DL-Threonine and DL-allothreonine showed a protective effect on various bacterial cells in the process of freeze-drying whereas L- and D-forms of them did not, probably owing to the difference in the physicochemical characteristics between L- (or D-) form and DL-form of the compounds in question. There was no difference in the protective activity between the optically active and inactive forms in the cases of serine, proline, tartaric acid and pyrrolidonecaboxylic acid.
