This peptide glossary defines the terms used on research peptide product pages, certificates of analysis and lab papers, from structure and synthesis to testing, handling, research terms and regulation. Each entry gives the definition first, cites its source and, where useful, links to a deeper guide. Every Disguised Alpha product is for laboratory research use only. Nothing on this page is guidance for use in people.
Jump to: Structure and chemistry, Synthesis and manufacturing, Testing and certificates, Formats and handling, Research terms, Regulatory and documents
Structure and chemistry
Amino acid
An amino acid carries an amino group and a carboxyl group; in the alpha-amino acids of peptides, both sit on one carbon with a side chain (IUPAC-IUB JCBN, 1984). Of over 140 in natural proteins, only selenocysteine and pyrrolysine are known additions to the standard 20 of the genetic code (Ambrogelly et al., 2007).
Amino acid residue
When amino acids combine into a peptide, the elements of water are removed, and what remains of each is called a residue (IUPAC-IUB JCBN, 1984). A residue therefore weighs one water, 18.02 Da, less than the free amino acid.
Peptide bond
A peptide bond is the amide bond joining one amino acid's carboxyl group to the next one's amino group; partial double-bond character keeps it planar (Chen et al., 2012). Links through a side-chain group, such as glutathione's gamma-glutamyl bond, are sometimes called isopeptide bonds (IUPAC-IUB JCBN, 1984).
Peptide
A peptide is any compound produced by amide formation between the carboxyl group of one amino acid and the amino group of another (IUPAC-IUB JCBN, 1984), typically 2 to 50 amino acids in a linear chain or ring (Hellinger et al., 2023). See what is a peptide?
Oligopeptide, polypeptide and protein
Peptides of fewer than about 10 to 20 residues may be called oligopeptides and longer ones polypeptides; defined chains of more than about 50 residues are usually called proteins, though authors differ (IUPAC-IUB JCBN, 1984). FDA authors put peptides at 40 or fewer amino acids (Naik et al., 2025).
Sequence, N-terminus and C-terminus
A peptide's sequence is the order of its residues, written from the N-terminus on the left to the C-terminus on the right. The N-terminal amino group is free or capped, for example by acetylation (Ac-); the C-terminal carboxyl group is free or converted to an amide (-NH₂) (IUPAC-IUB JCBN, 1984), as in oxytocin, CYIQNCPLG-NH₂.
One- and three-letter codes
Each amino acid has a three-letter symbol, such as Gln, and a one-letter code, such as Q, for compact sequences. B stands for Asp or Asn, Z for Glu or Gln and X for an unknown or other residue (IUPAC-IUB JCBN, 1984). See amino acid codes.
L- and D-amino acids
L and D name the two mirror-image configurations at an amino acid's alpha carbon. Glycine is achiral, and a symbol means L unless D is written (IUPAC-IUB JCBN, 1984). A D residue has the same formula and mass as its L form, so mass alone cannot reveal it.
Molecular formula
A molecular formula gives the number of atoms of each element in one molecule, in accord with its molecular mass; an empirical formula gives only proportions (IUPAC Green Book). Oxytocin is C₄₃H₆₆N₁₂O₁₂S₂ (PubChem CID 439302), and the reference table writes formulas in Hill order.
Molecular weight (average mass) and dalton
Molecular weight, or relative molecular mass, is a molecule's mass relative to one twelfth of a carbon-12 atom's mass, the unit called the dalton (Da) (IUPAC Green Book). Average mass uses standard atomic weights and suits weighing: BPC-157 is 1419.55 g/mol. See the molecular weight calculator.
Monoisotopic mass
Monoisotopic mass is the mass of a molecule built only from each element's most abundant isotope, such as carbon-12. Mass spectrometry often reports it, and it sits below the average mass: about 1418.70 against 1419.5 for BPC-157 (PubChem CID 9941957).
Net charge and isoelectric point (pI)
Net charge is the sum of a peptide's charges at a given pH, set by each group's pKa, such as 3.67 for aspartate and 10.40 for lysine in model pentapeptides (Thurlkill et al., 2006). The isoelectric point (pI) is the pH of zero net charge (Kozlowski, 2016).
