Time‐Resolved Energetics of Photoprocesses in Prokaryotic Phytochrome‐Related Photoreceptors

Time‐resolved photoacoustics ( PA ) is uniquely able to explore the energy landscape of photoactive proteins and concomitantly detects light‐induced volumetric changes (Δ V ) accompanying the formation and decay of transient species in a time window between ca. 20 ns and 5  μ s. Here, we report PA measurements on diverse photochromic bilin‐binding photoreceptors of prokaryotic origin: (1) the chromophore‐binding GAF 3 domain of the red (R)/green (G) switching cyanobacteriochrome 1393 ( Slr 1393g3) from Synechocystis ; (2) the red/far red (R/ FR ) Synechocystis Cph1 phytochrome; (3) full‐length and truncated constructs of Xanthomonas campestris bacteriophytochrome ( Xcc BphP), absorbing up to the NIR spectral region. In almost all cases, photoisomerization results in a large fraction of energy dissipated as heat (up to 90%) on the sub‐ns scale, reflecting the low photoisomerization quantum yield (<0.2). This “prompt” step is accompanied by a positive Δ V 1  =  5–12.5 mL mol −1 . Formation of the first intermediate is the sole process accessible to PA , with the notable exception of Slr 1393g3‐G for which Δ V 1  = +4.5 mL mol −1 is followed by a time‐resolved, energy‐conserving contraction Δ V 2  = −11.4 mL mol −1 , τ 2  = 180 ns at 2.4°C. This peculiarity is possibly due to a larger solvent occupancy of the chromophore cavity for Slr 1393g3‐G.