Mixed phospholipid-cardiotoxin monomolecular films studied by intrinsic polarized surface fluorescence

Surface fluorescence spectroscopy was used to study mixed phospholipid-cardiotoxin monomolecular films. Using polarized incident light we were able to detect the intrinsic fluorescence emission at a wavelength of 339 nm of the single Trp11 in the cardiotoxin molecule. Its fluorescence intensity increased continuously upon film compression up to a maximal value reached at 42 mN x m-1 in surface pressure. A relative polarization index can be used as an indication of the orientation of the indole ring. A transition at around 25 +/- 5 mN x m-1 in the orientation of the Trp11 relative to the plane of the interface was clearly evidenced during film compression, in agreement with our independent data showing a flipping of the whole toxin molecule (Bougis et al., (1981), Biochemistry 20, 4915-4920).