Photochemical energy storage and volume changes in the microsecond time range in bacterial photosynthesis — a laser induced optoacoustic study

Photoactivation and conformational gating for manganese binding and oxidation in bacterial reaction centers

The influence of illumination history of native bacterial reaction centers (BRCs) on the ability of binding and photo-induced oxidation of manganous ions was investigated in the pH range between 8.0 and 9.4. Binding of manganous ions to a buried site required 6 to 11-fold longer incubation periods, depending on the pH, in dark-adapted BRCs than in BRCs that were previously illuminated prior to manganese binding. The intrinsic electron transfer from the bound manganese ion to the photo-oxidized primary electron donor was found to be limited by a 2 to 5-fold slower precursor conformational step in the dark-adapted samples for the same pH range. The conformational gating could be eliminated by photoactivation, namely if the BRCs were illuminated prior to binding. Unlike in Photosystem II, photoactivation in BRCs did not involve cluster assembly. Photoactivation with manganese already bound was only possible at elevated detergent concentration. In addition, also exclusively in dark-adapted BRCs, a marked breaking point in the Arrhenius-plot was discovered around 15 °C at pH 9.4 indicating a change in the reaction mechanism, most likely caused by the change of orientation of the 2-acetyl group of the inactive bacteriochlorophyll monomer located near the manganese binding site.

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.

Modulation of the photocycle of a LOV domain photoreceptor by the hydrogen-bonding network

An extended hydrogen-bonding (HB) network stabilizes the isoalloxazine ring of the flavin mononucleotide (FMN) chromophore within the photosensing LOV domain of blue-light protein receptors, via interactions between the C(2)═O, N(3)H, C(4)═O, and N(5) groups and conserved glutamine and asparagine residues. In this work we studied the influence of the HB network on the efficiency, kinetics, and energetics of a LOV protein photocycle, involving the reversible formation of a FMN-cysteine covalent adduct. The following results were found for mutations of the conserved amino acids N94, N104, and Q123 in the Bacillus subtilis LOV protein YtvA: (i) Increased (N104D, N94D) or strongly reduced (N94A) rate of adduct formation; this latter mutation extends the lifetime of the flavin triplet state, i.e., adduct formation, more than 60-fold, from 2 μs for the wild-type (WT) protein to 129 μs. (ii) Acceleration of the overall photocycle for N94S, N94A, and Q123N, with recovery lifetimes 20, 45, and 85 times faster than for YtvA-WT, respectively. (iii) Slight modifications of FMN spectral features, correlated with the polarization of low-energy transitions. (iv) Strongly reduced (N94S) or suppressed (Q123N) structural volume changes accompanying adduct formation, as determined by optoacoustic spectroscopy. (v) Minor effects on the quantum yield, with the exception of a considerable reduction for Q123N, i.e., 0.22 vs 0.49 for YtvA-WT. The data stress the importance of the HB network in modulating the photocycle of LOV domains, while at the same time establishing a link with functional responses.

Hydrogen-bond network probed by time-resolved optoacoustic spectroscopy: photoactive yellow protein and the effect of E46Q and E46A mutations

The enthalpy and structural volume changes (delta Hi and delta Vi) produced upon photoinduced formation and decay of the red-shifted intermediate (pR = I1) in the photoactive yellow protein (WT-PYP) from Halorhodospira halophila and the mutated E46Q-PYP and E46A-PYP, were determined by laser-induced optoacoustic spectroscopy (LIOAS) using the two-temperatures method, at pH 8.5. These mutations alter the hydrogen bond between the phenolate oxygen of the chromophore and the residue at position 46. Hydrogen bonding is still possible in E46Q-PYP via the delta-NH2 group of glutamine, whereas it is no longer possible with the methyl group of alanine in E46A-PYP. In all three proteins, pR decays within hundreds of ns to micros into the next intermediate, pR'. The delta H values for the formation of pR (delta H pR) and for its decay into pR'(delta H pR-->pR') are negligibly affected by the E46Q and the E46A substitution. In all three proteins the large delta H pR value drives the photocycle. Whereas delta V pR is a similar contraction of ca. 15 ml mol(-1) for E46Q-PYP and WT-PYP, attributed to strengthening the hydrogen bond network (between 4 and 5 hydrogen bonds) inside the protein chromophore cavity, an expansion is observed for E46A-PYP, indicating just an enlargement of the chromophore cavity upon chromophore isomerization. The results are discussed in the light of the recent time-resolved room temperature, crystallographic studies with WT-PYP and E46Q-PYP. Formation of pR' is somewhat slower for E46Q-PYP and much slower for E46A-PYP. The structural volume change for this transition, delta V pR-->pR', is relatively small and positive for WT-PYP, slightly larger for E46Q-PYP, and definitely larger for the hydrogen-bond lacking E46A-PYP. This indicates a larger entropic change for this transition in E46A-PYP, reflected in the large pre-exponential factor for the pR to pR' decay rate constant determined in the 5-30 degrees C temperature range. This decay also shows an activation entropy that compensates the larger activation energy in E46A-PYP, as compared to the values for WT-PYP and E46Q-PYP.

