Glossary
Mannitol
Mannitol is a sugar alcohol added as a bulking agent to freeze-dried peptide vials. Why it forms a visible cake and what it means for your mass calculations.
Mannitol is a sugar alcohol with six carbons (C₆H₁₄O₆, 182.17 Da) and a very common bulking agent in freeze-dried products. It crystallises easily as it freezes, giving a firm, attractive cake that is mechanically robust and redissolves quickly — the reason it is found in so many lyophilised vials.
What mannitol does in a vial
Its primary role is to provide structure. Two or three milligrams of peptide on their own leave hardly any visible solid; with mannitol added, the same vial shows a proper white cake. This is by far the most frequent reason a vial “looks fuller” than its labelled mass suggests — and, conversely, why a vial without bulking agent can look alarmingly empty. Mannitol is also a non-reducing sugar alcohol, so, unlike glucose, it does not react with lysine side chains and leaves the peptide backbone untouched.
Two points of caution. Since mannitol crystallises instead of forming a glass, it protects fragile sequences during freezing less well than sucrose or trehalose, and some formulations therefore combine them. And its weight may or may not be counted in the labelled vial mass, so molarity calculations should always begin with the net peptide content given on the certificate of analysis. Background: lyophilisation explained.
Related terms
excipient · lyophilized · reconstitution guide