Thermodynamic Elucidation of Structural Stability of Deuterated Biological Molecules: Deuterated Phospholipid Vesicles in H2O

Thermodynamic studies of deuterated phospholipids in H20 were conducted using deuterated lipid bilayer yesides prepared from aqueous dispersions of 1,2-dimyristoyl-d54-, 1,2-dipalmitoyl-d62-, and 1,2-distearoyl-d70-phosphatidylcholines. The results obtained in combination with those previously reported in D2O made possible the evaluation of deuterium solvent and deuterium alkylchain substitution effects in deuterated phospholipids. These effects included the temperature (δtm), the enthalpy (δΔH), the free energy (δΔG), the entropy (δΔS), and the cooperative melting unit (δn) of the main lipid phase transition from the gel to the liquid-crystalline phase. To supplement thermodynamic observations, fluorescence steady-state polarization measurements were also performed. The results were examined to elucidate the nature of interactions in deuterated compounds. The deuterium solvent effect increases the structural stability of deuterated lipid vesicles, due to an enhancement of hydrophobic interaction in D2O. This observation is contrary to that previously reported for deuterated proteins, phycocyanins, where structural stability is reduced in D2O solvent. These findings therefore support the proposal that an alteration in hydrophobic interactions in deuterated proteins due to deuterium substitution is not likely to play a significant role in contributing to the structural instability of proteins.