Molecular Definition
Peptides are compounds consisting of two or more amino acids linked by peptide bonds. The peptide bond forms through a condensation reaction between the carboxyl group of one amino acid and the amino group of another, releasing a water molecule in the process.
Peptides are distinguished from proteins primarily by length. While definitions vary, peptides generally contain fewer than 50 amino acid residues, whereas proteins contain 50 or more. The term "polypeptide" typically refers to chains longer than dipeptides or tripeptides but shorter than proteins.
Amino Acid Composition
Twenty standard amino acids serve as the building blocks for peptide synthesis. Each amino acid contains an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain (R group) bonded to a central carbon atom.
The sequence of amino acids in a peptide, referred to as the primary structure, determines the peptide's chemical properties and biological activity. This sequence is written from the N-terminus (amino end) to the C-terminus (carboxyl end) by convention.
Standard Amino Acid Categories
- Nonpolar aliphatic: Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline
- Aromatic: Phenylalanine, Tyrosine, Tryptophan
- Polar uncharged: Serine, Threonine, Cysteine, Asparagine, Glutamine
- Positively charged: Lysine, Arginine, Histidine
- Negatively charged: Aspartate, Glutamate
Structural Characteristics
Peptide structure is described at multiple levels. Primary structure refers to the linear amino acid sequence. Secondary structure describes local folding patterns stabilized by hydrogen bonds, including alpha-helices and beta-sheets.
Shorter peptides may lack stable secondary structure in solution, existing as flexible chains with multiple conformations. Cyclic peptides, formed through bonds between residues in the chain, exhibit more constrained conformational behavior.
Physical Properties
Peptide molecular weight is determined by the sum of constituent amino acid masses minus the mass of water molecules released during bond formation. Molecular weight influences solubility, membrane permeability, and analytical detection methods.
Peptide solubility depends on amino acid composition. Sequences rich in charged or polar residues exhibit higher aqueous solubility, while hydrophobic sequences may require organic co-solvents for dissolution.
Research Applications
Peptides serve as research tools in receptor binding studies, enzyme substrate analysis, and protein interaction experiments. Their defined sequences allow for systematic structure-activity relationship investigations.
Synthetic peptides enable researchers to study specific protein fragments, receptor ligands, and signaling molecules under controlled laboratory conditions. Modifications to the peptide sequence or structure permit investigation of functional relationships.
Educational Notice
This material is provided for educational purposes related to laboratory research methodology. It does not constitute medical advice or guidance for human or veterinary applications. Researchers should consult primary literature and institutional guidelines for their specific applications.