In situ nonlinear optical spectroscopic study of the structural chirality in DPPC Langmuir monolayers at the air/water interface

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. Published under an exclusive license by AIP Publishing.

Automated summary

In this study, nonlinear optical spectroscopies were used to investigate the structural chirality in Langmuir monolayers composed of dipalmitoylphosphatidylcholine (DPPC). It was found that the chiral response decreased with increasing surface pressure, indicating the presence of chiral superstructures formed by achiral moieties. The chirality of the terminal CH3 groups showed a similar trend as that of the chiral response, while the chirality of the CH2 groups increased monotonically. A racemic DPPC monolayer showed no chiral response from the CH3 groups, but chiral response was observed from the CH2 groups, suggesting that the orientation of interfacial CH2 was independent of the molecular handedness. The results provide insights into the molecular mechanism underlying the structural chirality in DPPC monolayers.