S. Tristramnagle et al., MEASUREMENT OF CHAIN TILT ANGLE IN FULLY HYDRATED BILAYERS OF GEL PHASE LECITHINS, Biophysical journal, 64(4), 1993, pp. 1097-1109
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.