Relationship between the Antifreeze Activities and the Chemical Structures of Oligo- and Poly(glutamic acid)s

Antifreeze activities of various oligo- and poly(amino acid)s, more particularly oligo(glutamic acid)s (OGAs) and poly(glutamic acid)s (PGAs), were measured by differential scanning calorimetry (DSC). The polymers composed of acidic amino acids, such as PGA, had higher antifreeze activity than the other polymers. The antifreeze activities of the OGAs and PGAs tended to decrease as their molecular weights increased. However, their activities in the molecular weight range below 20 000 were higher than the antifreeze activity of glucose, which is known as a highly antifreeze substance. The antifreeze activities were only slightly affected by the optical isomerism and the peptide linkage type and decreased in the order Na salt = K salt ≫ Ca salt ≫ acidic form. It was considered that the high antifreeze activities of OGA and PGA are mainly caused by Coulomb's force of the dissociating counterions.