A blue light inducible two-component signal transduction system in the plant pathogen Pseudomonas syringae pv. tomato

The open reading frame PSPTO2896 from the plant pathogen Pseudomonas syringae pv. tomato encodes a protein of 534 amino acids showing all salient features of a blue light-driven two-component system. The N-terminal LOV (light, oxygen, voltage) domain, potentially binding a flavin chromophore, is followed by a histidine kinase (HK) motif and a response regulator (RR). The full-length protein (PST-LOV) and, separately, the RR and the LOV+HK part (PST-LOV(DeltaRR)) were heterologously expressed and functionally characterized. The two LOV proteins showed typical LOV-like spectra and photochemical reactions, with the blue light-driven, reversible formation of a covalent flavin-cysteine bond. The fluorescence changes in the lit state of full-length PST-LOV, but not in PST-LOV(DeltaRR), indicating a direct interaction between the LOV core and the RR module. Experiments performed with radioactive ATP uncover the light-driven kinase activity. For both PST-LOV and PST-LOV(DeltaRR), much more radioactivity is incorporated when the protein is in the lit state. Furthermore, addition of the RR domain to the fully phosphorylated PST-LOV(DeltaRR) leads to a very fast transfer of radioactivity, indicating a highly efficient HK activity and a tight interaction between PST-LOV(DeltaRR) and RR, possibly facilitated by the LOV core itself.

Entropy changes drive the electron transfer reaction of triplet flavin mononucleotide from aromatic amino acids in cation-organized aqueous media. A laser-induced optoacoustic study

The thermodynamic parameters for the formation of the free radicals upon electron transfer quenching of the flavin triplet state (3FMN) by tryptophan and tyrosine, Delta(FR)H and Delta(FR)V, were obtained in aqueous solution by the application of laser-induced optoacoustic spectroscopy at various temperatures. The Delta(FR)H and Delta(FR)V values include the electron transfer and charge separation steps plus the protonation of the FMN anion radical and the deprotonation of the amino-acid cation radical. A linear correlation was found between the Delta(FR)H and Delta(FR)V values for each of the amino acids in phosphate buffers of [CH3(CH2)3]4N+, Li+, NH4+, K+ and Cs+. The compensation between Delta(FR)H and Delta(FR)V within the salt series, and the independent evaluation of the Gibbs energy for electron transfer Delta(ET)G(o) afforded the entropy change, Delta(FR)S, for the reaction, different for the two amino acids. The values of Delta(FR)H, Delta(FR)V and Delta(FR)S in each buffer are mainly determined by the changes in strength and probably number of hydrogen bonds between the reacting partners and water produced along all steps leading to the radicals FMNH* and A*. The Delta(FR)V values linearly correlate with the tabulated entropy of organization of the water structure for the five cations, DeltaS(o)(cat). The entropy change upon formation of the free radicals, Delta(FR)S, quantitatively correlated to the Delta(FR)V value, drives the separation of the ion pair after the electron transfer reaction in the case of highly organizing cations. The ratio X = T Delta(FR)S/Delta(FR)V = (55 +/- 9) kJ cm(-3) for Trp as 3FMN quencher is smaller than X = (83 +/- 9) kJ cm(-3) for Tyr as quencher. These values are discussed in conjunction with the Marcus reorganization energy, as calculated from the Gibbs activation energy of the electron transfer process, which is independent of the salt present but different for each of the two quenchers.

