Introduction to the Analysis of Amino Acids
Structure and Nomenclature of Amino Acids
Amino acids are organic compounds that contain both the amine and carboxylic acid groups. The most common amino acids have a general structure in which a primary amine and a carboxy group are tied to the same carbon atom (Figure 1).
The carbon atom is also linked to a hydrogen atom and a -R group that confers to each amino acid particular properties. The -R group gives to each amino acid specific properties including how they behave during chromatographic separation. The structure of the -R group and the nomenclature of the more common amino acids are listed in the table below:
| Name | Symbol (3-letter) | Symbol (1-letter) | Molecular Weight | Side chain (R group) |
|---|---|---|---|---|
| Aspartic Acid | Asp | D | 133 | CH2-COOH |
| Glutamic Acid | Glu | E | 147 | CH2-CH2-COOH |
| Alanine | Ala | A | 89 | CH3 |
| Asparagine | Asn | N | 132 | CH2-CONH2 |
| Cysteine | Cys | C | 121 | CH2-SH |
| Cystine | CySS | 240 | CH2-S-S-CH2-CH(NH2)-COOH | |
| Glutamine | Gln | Q | 146 | CH2-CH2-CONH2 |
| Glycine | Gly | G | 75 | H |
| Isoleucine | Ile | I | 131 | CH(CH3)-CH2-CH3 |
| Leucine | Leu | L | 131 | CH2-CH(CH3)(CH3) |
| Methionine | Met | M | 149 | CH2-CH2-S-CH3 |
| Phenylalanine | Phe | F | 165 | CH2-C6H6 |
| Serine | Ser | S | 105 | CH2OH |
| Threonine | Thr | T | 119 | CHOH-CH3 |
| Tryptophan | Trp | W | 204 | CH2-C8H5N |
| Tyrosine | Tyr | Y | 181 | CH2-C6H5OH |
| Valine | Val | V | 117 | CH(CH3)(CH3) |
| Arginine | Arg | R | 174 | CH2-CH2-CH2-NH-C(NH)-NH2 |
| Histidine | His | H | 154 | CH2-C3H2N2 |
| Lysine | Lys | K | 146 | CH2-CH2-CH2-CH2-NH2 |
| Ornithine | Orn | O | 132 | CH2-CH2-CH2-NH2 |
The abbreviations for the amino acids, like those of the peptide nomenclature, are standard on an international level. Amino acids are found in their free state as well as combined in peptides (linear chains of amino acids) or proteins (also linked to a variety of substrates, for example carbohydrates).
All of these amino acids can be analyzed by means of chromatographic techniques, after appropriate sample preparation.
The bond occurs through the amine group of the first amino acid and the carboxy group of the other (Figure 2).
The bonding of two amino acids forms a dipeptide, of three amino acids a tripeptide, of different amino acids a polypeptide. A protein is a large polypeptide, with more than 20 amino acids.
The analysis of the amino acid sequence of polypeptides and proteins are carried out after hydrolysis of the peptide linkages.
Sample Preparation
Proteins generally contain 20 essential amino acids. In preparing protein samples for analysis, the principal idea is to break the peptide linkages between the amino acids without damaging the side chains.
With modern analytical instruments, sample preparation may require a much longer time than that required for the analysis. Still, the correct preparation technique is fundamental to the entire analytical procedure.
Some of the more common sample types requiring amino acid analysis are:
- pure protein, both natural and synthetic
- cereals and foods, where the essential protein level is important
- vegetable matter, for both nutritional and research use
Vapor phase hydrolysis with HCl is the most frequently used sample preparation technique. It is performed under reflux conditions or at 110 °C in sealed quartz tubes, from which air has been previously evacuated. The hydrolysis time varies from 18 to 72 hours, depending on the type of peptide linkages.
Hydrolysis under different conditions must be performed to obtain a complete screening of the amino acids present.
Hydrolysis: Continue to product overview, technical details, resource library, or accessories.
General: Go to the Milestone front page, or send us an inquiry.