Hydrophobicity
Hydrophobicity is the tendency of nonpolar groups to avoid water, in peptides mostly side chains such as those of leucine and phenylalanine. Peptides over 50% hydrophobic and under 25% charged dissolve efficiently in acetonitrile or DMF (Hoofnagle et al., 2016), and side-chain coefficients predict reversed-phase HPLC retention (Tripet et al., 2007).
Disulfide bridge
A disulfide bridge is a covalent bond between the sulfur atoms of two cysteines, formed by oxidizing their thiols, which removes two hydrogens (2.02 Da). It can close a ring, as in oxytocin, or join chains, as in insulin, whose A and B chains carry three disulfide bridges (Østergaard et al., 2020).
C-terminal amidation and N-terminal acetylation
C-terminal amidation turns the terminal carboxyl into an amide, removing a negative charge and 0.98 Da, as in oxytocin (Noki et al., 2024). N-terminal acetylation adds an acetyl group to the N-terminal amine, changing its charge, hydrophobicity and size (Ree et al., 2018) and adding 42.04 Da. See peptide modifications.
PEGylation
PEGylation attaches polyethylene glycol (PEG), a water-soluble polymer, to a peptide or protein, making it much larger. In mice, the terminal half-life of PEG in circulation rose from 18 minutes to 1 day as its molecular weight rose from 6,000 to 190,000 (Yamaoka et al., 1994).
Analog and fragment
An analog is a peptide modeled on a parent with residues or side chains changed (IUPAC-IUB calls these derivatives); a fragment is a partial sequence, named parent-(p-q)-peptide (IUPAC-IUB JCBN, 1984). BPC-157 is described as a 15-residue partial sequence of a gastric juice protein (Chang et al., 2011).
Salt form and counterion
A counterion is an ion of opposite charge paired with a charged group, so the solid in a vial is a salt form. Peptides often occur as salts with different counter-ions (Sikora et al., 2020), such as acetate, trifluoroacetate or chloride (Mrozik et al., 2012). See TFA vs acetate salts.
TFA salt
A TFA salt pairs a peptide with trifluoroacetate. Trifluoroacetic acid cleaves peptides from the resin and serves as an ion-pairing reagent in purification, so peptides come out as TFA salts (Erckes et al., 2025). TFA salts of three peptides gave less osteoblast proliferation than hydrochloride salts (Cornish et al., 1999).
Acetate and hydrochloride salts
Acetate and hydrochloride salts replace trifluoroacetate with acetate or chloride, by freeze-drying from hydrochloric acid, reversed-phase HPLC, ion-exchange resin or deprotonation and reprotonation (Roux et al., 2008); in one study 10 mM HCl was optimal for TFA removal and left purity unchanged (Erckes et al., 2025).
Synthesis and manufacturing
Solid-phase peptide synthesis (SPPS)
Solid-phase peptide synthesis (SPPS) builds a peptide step by step on a chain anchored to an insoluble support, so impurities rinse away between steps. Bruce Merrifield developed it (Mitchell, 2008), winning the 1984 Nobel Prize in Chemistry (Nobel Prize). See how research peptides are made.
Fmoc chemistry
Fmoc chemistry protects each incoming amino acid's amino group with Fmoc, removed each cycle by the mild base piperidine; this permits tert-butyl side-chain protection and a resin link cleaved by mild acid, with no final hydrogen fluoride step (Chang and Meienhofer, 1978). It is now the method of choice (Behrendt et al., 2016).
Resin
In SPPS, the resin is the insoluble support carrying the growing chain. A linker joins the peptide to it and protects the C-terminal carboxyl group until the peptide is cleaved off (Noki et al., 2024), while each step's by-products are simply washed away (Behrendt et al., 2016).
Coupling
Coupling forms each new peptide bond: the incoming amino acid's carboxyl group, activated for example by DIC with HOBt or by a uronium reagent with a base, reacts with the chain's free amine (Behrendt et al., 2016). Incomplete reactions leave deletion peptides (Merrifield et al., 1988).
