Excited-State Processes in Phycocyanobilin Studied by Femtosecond Spectroscopy

Assembly of allophycocyanin from individual subunits: involvement of apo-form in the formation of trimers

In cyanobacteria and red algae, allophycocyanin (APC), as well as other phycobiliproteins, is involved in the energy transfer of photosystems. Since APC is a potent fluorescent protein for imaging and biomedical applications, it is necessary to obtain purified protein in large quantities, which is currently possible by biosynthesis in bacterial systems. Here we emphasize the challenges of obtaining the trimeric form of the protein from α-APC and β-APC subunits of allophycocyanin in vitro. This approach allowed us to study the individual subunits and to perform assembly of allophycocyanin trimers in vitro. Using different spectroscopic techniques, we detected the heterogeneity of the synthesized β-APC and showed the possibility that not only holo-forms may be involved in trimer formation. Data allowed us to provide additional arguments in favor of excitonic coupling of chromophores in APC trimers.

Dyeing Non-Recyclable Polyethylene Plastic with Photoacid Phycocyanobilin from Spirulina Algae: Ultrafast Photoluminescence Studies

Despite the enormous environmental damage caused by plastic waste, it makes up over one-third of globally produced plastics. Polyethylene (PE) wastes have low recycling but high production rates. Towards the construction of ionic solar cells from PE, the present work describes the loading of a bioactive photoacid phycocyanobilin (PCB) dye from the pigment of Spirulina blue-green algae (as a natural resource) on low-density polyethylene (LDPE) plastic film. Dyeing was confirmed by X-ray photoelectron spectroscopy (XPS). Upon excitation of the Soret-band (400 nm), the photoluminescence (PL) spectra of PCB in neat solvents revealed two prominent emission peaks at 450-550 and 600-700 nm. The first band assigned to bilirubin-like (PCB_BR) species predominated the spectral profile in the highly rigid solvent glycerol and upon loading 0.45 % (w/w) of the dye on plastic. The photoluminescence excitation (PLE) spectra of PCB for the second region (Q-band) at 672 nm in the same solvents confirmed the ground state heterogenicity previously associated with the presence of PCB_A (neutral), PCB_B (cationic), and PCB_C (anionic) conformers. Time-resolved photoluminescence (TRPL) measurements induced via excitation of all PCB species at 510 nm in methanol revealed three-lifetime components with τ₁ = ~0.1 ns and τ₂ = ~2 ns associated with PCB_BR species and τ₃ = ~5 ns pertinent to the long-living photoproduct X*. Decay-associated spectra (DAS) analysis of the photoluminescence transient spectra of the final dyed films in the solid-state confirmed the improved generation of the long-living photoproduct as manifested in a significant increase in the PL intensity (~100-fold) and lifetime value (~90 ns) in the Q-region upon loading 6.92 % (w/w) of the dye on plastic. The photoproduct species were presumably assigned to the deprotonated PCB species, suggesting improved ionic mobility. The potential implementation of the PCB-sensitized PE solid wastes for the fabrication of ionic solar cells is discussed.

Anti-Stokes fluorescence excitation reveals conformational mobility of the C-phycocyanin chromophores

The structural organization of natural pigment-protein complexes provides a specific environment for the chromophore groups. Yet, proteins are inherently dynamic and conformationally mobile. In this work, we demonstrate the heterogeneity of chromophores of C-phycocyanin (C-PC) from Arthrospira platensis. Part of the population of trimeric C-PC is subject to spontaneous disturbances of protein-protein interactions resulting in increased conformational mobility of the chromophores. Upon fluorescence excitation in the visible range, the spectral signatures of these poorly populated states are masked by bulk chromophore states, but the former could be clearly discriminated when the fluorescence is excited by near-infrared quanta. Such selective excitation of conformationally mobile C-PC chromophores is due to the structure of their S₁ level, which is characterized by a significantly broadened spectral line. We demonstrate that the anti-Stokes C-PC fluorescence is the result of single-photon absorption. By combining spectral and structural methods, we characterize four distinct states of C-PC chromophores emitting at 620, 650, 665, and 720 nm and assigned the fast component in the anti-Stokes fluorescence decay kinetics in the range of 690-750 nm to the chromophores with increased conformational mobility. Our data suggest that the spectral and temporal characteristics of the anti-Stokes fluorescence can be used to study protein dynamics and develop methods to visualize local environment parameters such as temperature.

