The four rings of the bilin are labelled A-D. Carbon atoms before and one picosecond after photoactivation are shown in grey and cyan, respectively. Water molecules before and after photoactivation are shown in deep red and bright red, respectively. Otherwise, oxygen, nitrogen and sulphur atoms are shown in red, blue and yellow, respectively. The ~50° rotation of the D-ring is indicated by the red arrow. The pyrrole water molecule (PW) above the nitrogen atoms of the A-, B- and C-rings disappears on photoactivation. Hydrogen bonds (yellow dashes) between the bilin and amino acid side chains in the rest of the phytochrome are also broken. Figure prepared by the author using PyMol from data provided by Claesson et al.