DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine), Lot
#160PC-198, was purchased from Avanti Polar Lipids (Alabaster, AL), and
no lysolecithin was found using thin layer chromatography. Polyvinylpyrrolidone
(PVP, MW = 40,000) was purchased from Sigma Chemical Co. (Milwaukee, WI)
and dried in a 70 
C
oven overnight. PVP solutions from 5 to 45% (w:w) were prepared by mixing
dried PVP with distilled, deionized water. PVP solutions were added to
dried DPPC in a 3:1 (for 30% PVP and below) or 4:1 (for 35% PVP and above)
weight ratio. The samples were cycled in 5 minute intervals between 70
C and 5 
C
three times with vortexing, then held at 50 
C
for 3-4 hours and finally allowed to equilibrate overnight at room temperature.
The samples were loaded into carefully cleaned thin walled 1.0 or 1.5 mm
flint glass X-ray capillaries (Charles Supper Co., Cambridge, MA) flame-sealed
at one end. The capillaries were centrifuged for 10 min at 1,100 g
to remove air bubbles. At PVP concentrations of 30% and above, the lipid
dispersions centrifuged up instead of down at 0 
C.
The capillaries were then flame sealed at the other end and loaded into
cassettes with 14 slots/cassette. Both ends of the capillaries were embedded
in slabs of silicone sealer to insure against dehydration. These cassettes
fit directly into a custom cassette holder which was contained in a cylindrical
aluminum sample chamber with two 1.5 
m
thick mylar windows for entry and exit of X rays. Temperature was controlled
by a Lake Shore Cryotronics Model DRC-91C Temperature Controller (Westerville,
Ohio) which responded to a 1000 ohm platinum resistance thermometer in
the center of the sample cassette (Rosemount, Eagen, MN). Temperature stability
was 
0.02 
C.
The chamber was attached to X-Y-Z motorized translations to move the samples
in the X-ray beam.