Ultrafast proton release reaction and primary photochemistry of phycocyanobilin in solution observed with fs-time-resolved mid-IR and UV/Vis spectroscopy

Deactivation processes of photoexcited (λ_ex = 580 nm) phycocyanobilin (PCB) in methanol were investigated by means of UV/Vis and mid-IR femtosecond (fs) transient absorption (TA) as well as static fluorescence spectroscopy, supported by density-functional-theory calculations of three relevant ground state conformers, PCB_A, PCB_B and PCB_C, their relative electronic state energies and normal mode vibrational analysis. UV/Vis fs-TA reveals time constants of 2.0, 18 and 67 ps, describing decay of PCB_B*, of PCB_A* and thermal re-equilibration of PCB_A, PCB_B and PCB_C, respectively, in line with the model by Dietzek et al. (Chem Phys Lett 515:163, 2011) and predecessors. Significant substantiation and extension of this model is achieved first via mid-IR fs-TA, i.e. identification of molecular structures and their dynamics, with time constants of 2.6, 21 and 40 ps, respectively. Second, transient IR continuum absorption (CA) is observed in the region above 1755 cm^-1 (CA1) and between 1550 and 1450 cm^-1 (CA2), indicative for the IR absorption of highly polarizable protons in hydrogen bonding networks (X-H^…Y). This allows to characterize chromophore protonation/deprotonation processes, associated with the electronic and structural dynamics, on a molecular level. The PCB photocycle is suggested to be closed via a long living (> 1 ns), PCB_C-like (i.e. deprotonated), fluorescent species.

The interplay between chromophore and protein determines the extended excited state dynamics in a single-domain phytochrome

Phytochromes are a diverse family of bilin-binding photoreceptors that regulate a wide range of physiological processes. Their photochemical properties make them attractive for applications in optogenetics and superresolution microscopy. Phytochromes undergo reversible photoconversion triggered by the Z ⇄ E photoisomerization about the double bond in the bilin chromophore. However, it is not fully understood at the molecular level how the protein framework facilitates the complex photoisomerization dynamics. We have studied a single-domain bilin-binding photoreceptor All2699g1 (Nostoc sp. PCC 7120) that exhibits photoconversion between the red light-absorbing (P_r) and far red-absorbing (P_fr) states just like canonical phytochromes. We present the crystal structure and examine the photoisomerization mechanism of the P_r form as well as the formation of the primary photoproduct Lumi-R using time-resolved spectroscopy and hybrid quantum mechanics/molecular mechanics simulations. We show that the unusually long excited state lifetime (broad lifetime distribution centered at ∼300 picoseconds) is due to the interactions between the isomerizing pyrrole ring D and an adjacent conserved Tyr142. The decay kinetics shows a strongly distributed character which is imposed by the nonexponential protein dynamics. Our findings offer a mechanistic insight into how the quantum efficiency of the bilin photoisomerization is tuned by the protein environment, thereby providing a structural framework for engineering bilin-based optical agents for imaging and optogenetics applications.

Charge transfer states in phycobilisomes

Phycobilisomes (PBs) absorb light and supply downstream photosynthetic processes with excitation energy in many cyanobacteria and algae. In response to a sudden increase in light intensity, excess excitation energy is photoprotectively dissipated in PBs by means of the orange carotenoid protein (OCP)-related mechanism or via a light-activated intrinsic decay channel. Recently, we have identified that both mechanisms are associated with far-red emission states. Here, we investigate the far-red states involved with the light-induced intrinsic mechanism by exploring the energy landscape and electro-optical properties of the pigments in PBs. While Stark spectroscopy showed that the far-red states in PBs exhibit a strong charge-transfer (CT) character at cryogenic temperatures, single molecule spectroscopy revealed that CT states should also be present at room temperature. Owing to the strong environmental sensitivity of CT states, the knowledge gained from this study may contribute to the design of a new generation of fluorescence markers.

