Indole Adsorption to a Lipid Monolayer Studied by Optical Second Harmonic Generation

Adsorption of indole from an aqueous subphase to a lipid (DPPC) monolayer at the air/water interface is studied by nonresonant second harmonic generation (SHG) with lambda = 800 nm and by observation of compression isotherms with a Langmuir trough. The nonlinear susceptibilities of the monolayers have been carefully measured, including corrections for the contributions of the lipid monolayer and subphase, calibration of absolute values by comparison with a quartz reference, and measurement of absolute phase by comparison with a clean air/water interface. Details of the calibrations involving z-cut quartz and the clean air/water interface are presented. The number density of adsorbed indole molecules has been estimated by comparison of nonlinear susceptibilities with a reference monolayer of hexadecyl 3-indoleacetate. The extent of adsorption of indole to the lipid monolayer is maximum at low surface pressure, decreasing from an indole/DPPC ratio near 2:1 as the monolayer is compressed. The free energy change for adsorption of indole to the monolayer is estimated as -8.1 kcal/mol, which is similar to a result reported previously for a lipid bilayer. Information on the orientation of adsorbed indole is derived by comparison of observed ratios of nonlinear susceptibilities with calculated ratios for model indole monolayers with assumed orientational distributions. Measurement and analysis procedures are described in detail for effective nonlinear susceptibilities in a s/p polarization basis. The calculations use as input the molecular hyperpolarizability tensor of indole, which is obtained by using several computational approaches including sum-over-states calculations and time-dependent Hartree-Fock and density functional methods. The analysis shows that the pyrrole ring of indole points toward the water subphase, and the tilt angle of the long axis of indole increases from near 50 degrees to near 60 degrees as the monolayer is compressed. The analysis also suggests that the plane of indole lies more nearly parallel than perpendicular to the plane of the interface.