Mannitol is a six-carbon sugar alcohol (C₆HₔO₆, 182.17 Da) widely used as a bulking agent in freeze-dried formulations. It crystallises readily during freezing and produces a firm, elegant cake with good mechanical strength and fast redissolution, which is why it appears so often in lyophilised vials.
Why mannitol matters in peptide research
Its main job is structural. A vial containing two or three milligrams of peptide and nothing else yields almost no visible solid; add mannitol and the same vial produces a proper white cake. This is the single most common explanation for a vial that “looks fuller” than its labelled mass, and equally for the alarm when an unbulked vial looks empty. Mannitol is also non-reducing, so unlike glucose it will not react with lysine side chains, and it is inert to the peptide backbone.
Two caveats follow. Because mannitol crystallises rather than forming a glass, it is a weaker cryoprotectant than sucrose or trehalose for fragile sequences, and formulations sometimes combine them. And its mass may or may not be included in labelled vial mass, so any molarity work should start from net peptide content on the certificate of analysis. Background: lyophilisation explained.