DETERMINATION OF THE HYDROCARBON CORE STRUCTURE OF FLUID DIOLEOYLPHOSPHOCHOLINE (DOPC) BILAYERS BY X-RAY-DIFFRACTION USING SPECIFIC BROMINATION OF THE DOUBLE-BONDS - EFFECT OF HYDRATION
K. Hristova et Sh. White, DETERMINATION OF THE HYDROCARBON CORE STRUCTURE OF FLUID DIOLEOYLPHOSPHOCHOLINE (DOPC) BILAYERS BY X-RAY-DIFFRACTION USING SPECIFIC BROMINATION OF THE DOUBLE-BONDS - EFFECT OF HYDRATION, Biophysical journal, 74(5), 1998, pp. 2419-2433
Changes in the structure of the hydrocarbon core (HC) of fluid lipid b
ilayers can reveal how bilayers respond to the partitioning of peptide
s and other solutes (Jacobs, R. E., and S. H. White. 1989. Biochemistr
y. 28:3421-3437). The structure of the HC of dioleoylphosphocholine (D
OPC) bilayers can be determined from the transbilayer distribution of
the double-bonds (Wiener, M. C., and S. H. White. 1992. Biophys. J. 61
:434-447). This distribution, representing the time-averaged projectio
n of the double-bond positions onto the bilayer normal (z), can be obt
ained by means of neutron diffraction and double-bond specific deutera
tion (Wiener, M. C., G. I. King, and S. H. White. 1991. Biophys. J. 60
:568-576), For fully resolved bilayer profiles, a close approximation
of the distribution could be obtained by x-ray diffraction and isomorp
hous bromine labeling at the double-bonds of the DOPC sn-2 acyl chain
(Wiener, M. C., and S. H. White. 1991. Biochemistry. 30:6997-7008). We
have modified the bromine-labeling approach in a manner that permits
determination of the distribution in under-resolved bilayer profiles o
bserved at high water contents. We used this new method to determine t
he transbilayer distribution of the double-bond bromine labels of DOPC
over a hydration range of 5.4 to 16 waters per lipid, which reveals h
ow the HC structure changes with hydration. We found that the transbil
ayer distributions of the bromines can be described by a pair of Gauss
ians of 1/e half-width A(Br) located at z = +/-Z(Br) relative to the b
ilayer center. For hydrations from 5.4 waters up to 9.4 waters per lip
id, Z(Br) decreases from 7.97 +/- 0.27 Angstrom to 6.59 +/- 0.15 Angst
rom, while A(Br) increased from 4.62 +/- 0.62 Angstrom to 5.92 +/- 0.3
7 Angstrom, consistent with the expected hydration-induced decrease in
HC thickness and increase in area per lipid. After the phosphocholine
hydration shell was filled at similar to 12 waters per lipid, we obse
rved a shift in Z(Br) to similar to 7.3 Angstrom, indicative of a dist
inct structural change upon completion of the hydration shell. For hyd
rations of 12-16 waters per lipid, the bromine distribution remains co
nstant at Z(Br) = 7.33 +/- 0.25 Angstrom and A(Br) = 5.35 +/- 0.5 Angs
trom. The absolute-scale structure factors obtained in the experiments
provided an opportunity to test the so-called fluid-minus method of s
tructure-factor scaling. We found that the method is quite satisfactor
y for determining the phases of structure factors, but not their absol
ute values.