Amino Acids & Peptides Introduction:

Both D-amino acids and non-α-amino acids occur in nature, but only L-α-amino acids are present in proteins. All amino acids possess at least two weakly acidic functional groups, R-NH3+ & R-COOH. Many also possess additional weakly acidic functional groups such as -OH, -SH, guanidino, or imidazole groups.

While proteins contain only L- α -amino acids, microorganisms elaborate peptides that contain both D- and L-α-amino acids. Several of these peptides are of therapeutic value, including the antibiotics bacitracin and gramicidin A and the antitumor agent bleomycin. Certain other microbial peptides are toxic.

Neither humans nor any other higher animals can synthesize 10 of the 20 common L- α -amino acids in amounts adequate to support infant growth or to maintain health in adults.

The Genetic Code Specifies20 L-Amino Acids Of the over 300 naturally occurring amino acids, 20 constitute the monomer units of proteins

Amino Acids May Have Positive, Negative,or Zero Net Charge Charged and uncharged forms of the ionizable -COOH and -NH3 + weak acid group s

pKa Values Express the Strengths of Weak Acids.At Its Isoelectric pH (pI), an Amino Acid Bears No Net Charge.

The isoelectric, species is the form of a molecule that has an equal number of positive and negative charges and thus is electrically neutral. The isoelectric pH, also called the pI, is the pH midway between pKa values on either side of the isoelectric species. For an amino acid such as alanine that has only two dissociating groups,there is no ambiguity. The first pKa (R-COOH) is 2.35 and the second pKa (R- NH3 +) is The isoelectric pH (pI) of alanine thus is:

For polyfunctional acids, pI is also the pH midway between the pKa values on either side of the isoionic species. For example, the pI for aspartic acid is: For lysine, pI is calculated from:

The Solubility and Melting Points of Amino Acids Reflect Their Ionic Character