Cooperative Adsorption of Trehalose to DPPC Monolayers at the Water-Air Interface Studied with Vibrational Sum...

A combination of surface tension, surface-specific vibrational spectroscopy, and differential scanning calorimetry experiments was performed to examine the ability of lipid films to enrich interfacial organic content by attracting soluble, neutral saccharides from bulk aqueous solution. This "cooperative adsorption" hypothesis has been proposed as a possible source of the high organic fractions found in sea spray aerosols and is believed to be responsible for cryoprotection in some organisms. Experiments described in this work show that the neutral disaccharide trehalose (Tre) is drawn to lipid films composed of dipalmitoylphosphatidylcholine (DPPC), a saturated lipid that is a major component of most eukaryotic cells. The effects of Tre on DPPC monolayer structure and organization were tested with tightly packed monolayers in the two-dimensional solid phase (40 Ų/molecule) and more expanded monolayers in the two-dimensional liquid condensed phase (55 Ų/molecule). Surface tension data show that DPPC monolayer behavior remains largely unchanged until Tre bulk concentrations are sufficiently high (≥50 mM). In contrast, surface-specific vibrational sum frequency spectra show that when Tre bulk concentrations are ≥10 mM, DPPC monolayers in their liquid condensed state (55 Ų/molecule) became more ordered, implying relatively strong noncovalent interactions between the two species. Tre also induces changes in DPPC bilayer behavior as evidenced by a gel-to-liquid crystalline phase transition temperature that increases with increasing Tre concentration. This result suggests that Tre associates with the DPPC headgroups in very specific ways leading to partial dehydration. Together, these results support the cooperative adsorption mechanism under some circumstances, namely, that there is a minimum aqueous phase Tre concentration required to induce observable structural changes in a lipid monolayer and that these effects are most pronounced with DPPC monolayers in their liquid condensed state compared to those of a tightly packed two-dimensional solid.