Investigation of frozen protease-catalyzed peptide synthesis systems — a differential scanning calorimetry and electron microscopy approach

The internal structure of a protease-catalyzed frozen aqueous peptide synthesis system was studied by freeze-fracture electron microscopy. Distinct lense-like liquid microinclusions differing in size from 0.25 to 1.7 μm were observed. Differential scanning calorimetry was used to determine the amount of unfrozen water per molecule of peptide reactant. Comparison of the results with data obtained previously by examination of the same peptide synthesis system using 1H NMR relaxation time technique showed that DSC is the more reliable method for this special purpose. Furthermore, the amount of unfrozen water in a frozen α-chymotrypsin solution determined by magic angle spinning NMR in previous investigations was confirmed by the calorimetric method.