Unique contrast patterns from resonance-enhanced chiral SHG of cell membranes

Chirality can produce novel nonlinear optical effects that may form the basis for new imaging contrast agents. In this paper, we developed a new chiral chromophore 2, which is the dimer of a known voltage sensitive dye, monomer 1, with the chirality originating from the twisted orientation between two subunits. Racemic dimer and monomer 1 were used as the references to study the effect of chirality in SHG microscopy of live cells. All these dyes selectively stain the outer leaflets of cell membranes, producing strong resonance-enhanced SHG images. At the symmetric junction between two adherent cells, monomer or racemic dimer SHG is forbidden due to centrosymmetry, and indeed little SHG was observed (10 +/- 1% relative to nonjunction). When stained with the chiral dimer, the junction is no longer centrosymmetric and much stronger SHG was observed (39 +/- 4% relative to nonjunction). Plane polarized light produces highly polarized images of spherical cells stained with racemic dye, but for the chiral dye, the polarized pattern is largely eliminated by the chiral SHG emanating from the subresolution membrane convolutions.