Photoinduced Electron Transfer to Triplet Flavins. Correlation between the Volume Change-Normalized Entropic Term and the Marcus Reorganization Energy†

The data obtained through the application of nanosecond laser-induced optoacoustic spectroscopy (LIOAS) to several electron donor-acceptor pairs in aqueous solution were analyzed together with the respective experimentally determined Marcus reorganization energy. Acceptors were the flavin mononucleotide and flavin-adenine dinucleotide triplet states (3FMN and 3FAD) and donors were tryptophan, tyrosine, histidine, triethanolamine, and ethylenediaminetetraacetic acid. The respective calculated Gibbs energy for electron transfer, Delta(ET)G degrees , was used together with the enthalpy change for the formation of free radicals, Delta(FR)H, obtained from the LIOAS data, to derive the entropy change for the formation of the radicals, Delta(FR)S. In all cases, variation of the monovalent cations, i.e., [CH3(CH2)3]4N+, Li+, NH4+, K+, and Cs+, resulted in variation of the enthalpy change, Delta(FR)H, and in the structural volume change, Delta(FR)V, for the free-radical production, both derived from LIOAS. Delta(FR)H and Delta(FR)V linearly correlated with each other within the cation series. From this correlation the respective entropic term TDelta(FR)S was derived as well as the ratio X = TDelta(FR)S/Delta(FR)V for each of the pairs. X linearly correlated with the respective total Marcus reorganization energy, lambda, for all systems analyzed. This observation underlines the concept that both lambda and Delta(FR)V respond to the same phenomena. The correlation also offers an experimental approach for the understanding at a molecular level of the origin of the lambda values as well as for their evaluation.

Acid−Base Equilibria in 5,10,15,20-Tetrakis(4-sulfonatophenyl)chlorin: Role of Conformational Flexibility

The acid-base equilibria in 5,10,15,20-tetrakis(4-sulfonatophenyl)chlorin were studied in aqueous solution and compared with the respective data for the corresponding porphyrin. The reduction of the pyrrole ring in the tetrapyrrolic macrocycle noticeably influences both free base/monoprotonated and mono-/diprotonated species equilibria. In strong acidic solutions protonation of 4-sulfonatophenyl groups takes place in addition to protonation of the macrocycle core. The photophysical properties of all ionic forms are influenced by an enhanced rate of internal S1 --> S0 conversion, leading to about 50% and 90% deactivation through this channel for the free base and diprotonated species, respectively. The enhancement of the rate of the radiationless transitions is explained by an increased conformational flexibility of the chlorin macrocycle with respect to that of a porphyrin. Structural volume change measurements with laser-induced optoacoustic spectroscopy support this explanation. The contraction upon triplet state formation of the free base is about one-half of that measured for the corresponding porphyrin. This contraction should be due to intramolecular structural rearrangements of the macrocycle to adopt a minimum energy conformation in case of the chlorin. On the contrary, for the more rigid porphyrin macrocycle the interactions of the molecule with the solvent environment play a more important role. The diprotonated forms of both porphyrin and chlorin show a high radiationless S1 --> S0 conversion rate and seem to have a similar conformational flexibility. In agreement with previous calculations, the conformational flexibility of the diprotonated forms appears to be higher than that of the free base molecule.

Initial characterization of a blue-light sensing, phototropin-related protein from Pseudomonas putida: a paradigm for an extended LOV construct

The open reading frame PP2739 from Pseudomonas putida KT2440 encodes a 151 amino acid protein with sequence similarity to the LOV domains of the blue-light sensitive protein YtvA from Bacillus subtilis and to the phototropins (phot) from plants. This sensory box LOV protein, PpSB2-LOV, comprises a LOV core, followed by a C-terminal segment predicted to form an alpha-helix, thus constituting a naturally occurring paradigm for an extended LOV construct. The recombinant PpSB2-LOV shows a photochemistry very similar to that of YtvA and phot-LOV domains, yet the lifetime for the recovery dark reaction, taurec=114 s at 20 degrees C, resembles that of phot-LOV domains (5-300 s) and is much faster than that of YtvA or YtvA-LOV (>3000 s). Time-resolved optoacoustics reveals phot-like, light-driven reactions on the ns-micros time window with the sub-nanosecond formation of a flavin triplet state (PhiT=0.46) that decays into the flavin-cysteine photoadduct with 2 micros lifetime (Phi390=0.42). The fluorescence spectrum and lifetime of the conserved W97 resembles the corresponding W103 in full-length YtvA, although the quantum yield, PhiF, is smaller (about 55% of YtvA) due to an enhanced static quenching efficiency. The anisotropy of W97 is the same as for W103 in YtvA (0.1), and considerably larger than the value of 0.06, found for W103 in YtvA-LOV. Different to YtvA and YtvA-LOV, the fluorescence for W97 becomes larger upon photoproduct formation. These data indicate that W97 is located in a similar environment as W103 in full-length YtvA, but undergoes larger light-driven changes. It is concluded that the protein segment located C-terminally to the LOV core (analogous to an interdomain linker) is enough to confer to the conserved tryptophan the fluorescence characteristics typical of full-length YtvA. The larger changes experienced by W97 upon light activation may reflect a larger conformational freedom of this protein segment in the absence of a second domain.

