Probing ordered lipid assemblies with polarized third-harmonic-generation microscopy

Abstract : Ordered lipid assemblies are responsible for important physiological functions including skin barrier and axon conductivity. However, techniques commonly used to probe molecular order such as X-ray scattering and nuclear magnetic resonance are not suited for in-situ tissue studies. Here, we identify and characterize a novel contrast mechanism in nonlinear optical microscopy which is sensitive to molecular ordering in multilamellar lipid vesicles (MLVs) and in samples obtained from human skin biopsy: polarized third-harmonic generation (P-THG). We develop a multiscale theoretical framework to calculate the anisotropic, nonlinear optical response of lipid arrays as a function of molecular order. This analysis reveals that conserved carbon-carbon bond and aliphatic tail directionality are the atomic- and molecular-scale sources of the observed P-THG response, respectively. Agreement between calculations and experiments on lipid droplets and MLVs validates the use of P-THG as a probe of lipid ordering. Finally, we show that P-THG can be used to map molecular ordering in the multilamellar, intercorneocyte lipid matrix of the stratum corneum of human skin. These results provide the foundation for the use of P-THG in probing molecular order and highlight a novel biomedical application of multiphoton microscopy in an optically accessible tissue relevant to monitoring lipid-related disorder.
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Maxwell Zimmerley, Pierre Mahou, Delphine Débarre, Marie-Claire Schanne-Klein, Emmanuel Beaurepaire. Probing ordered lipid assemblies with polarized third-harmonic-generation microscopy. Physical Review X, American Physical Society, 2013, 3 (1), pp.11002. ⟨10.1103/PhysRevX.3.011002⟩. ⟨hal-00324248⟩

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