Peptides vs. Proteins

Both are chains of amino acids. Peptides are short; proteins are long and fold into complex three-dimensional structures that determine their function.

That structural difference matters for manufacturing and for stability. Peptides can be built by chemical synthesis; most proteins must be produced in living cells.

Why fragments are not the whole molecule

A fragment of a protein is not a smaller version of it. TB-500 relates to thymosin beta-4, and AOD-9604 to growth hormone, but neither reproduces the parent molecule's full activity — one reason evidence for the parent should never be read as evidence for the fragment.