Protonation of the Biliverdin IXα Chromophore in the Red and Far-Red Photoactive States of a Bacteriophytochrome

The
tetrapyrrole chromophore biliverdin IXα (BV) in the bacteriophytochrome
from Deinococcus radiodurans (DrBphP)
is usually assumed to be fully protonated, but this assumption has
not been systematically validated by experiments or extensive computations.
Here, we use force field molecular dynamics simulations and quantum
mechanics/molecular mechanics calculations with density functional
theory and XMCQDPT2 methods to investigate the effect of the five
most probable protonation forms of BV on structural stability, binding
pocket interactions, and absorption spectra in the two photochromic
states of DrBphP. While agreement with X-ray structural data and measured
UV/vis spectra suggest that in both states the protonated form of
the chromophore dominates, we also find that a minor population with
a deprotonated D-ring could contribute to the red-shifted tail in
the absorption spectra.