Light-induced linear dichroism in photoreversibly photochromic sensor pigments 1. Theory

With a photoreversibly photochromic regulator pigment such as phytochrome, linear action dichroism could theoretically be obtained after photoselection even if the molecules are initially randomly oriented. If randomly oriented Pr (red-absorbing phytochrome) (plant photoregulators) molecules are partially converted to Pfr (far-red absorbing phytochrome) molecules by plane-polarized red light, those molecules will preferentially be converted which have their red transition moments nearly parallel to the electric vector of the red light. The effect of subsequent plane-polarized far-red light will depend on the plane of polarization. A general theory is developed for how this can be used to determine whether or not the transition moment changes direction during conversion. The pigment need not be isolated, since only physiological reactions (such as germination or chromatic adaptation) are measured.