In situ nonlinear optical spectroscopic study of the structural chirality in DPPC Langmuir monolayers at the...

We conduct a molecular study on the structural chirality in Langmuir monolayers composed of dipalmitoylphosphatidylcholine (DPPC) using in situ nonlinear optical spectroscopies, including second harmonic generation (SHG) and sum frequency generation (SFG). Chiral SHG response is observed from L-DPPC monolayers at moderate surface pressures and almost vanishes at a high surface pressure. SFG spectra of L-DPPC monolayers show chiral features that can be assigned to the terminal CH₃ groups and the CH₂ groups attached to the chiral center atom. This means that these achiral moieties form chiral superstructures at the interface. Along with increasing surface pressure, the structural chirality of CH₃ groups shows a similar trend as that of chiral SHG, but CH₂ chirality increases monotonically. Furthermore, in a racemic DPPC monolayer with a moderate surface pressure, both chiral SHG and chiral SFG of CH₃ groups are absent, whereas chiral SFG of CH₂ groups is clearly present, indicating that L- and D-DPPC are diastereomers at the air/water interface and interfacial CH₂ prefers a certain orientation regardless of the molecular handedness. A molecular mechanism is proposed to explain the origin of the structural chirality in DPPC monolayers.