MEASUREMENT OF CHAIN TILT ANGLE IN FULLY HYDRATED BILAYERS OF GEL PHASE LECITHINS

The tilt angle theta(tilt) of the hydrocarbon chains has been determin ed for the fully hydrated gel phase of a series of saturated lecithins . Oriented samples were prepared on glass substrates and hydrated with supersaturated water vapor. Evidence for full hydration was the same intensity pattern of the low angle lamellar peaks and the same lamella r repeat D as unoriented multilamellar vesicles. Tilting the sample pe rmitted observation of all the wide angle arcs necessary to verify the theoretical diffraction pattern corresponding to tilting of the chain s towards nearest neighbors. The length of the scattering unit corresp onds to two hydrocarbon chains, requiring each bilayer to scatter cohe rently rather than each monolayer. For DPPC, theta(tilt) was determine d to be 32.0 +/- 0.5-degrees at 19-degrees-C, slightly larger than pre vious direct determinations and considerably smaller than the value re quired by recent gravimetric measurements. This new value allows more accurate determinations of a variety of structural parameters, such as area per lipid molecule, A = 47.2 +/- 0.5 angstrom2, and number of wa ter molecules of hydration, n(w) = 11.8 +/- 0.7. As the chain length n of the lipids was increased from 16 to 20 carbons, the parameters A a nd n(w) remained constant, suggesting that the headgroup packing is at its excluded volume limit for this range. However, theta(tilt) increa sed by 3-degrees and the chain area A(c) decreased by 0.5 angstrom2. T his behavior is explained in terms of a competition between a bulk fre e energy term and a finite or end effect term.