Internal dynamics and protein–matrix coupling in trehalose-coated proteins

We review recent studies on the role played by non-liquid, water-containing matrices on the dynamics and structure of embedded proteins. Two proteins were studied, in water-trehalose matrices: a water-soluble protein (carboxy derivative of horse heart myoglobin) and a membrane protein (reaction centre from Rhodobacter sphaeroides). Several experimental techniques were used: Mossbauer spectroscopy, elastic neutron scattering, FTIR spectroscopy, CO recombination after flash photolysis in carboxy-myoglobin, kinetic optical absorption spectroscopy following pulsed and continuous photoexcitation in Q(B) containing or Q(B) deprived reaction centre from R. sphaeroides. Experimental results, together with the outcome of molecular dynamics simulations, concurred to give a picture of how water-containing matrices control the internal dynamics of the embedded proteins. This occurs, in particular, via the formation of hydrogen bond networks that anchor the protein surface to the surrounding matrix, whose stiffness increases by lowering the sample water content. In the conclusion section, we also briefly speculate on how the protein-matrix interactions observed in our samples may shed light on the protein-solvent coupling also in liquid aqueous solutions.

Recording of blue light-induced energy and volume changes within the wild-type and mutated phot-LOV1 domain from Chlamydomonas reinhardtii

The time-resolved thermodynamics of the flavin mononucleotide (FMN)-binding LOV1 domain of Chlamydomonas reinhardtii phot (phototropin homolog) was studied by means of laser-induced optoacoustic spectroscopy. In the wild-type protein the early red-shifted intermediate LOV(715) exhibits a small volume contraction, DeltaV(715) = -1.50 ml/mol, with respect to the parent state. LOV(715) decays within few micro s into the covalent FMN-Cys-57 adduct LOV(390), that shows a larger contraction, DeltaV(390) = -8.8 ml/mol, suggesting a loss of entropy and conformational flexibility. The high energy content of LOV(390), E(390) = 180 kJ/mol, ensures the driving force for the completion of the photocycle and points to a strained photoreceptor conformation. In the LOV-C57S mutated protein the photoadduct is not formed and DeltaV(390) is undetected. Large effects on the measured DeltaVs are observed in the photochemically competent R58K and R58K/D31Q mutated proteins, with DeltaV(390) = -2.0 and -1.9 ml/mol, respectively, and DeltaV(715) approximately 0. The D31Q and D31N substitutions exhibit smaller but well-detectable effects. These results show that the photo-induced volume changes involve the protein region comprising Arg-58, which tightly interacts with the FMN phosphate group.

Time-resolved photothermal methods: accessing time-resolved thermodynamics of photoinduced processes in chemistry and biology

Photothermal methods are currently being employed in a variety of research areas, ranging from materials science to environmental monitoring. Despite the common term which they are collected under, the implementations of these techniques are as diverse as the fields of application. In this review, we concentrate on the recent applications of time-resolved methods in photochemistry and photobiology.

Listening to the blue: the time-resolved thermodynamics of the bacterial blue-light receptor YtvA and its isolated LOV domain