Cleavage and crude peptide
Cleavage frees the finished chain from the resin and strips side-chain protection, in Fmoc chemistry with a trifluoroacetic acid cocktail and scavengers (King et al., 1990). What comes off is the crude peptide: the target plus deletion and insertion sequences, diastereomers, oxidized forms and other by-products (D'Hondt et al., 2014).
Preparative HPLC purification
Preparative HPLC purification separates the target from crude by-products, usually by reversed-phase HPLC. In one example, fractions from up to 200 mg of a crude 26-residue peptide were pooled as hydrophilic impurities, pure product and hydrophobic impurities, then freeze-dried (Mant et al., 2007).
Lyophilization (freeze-drying) and lyophilized cake
Lyophilization, or freeze-drying, removes water from a frozen sample by sublimation and desorption in three steps: freezing, primary drying and secondary drying (Roy and Gupta, 2004). It leaves a porous solid, often a cake, and a non-ideal cake can have no impact on quality (Patel et al., 2017). See what does lyophilized mean?
Batch, lot and lot number
A batch, or lot, is a quantity of material produced in a process or series of processes so that it is expected to be homogeneous within specified limits; its lot number identifies it and traces its production and distribution history (FDA ICH Q7 guidance). The COA portal lists certificates by lot.
Testing and certificates
Certificate of analysis (COA)
A certificate of analysis (COA) reports the testing of one batch. FDA's ICH Q7 guidance says it should name the material and batch number, list each test with acceptance limits and numerical results, and be dated and signed by quality staff (FDA ICH Q7 guidance). See what is a COA?
HPLC and reversed-phase HPLC
High-performance liquid chromatography (HPLC) pushes a dissolved sample through a packed column so its components separate and reach a detector at different times. Reversed-phase HPLC, on C18 or C8 columns with water, acetonitrile and TFA, is the most widely used mode for peptides (Mant et al., 2007). See HPLC purity testing.
HPLC purity (area percent)
HPLC purity is the main peak's share of the total peak area, usually from a UV detector: 99% means other peaks add up to 1%. It depends on column, acidic modifier and temperature (Stalmans et al., 2016) and ignores counter-ions: swapping TFA for chloride left purity unchanged (Erckes et al., 2025).
Chromatogram and retention time
A chromatogram is the detector's trace of a run against time, each separated component appearing as a peak. Retention time is when a component's peak appears under fixed conditions; matched to a reference it is limited evidence of identity, and peptide retention times can be predicted from composition (Tripet et al., 2007).
Identity test, mass spectrometry and LC-MS
An identity test checks that a material is the compound on the label. Mass spectrometry, which measures ions by mass-to-charge ratio, answers most directly: measured mass is checked against calculated mass (Stoppacher et al., 2013); electrospray ionization made it practical for large biomolecules (Fenn et al., 1989). LC-MS adds chromatography first and can identify a peptide and its impurities (Stalmans et al., 2016).
Net peptide content
Net peptide content is the share of a peptide powder's mass that is peptide, the rest being water, counter-ions and other impurities. It is not purity: peptides over 99% pure by HPLC held 62 to 88% peptide (Wang et al., 2012). This site's certificates give the measured amount in the vial. See fill accuracy.
Endotoxin and endotoxin units (EU)
Endotoxins are lipopolysaccharides from the outer membrane of Gram-negative bacteria, released after the cells die; sterilizing filters do not remove them, and results are given in endotoxin units (EU), a unit of endotoxin activity (FDA). See endotoxin testing.
LAL test
The LAL test detects endotoxin with Limulus amebocyte lysate, an extract of horseshoe crab blood cells whose protease cascade, triggered by endotoxin, forms a gel (Iwanaga, 2007). USP <85> covers gel-clot and photometric methods (FDA guidance, 2026); in a dispute, the gel-clot limit test decides (FDA Q4B Annex 14).
Recombinant factor C (rFC)
Recombinant factor C (rFC) is a lab-made copy of factor C, the zymogen that primes the horseshoe crab clotting cascade (Ding and Ho, 2010). Under USP <86>, rFC tests are alternative tests unless a monograph specifies otherwise (USP), and rFC does not recognize the factor G pathway that lysate detects (Bolden et al., 2020).
