Mechanisms of interaction of amino acids with phospholipid bilayers during freezing

In this study we compare the ability of various amino acids to protect small unilamellar vesicles against damage during freeze/thaw. Liposomes were composed of 75% palmitoyloleoyl phosphatidylcholine and 25% phosphatidylserine. Damage to liposomes frozen in liquid nitrogen and thawed at 20 degrees C was assessed by resonance energy transfer. Cryoprotection by numerous amino acids was compared in the presence and absence of 350 mM NaCl. The majority of amino acids with hydrocarbon side chains increased membrane damage during freeze/thaw regardless of the presence of salt. However, amino acids with hydrocarbon side chains of less than three carbons long, e.g. glycine, alanine, and 2-aminobutyric acid, were cryoprotective only in the presence of salt. We suggest that NaCl selectively increases the solubility of such amino acids, allowing them to act as cryoprotectants. In contrast, amino acids with side chains containing charged amine groups were cryoprotective regardless of the presence of salt. The degree of charge on the second amine group is shown to be important for cryoprotection by these molecules. We present evidence that suggests an interaction between the positively charged, second amine group of the amino acid, and the negatively charged phospholipid headgroup.