Every compound in this database is referenced strictly for in vitro research and laboratory use. None are approved for human consumption, therapeutic use, or veterinary application. This database is an educational reference only.
What Molecular Weight Means Here
Molecular weight is the mass of one mole of a compound, expressed in grams per mole (g/mol), equivalent to daltons (Da) for these purposes. For a peptide, it is essentially the sum of the masses of its constituent amino acids, minus a water molecule for each peptide bond formed, plus the mass of any chemical modifications.
This database lists free-base average molecular weight — the mass of the peptide molecule alone, with no salt counterions, no bound water, and no other associated molecules. This is the value cataloged in chemical databases such as PubChem, and it is the correct reference figure. But it is not the same as the mass of powder that arrives in a vial — a distinction this page exists to make clear.
This page is the mass-focused companion to the broader Peptide Database. Where that page indexes peptides by class and function, this one sorts them by mass and goes deep on what mass actually means in practice.
Master Table — Sorted by Mass
All 45+ compounds, ordered from lightest to heaviest free-base molecular weight. Sorting by mass makes the size classes visible — the tripeptides cluster at the top, the engineered proteins at the bottom.
| Compound | MW (g/mol) | Amino Acids | Size Class | Class / Use |
|---|---|---|---|---|
| 5-Amino-1MQ | ~174.2 | Non-peptide | Small molecule | NNMT inhibitor |
| Glutathione | 307.3 | 3 | Tripeptide | Antioxidant |
| Methylene Blue | 319.9 | Non-peptide | Small molecule | Redox research |
| GHK-Cu | 340.4 / ~403.9* | 3 | Tripeptide | Healing / skin |
| KPV | 342.4 | 3 | Tripeptide | Anti-inflammatory |
| Epitalon | 390.3 | 4 | Tetrapeptide | Longevity |
| MK-677 (Ibutamoren) | 528.7 | Non-peptide | Small molecule | Oral GH secretagogue |
| SS-31 (Elamipretide) | 639.8 | 4 | Tetrapeptide | Mitochondrial |
| NAD+ | 663.4 | Non-peptide | Small molecule | Cellular metabolism |
| Ipamorelin | 711.9 | 5 | Pentapeptide | GH secretagogue |
| Selank | 751.9 | 7 | Heptapeptide | Nootropic |
| Semax | 813.9 | 7 | Heptapeptide | Nootropic |
| GHRP-2 | 817.9 | 6 | Hexapeptide | GH secretagogue |
| N-Acetyl Selank Amidate | ~834 | 7 | Heptapeptide | Nootropic (stabilized) |
| DSIP | 848.9 | 9 | Nonapeptide | Sleep research |
| GHRP-6 | 873.0 | 6 | Hexapeptide | GH secretagogue |
| Hexarelin | 887.0 | 6 | Hexapeptide | GH secretagogue |
| Thymosin β4 Fragment (TB-500, true) | 889.0 | 7 | Heptapeptide | Actin-binding motif |
| Melanotan II | 1,024.2 | 7 | Cyclic heptapeptide | Melanocortin agonist |
| PT-141 (Bremelanotide) | 1,025.2 | 7 | Cyclic heptapeptide | Melanocortin agonist |
| Gonadorelin | 1,182.3 | 10 | Decapeptide | GnRH analog |
| Kisspeptin-10 | 1,302.5 | 10 | Decapeptide | Reproductive research |
| BPC-157 | 1,419.5 | 15 | Pentadecapeptide | Healing / gut repair |
| Pentadeca Arginate | ~1,690 | 15 | Pentadecapeptide | Healing (BPC analog) |
| AOD-9604 | 1,815.1 | 16 | Small peptide | Lipolysis research |
| HGH Fragment 176-191 | 1,817.1 | 16 | Small peptide | Lipolysis research |
| MOTS-c | 2,174.6 | 16 | Small peptide | Mitochondrial |
| Humanin | 2,687.0 | 24 | Mid-size peptide | Mitochondrial |
| MGF | ~2,867 | 24 | Mid-size peptide | Muscle repair |
| Thymosin Alpha-1 | 3,108.3 | 28 | Mid-size peptide | Immune modulation |
| VIP | 3,325.8 | 28 | Mid-size peptide | Vasoactive / immune |
| Sermorelin | 3,357.9 | 29 | Mid-size peptide | GHRH analog |
| CJC-1295 (no DAC) | 3,367.9 | 29 | Mid-size peptide | GHRH analog |
| CJC-1295 (with DAC) | 3,647.2 | 30 | Mid-size peptide | Long-acting GHRH analog |
| Cagrilintide | ~3,748 | ~37 | Large peptide | Amylin analog |
| Semaglutide | 4,113.6 | 31 | Large peptide | GLP-1 agonist |
