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Amino Acids

Amino Acids Characteristics and Structures

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This is the general structure of an amino acid.

This is the general structure of an amino acid. R denotes the functional group of the amino acids. The diagram depicts the ionization of an amino acid at pH = 7.4.

Todd Helmenstine

Amino acids are a type of organic acid that contain both a carboxyl group (COOH) and an amino group (NH2). The general formula for an amino acid is given below. Although the neutrally-charged structure is commonly written, it is inaccurate because the acidic COOH and basic NH2 groups react with one another to form an internal salt called a zwitterion. The zwitterion has no net charge; there is one negative (COO-) and one positive (NH3+) charge.

There are 20 amino acids derived from proteins. While there are several methods of categorizing them, one of the most common is to group them according to the nature of their side chains.

Nonpolar Side Chains
There are eight amino acids with nonpolar side chains. Glycine, alanine, and proline have small, nonpolar side chains and are all weakly hydrophobic. Phenylalanine, valine, leucine, isoleucine, and methionine have larger side chains and are more strongly hydrophobic.

Polar, Uncharged Side Chains
There are also eight amino acids with polar, uncharged side chains. Serine and threonine have hydroxyl groups. Asparagine and glutamine have amide groups. Histidine and tryptophan have heterocyclic aromatic amine side chains. Cysteine has a sulfhydryl group. Tyrosine has a phenolic side chain. The sulfhydryl group of cysteine, phenolic hydroxyl group of tyrosine, and imidazole group of histidine all show some degree of pH-dependent ionization.

Charged Side Chains
There are four amino acids with charged side chains. Aspartic acid and glutamic acid have carboxyl groups on their side chains. Each acid is fully ionized at pH 7.4. Arginine and lysine have side chains with amino groups. Their side chains are fully protonated at pH 7.4.

This table shows amino acid names, three- and one-letter standard abbreviations, and linear structures (atomis in bold text are bonded to each other). Click on the amino acid name for its Fischer projection formula.

Table of the Amino Acids

Name Abbreviation Linear Structure
Alanine ala A CH3-CH(NH2)-COOH
Arginine arg R HN=C(NH2)-NH-(CH2)3-CH(NH2)-COOH
Asparagine asn N H2N-CO-CH2-CH(NH2)-COOH
Aspartic Acid asp D HOOC-CH2-CH(NH2)-COOH
Cysteine cys C HS-CH2-CH(NH2)-COOH
Glutamic Acid glu E HOOC-(CH2)2-CH(NH2)-COOH
Glutamine gln Q H2N-CO-(CH2)2-CH(NH2)-COOH
Glycine gly G NH2-CH2-COOH
Histidine his H NH-CH=N-CH=C-CH2-CH(NH2)-COOH
Isoleucine ile I CH3-CH2-CH(CH3)-CH(NH2)-COOH
Leucine leu L (CH3)2-CH-CH2-CH(NH2)-COOH
Lysine lys K H2N-(CH2)4-CH(NH2)-COOH
Methionine met M CH3-S-(CH2)2-CH(NH2)-COOH
Phenylalanine phe F Ph-CH2-CH(NH2)-COOH
Proline pro P NH-(CH2)3-CH-COOH
Serine ser S HO-CH2-CH(NH2)-COOH
Threonine thr T CH3-CH(OH)-CH(NH2)-COOH
Tryptophan trp W Ph-NH-CH=C-CH2-CH(NH2)-COOH
Tyrosine tyr Y HO-Ph-CH2-CH(NH2)-COOH
Valine val V (CH3)2-CH-CH(NH2)-COOH
Related Video
Introduction to Amino Acids

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