Phytochromobilin C15-Z,syn → C15-E,anti isomerization: concerted or stepwise?

Abstract

The C15-Z,syn → C15-E,anti isomerization of phytochromobilin that underlies the photoactivation of phytochrome, the plant photoreceptor responsive to red and far-red light, is investigated by means of quantum chemical methods. By calculating ground and excited-state potential energy surfaces for a phytochromobilin model comprising the full tetrapyrrolic skeleton, and taking into consideration rotations around the C14−C15 and C15C16 bonds constituting the methine bridge between pyrrole rings C and D, it is found that a stepwise Z → E, syn → anti mechanism is energetically preferable over a concerted Z,syn → E,anti mechanism. In particular, on the basis of the calculated potential energy surfaces, it is proposed that the primary photochemical reaction involves a Z → E isomerization only, and that the subsequent syn → anti isomerization proceeds thermally.