| LL-37 | 4,493 | 37 | Large peptide | Antimicrobial / immune |
| Retatrutide | ~4,731 | 39 | Large peptide | Triple incretin agonist |
| Tirzepatide | 4,813.5 | 39 | Large peptide | Dual GIP/GLP-1 agonist |
| Thymosin β4 (sold as TB-500) | 4,921 | 43 | Large peptide | Tissue repair |
| Tesamorelin | 5,135.9 | 44 | Large peptide | GHRH analog |
| IGF-1 DES | ~7,372 | 67 | Engineered protein | IGF-1 variant |
| IGF-2 | ~7,500 | 67 | Engineered protein | Growth factor |
| IGF-1 LR3 | ~9,111 | 83 | Engineered protein | Long-acting IGF-1 analog |
* GHK-Cu: first value is the free tripeptide (Gly-His-Lys), second is the copper complex. Values prefixed "~" are approximate — for newer compounds or those whose mass varies by formulation. Cerebrolysin and Thymalin are excluded from this table as they are peptide mixtures, not single molecules with a defined mass.
Peptides by Size Class
Sorting by mass exposes a useful structure. Peptides cluster into rough size bands, and size band is a practical proxy for how a compound behaves in handling and storage.
| Size Class | Approx. MW Range | Examples | Practical Tendency |
|---|---|---|---|
| Tripeptides | 300–350 | GHK-Cu, KPV, Glutathione | Very stable; tolerate handling well |
| Tetra- to heptapeptides | 390–900 | Epitalon, SS-31, Ipamorelin, Semax, Selank | Stable; simple linear structures |
| Cyclic heptapeptides | ~1,024–1,025 | PT-141, Melanotan II | Cyclic structure adds stability |
| Small peptides (10–16 aa) | 1,180–2,200 | BPC-157, AOD-9604, MOTS-c | Generally robust |
| Mid-size peptides (24–30 aa) | 2,600–3,650 | Thymosin Alpha-1, Sermorelin, CJC-1295 | More fragile; GHRH analogs degrade faster once reconstituted |
| Large peptides (31–44 aa) | 3,700–5,200 | Semaglutide, Tirzepatide, Thymosin β4, Tesamorelin | Complex structure; lipidated GLP-1s are stabilized by design |
| Engineered proteins (60+ aa) | 7,000–9,200 | IGF-1 LR3, IGF-1 DES, IGF-2 | Most structurally complex; handle with most care |
Salt Forms: TFA vs Acetate
Synthetic peptides are almost never isolated as a pure free base. The purification process leaves them as a salt — the peptide paired with counterions. The two forms you will encounter are trifluoroacetate (TFA) salt and acetate salt.
This matters for one reason: the salt adds mass. A milligram of "peptide powder" is a milligram of peptide-plus-counterions-plus-bound-water — not a milligram of peptide free base. The free-base molecular weights in the table above describe the peptide molecule; the powder in the vial weighs more per molecule.
| Aspect | TFA Salt | Acetate Salt |
|---|---|---|
| Origin | Default from standard HPLC purification | Often produced by salt exchange after purification |
| Counterion mass | Heavier per counterion | Lighter per counterion |
| Effect on net peptide content | Lower net peptide per mg of powder | Higher net peptide per mg of powder |
| Where it's noted | Certificate of analysis | Certificate of analysis |
Why a 5 mg Vial Isn't 5 mg of Peptide
This is the single most useful thing to understand about peptide mass, and most references skip it.
When a vial is labeled 5 mg, that figure usually describes the gross weight of the lyophilized powder in the vial. That powder is not pure peptide free base. It is:
- The peptide itself (the free-base mass)
- Plus salt counterions (TFA or acetate)
- Plus bound water — peptides are hygroscopic and retain water even after lyophilization
The result: the net peptide content — the mass of actual peptide free base — is typically somewhat lower than the labeled gross weight. A manufacturer states the issue plainly: batch-specific molecular weights vary from batch to batch due to the degree of hydration, which affects the solvent volume required to prepare a given concentration.
When You Actually Need Molecular Weight
A point of clarification, because it is widely misunderstood: you do not need molecular weight for standard reconstitution.
