Indole adsorption to a lipid monolayer studied by optical second harmonic generation
1Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario, K1A 0R6, Canada.
The Journal of Physical Chemistry. B
|July 16, 2009
Summary
This study investigates indole adsorption onto lipid monolayers using nonlinear optics and surface pressure measurements. Indole adsorption is highest at low surface pressure, with its orientation shifting as the monolayer compresses.
Area of Science:
- Surface science
- Physical chemistry
- Biophysics
Background:
- Lipid monolayers at interfaces are model systems for biological membranes.
- Indole is a biologically relevant molecule found in various compounds.
- Understanding molecule-lipid interactions is crucial for drug delivery and biomaterial design.
Purpose of the Study:
- To quantify indole adsorption to a dipalmitoylphosphatidylcholine (DPPC) monolayer.
- To determine the orientation of adsorbed indole molecules.
- To investigate the effect of surface pressure on indole adsorption and orientation.
Main Methods:
- Nonresonant second harmonic generation (SHG) spectroscopy with lambda = 800 nm.
- Langmuir trough measurements of compression isotherms.
- Computational methods (sum-over-states, TDHF, DFT) for molecular hyperpolarizability.
Main Results:
- Indole adsorption is maximal at low surface pressure, with an indole/DPPC ratio near 2:1.
- The free energy of adsorption is estimated at -8.1 kcal/mol.
- Adsorbed indole orients with the pyrrole ring towards the aqueous subphase, and its tilt angle increases with compression.
Conclusions:
- Indole readily adsorbs to DPPC monolayers, with significant energetic favorability.
- Surface pressure influences both the extent of adsorption and the orientation of indole.
- The findings provide insights into molecular interactions at lipid-water interfaces.
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