Influence of the crystalline state on photoinduced dynamics of photoactive yellow protein studied by ultraviolet-visible transient absorption spectroscopy.

Time-resolved ultraviolet-visible spectroscopy was used to characterize the photocycle transitions in single crystals of wild-type and the E-46Q mutant of photoactive yellow protein (PYP) with microsecond time resolution. The results were compared with the results of similar measurements on aqueous solutions of these two variants of PYP, with and without the components present in the mother liquor of crystals. The experimental data were analyzed with global and target analysis. Distinct differences in the reaction path of a PYP molecule are observed between these conditions when it progresses through its photocycle. In the crystalline state i), much faster relaxation of the late blue-shifted photocycle intermediate back to the ground state is observed; ii), this intermediate in crystalline PYP absorbs at 380 nm, rather than at 350-360 nm in solution; and iii), for various intermediates of this photocycle the forward reaction through the photocycle directly competes with a branching reaction that leads directly to the ground state. Significantly, with these altered characteristics, the spectroscopic data on PYP are fully consistent with the structural data obtained for this photoreceptor protein with time-resolved x-ray diffraction analysis, particularly for wild-type PYP.

On the mechanism of activation of the BLUF domain of AppA.

AppA, a transcriptional antirepressor, regulates the steady expression of photosynthesis genes in Rhodobacter sphaeroides in response to high-intensity blue light and to redox signals. Its blue-light sensing is mediated by an N-terminal BLUF domain, a member of a novel flavin fold. The photocycle of this domain (AppA(5-125)) includes formation of a slightly red-shifted long-lived signaling state, which is formed directly from the singlet excited state of the flavin on a subnanosecond time scale [Gauden et al. (2005) Biochemistry 44, 3653-3662]. The red shift of the absorption spectrum of this signaling state has been attributed to a rearrangement of its hydrogen-bonding interactions with the surrounding apoprotein. In this study we have characterized an AppA mutant with an altered aromatic amino acid: W104F. This mutant exhibits an increased lifetime of the singlet excited state of the flavin chromophore. Most strikingly, however, it shows a 1.5-fold increase in its quantum yield of signaling state formation. In addition, it shows a slightly increased rate of ground-state recovery. On top of this, the presence of imidazole, both in this mutant protein and in the wild-type BLUF domain, significantly accelerates the rate of ground-state recovery, suggesting that this rate is limited by rearrangement of (a) hydrogen bond(s). In total, an approximately 700-fold increase in recovery rate has been obtained, which makes the W104F BLUF domain of AppA, for example, suitable for future analyses with step-scan FTIR. The rate of ground-state recovery of the BLUF domain of AppA follows Arrhenius kinetics. This suggests that this domain itself does not undergo large structural changes upon illumination and that the structural transitions in full-length AppA are dominated by interdomain rearrangements.

Photocycle of the flavin-binding photoreceptor AppA, a bacterial transcriptional antirepressor of photosynthesis genes.

The flavoprotein AppA from Rhodobacter sphaeroides contains an N-terminal domain belonging to a new class of photoreceptors designated BLUF domains. AppA was shown to control photosynthesis gene expression in response to blue light and oxygen tension. We have investigated the photocycle of the AppA BLUF domain by ultrafast fluorescence, femtosecond transient absorption, and nanosecond flash-photolysis spectroscopy. Time-resolved fluorescence experiments revealed four components of flavin adenine dinucleotide (FAD) excited-state decay, with lifetimes of 25 ps, 150 ps, 670 ps, and 3.8 ns. Ultrafast transient absorption spectroscopy revealed rapid internal conversion and vibrational cooling processes on excited FAD with time constants of 250 fs and 1.2 ps, and a multiexponential decay with effective time constants of 90 ps, 590 ps, and 2.7 ns. Concomitant with the decay of excited FAD, the rise of a species with a narrow absorption difference band near 495 nm was detected which spectrally resembles the long-living signaling state of AppA. Consistent with these results, the nanosecond flash-photolysis measurements indicated that formation of the signaling state was complete within the time resolution of 10 ns. No further changes were detected up to 15 micros. The quantum yield of the signaling-state formation was determined to be 24%. Thus, the signaling state of the AppA BLUF domain is formed on the ultrafast time scale directly from the FAD singlet excited state, without any apparent intermediate, and remains stable over 12 decades of time. In parallel with the signaling state, the FAD triplet state is formed from the FAD singlet excited state at 9% efficiency as a side reaction of the AppA photocycle.

Frequency-resolved pump-probe characterization of femtosecond infrared pulses.

A novel method for ultrashort IR pulse characterization is presented. The technique utilizes a frequency-resolved pump-probe geometry that is common in applications of ultrafast spectroscopy, without any modifications of the setup. The experimental demonstration of the method was carried out to characterize 70-fs IR pulses centered at 3 microm .