Peptides are short chains of amino acids linked by peptide bonds. Their composition — the specific amino acids and their arrangement — determines their physical properties, chemical behavior, and biological function.
Amino Acid Building Blocks
The 20 Standard Amino Acids
All naturally occurring peptides are built from 20 standard amino acids, each with a unique side chain (R group) that determines its properties:
Nonpolar (Hydrophobic):
Glycine, Alanine, Valine, Leucine, Isoleucine, Proline, Phenylalanine, Tryptophan, Methionine
Polar (Uncharged):
Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine
Positively Charged:
Lysine, Arginine, Histidine
Negatively Charged:
Aspartic acid, Glutamic acid
Non-Standard Amino Acids
Some peptides contain modified or non-standard amino acids:
- D-amino acids (mirror images of natural L-forms)
- Modified amino acids (phosphorylated, glycosylated)
- Synthetic amino acids not found in nature
Peptide Bond Formation
The peptide bond forms through a condensation reaction:
- The carboxyl group (−COOH) of one amino acid reacts with the amino group (−NH₂) of the next
- A water molecule is released
- The resulting amide bond (−CO−NH−) is the peptide bond
Properties of the Peptide Bond
- Partial double-bond character — restricts rotation, creating a planar geometry
- Trans configuration — most peptide bonds adopt the trans arrangement (except before proline)
- Stability — resistant to hydrolysis under physiological conditions
How Composition Affects Properties
Solubility
- Hydrophilic residues increase water solubility
- Hydrophobic peptides may require organic co-solvents
- Charged residues are pH-dependent in their solubility effects
Stability
- Methionine and cysteine are susceptible to oxidation
- Asparagine can deamidate, especially at Asn-Gly sequences
- Tryptophan is sensitive to light degradation
Structure
- Proline introduces rigid kinks in the chain
- Cysteine can form disulfide bridges
- Glycine provides maximum flexibility
Sequence Notation
Peptides are written from N-terminus (amino end) to C-terminus (carboxyl end):
H-Ala-Gly-Phe-Leu-OH or using single-letter codes: AGFL
Why Composition Matters
At Evolve Aminos, we understand that precise peptide composition is fundamental. Even a single amino acid change can dramatically alter a peptide's properties. Our rigorous quality control ensures exact composition as specified.