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 (ΔV) accompanying the formation and decay of transient species in a time window between ca. 20 ns and 5 μs. Here, we report PA measurements on diverse photochromic bilin-binding photoreceptors of prokaryotic origin: (1) the chromophore-binding GAF3 domain of the red (R)/green (G) switching cyanobacteriochrome 1393 (Slr1393g3) from Synechocystis; (2) the red/far red (R/FR) Synechocystis Cph1 phytochrome; (3) full-length and truncated constructs of Xanthomonas campestris bacteriophytochrome (XccBphP), 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 ΔV1  = 5-12.5 mL mol-1 . Formation of the first intermediate is the sole process accessible to PA, with the notable exception of Slr1393g3-G for which ΔV1  = +4.5 mL mol-1 is followed by a time-resolved, energy-conserving contraction ΔV2  = -11.4 mL mol-1 , τ2  = 180 ns at 2.4°C. This peculiarity is possibly due to a larger solvent occupancy of the chromophore cavity for Slr1393g3-G.