Label-Free Optical Method for Quantifying Molecular Transport Across Cellular Membranes In Vitro.

We demonstrate a nonlinear optical method for the label-free quantification of membrane transport rates of small/medium size molecules in living cells. Specifically, second-harmonic generation (SHG) laser scattering permits surface-specific characterization of transport across membranes. Unfortunately, most biologically relevant molecules are SHG-inactive. In the interest of extending this methodology for characterizing transport of any molecule, we monitor the SHG produced from an SHG-active reference molecule, in the presence of an SHG-inactive target molecule-of-interest as both molecules compete to cross a membrane. Of significance, the SHG-inactive target transport rate can be deduced as a perturbation in the measured transport rate of the reference. As proof-of-principle, we examine competitive transport of the strongly SHG-active cation, malachite green (MG), in the presence of a weakly SHG-active dication, propidium (Pro), across the outer-membrane protein channels in living bacteria. Comparison of the extracted and directly measured Pro transport rates validates the effectiveness of the method.