Neutral amino acids are a group of amino acids that carry no net electrical charge at a physiological pH (around 7). They are classified as either nonpolar (hydrophobic) or polar (hydrophilic) based on their side chains' interaction with water.
Nonpolar Neutral Amino Acids
These amino acids have hydrophobic side chains, meaning they tend to avoid water. Examples include:
- Glycine (Gly, G): The simplest amino acid with a single hydrogen atom as its side chain.
- Alanine (Ala, A): Has a methyl group as its side chain.
- Valine (Val, V): Features a branched isopropyl side chain.
- Leucine (Leu, L): Possesses a branched isobutyl side chain.
- Isoleucine (Ile, I): Another branched-chain amino acid with a slightly different structure than leucine.
- Methionine (Met, M): Contains a thioether group in its side chain.
- Proline (Pro, P): Unique in its cyclic structure, making it less flexible than other amino acids.
- Phenylalanine (Phe, F): Has a benzene ring as its side chain.
- Tryptophan (Trp, W): Contains an indole ring in its side chain.
Polar Neutral Amino Acids
These amino acids have hydrophilic side chains, which means they interact well with water. Examples include:
- Serine (Ser, S): Contains a hydroxyl group (-OH) in its side chain.
- Threonine (Thr, T): Similar to serine, but with an additional methyl group.
- Tyrosine (Tyr, Y): Possesses a hydroxyl group attached to a benzene ring.
- Cysteine (Cys, C): Contains a thiol group (-SH) that can form disulfide bonds.
- Asparagine (Asn, N): Has an amide group in its side chain.
- Glutamine (Gln, Q): Similar to asparagine, but with an extra methylene group.
Note: While histidine is sometimes categorized as neutral, its imidazole side chain can carry a positive charge at a lower pH. Therefore, its categorization can be context-dependent. The classification of amino acids as "neutral" typically refers to their charge at physiological pH.
The references provided confirm these examples and classifications. Further research on amino acid properties will provide a more complete understanding.
