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

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.