Which factors determine the acidity of the phytochromobilin chromophore of plant phytochrome?

Quantum chemical calculations aimed at identifying the factors controlling the acidity of phytochromobilin, the tetrapyrrole chromophore of the plant photoreceptor phytochrome, are reported. Phytochrome is converted from an inactive (Pr) to an active form (Pfr) through a series of events initiated by a Z -> E photoisomerization of phytochromobilin, forming the Lumi-R intermediate, and much controversy exists as to whether the protonation state of the chromophore (cationic in Pr with all nitrogens protonated) changes during the photoactivation. Here, relative ground (S-0) and excited-state (S-1) pK(a)s of all four pyrrole moieties of phytochromobilin in all 64 possible configurations with respect to the three methine bridges are calculated in a protein-like environment, using a recently benchmarked level of theory. Accordingly, the relationships between acidity and chromophore geometry and charge distribution, hydrogen bonding, and light absorption are investigated in some detail, and discussed in terms of possible mechanisms making a proton transfer reaction more probable along the Pr -> Pfr reaction than in the parent cationic Pr state. It is found that charge distribution in the cationic species, intra-molecular hydrogen bonding in the neutral, and hydrogen bonding with two highly conserved aspartate and histidine residues have a significant effect on the acidity, while overall chromophore geometry and electronic state are less important factors. Furthermore, based on the calculations, two processes that may facilitate a proton transfer by substantially lowering the pK(a)s relative to their Pr values are identified: (i) a thermal Z,anti -> Z,syn isomerization at C5, occurring after formation of Lumi-R; (ii) a perturbation of the hydrogen bonding network which in Pr comprises the nitrogens of pyrroles A, B and C and the two aspartate and histidine residues.