Glass formation of a DMSO-water mixture probed with a photosynthetic pigment

Despite their extensive industrial usage, glass-forming liquids are not fully understood, and methods to investigate their dynamical heterogeneity are sought after. Here we show how the appearance of a second component in the visible absorption spectrum of a photosynthetic pigment upon cooling can be used to probe the glass transition of a dimethylsulfoxide-water mixture. The changes in the relative ratio of the two components with respect to temperature follow a sigmoid curve, and we show that the second component arises due to protonation of the pigment at low temperatures. Furthermore, from visible transient absorption spectra we show that, unlike the first component, the dynamics of the second component slows down significantly at lower temperatures, suggesting that there are two distinct environments with fast and slow fluctuations. Our results therefore enable a new method to characterize the dynamical heterogeneity of glass-forming liquids.

Dissecting pigment architecture of individual photosynthetic antenna complexes in solution

Oligomerization plays a critical role in shaping the light-harvesting properties of many photosynthetic pigment-protein complexes, but a detailed understanding of this process at the level of individual pigments is still lacking. To study the effects of oligomerization, we designed a single-molecule approach to probe the photophysical properties of individual pigment sites as a function of protein assembly state. Our method, based on the principles of anti-Brownian electrokinetic trapping of single fluorescent proteins, step-wise photobleaching, and multiparameter spectroscopy, allows pigment-specific spectroscopic information on single multipigment antennae to be recorded in a nonperturbative aqueous environment with unprecedented detail. We focus on the monomer-to-trimer transformation of allophycocyanin (APC), an important antenna protein in cyanobacteria. Our data reveal that the two chemically identical pigments in APC have different roles. One (α) is the functional pigment that red-shifts its spectral properties upon trimer formation, whereas the other (β) is a "protective" pigment that persistently quenches the excited state of α in the prefunctional, monomer state of the protein. These results show how subtleties in pigment organization give rise to functionally important aspects of energy transfer and photoprotection in antenna complexes. The method developed here should find immediate application in understanding the emergent properties of other natural and artificial light-harvesting systems.

Single-molecule spectroscopy and femtosecond transient absorption studies on the excitation energy transfer process in ApcE(1–240) dimers

The red-shifted absorption of ApcE dimers results from extending chromophore conformation, which does not depend on strong exction coupling.

Active and silent chromophore isoforms for phytochrome Pr photoisomerization: An alternative evolutionary strategy to optimize photoreaction quantum yields

Photoisomerization of a protein bound chromophore is the basis of light sensing of many photoreceptors. We tracked Z-to-E photoisomerization of Cph1 phytochrome chromophore PCB in the Pr form in real-time. Two different phycocyanobilin (PCB) ground state geometries with different ring D orientations have been identified. The pre-twisted and hydrogen bonded PCB(a) geometry exhibits a time constant of 30 ps and a quantum yield of photoproduct formation of 29%, about six times slower and ten times higher than that for the non-hydrogen bonded PCB(b) geometry. This new mechanism of pre-twisting the chromophore by protein-cofactor interaction optimizes yields of slow photoreactions and provides a scaffold for photoreceptor engineering.

Watching conformational- and photo-dynamics of single fluorescent proteins in solution

Observing the dynamics of single biomolecules over prolonged time periods is difficult to achieve without significantly altering the molecule through immobilization. It can, however, be accomplished using the Anti-Brownian ELectrokinetic (ABEL) Trap, which allows extended investigation of solution-phase biomolecules - without immobilization -through real-time electrokinetic feedback. Here we apply the ABEL trap to study an important photosynthetic antenna protein, Allophycocyanin (APC). The technique allows the observation of single molecules of solution-phase APC for more than one second. We observe a complex relationship between fluorescence intensity and lifetime that cannot be explained by simple static kinetic models. Light-induced conformational changes are shown to occur and evidence is obtained for fluctuations in the spontaneous emission lifetime, which is typically assumed to be constant. Our methods provide a new window into the dynamics of fluorescent proteins and the observations are relevant for the interpretation of in vivo single-molecule imaging experiments, bacterial photosynthetic regulation, and biomaterials for solar energy harvesting.

