Probing bacteriorhodopsin photochemistry with nonlinear optics. Comparing the second harmonic generation of bR and the photochemically induced intermediate K

The nonlinear optical properties of the bacteriorhodopsin chromophore in the bR568 and K states are investigated by second harmonic generation. The comparison of amplitudes and phases of the second-order nonlinear optical polarizabilities of the retinal chromophore in the two states has revealed a noticeable increase of the induced dipole of the retinal as a result of the bR568 → K transition. The results have been explained in terms of recent theoretical understandings of the nonlinear optical properties of polyenes. Within the context of these understandings we have discussed the molecular origins of the light-induced color changes and the possible mechanism of photon energy storage observed in this protein.