Peptide Glossary: Essential Definitions and Terminology

Acetamidomethyl (Acm) – A thiol protecting group commonly used in peptide synthesis for orthogonal protection of cysteine residues during disulfide bond formation. Active Ester – Reactive amino acid derivatives such as pentafluorophenyl (OPfp) and hydroxysuccinimido (OSu or NHS) esters that find broad application in peptide synthesis. Aggregation – The tendency of growing peptide chains to associate with each other during synthesis, often leading to reduced coupling efficiency and impaired synthesis quality. The main cause of failure for peptide chemosynthesis. Alpha-amino Acid – The building blocks from which peptides are constructed, containing both amine and carboxyl functional groups. Amino Acid – Any molecule that contains both amine and carboxyl functional groups. The fundamental building blocks of peptides and proteins. Antimicrobial Peptides (AMPs) – Peptides that possess antimicrobial activity, frequently cationic and associate with anionic LPS located on the outer membranes of Gram-negative bacteria. Aspartimide Formation – A side reaction where aspartic acid residues can form cyclic imide structures during peptide synthesis, particularly problematic when DBU is used for Fmoc deprotection.

C-terminus – The carboxy-terminal end of a peptide chain. Chemical peptide synthesis most commonly starts at the carboxyl end of the peptide and proceeds toward the amino-terminus. Capping – An optional process where the N-terminal amino group is blocked to prevent the obtained peptide from further reaction. Carbodiimides – Coupling reagents including DCC, DIC, and EDC·HCl commonly used to prepare amides, esters and acid anhydrides from carboxylic acids. CBz (Carboxybenzyl) – A protecting group with a benzyl group that can be removed using catalytic hydrogenation (Pd-C, H₂). Cell-Penetrating Peptides (CPPs) – Short chains of 5–30 amino acid residues widely used for effective intracellular transportation of biologically active drug molecules. COMU – A safer and more efficient coupling reagent than benzotriazole-based reagents, requiring only one equivalent of base rather than two equivalents needed for HATU and HBTU. Coupling Reagents – Chemical compounds used to facilitate the formation of amide bonds between amino acids in peptide synthesis. Examples include HATU, HBTU, HCTU, DIC, and PyBOP. Cyclic Peptides – Peptides in which the amino acid sequence forms a ring structure rather than a straight chain, such as the antibiotics tyrocidin and gramicidin.

EDC – A water-soluble carbodiimide coupling reagent used in peptide synthesis. Epimerization – The unwanted conversion of L-amino acids to D-amino acids during coupling reactions.

Glycopeptides – Peptides containing carbohydrate modifications.

HBTU – A HOBt-based uronium coupling reagent. HCTU – A coupling reagent that remains colorless through long synthesis sequences and has greater stability. HPLC – The primary method for peptide purification and analysis, typically using C18 reverse phase columns. Hydrazine – A reagent used to remove Dde and ivDde protecting groups.

Linker – Bifunctional molecules anchoring the growing peptide to the insoluble carrier. Liquid-Phase Peptide Synthesis (LPPS) – A classical approach to peptide synthesis that is slow and labor-intensive because the product must be manually removed from reaction solution after each step.

N-terminus – The amino-terminal end of a peptide chain. Peptides are biologically synthesized from N-terminus to C-terminus, opposite to chemical synthesis. Native Chemical Ligation – A ligation approach where two shorter fully deprotected synthetic peptides can be joined in solution to access longer peptide lengths. Ninhydrin Test – A qualitative test used to monitor coupling reactions during peptide synthesis by detecting free amino groups.

Peptide – Two or more amino acids chained together by peptide bonds, typically 2 to 50 amino acids. Peptide Bond – A covalent chemical bond formed between two consecutive amino acid monomers. Peptide Library – A systematic combination of different peptides in large numbers. Peptide Mapping – An analytical technique used to identify and characterize proteins by breaking them down into smaller peptide fragments. Peptide Synthesis – A biological or chemical process in which amino acids are added stepwise to a chain. Piperidine – A secondary amine commonly used at 20% concentration in DMF for Fmoc deprotection. Polypeptides – Peptides typically composed of more than ten amino acids. Post-Translational Modifications (PTMs) – Chemical modifications to amino acids that occur after protein synthesis. Protecting Groups – Chemical groups used to prevent unintended reactions during peptide synthesis. Pseudoprolines – Specialized units used in difficult peptide synthesis to reduce aggregation. PyBOP – A phosphonium-based coupling reagent.

Side-Chain Protection – Protection of reactive side chains on amino acids during peptide synthesis using groups like tert-butyl ethers, trityl ethers, or benzyl ethers. Solid-Phase Peptide Synthesis (SPPS) – The most common method of peptide synthesis where the peptide is anchored by its C-terminus to an insoluble polymer. Solution-Phase Synthesis – Traditional peptide synthesis method conducted in solution. Stapled Peptides – Conformationally constrained peptides created using unnatural alkenyl amino acids for peptide ‘stapling’.

Unnatural Amino Acids (UAAs) – Non-proteinogenic amino acids designed for specific research functions. Uronium Salts – A class of coupling reagents including HATU, HBTU, and HCTU.