YtvA is a bacterial flavo-protein related to plant phototropin. The photochemistry of YtvA and of its isolated LOV domain (YtvA-LOV) has been characterized by optical, mass spectrometric and photocalorimetric methods. The energy content (E390) of the FMN-C4a-thiol photoadduct (YtvA390 and YtvA-LOV390) and its structural volume change (deltaV390), with respect to the parent state, have been determined by means of Laser Induced Optoacoustic Spectroscopy (LIOAS). The high value of E390, 136 and 115 kJ mol(-1), respectively, ensures a large driving force for the dark recovery to the unphotolyzed state and points to a strained conformation of the protein or/and the chromophore in the photoadduct. The value of deltaV390 is significantly different for the two proteins, deltaV390 = -12.5 ml mol(-1) in YtvA and -17.2 ml mol(-1) in YtvA-LOV. The kinetics of the dark recovery reaction for YtvA-LOV is slower than for full-length YtvA, with taurec = 3900 and 2600 s at 25 degrees C, respectively, and shows a different temperature dependence. A similarly slow kinetics can be induced in YtvA by high ionic strength. Minor differences are observed in the fluorescence and photoadduct formation quantum yield. The overall stability is higher for YtvA than for YtvA-LOV. The data as a whole are indicative of an interaction between the two domains of YtvA, most probably mediated by electrostatic interactions that renders the full-length protein a compact and more rigid unit. The results reported here support the idea that the formation of the photoadduct changes the dynamics of the protein, depending on the conformational flexibility of the parent state. Flashing of the photoadduct induces a negligible deltaV, with 96% of the excitation energy dissipated as heat in <20 ns, indicating that the photoadduct does not undergo a photocycle on the LIOAS time scale, or that the photoinduced reactions occur with very low yield.

Structural changes upon excitation of D1-D2-Cyt b559 photosystem II reaction centers depend on the beta-carotene content

Different preparations of D1-D2-Cyt b559 complexes from spinach with different beta-carotene (Car) content [on average from <0.5 to 2 per reaction center (RC)] were studied by means of laser-induced optoacoustic spectroscopy. phiP680(+)Pheo(-) does not depend on the preparation (or on the Car content) inasmuch as the magnitude of the prompt heat (produced within 20 ns) does not vary for the different samples upon excitation at 675 and 620 nm. The energy level of the primary charge-separated state, P680(+)Pheo(-), was determined as EP680(+)Pheo(-) = 1.55 eV. Thus, an enthalpy change accompanying charge separation from excited P680 of deltaH*P680Pheo-->P680(+)Pheo(-) = -0.27 eV is obtained. Calculations using the heat evolved during the time-resolved decay of P680(+)Pheo(-) (< or = 100 ns) affords a triplet (3[P680Pheo]) quantum yield phi3[P680Pheo] = 0.5 +/- 0.14. The structural volume change, deltaV1, corresponding to the formation of P680(+)Pheo(-), strongly depends on the Car content; it is ca. -2.5 A3 molecule(-1) for samples with <0.5 Car on average, decreases (in absolute value) to -0.5 +/- 0.2 A3 for samples with an average of 1 Car, and remains the same for samples with two Cars per RC. This suggests that the Car molecules induce changes in the ground-state RC conformation, an idea which was confirmed by preferential excitation of Car with blue light, which produced different carotene triplet lifetimes in samples with 2 Car compared to those containing less carotene. We conclude that the two beta-carotenes are not structurally equivalent. Upon blue-light excitation (480 nm, preferential carotene absorption) the fraction of energy stored is ca. 60% for the 9Chl-2Car sample, whereas it is 40% for the preparations with one or less Cars on average, indicating different paths of energy distribution after Car excitation in these RCs with remaining chlorophyll antennae.

First evidence for phototropin-related blue-light receptors in prokaryotes

A prokaryotic protein, YtvA from Bacillus subtilis, was found to possess a light, oxygen, voltage (LOV) domain sharing high homology with the photoactive, flavin mononucleotide (FMN)-binding LOV domains of phototropins (phot), blue-light photoreceptors for phototropism in higher plants. Computer-based three-dimensional modeling suggests that YtvA-LOV binds FMN in a similar pocket as phot-LOVs. Recombinant YtvA indeed exhibits the same spectroscopical features and blue-light-induced photochemistry as phot-LOVs, with the reversible formation of a blue-shifted photoproduct, assigned to an FMN-cysteine thiol adduct (Thio383). By means of laser-flash photolysis and time-resolved optoacoustic experiments, we measured the quantum yield of formation for Thio383, Phi(Thio) = 0.49, and the enthalpy change, DeltaH(Thio) = 135 kJ/mol, with respect to the parent state. The formation of Thio383 is accompanied by a considerable volume contraction, DeltaV(Thio) = -13.5 ml/mol. Similar to phot-LOVs, Thio383 is formed from the decay of a red-shifted transient species, T650, within 2 micros. In both YtvA and free FMN, this transient has an enthalpy content of approximately 200 kJ/mol, and its formation is accompanied by a small contraction, DeltaV(T) approximately -1.5 ml/mol, supporting the assignment of T650 to the FMN triplet state, as suggested by spectroscopical evidences. These are the first studies indicating that phototropin-related, blue-light receptors may exist also in prokaryotes, besides constituting a steadily growing family in plants.

