The photoactivation of the plant photoreceptor phytochrome is governed by a red-light-induced C15-Z -> C15-E isomerization of the tetrapyrrolic chromophore phytochromobilin. From the viewpoint of experimental studies, ambiguity prevails as to whether the photoactivation involves a proton transfer from the chromophore to the surrounding protein. Here, we report a theoretical study addressing the effect of phytochromobilin protonation state on its photoisomerization by means of quantum chemical calculations. It is found that neutral forms of the chromophore are much less likely to photoisomerize than the parent, protonated form - a finding which supports the view that phytochromobilin remains protonated during phytochrome photoactivation. (c) 2005 Elsevier B.V. All rights reserved.