Peptides and proteins compared

Production

Short peptides are built residue by residue on a solid support, which is precise and scales to sequences of a few dozen amino acids. Beyond that, the yield of any stepwise chemical process falls away: a small loss at each of two hundred steps compounds into very little product.

Larger molecules are therefore produced biologically, by inserting the gene into an expression system and letting a cell build the chain. That gives a correctly assembled sequence but introduces a different problem — separating the product from everything else the cell made.

Structure and fragility

A short peptide is usually flexible, without a fixed shape. A protein folds into a specific conformation, and its activity depends on that fold being intact. Heat, agitation, extremes of pH or repeated freezing can unfold it, and an unfolded protein may be chemically intact yet functionally useless.

This is why proteins are handled with more care: gently, cold, and with freeze-thaw cycling minimised.

Analysis

Both are checked by chromatography and mass spectrometry, but the tolerances differ. A 1,400 Da peptide has its mass confirmed to within a fraction of a dalton; a 22,000 Da protein carries more mass heterogeneity, including glycosylation in some cases, so identity confirmation is a broader statement.