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

The adsorption and orientation of a dye molecule to a planar, glass-support ed phospholipid bilayer, a widely-used model membrane for biological studie s, was measured with second harmonic spectroscopy. The results indicate tha t the positively charged dye binds at an angle of 19 degrees from the norma l to the membrane plane. These results open the possibility of obtaining st ructural information on a variety of other adsorbed molecules at phospholip id membranes, including proteins, organic molecules and drugs. The nonlinea r optical properties of the dye molecule are also demonstrated for the firs t time and indicate that others in the oxaxole family may be useful for sec ond harmonic generation studies. (C) 2000 Elsevier Science B.V. All rights reserved.