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Abstract
Small-angle x-ray diffraction has been applied in locating either butylated hydroxytoluene
(BHT) or delta-tocopherol and their brominated analogues at a concentration of 40
mol% in oriented bilayers of dipalmitoylphosphatidylcholine (DPPC) or DPPC + 15 mol%
cholesterol at 20 degrees C. Phases were determined using swelling experiments with
structure factors plotted in reciprocal space, creating a relatively smooth curve
as the amount of water between the bilayers was changed. Continuous Fourier transforms
were also calculated using sampling theory (Shannon, C. E. 1949. Proc. Inst. Radio
Engrs. NY. 37:10-21) to further test the consistency of the phase assignments. Fourier
synthesis of structure factors resulted in absolute electron density profiles for
different bilayers to a resolution of 5-6 A. In addition, difference Patterson maps
were constructed to confirm the positions of the bromine atoms in the unit cell. Analysis
of the data indicates the following: (a) The BHT molecules are dispersed throughout
the alkyl-chain region in DPPC samples with and without cholesterol. (b) The chromanol
ring of delta-tocopherol is in the vicinity of the glycerol backbone-headgroup region
in samples of DPPC or DPPC + 15 mol% cholesterol. (c) Difference Patterson maps confirm
the localization of bromine atoms in the various delta-tocopherol samples and lack
of bromine localization in the various BHT samples.