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Nonpolar and Uncharged Polar Amino Acids
Amino acids differ from one another based on their side chain group. Eight of the twenty amino acids fall into the hydrophobic category because their side chains contain non-polar groups. Alanine, valine, leucine and isoleucine all contain straight-chain hydrocarbon side groups. Methionine contains a straight chain hydrocarbon group that has a sulfur atom. Sulfur has the same electronegativity as carbon, which makes the methionine also non-polar. Phenylalanine, tyrosine and tryptophan all contain non-polar aromatic rings. Tyrosine and tryptophan are slightly less hydrophobic and slightly more reactive than phenylalanine because they contain an -OH and -NH group, respectively. Another amino acid that is actually hydrophobic is proline. However, proline falls into its own category because it is the only amino acid that contains the side chain group bound to the alpha carbon and the nitrogen. This creates a five-membered ring that is structurally obstructive. Glycine, the smallest amino acid, is actually achiral because it contains a hydrogen atom as the side chain. As a result of the small nature of the hydrogen side chain on glycine, it can interact with hydrophobic and hydrophilic environments. There are five amino acids that are polar but uncharged. These include serine, threonine, asparagine, glutamine and cysteine. Cysteine contains a thiol group that is responsible for creating disulfide bridges.
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