To guarantee the immobilization of urease in the air–water
interface, EPS were employed as the aqueous subphase. This system
can adsorb at the DODAB monolayers, which was already
shown in the literature under similar conditions [6,7]. Comparing
the isotherms for pure DODAB monolayers shown in Fig. 1A
to the isotherms for DODAB-ESP monolayers shown in Fig. 1B, the
isotherm for DODAB-EPS is shifted to smaller areas when the areas
for the same surface pressures are compared. This indicates the condensation
of the monolayer due to the minimization of the lateral
electrostatic repulsions of the lipid monolayer. This is related to
the fact that DODAB is positively charged and EPS are negatively
charged under the conditions employed in this study. In addition, the shape of the curves changes when EPS are incorporated,
which indicates an alteration in the compressibility properties of
the monolayer. The isotherms in Fig. 1B show that the DODABEPS-
urease system is more expanded than that without urease
above 16 mN/m, which is most probably not due to urease penetration
into the DODAB-EPS system but rather to a change in the
supramolecular arrangement of the monolayer. In addition, the
phase transition observed in the DODAB-EPS curve at that surface
pressure may be interpreted as a molecular reorientation of the
system, which does not occur when urease is incorporated.