Electron transfer kinetics in photosynthetic reaction centers embedded in trehalose glasses: trapping of conformational substates at room temperature

We report on room temperature electron transfer in the reaction center (RC) complex purified from Rhodobacter sphaeroides. The protein was embedded in trehalose-water systems of different trehalose/water ratios. This enabled us to get new insights on the relationship between RC conformational dynamics and long-range electron transfer. In particular, we measured the kinetics of electron transfer from the primary reduced quinone acceptor (Q(A)(-)) to the primary photo oxidized donor (P(+)), by time-resolved absorption spectroscopy, as a function of the matrix composition. The composition was evaluated either by weighing (liquid samples) or by near infrared spectroscopy (highly viscous or solid glasses). Deconvolution of the observed, nonexponential kinetics required a continuous spectrum of rate constants. The average rate constant (<k> = 8.7 s(-1) in a 28% (w/w) trehalose solution) increases smoothly by increasing the trehalose/water ratio. In solid glasses, at trehalose/water ratios > or = 97%, an abrupt <k> increase is observed (<k> = 26.6 s(-1) in the driest solid sample). A dramatic broadening of the rate distribution function parallels the above sudden <k> increase. Both effects fully revert upon rehydration of the glass. We compared the kinetics observed at room temperature in extensively dried water-trehalose matrices with the ones measured in glycerol-water mixtures at cryogenic temperatures and conclude that, in solid trehalose-water glasses, the thermal fluctuations among conformational substates are inhibited. This was inferred from the large broadening of the rate constant distribution for electron transfer obtained in solid glasses, which was due to the free energy distribution barriers having become quasi static. Accordingly, the RC relaxation from dark-adapted to light-adapted conformation, which follows primary charge separation at room temperature, is progressively hindered over the time scale of P(+)Q(A)(-) charge recombination, upon decreasing the water content. In solid trehalose-water glasses the electron transfer process resulted much more affected than in RC dried in the absence of sugar. This indicated a larger hindering of the internal dynamics in trehalose-coated RC, notwithstanding the larger amount of residual water present in comparison with samples dried in the absence of sugar.

Thermodynamics of the early steps in the photocycle of Natronobacterium pharaonis halorhodopsin. Influence of medium and of anion substitution

The enthalpy (delta H) and structural volume changes (delta V) associated with the formation and decay of the early intermediate K600 in the photocycle of Natronobacterium pharaonis halorhodopsin (pHR), an inward-directed anion pump, were obtained by laser-induced optoacoustic spectroscopy. A large expansion is associated with K600 formation, its value depending on the medium and on the anion (Cl-, NO3-, Br-, I-). A smaller expansion is associated with K600 decay to L520. A contraction is found for the same step in the case of the azide-loaded pHR which is an efficient outward-directed proton pump. Thus, the conformational changes in L520 determine the direction and sign of charge translocation. The linear correlation between delta H and delta V for chloride-loaded pHR observed upon mild medium variations is attributed to enthalpy-entropy compensation effects and allows the calculation of the free-energy changes, delta GK = (97 +/- 16) kJ/mol and delta GKL = -(2 +/- 2) kJ/mol. Different from other systems, delta S correlates negatively with delta V in the first steps of the pHR photocycle. Thus, the space around the anion becomes larger and more rigid during each of these two steps. The photocycle quantum yield was 0.52 for chloride-pHR as measured by laser flash photolysis.

Thermal and structural changes of photosynthetic reaction centers characterized by photoacoustic detection with a broad frequency band hydrophone

