Mr. Morrow et al., THERMAL HISTORY ALTERS CHOLESTEROL EFFECT ON TRANSITION OF 1-PALMITOYL-2-LINOLEOYL PHOSPHATIDYLCHOLINE, Biophysical journal, 71(6), 1996, pp. 3207-3214
The effect of cholesterol on the bilayer phase behavior of heteroacid
phosphatidylcholines with one unsaturated fatty acid depends on the na
ture of the unsaturated chain. Previous differential scanning calorime
try (DSC) studies showed that -palmitoyl-2-linoleoyl-sn-glycero-3-phos
phocholine (16:0-18:2 PC) had a broad, weak transition at about -18 de
grees C, which was effectively eliminated by less than 15 mol % choles
terol. Phospholipids with greater and lesser degrees of unsaturation d
isplayed stronger phase transitions and less sensitivity to cholestero
l. In this work, deuterium nuclear magnetic resonance has been used to
examine the phase behavior of opalmitoyl-2-linoleoyl-sn-glycero-3-pho
sphocholine (16:0-18:2 PC-d(31)) alone, and with 15 mol % cholesterol.
The behavior is found to be sensitive to sample thermal history. Mode
rately fast cooling (1 degrees/h) results in a continuous phase change
from a fluid to an ordered phase in the pure lipid. Under similar coo
ling conditions, the sample containing cholesterol displays increased
chain order and a continuous phase change with no apparent isothermal
transition, However, when these systems are cooled at a reduced rate (
0.3 degrees/h), the continuous phase change is pre-empted by a sharp t
ransition into a more ordered phase that gives a deuterium spectrum ha
ving intensity at a value of the quadrupole-splitting characteristic o
f a rigid lattice system. In the pure lipid, this transition effective
ly coincides with the center of the continuous phase change. Addition
of 15 mol % cholesterol lowers the temperature of this sharp transitio
n by about 3 degrees C. These observations provide some insights into
the behavior of this system seen using differential scanning calorimet
ry. Results of deuteron transverse relaxation measurements under these
conditions are also reported.