The interaction of L-phenylalanine with a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer at the air-water interface was explored using a combination of experimental techniques and molecular dynamics (MD) simulations. By means of Langmuir trough methods and Brewster angle microscopy, L-phenylalanine was shown to significantly alter the interfacial tension and the surface domain morphology of the DPPC film. In addition, confocal microscopy was used to explore the aggregation state of L-phenylalanine in the bulk aqueous phase. Finally, MD simulations were performed to gain molecular-level information on the interactions of L-phenylalanine and DPPC at the interface. Taken together, these results show that L-phenylalanine intercalates into a DPPC film at the air-water interface, thereby affecting the surface tension, phase morphology, and ordering of the DPPC film. The results are discussed in the context of biological systems and the mechanism of diseases such as phenylketonuria.

The exposure of organic-coated marine aerosols containing cholesterol (Chol) to radiation and/or an oxidizing atmosphere results in the formation of oxidized derivatives or oxysterols and will likely change aerosol surface properties. However, the intermolecular interactions between oxysterols and other lipid components and their influence on the surface properties of marine aerosols are not well-known. To address this question, the interfacial behavior and domain morphology of model Langmuir monolayers of two ring-substituted oxysterols, 7-ketocholesterol (7-KChol) and 5β,6β-epoxycholesterol (5,6β-EChol), mixed with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were investigated by means of compression isotherms and Brewster angle microscopy (BAM) over a broad range of surface pressures and sterol molar ratios. Mixed DPPC/cholesterol (Chol) monolayers were also measured for comparison. The results of compression experiments showed that the condensing effect induced on mixed DPPC/sterol monolayers at low surface pressures and for intermediate molar ratios (0.3 ≤ X(sterol) ≤ 0.7) was weaker for oxysterols than for Chol. Additionally, mixed DPPC/oxysterol monolayers exhibited markedly smaller (∼2-3-fold) interfacial rigidity. Examination of the excess free energy of mixing further revealed that DPPC monolayers containing 7-KChol and Chol were thermodynamically more stable at high surface pressures than those with 5,6β-EChol, indicating that the strength of interactions between DPPC and 5,6β-EChol was the smallest. Finally, BAM images in the LE-LC phase of DPPC revealed that in comparison to Chol the addition of small amounts of oxysterols results in larger and less numerous domains, showing that oxysterols are not as effective in fluidizing the condensed phase of DPPC. Taken together, these results suggest that the strength of van der Waals interactions of DPPC alkyl chains with sterols follows the sterol hydrophobicity, with Chol being the most hydrophobic and oxysterols more hydrophilic due to their ketone and epoxy moieties. The difference in the condensing ability and stability of 7-KChol and 5,6β-EChol on DPPC likely originates from the distinct molecular structure and position of oxidation on the steroid nucleus. As suggested by recent MD simulations, depending on the oxidation position, ring-substituted oxysterols have a broader angular distribution of orientation than Chol in bilayers, which could be responsible for the observed reduction in condensing ability.