantage of the natural chiral properties of cellulose and triacetylcellulose resulting from the helical structure of the polymers (98). Amino acid derivatives have been resolved on silica gel layers impregnated with chiral acids or bases, for example. 

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