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Amino Acids

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    Amino acids are the basic building blocks of proteins. There are twenty naturally occuring amino acids, and they are all characterized by an anime group, a carboxylic acid group, and an unique side chain (or R-group) that gives each one its specific properties.


    The α-carbon of an amino acid is chiral, so each one has L and D isomers. All protein amino acids, however, only exist in their L forms. While the figure to the left depicts the correct functional groups, amino acids are conventionally written in their "zwitterionic" forms, where the amine group is protonated and the carboxyl group is deprotonated due to the pH level of most bodily fluids.


    Amino acids are amphoteric, which means they have acidic and basic tendencies. The carboxyl group is able to lose a proton and the amine group is able to accept a proton. Amino acids are also ionic in character, and behave as ampholytes, meaning they move to their isoelectric points when placed in a pH gradient under an electric field.

    Peptide Bonds

    Amino acids connect with each other by forming peptide bonds involving the amino and carboxyl groups. These bonds are the result of a dehydration reaction, and are planar and partially ionic due to their resonance hybrids.

    List of Amino Acids

    Glycine and alanine are the simplest amino acids. They are non-polar and neutral. Glycine is hydrophilic, and alanine is hydrophobic.

    Glycine, Gly, G Alanine, Ala, A

    Valine, leucine, isoleucine, methionine, and proline are non-polar, neutral, and aliphatic. All are hydrophobic except proline.

    Valine, Val, V Leucine, Leu, L Isoleucine, Ile, I Methionine, Met, M Proline, Pro, P

    Phenylalanine and tryptophan are non-polar, neutral, and aromatic. Phenylalanine is hydrophobic, and tryptophan is hydrophilic.

    Phenylalanine, Phe, F Tryptophan, Trp, W

    Serine, threonine, tyrosine, asparagine, and glutamine are polar and neutral. All are hydrophilic.

    Serine, Ser, S Threonine, Thr, T Tyrosine, Tyr, Y Asparagine, Asn, N Glutamine, Gln, Q

    Cysteine is neutral, non-polar, and hydrophobic.

    Cysteine, Cys, C

    Lysine, arginine, and histidine are polar, positively charged, and hydrophilic.

    Lysine, Lys, K Arginine, Arg, R Histidine, His, H

    Aspartate and glutamate are polar, negatively charged, and hydrophilic.

    Aspartate, Asp, D Glutamate, Glu, E

    Amino Acids is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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