TIME-RESOLVED 2ND-HARMONIC GENERATION IN THE RANDOMLY ORIENTED PURPLEMEMBRANE

We have observed second harmonic generation (SHG) at 532 nm in randoml y oriented suspensions:of the purple membrane of Halobacterium halobiu m. The intensity of the second harmonic can be switched by photoexcita tion of the bacteriorhodopsin (BR) chromophore at 570 nm. The SHG inte nsity decreases in <1 mu s and reaches a minimum in <1 ms with the for mation of the M state of BR. For dilute samples, we have observed phot oinduced changes in the SHG intensity of up to 90%. At low concentrati ons, the second harmonic intensity in light-adapted BR depends linearl y on concentration, while showing saturation at higher concentrations. The linear concentration dependence Shows that coherence of the secon d-order polarization between membrane particles is weak. At the low co ncentrations used, the SHG time dependence cannot be attributed to abs orption of the 532-nm second harmonic. The fact that the SHG signal is almost entirely abolished with the formation of the M state suggests that the BR signal is strongly resonance enhanced by a two-photon reso nance at 532 nm.