Ground-state heterogeneity and vibrational energy redistribution in bacterial phytochrome observed with femtosecond 2D IR spectroscopy

Phytochromes belong to a group of photoreceptor proteins containing a covalently bound biliverdin chromophore that inter-converts between two isomeric forms upon photoexcitation. The existence and stability of the photocycle products are largely determined by the protein sequence and the presence of conserved hydrogen-bonding interactions in the vicinity of the chromophore. The vibrational signatures of biliverdin, however, are often weak and obscured under more intense protein bands, limiting spectroscopic studies of its non-transient signals. In this study, we apply isotope-labeling techniques to isolate the vibrational bands from the protein-bound chromophore of the bacterial phytochrome from Deinococcus radiodurans. We elucidate the structure and ultrafast dynamics of the chromophore with 2D infra-red (IR) spectroscopy and molecular dynamics simulations. The carbonyl stretch vibrations of the pyrrole rings show the heterogeneous distribution of hydrogen-bonding structures, which exhibit distinct ultrafast relaxation dynamics. Moreover, we resolve a previously undetected 1678 cm⁻¹ band that is strongly coupled to the A- and D-ring of biliverdin and demonstrate the presence of complex vibrational redistribution pathways between the biliverdin modes with relaxation-assisted measurements of 2D IR cross peaks. In summary, we expect 2D IR spectroscopy to be useful in explaining how point mutations in the protein sequence affect the hydrogen-bonding structure around the chromophore and consequently its ability to photoisomerize to the light-activated states.