Changes in the structure of the hydrocarbon core (HC) of fluid lipid bilayers can reveal how bilayers respond to the partitioning of peptides and other solutes (Jacobs, R. E., and S. H. White. 1989. Biochemistry. 28 3421- 3437). The structure of the HC of dioleoylphosphocholine (DOPC) 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 projection of the double-bond positions onto the bilayer normal (z), can be obtained by means of neutron diffraction and double-bond specific deuteration (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 isomorphous 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 observed at high water contents. We used this new method to determine the transbilayer distribution of the double-bond bromine labels of DOPC over a hydration range of 5.4 to 16 waters per lipid, which reveals how the HC structure changes with hydration. We found that the transbilayer distributions of the bromines can be described by a pair of Gaussians of 1/e half-width A(Br) located at z = ±Z(Br) relative to the bilayer center. For hydrations from 5.4 waters up to 9.4 waters per lipid, Z(Br) decreases from 7.97 ± 0.27 Å to 6.59 ± 0.15 Å, while A(Br) increased from 4.62 ± 0.62 Å to 5.92 ± 0.37 Å, consistent with the expected hydration-induced decrease in HC thickness and increase in area per lipid. After the phosphocholine hydration shell was filled at ~12 waters per lipid, we observed a shift in Z(Br) to ~7.3 Å, indicative of a distinct structural change upon completion of the hydration shell. For hydrations of 12-16 waters per lipid, the bromine distribution remains constant at Z(Br) = 7.33 ± 0.25 Å and A(Br) = 5.35 ± 0.5 Å. The absolute- scale structure factors obtained in the experiments provided an opportunity to test the so-called fluid-minus method of structure-factor scaling. We found that the method is quite satisfactory for determining the phases of structure factors, but not their absolute values.
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