> For the complete documentation index, see [llms.txt](https://docs.rplpeptides.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.rplpeptides.com/docs/glossary/peptide-chemistry/amino-acid.md). # Amino Acid ## Definition An amino acid is an organic compound containing both an amino group (-NH₂) and a carboxyl group (-COOH), along with a unique side chain (R-group). Amino acids are the building blocks of peptides and proteins. ## Overview Twenty standard amino acids are encoded by the genetic code and serve as the fundamental units from which all peptides and proteins are constructed. Each amino acid has a unique side chain that determines its chemical properties — hydrophobic, hydrophilic, acidic, basic, or aromatic — and influences peptide behavior, stability, and biological activity. ## Technical Explanation **Basic Structure:** Every amino acid shares a central carbon (α-carbon) bonded to: * An amino group (-NH₂) * A carboxyl group (-COOH) * A hydrogen atom (-H) * A variable side chain (-R) **Classification by Side Chain:** * Hydrophobic: Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Tryptophan, Proline * Hydrophilic (Polar): Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine * Acidic: Aspartic Acid, Glutamic Acid * Basic: Lysine, Arginine, Histidine **Role in Peptide Design:** The sequence and properties of amino acids determine peptide folding, solubility, stability, receptor binding, and biological activity. Modifications can be introduced at specific residues to enhance these properties. ## Importance in Peptide Documentation Amino acid composition is a core identity parameter confirmed by mass spectrometry and amino acid analysis. Specifications on a COA or TDS reference the expected amino acid sequence. Purity assessment depends on knowing the correct amino acid composition. ## Related Terms * [Peptide](/docs/glossary/peptide-chemistry/peptide.md) * [Peptide Sequence](/docs/glossary/peptide-chemistry/peptide-sequence.md) * [Molecular Weight](/docs/glossary/chemical-terms/molecular-weight.md) * [Modification](/docs/glossary/peptide-chemistry/modification.md) ## Frequently Asked Questions **Q: Why do D-amino acids matter in research peptides?** A: D-amino acids can increase resistance to enzymatic degradation, extending the half-life of peptides in biological systems. Some research peptides incorporate D-amino acids for this reason. **Q: Can amino acid substitutions change peptide activity?** A: Yes — even a single amino acid substitution can dramatically alter receptor binding, stability, solubility, and biological activity. This is the basis for designing peptide analogs. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.rplpeptides.com/docs/glossary/peptide-chemistry/amino-acid.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.