Subpicosecond midinfrared spectroscopy of the Pfr reaction of phytochrome Agp1 from Agrobacterium tumefaciens

Phytochromes are light-sensing pigments found in plants and bacteria. For the first time, the P(fr) photoreaction of a phytochrome has been subject to ultrafast infrared vibrational spectroscopy. Three time constants of 0.3 ps, 1.3 ps, and 4.0 ps were derived from the kinetics of structurally specific marker bands of the biliverdin chromophore of Agp1-BV from Agrobacterium tumefaciens after excitation at 765 nm. VIS-pump-VIS-probe experiments yield time constants of 0.44 ps and 3.3 ps for the underlying electronic-state dynamics. A reaction scheme is proposed including two kinetic steps on the S(1) excited-state surface and the cooling of a vibrationally hot P(fr) ground state. It is concluded that the upper limit of the E-Z isomerization of the C(15) = C(16) methine bridge is given by the intermediate time constant of 1.3 ps. The reaction scheme is reminiscent of that of the corresponding P(r) reaction of Agp1-BV as published earlier.

Sub-picosecond mid-infrared spectroscopy of phytochrome Agp1 from Agrobacterium tumefaciens

The photoinduced primary reaction of the biliverdin binding phytochrome Agp1 (Agp1-BV) from Agrobacterium tumefaciens was investigated by sub-picosecond time-resolved Vis pump-IR probe spectroscopy. Three time constants of tau(1)=0.7+/-0.05 ps, tau(2)=3.3+/-0.2 ps and tau(3)=33.3+/-1.5 ps could be isolated from the dynamics of structurally specific marker bands of the BV chromophore. These results together with those of accompanying sub-picosecond Vis pump-Vis probe spectroscopy allow the extension of the reaction scheme for the primary process by a vibrationally excited electronic ground state. The isomerization at the C15=C16 bond occurs within the lifetime of the excited electronic state. A quantum yield of 0.094 for the primary reaction is determined, suggesting that the quantum yield of formation of the P(fr) far-red-absorbing form is already established in the primary photoreaction of the P(r) (red-absorbing) form.

The excited-state chemistry of phycocyanobilin: a semiempirical study

Based on previous time-resolved absorption studies, phycocyanobilin undergoes a photoreaction from an A- into a B- and C-form, with the latter two photoproducts showing absorption spectra red-shifted from A. To identify the molecular mechanism involved in the excited-state reactions, the structural origin of the red shift in the absorption spectra is investigated. Using semiempirical AM1 calculations that include configuration interaction by pair doubles excitation configuration interaction, the absorption spectra of different conformers as well as different protonation states were calculated. The results clearly indicate a pronounced red shift in the spectra of structures either protonated or deprotonated at the basic/acidic centres of the tetrapyrrole chromophore whereas, in contrast, conformational changes alone result in a blue shift. Furthermore, it is shown by quantum chemical calculations that the basicity of phycocyanobilin is much higher in the excited than in the ground state, with a decrease in the excited-state pK(B)* of approximately 9.5 units. The acidity is only slightly enhanced with a drop in pK(A)* of only approximately 1.6 units. From these findings, a reaction model for the excited-state processes in phycocyanobilin is proposed. According to this model, photoexcitation of phycocyanobilin triggers an excited-state proton transfer giving rise to the formation of a protonated species. In parallel, the local increase in the medium pH associated with protonation then forwards a deprotonation at an acidic NH-group so that in effect both protonated and deprotonated phycocyanobilin would arise from the initial photoreaction and account for the observed red shift in the spectra of the B- and C-forms.