Sterility test
A sterility test looks for viable bacteria and fungi. USP <71> is the compendial culture method; in a 118-organism study, it outperformed one automated system but not another (England et al., 2019). Most sterility results on Disguised Alpha certificates are by PCR, which detects microbial DNA rather than growth. See the lab testing overview.
Third-party testing and ISO/IEC 17025
Third-party testing means a lab independent of the manufacturer and seller ran the analysis; the labs behind Disguised Alpha certificates are unaffiliated with it. ISO/IEC 17025:2017 sets general requirements for the competence of testing and calibration laboratories (FDA), and ILS Laboratories is accredited to it. See choosing a supplier.
Formats and handling
Reconstitution and diluent
Reconstitution is dissolving a lyophilized material in a measured volume of diluent, such as sterile or bacteriostatic water: 10 mg in 2 mL gives 5 mg/mL. Acidic peptides dissolve best in basic solutions, basic peptides in acidic ones (Hoofnagle et al., 2016). See the reconstitution guide and calculator.
Bacteriostatic water and sterile water
Bacteriostatic water is sterile water containing antimicrobial agents that suppress microbial growth (Hu et al., 2023); Disguised Alpha's uses 0.9% benzyl alcohol, one of the two most common preservatives in peptide and protein products (Meyer et al., 2007). Sterile water has no antimicrobial or other substance added (FDA). See the bac water guide and bacteriostatic vs sterile water.
Aliquot and freeze-thaw cycle
An aliquot is a measured portion of a larger sample; single-use aliquots spare a stock repeated freeze-thaw cycles. Freezing's chief stresses are low temperature, freeze-concentration and ice formation (Bhatnagar et al., 2007), and cysteine, methionine and tryptophan oxidize faster during freeze-thaw cycles (Hoofnagle et al., 2016). See the storage guide.
Hygroscopic and water content
Hygroscopic means taking up moisture from the air, as most lyophilized substances do (Lam and Patapoff, 2011). Water content is commonly measured by Karl Fischer titration (Wang et al., 2020); in lyophilized insulin at 35°C, degradation rose with it (Strickley and Anderson, 1996). Lyophilized powder brought to room temperature in a desiccator before reconstitution avoids water uptake (Hoofnagle et al., 2016).
Stability and degradation
Stability is how well a material keeps its identity, purity and content under stated conditions. Degradation is chemical breakdown: asparagine deamidates at rates set by pH, temperature and buffer (Patel and Borchardt, 1990), methionine oxidizes to its sulfoxide (Li et al., 1995), and acid hydrolyzes Asp-Gly bonds (Oliyai and Borchardt, 1993).
Solubility
Solubility is how much of a material dissolves in a given solvent, pH and temperature; a fully dissolved peptide gives a clear solution without flecks or cloudiness (Hoofnagle et al., 2016). In one protein study, the pH of minimum solubility moved with the isoelectric point (Shaw et al., 2001). See peptide solubility.
Research terms
Receptor and ligand
A receptor is a protein that binds a specific signaling molecule and turns binding into a change in cell activity; anything that binds it is a ligand. Affinity describes how well a ligand binds, and intrinsic efficacy what it does to receptor activity (Berg and Clarke, 2018).
Agonist, partial agonist and antagonist
An agonist binds a receptor and activates it, a full agonist typically to the maximal response a system can produce and a partial agonist submaximally. An antagonist has affinity but zero intrinsic efficacy: it binds without a response and blocks agonists (Berg and Clarke, 2018). Oxytocin is an agonist at the oxytocin receptor.
Allosteric modulator
An allosteric modulator binds a receptor at a site distinct from the one its endogenous ligand uses and changes the receptor's response to that ligand; at G protein-coupled receptors, such ligands can modulate the effects of diverse endogenous ligands and bias signaling (Wootten et al., 2013).
EC50 and IC50
IC50 is the concentration of an inhibitor causing 50 percent inhibition (Cheng and Prusoff, 1973). EC50 is its counterpart for an effect: in a four-parameter logistic fit, the relative EC50 or IC50 is the concentration at a response midway between the lower and upper plateaus (Sebaugh, 2011).
