Tirzepatide

1 Identity

Acylated synthetic lipopeptide, not a plain peptide. A 39-residue single-chain backbone of GIP-analog design, C-terminally amidated, carrying two 2-aminoisobutyric acid substitutions, with a C20 fatty diacid conjugated to a lysine side chain through a gamma-glutamate spacer and two AEEA units. Four structural features that a residue count conceals and that a plain-peptide specification is blind to: Aib at positions 2 and 13; the C-terminal amide; the acylation itself; and the position of the acylation, since the chain carries a second, unmodified lysine. Contained in the FDA-approved drug products Mounjaro and Zepbound (NDA 217806), both Eli Lilly. Gray-market shorthand is recorded here so the regulatory watch can recognize listings, and is used nowhere in this company's records, certificates, labels or invoices: "twincretin", "MHC-2 TRZ".

Sequence

Tyr1-Aib2-Glu3-Gly4-Thr5-Phe6-Thr7-Ser8-Asp9-Tyr10-Ser11-Ile12-Aib13-Leu14-Asp15-Lys16-Ile17-Ala18-Gln19-Lys20-Ala21-Phe22-Val23-Gln24-Trp25-Leu26-Ile27-Ala28-Gly29-Gly30-Pro31-Ser32-Ser33-Gly34-Ala35-Pro36-Pro37-Pro38-Ser39-NH2, with the Lys20 side-chain N-epsilon bearing (AEEA)-(AEEA)-(gamma-Glu)-(C20 diacid), written out as Lys20(epsilon)-NH-[AEEA-AEEA-gamma-Glu-CO(CH2)18COOH]. Two AEEA units, not one; that is the feature that separates this molecule's linker from retatrutide's. Aib at positions 2 and 13 only. Lys16 is present and unmodified, and that is the positional-isomer risk. Thirty-nine residues, no disulfides. Supplied commercially as the acetate salt.

Molecular formula

C225H348N48O68 (free base). Des-acyl 39-mer backbone C188H283N45O56. The acyl side chain contributes C37H65N3O12 net, after replacing one hydrogen on the lysine epsilon-amine.

Average mass

Average mass 4813.527 Da (C225H348N48O68), computed from the formula. The FDA-approved Zepbound labeling, revision 02/2026, states the same formula and a molecular weight of 4813.53 Da. That is 0.003 Da from the figure above, or 0.6 ppm at 4.8 kDa. Des-acyl backbone 4069.591 Da, giving a des-acylation interval of 743.936 Da average. Mono-acetate salt 4873.579 Da.

Monoisotopic mass

4810.52486 Da neutral (C225H348N48O68); [M+H]+ 4811.53213. At this mass the measurement that matters is the deconvoluted multiply-charged envelope, and identity work requires not less than 30,000 resolving power, because the C-terminal amide versus free acid difference is 0.984 Da and is invisible below that. Des-acyl backbone 4067.06803, giving a des-acylation interval of 743.4568 Da monoisotopic.

Salt / variant note

All figures computed on the IUPAC 2021 abridged table. Wrong-molecule substitutions, each separable by arithmetic on one calculator: semaglutide C187H291N45O59, 4113.641 / 4111.11538, 699.886 Da lighter, the cheaper API, the same colorless cake, and the single most documented substitution in this channel, so a certificate showing about 4113 is not this article; retatrutide C221H342N46O68, 4731.421 / 4728.47176, only 82.106 Da away, 1.7 percent at 4.8 kDa, which a nominal-mass MALDI trace with a broad peak will not cleanly separate; liraglutide C172H265N43O51, 3751.262 / 3748.94646. Within-molecule variants, each checkable against the supplier's own declared parent mass with no reference standard in hand. Des-acyl tirzepatide, the bare 39-mer carrying both Aib substitutions and the C-terminal amide but no side chain, C188H283N45O56, 4069.591 / 4067.06803, an interval of 743.936 Da average and 743.4568 Da monoisotopic below parent; it is the direct product of a failed or omitted conjugation and it is invisible to a backbone-only identity check, because the backbone is precisely what such a check confirms. Over-acylated species carrying a second side chain on Lys16, C262H413N51O80, 5557.463 / 5553.98168, the same interval above parent. Positional isomer acylated at Lys16 rather than Lys20: identical formula, identical exact mass at every decimal place, undetectable by any mass spectrometer at any resolving power, and found only by a peptide map or by a chromatographic method demonstrated to resolve the two isomers against a spiked standard. Des-amido, C-terminal free acid instead of the specified amide, C225H347N47O69, 4814.511 / 4811.50887, exactly plus 0.984 Da and invisible below about 30,000 resolving power. Aib substitution: alanine for Aib at position 2 or 13 is minus 14.0157 Da monoisotopic per position and minus 28.03 for both; Aib is non-proteinogenic, costs more and couples worse than alanine, so substituting it is the cheapest way to make something that resembles the molecule. Linker error: a one-AEEA linker gives a side chain 145.16 Da lighter and is retatrutide's architecture, not this one's. Salt form: the acetate is the commercial default, mono-acetate 4873.579, and adds 60.05 Da per counterion to the labeled mass basis.

2 Class & testing panel

Form

Single article

Testing panel

P2 — panel definitionwith mandatory P3 additions

3 Primary sources & evidence

The index reports the design and provenance of the literature, not a conclusion about effect.

A large sponsor-controlled clinical program exists and is recorded by study type, size and identifier only, with no reported outcome restated. Preclinical receptor pharmacology with first-in-human proof of concept: Coskun T et al., Mol Metab 2018, PMID 30473097. Human mechanistic study: Urva S et al., Diabetes Obes Metab 2020, PMID 32519795. Human randomized phase 3 trials: SURMOUNT-1, Jastreboff AM et al., NEJM 2022, PMID 35658024, 72 weeks; SURMOUNT-5, Aronne LJ et al., NEJM 2025, PMID 40353578, active comparator; SURMOUNT-OSA, Malhotra A et al., NEJM 2024, PMID 38912654; SUMMIT, Packer M et al., NEJM 2025, PMID 39555826, n=731. The SURPASS phase 3 program supported the 2022 approval; individual SURPASS PMIDs were not independently verified and none is printed here, because an unverified identifier is worse than none. The molecule also serves as active comparator in registered phase 3 trials of other compounds, including NCT06662383, NCT06221969, NCT06534411 and NCT06131437. The approved labeling carries a boxed warning and a warnings-and-precautions list; their content is not reproduced here, and the label at revision 02/2026 is the authoritative text. The entire human evidence base concerns investigational or approved material made under an approved application, not any article sold in the research-chemical market, and no independent published identity confirmation, impurity profile or stereochemical analysis of gray-market "tirzepatide" was located.

4 Storage & specification

Storage

Lyophilized powder at minus 20 degrees C plus or minus 5 degrees C, desiccated and protected from light, with the C20 fatty diacid side chain making the article surface-active and prone to adsorptive loss in dilute solution, so glass and low-binding plasticware rather than ordinary polypropylene.

Shelf life

The interval is dated on the acyl side chain rather than on gross purity, because the two attributes that move first on an acylated incretin analog are des-acylation at the 743.936 Da interval and hydrolytic loss of the C-terminal amide at 0.984 Da, and neither is visible in an area-percent figure. Each lot is placed on a stability protocol that tracks those two mass intervals, and the interval a lot carries is the one that protocol supports rather than one read off an area-percent purity figure.