Monolayer Properties of 1,3-Diamidophospholipids
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Abstract
While nature provides an endless
variety of phospholipids presenting
hydrolyzable ester linkages for the 1,2-positioned hydrocarbon tails,
we designed and synthesized 1,3-diamidophospholipids which contain
stable fatty acid amides. These new phospholipids form faceted unilamellar
vesicles with mechanosensitive properties. Aiming to understand the
mechanism responsible for this behavior at a molecular level, we investigated
the 1,3-diamidophospholipid family in monolayers, a simplified model
membrane system. Langmuir isotherms combined with <i>in situ</i> grazing incidence X-ray diffraction (GIXD), specular X-ray reflectivity
(XR), and infrared reflection–absorption spectroscopy (IRRAS)
allowed the characterization of the monolayers from a structural and
thermodynamical point of view. The existence of strong headgroup interactions
due to the formation of a hydrogen-bonding network was clearly revealed
by IRRAS and by the high rigidity of the monolayers. GIXD showed that
only the longer chain compounds of the series (Pad-PC-Pad (1,3-dipalmitamidopropan-2-phosphocholine)
and Sad-PC-Sad (1,3-distearamidopropan-2-phosphocholine) were able
to form ordered monolayers. The chains are strongly tilted in a rigid
lattice formed due to these hydrogen-bonding interactions between
the headgroups. The thermodynamical analysis leads to a critical temperature
of the monolayer which is clearly different from the main phase transition
temperature in bulk, indicating that there must be a different structural
arrangement of the 1,3-diamidophospholipids in monolayers and in bilayers