A peptide is a short chain of amino acids linked together by peptide bonds. “Short” is relative — most research peptides run anywhere from 2 to about 50 amino acids. Once a chain gets much longer and folds into a complex 3D structure, it’s usually called a protein instead. The distinction isn’t a hard scientific line so much as a useful convention: insulin (51 amino acids) is sometimes called a peptide hormone and sometimes a small protein, depending on who’s writing.

Not everything sold alongside peptides is actually a peptide. Compounds like MK-677 and 5-Amino-1MQ are small organic molecules — they aren’t built from amino acids at all, but they’re grouped commercially with peptides because they act on similar biological pathways (growth-hormone signaling and cellular metabolism, respectively). Knowing which category a compound falls into matters for research purposes: peptides typically can’t survive oral digestion intact, which is why most are studied via injection or intranasal routes, while small molecules like MK-677 are stable enough to study orally.

Peptides in solution are chemically unstable over time — they’re vulnerable to hydrolysis (breakdown by water), oxidation, and bacterial contamination. Lyophilization (freeze-drying) removes the water content while the peptide is frozen, leaving a stable powder that can be stored for much longer periods, typically refrigerated or frozen. That’s why almost every compound in the Signal Library ships as a lyophilized powder that gets reconstituted with a diluent immediately before use in a research protocol — more on that in the Reconstitution Basics lesson.

Some research peptides are synthetic copies of molecules the body already makes — growth-hormone secretagogues like Ipamorelin mimic the natural hormone ghrelin, and IGF-LR3 is an engineered version of a hormone your liver already produces. Others are fully synthetic designs with no natural counterpart, built in a lab to test a specific biological hypothesis — FOXO4-DRI is a good example, engineered specifically to disrupt one protein-protein interaction in senescent cells. Both categories show up throughout the Signal Library, and knowing which type you’re looking at tells you a lot about what kind of research literature to expect.