Quantum dynamics of excited states of Phytochromes: an important class of protein chromophore

I have been learning a little more about phytochromes which are a important class of photo-active biomolecules.  This was stimulated by my previous post about single molecule spectroscopy studies of a photosynthetic protein containing phytochromes. 

I mention here two recent PNAS papers that I found particularly interesting. I just include some key text and figures from the papers.

Ultrafast excited-state isomerization in phytochrome revealed by femtosecond stimulated Raman spectroscopy

Photochemical interconversion between the red-absorbing (P_r) and the far-red-absorbing (P_fr) forms of the photosensory protein phytochrome initiates signal transduction in bacteria and higher plants. The P_r-to-P_frtransition commences with a rapid Z-to-E photoisomerization at the C₁₅C₁₆ methine bridge of the bilin.

This paper reports evidence that this isomerization occurs on the excited state potential energy surface.

Distinct classes of red/far-red photochemistry within the phytochrome superfamily

Phytobilin chromophores can underdo two further possible rotations of the end rings following absorption of red light. Different proteins favour different rotations of these D rings.

The associated quantum chemistry has been considered in Deciphering Intrinsic Deactivation/Isomerization Routes in a Phytochrome Chromophore Model