Ultrafast dynamics of phytochrome from the cyanobacterium synechocystis, reconstituted with phycocyanobilin and phycoerythrobilin

Femtosecond time-resolved transient absorption spectroscopy was employed to characterize for the first time the primary photoisomerization dynamics of a bacterial phytochrome system in the two thermally stable states of the photocycle. The 85-kDa phytochrome Cph1 from the cyanobacterium Synechocystis PCC 6803 expressed in Escherichia coli was reconstituted with phycocyanobilin (Cph1-PCB) and phycoerythrobilin (Cph1-PEB). The red-light-absorbing form Pr of Cph1-PCB shows an approximately 150 fs relaxation in the S(1) state after photoexcitation at 650 nm. The subsequent Z-E isomerization between rings C and D of the linear tetrapyrrole-chromophore is best described by a distribution of rate constants with the first moment at (16 ps)(-1). Excitation at 615 nm leads to a slightly broadened distribution. The reverse E-Z isomerization, starting from the far-red-absorbing form Pfr, is characterized by two shorter time constants of 0.54 and 3.2 ps. In the case of Cph1-PEB, double-bond isomerization does not take place, and the excited-state lifetime extends into the nanosecond regime. Besides a stimulated emission rise time between 40 and 150 fs, no fast relaxation processes are observed. This suggests that the chromophore-protein interaction along rings A, B, and C does not contribute much to the picosecond dynamics observed in Cph1-PCB but rather the region around ring D near the isomerizing C(15) [double bond] C(16) double bond. The primary reaction dynamics of Cph1-PCB at ambient temperature is found to exhibit very similar features as those described for plant type A phytochrome, i.e., a relatively slow Pr, and a fast Pfr, photoreaction. This suggests that the initial reactions were established already before evolution of plant phytochromes began.

Enantioselective syntheses of (13)C-labeled (2R)- and (2S)-phytochromobilin dimethyl ester

(2R)- and (2S)-phytochromobilin dimethyl ester have been prepared in enantiomerically pure form, specifically (13)C-labeled at C(10) or C(15).

First steps in the phytochrome phototransformation: a comparative femtosecond study on the forward (Pr --> Pfr) and back reaction (Pfr --> Pr)

The primary light-induced events in the reversible Pr right harpoon over left harpoon Pfr phototransformation are investigated by femtosecond absorption spectroscopy using a pump-probe technique. After the selective electronic excitation of Pr and Pfr with pulses at 610 and 730 nm, respectively, the transient absorption spectra were measured as a function of the delay time and subjected to a global fit analysis. As a result of this analysis, the decay-associated spectra of the kinetic components involved in the formation of the first photoproducts in the forward and back reaction are obtained. These spectra provide a more detailed understanding of the primary stages in the light-induced transformations. In addition, the influence of the solvent viscosity on the initial reaction steps was studied. In each direction of reaction, a short-lifetime component is found to be strongly viscosity-dependent, indicating that the primary photochemistry encompasses intramolecular motions of the chromophore or its proximal amino acid side chains. H-D exchange has no significant effect on the kinetics of the initial photoprocesses. This suggests that the isomerization reaction in both directions is not accompanied by a rate-limiting proton transfer.

New syntheses of the C,D-ring pyrromethenones of phytochrome and phycocyanin

Pyrromethenone 7, the C,D-ring segment of phytochrome (Pr, 4), has been prepared in an efficient fashion employing three new strategies. Each of these has potential advantages for the synthesis of labeled material. Our first approach is related to the Gossauer synthesis, with the difference that strong alkali is avoided in the condensation of the C- and D-ring components 8 and 17. The key silyloxypyrrole 17 was readily prepared on multigram scales beginning with inexpensive butyrolactone (10). A second synthesis began with 2-acetylbutyrolactone (41). The key steps involved conversion of 41 to the Z-enoltriflate 42, followed by Pd(0)-catalyzed coupling with trimethylsilylacetylene, p-chlorophenylselenide ring opening, and finally, amidation to afford the ring-D synthon 45 having the proper geometry and oxidation state for conversion to 7. Sonogashira coupling of 45 with the iodopyrrole 22, followed by oxidative elimination, and F(-)-induced 5-exo-dig cyclization of the resultant pyrroloalkyne 47, then completed the synthesis. In similar fashion, we have also prepared pyrromethenone 6, the C,D-ring segment of phycocyanin (2).