J. Biol. Chem., Vol. 266, Issue 21, 13602-13606, Jul, 1991
Freezing of phosphocholine headgroup in fully hydrated sphingomyelin bilayers and its effect on the dynamics of nonfreezable water at subzero temperatures
WG Wu, LM Chi, TS Yang and SY Fang
Institute of Life Sciences, National Tsing Hua University, Hsinchu, Taiwan, Republic of China.
Differential scanning calorimetry (DSC) and nuclear magnetic resonance
(NMR) spectroscopy are applied to characterize the nonfreezable water
molecules in fully hydrated D2O/sphingomyelin at temperatures below 0
degrees C. Upon cooling, DSC thermogram displays two thermal transitions
peaked at -11 and -34 degrees C. The high-temperature exothermic transition
corresponds to the freezing of the bulk D2O, and the low-temperature
transition, which has not previously been reported, can be ascribed to the
freezing of the phosphocholine headgroup in the lipid bilayer. The dynamics
of nonfreezable water are also studied by 2H NMR T1 (spin-lattice
relaxation time) and T2e (spin-spin relaxation time obtained by two pulse
echo) measurements at 30.7 MHz and at temperatures down to -110 degrees C.
The temperature dependence of the T1 relaxation time is characterized by a
distinct minimum value of 2.1 +/- 0.1 ms at -30 degrees C. T2e is
discontinuous at temperature around -70 degrees C, indicating another
freezing-like event for the bound water at this temperature. Analysis of
the relaxation data suggest that nonfreezable water undergoes both fast and
slow motions at characteristic NMR time scales. The slow motions are
affected when the lipid headgroup freezes.
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