Deuterium isotope effect on the stability of molecules: phospholipids

Abstract

To examine the deuterium isotope effect on the stability of phospholipids, the phase transition properties of bilayer vesicles, prepared from deuterated phospholipids, were investigated and compared with those of nondeuterated (normal) phospholipids. Three lipids, 1,2-distearoyl-, 1,2-dipalmitoyl-, and 1,2-dimyristoylphosphatidylcholines, were studied. Differential scanning microcalorimetry showed that, unlike a sharp phase transition observed in a nondeuterated lipid, a deuterated lipid exhibits a broad transition which is associated with a lower temperature (t/sub m/) and small enthalpy (..delta..H/sub m/) and entropy (..delta..S/sub m/) of the transition. The magnitude of t/sub m/ in deuterated lipid is 3-5/sup 0/C lower than in the corresponding nondeuterated lipid. Deuteration causes the magnitude of ..delta..H/sub m/ and ..delta..S/sub m/ to be decreased by 14-35%. For each CH/sub 2/ group, the deuterium substitution in general decreases ..delta..H/sub m/, ..delta..G/sub m/ (Gibbs energy), and ..delta..S/sub m/ by 272 J/mol, 15.4 J/mol, and 0.7 J/mol deg), respectively. Steady-state fluorescence polarization (P) measurements have demonstrated that, at a temperature above t/sub m/ (liquid-crystalline phase), deuterated and nondeuterated lipids essentially have a same magnitude of P, while at a temperature below t/sub m/ (gel phase), deuterated lipid has a lower P than nondeuterated lipid. Consequently, deuterated lipid has a lower magnitude of ..delta..P more » (difference in P below t/sub m/), indicating that the structural differences between gel and liquid-crystalline phase is less profound in deuterated lipids. The melting properties of deuterated and nondeuterated fatty acids corresponding to these three lipids were also examined. 20 references, 3 figures, 1 table. « less