Second harmonic spectroscopy: detection and orientation of molecules at a biomembrane interface

The adsorption and orientation of a dye molecule to a planar, glass-supported phospholipid bilayer, a widely-used model membrane for biological studies, was measured with second harmonic spectroscopy. The results indicate that the positively charged dye binds at an angle of 19 degrees from the normal to the membrane plane. These results open the possibility of obtaining structural information on a variety of other adsorbed molecules at phospholipid membranes, including proteins, organic molecules and drugs. The nonlinear optical properties of the dye molecule are also demonstrated for the first time and indicate that others in the oxaxole family may be useful for second harmonic generation studies.