Routine reconstitution math is purely mass-and-volume: milligrams of peptide divided by milliliters of bacteriostatic water gives concentration; the rest follows from there. Molecular weight never enters that calculation. The full set of formulas is in the Reconstitution Reference.
Molecular weight becomes necessary only when you need to work in molar units:
- Converting a mass concentration (mg/mL) to a molar concentration (mM, µM)
- Preparing a solution to a specified molarity for a receptor-binding or cell-culture assay
- Comparing two peptides on a mole-for-mole basis rather than a mass basis
The conversion itself is straightforward: molar concentration = mass concentration ÷ molecular weight. For a 1 mg/mL solution of BPC-157 (MW 1,419.5), that is 1 ÷ 1,419.5 ≈ 0.0007 mol/L, or about 0.70 mM.
Verified Molecular Formulas
Molecular formula — the exact atom count of a molecule — is published and stable for well-characterized peptides. For two of the most-referenced compounds in this database, the formulas are firmly established:
For other compounds, molecular formula is best taken directly from the certificate of analysis or a primary chemical database such as PubChem for the specific form (free base vs salt) you are working with. Because the formula changes with salt form and any modification, this database does not list a formula column for every compound — an incorrect formula is worse than no formula. Individual compound pages, as they publish, will carry the verified formula for each peptide alongside its PubChem CID.
Frequently Asked Questions
What is the free-base molecular weight of a peptide?
The free-base molecular weight is the mass of the peptide molecule itself, with no counterions, water, or other associated molecules included. It is the value published in chemical databases like PubChem and is the standard reference figure. The actual mass of a supplied peptide vial is usually higher because it includes salt counterions and bound water.
Why does a 5mg peptide vial not contain exactly 5mg of peptide?
A vial labeled 5 mg typically refers to the gross weight of the lyophilized powder, which includes the peptide plus its salt counterions and any bound water. Depending on the salt form, the net peptide content — the peptide free base alone — can be meaningfully lower than the labeled weight. The certificate of analysis lists the actual peptide content for that batch.
What is the difference between TFA salt and acetate salt peptides?
Peptides are commonly supplied as either a trifluoroacetate (TFA) salt or an acetate salt, depending on the purification process. The salt form adds mass beyond the peptide free base and slightly changes the net peptide content per milligram of powder. TFA is heavier per counterion than acetate. The salt form should be listed on the certificate of analysis.
Do I need molecular weight to reconstitute a peptide?
No. Standard reconstitution math uses only the labeled peptide mass in milligrams and the volume of bacteriostatic water in milliliters. Molecular weight is only needed for molar calculations, such as converting between mass concentration and molar concentration, which is uncommon in routine research handling.
Why do molecular weights differ slightly between databases?
Small differences usually come from whether a source lists the free-base mass or a salt-form mass, from rounding conventions, or from using average versus monoisotopic mass. For consistency, this database lists free-base average molecular weight. Always defer to the certificate of analysis for the specific batch when precision matters.
What is the heaviest research peptide?
Among commonly researched peptides, IGF-1 LR3 is the heaviest at roughly 9,111 g/mol — an 83-amino-acid engineered protein. The GLP-1 class agonists Tirzepatide and Semaglutide are also large, at roughly 4,800 and 4,100 g/mol respectively, as is Thymosin Beta-4 at roughly 4,921 g/mol.
Is molecular weight the same as molecular mass?
In everyday research use they are used interchangeably and expressed in g/mol or daltons. Strictly, molecular weight is a relative, dimensionless quantity and molecular mass is an absolute mass, but for peptide handling the distinction is not practically important. This database uses average molecular weight throughout.
How is a peptide's molecular weight calculated?
It is the sum of the average residue masses of every amino acid in the chain, minus one water molecule (18 g/mol) for each peptide bond, plus the mass of any modifications such as acetylation, amidation, or a lipid side chain. For modified peptides like the lipidated GLP-1 agonists, the modification adds substantial mass beyond the amino acid backbone.
All compounds in this database are referenced strictly for in vitro research and laboratory use. None are approved by the FDA for human consumption, therapeutic use, or veterinary application. This database is provided for educational and reference purposes only and does not constitute medical advice.
Molecular weights are free-base average values aggregated from PubChem, published literature, and manufacturer technical documentation. Values prefixed "~" are approximate. For any precise work, confirm against the certificate of analysis for the specific batch and salt form.