Bm. Discher et al., Phase separation in monolayers of pulmonary surfactant phospholipids at the air-water interface: Composition and structure, BIOPHYS J, 77(4), 1999, pp. 2051-2061
The phase behavior of monolayers containing the complete set of purified ph
ospholipids (PPL) obtained from calf surfactant was investigated as a model
for understanding the phase transitions that precede compression of pulmon
ary surfactant to high surface pressure. During compression, both fluoresce
nce microscopy and Brewster angle microscopy (BAM) distinguished domains th
at separated from the surrounding film. Quantitative analysis of BAM graysc
ales indicated optical thicknesses for the PPL domains that were similar to
the liquid condensed phase for dipalmitoyl phosphatidylcholine (DPPC), the
most abundant component of pulmonary surfactant, and higher and less varia
ble with surface pressure than for the surrounding film. BAM also showed th
e optical anisotropy that indicates long-range orientational order of tilte
d lipid chains for the domains, but not for the surrounding film. Fluoresce
nce microscopy shows that addition of DPPC to the PPL increased the area of
the domains. At fixed surface pressures from 20-40 mN/m, the total area of
each phase grew in proportion with the mol fraction of DPPC. This constant
variation allowed analysis of the DPPC mol fraction in each phase, constru
ction of a simple phase diagram, and calculation of the molecular area for
each phase. Our results indicate that the phase surrounding the domains is
more expanded and compressible, and contains reduced amounts of DPPC in add
ition to the other phospholipids. The domains contain a mol fraction for DP
PC of at least 96%.