TSMS-PC-004
Primary Structure: How Sequence Defines Peptide Identity Study Guide
School of Peptide Chemistry · Peptide Chemistry Foundations · Intermediate
Open full lessonLearning objectives
- Define primary structure.
- Explain sequence direction and numbering.
- Describe terminal and side-chain modifications.
- Distinguish composition from sequence.
- Understand why intact mass alone may be insufficient for full identity confirmation.
Executive summary
Primary structure is the exact linear order of amino-acid residues together with defined terminal states, stereochemistry, and covalent modifications.
A sequence is not fully specified until the following are clear:
- residue order,
- N-terminal state,
- C-terminal state,
- stereochemistry,
- disulfide connectivity where applicable,
- side-chain modifications,
- isotopic or reporter labels.
Two molecules may share the same amino-acid composition and molecular mass while differing in residue order or stereochemistry.
A sequence is not fully specified until the following are clear:
- residue order,
- N-terminal state,
- C-terminal state,
- stereochemistry,
- disulfide connectivity where applicable,
- side-chain modifications,
- isotopic or reporter labels.
Two molecules may share the same amino-acid composition and molecular mass while differing in residue order or stereochemistry.
Key takeaways
- Primary structure defines peptide identity at the covalent level.
- Sequence direction is N-to-C.
- Termini, stereochemistry, and modifications must be specified.
- Intact mass is necessary evidence but may not be sufficient.
- Sequence knowledge predicts analytical and stability behavior.
Self-review questions
- Why is sequence direction important?
- Name two terminal modifications.
- Why can intact mass fail to distinguish sequence variants?
- What does amino-acid analysis establish?
- How can sequence predict degradation risk?
Use the full lesson to verify your answers.