Half-life
Half-life (t½) is the time for a concentration to fall to half its starting value (IUPAC Green Book). In one stability study, an Asn-Gly model hexapeptide deamidated with a half-life of 1.4 days at 37°C and pH 7.4 (Geiger and Clarke, 1987).
Preclinical, in vitro, in vivo and ex vivo
Preclinical research is laboratory and animal work done before clinical studies; FDA names its two types as in vitro and in vivo (FDA). In vitro means outside a living organism, as in cell culture, in vivo inside one, and ex vivo in removed tissue such as precision-cut slices (Li et al., 2016).
Cell line
A cell line is a population of cells, derived from tissue, that can be grown and passaged in culture, some indefinitely. Guidelines list misidentification, contamination such as mycoplasma, and genetic or phenotypic instability as recurring problems and recommend short tandem repeat (STR) profiling for authentication (Geraghty et al., 2014).
Peptide bioregulator
Peptide bioregulator is the name V. Kh. Khavinson's group in St Petersburg gives to small peptides isolated from organs and tissues and to their synthesized di-, tri- and tetrapeptide analogues (Anisimov and Khavinson, 2010). It is that group's label, not a regulatory category. See peptide bioregulators.
Secretagogue
A secretagogue is a substance that makes a gland or cell secrete. Growth hormone secretagogues make the pituitary release growth hormone; the name was given to small synthetic molecules acting through GHS-R, a G protein-coupled receptor (Kojima et al., 1999). See growth hormone secretagogues.
Regulatory and documents
Research use only (RUO)
Research use only (RUO) marks material for laboratory research, not clinical or diagnostic use. Under FDA's labeling rule, an in vitro diagnostic product in the laboratory research phase carries the statement "For Research Use Only. Not for use in diagnostic procedures." (FDA guidance, 2013) All Disguised Alpha products are research use only. See the RUO policy.
GMP
GMP, good manufacturing practice, is the part of quality assurance that ensures medicinal products are consistently produced and controlled to the quality standards appropriate to their intended use, covering production, quality control, validated processes, personnel, premises and materials (WHO). FDA's ICH Q7 guidance applies it to active pharmaceutical ingredients.
Pharmacopeia (USP, Ph. Eur.)
A pharmacopeia is an official compendium of quality standards; its monographs set tests and acceptance criteria, such as identification, purity and assay. The United States Pharmacopeia (USP) and European Pharmacopoeia (Ph. Eur.) differed widely on peptide specifications (Vergote et al., 2009), though their endotoxin chapters are conditionally interchangeable in the ICH regions (FDA Q4B Annex 14).
FDA approval
FDA approval is the US Food and Drug Administration's permission to market a specific product, such as a new drug or biological product, after it evaluates the maker's test results and ability to meet federal quality standards. It belongs to the product: FDA does not approve facilities independently, and registration or listing does not denote approval (FDA).
WADA Prohibited List
The WADA Prohibited List names the substances and methods prohibited in sport, at all times or in competition only. The World Anti-Doping Agency updates it at least annually, publishing in October for 1 January, and its examples are not exhaustive (WADA).
Safety data sheet (SDS)
A safety data sheet (SDS), formerly a material safety data sheet, catalogs a chemical's information and hazards in 16 sections (Allen, 2017). US OSHA requires makers, distributors and importers to provide one for each hazardous chemical, covering properties, hazards, protective measures and handling precautions (OSHA).
PubChem CID
A PubChem CID (Compound ID) identifies a record in PubChem's Compound database, the NIH resource of unique standardized chemical structures; a new structure gets a new CID (Kim et al., 2016). Salts can have their own: oxytocin is CID 439302, oxytocin acetate CID 12004215. See the reference table.
INN and development code name
An International Nonproprietary Name (INN) is a unique generic name WHO selects for a pharmaceutical substance; INNs are public property, pharmacologically related substances share a stem, and salts take modified names (WHO). Where no INN is known, the WADA list uses IUPAC chemical names (WADA). Some compounds also carry development codes: PubChem lists PL-14736 for BPC-157 (CID 9941957).