Photoacoustic measurements using a broad frequency band hydrophone were carried out in photosynthetic reaction centers (RC) isolated from Rhodobacter sphaeroides R-26 purple bacteria. Data were extracted on enthalpy and volume changes accompanying the primary steps after excitation in the range of 0-500 microseconds aimed at further characterizing the thermodynamic properties of the RC. Quinone titration showed that the volume contraction accompanying the electron transport is sensitive to the molecular species occupying the secondary quinone site. delta VM = 14.4, 7.7 and 4.3 cm3 molar volume contractions were calculated from the measured parameters for 1, 2 and 0.07 quinone/RC after light excitation. Comparing the enthalpy changes (delta H) to the Gibbs free energy data in the literature, a rather large (26%) entropic contribution to the free energy changes (delta G) is estimated for the P*QAQB-->P+QA-QB electron transport (where QA and QB represent primary and secondary quinones, respectively). This is in contrast to previous estimations that delta G = delta H in these processes. On the other hand, only a small (4%) entropic contribution to the delta G of the P*QAQB-->P+QAQB- process is estimated, in agreement with the literature data. Our results are in good agreement with the data obtained earlier (Edens et al. [2000] J. Am. Chem. Soc. 122, 1479-1485).

Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers: a pulsed photoacoustic study of electron transfer in photosystem I reveals a similarity to bacterial reaction centers in both volume change and entropy

The thermodynamic properties of electron transfer in biological systems are far less known in comparison with that of their kinetics. In this paper the enthalpy and entropy of electron transfer in the purified photosystem I trimer complexes from Synechocystis sp. PCC 6803 have been studied, using pulsed time-resolved photoacoustics on the 1 micros time scale. The volume contraction of reaction centers of photosystem I, which results directly from the light-induced charge separation forming P(700+F(A)/F(B-) from the excited-state P700*, is determined to be -26 +/- 2 A3. The enthalpy of the above electron-transfer reaction is found to be -0.39 +/- 0.1 eV. Photoacoustic estimation of the quantum yield of photochemistry in the purified photosystem I trimer complex showed it to be close to unity. Taking the free energy of the above reaction as the difference of their redox potentials in situ allows us to calculate an apparent entropy change (TDeltaS) of +0.35 +/- 0.1 eV. These values of DeltaV and TDeltaS are similar to those of bacterial reaction centers. The unexpected sign of entropy of electron transfer is tentatively assigned, as in the bacterial case, to the escape of counterions from the surface of the particles. The apparent entropy change of electron transfer in biological system is significant and cannot be neglected.

Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers: volume change, enthalpy, and entropy of electron-transfer reactions in the intact cells of the cyanobacterium Synechocystis PCC 6803

The volume and enthalpy changes for charge transfer in the 0.1-10 micros time window in photosynthetic reaction centers of the intact cells of Synechocystis PCC 6803 were determined using pulsed, time-resolved photoacoustics. This required invention of a method to correct for the cell artifact at the temperature of maximum density of water caused by the heterogeneous system. Cells grown under either white or red light had different PS I/PS II molar ratios, approximately 3 and approximately 1.7, respectively, but invariable action spectra and effective antenna sizes of the photosystems. In both cultures, the photoacoustic measurements revealed that their thermodynamic parameters differed strongly in the spectral regions of predominant excitation of PS I (680 nm) and PS II (625 nm). On correcting for contribution of the two photosystems at these wavelengths, the volume change was determined to be -27 +/- 3 and -2 +/- 3 A3 for PS I and PS II, respectively. The energy storage on the approximately 1 micros time scale was estimated to be 80 +/- 15% and 45 +/- 10% per trap in PS I and PS II, respectively. These correspond to enthalpies of -0.33 +/- 0.2 and -1 +/- 0.2 eV for the assumed formation of ion radical pairs P700+F(AB-) and Y(Z*)P680Q(A-), respectively. Taking the free energy of the above reactions as the differences of their redox potentials in situ, apparent entropy changes were estimated to be +0.4 +/- 0.2 and -0.2 +/- 0.2 eV for PS I and PS II, respectively. These values are similar to that obtained in vitro for the purified reaction center complexes on the microsecond time scale [Hou et al. (2001) Biochemistry 40, 7109-7116, 7117-7125]. The constancy of these thermodynamic values over a 2-fold change of the ratio of PS I/PS II is support for this method of in vivo analysis. Our pulsed PA method can correct the "cell" or heterogeneous artifact and thus opens a new route for studying the thermodynamics of electron transfer in vivo.

Evidence for protein dielectric relaxations in reaction centers associated with the primary charge separation detected from Rhodospirillum rubrum chromatophores by combined photovoltage and absorption measurements in the 1-15 ns time range

Fast photovoltage measurements in Rhodospirillum rubrum chromatophores in the nanosecond time range, escorted by time-resolved absorption measurements, are described. Under reducing conditions, the photovoltage decayed significantly faster than the spectroscopically detected charge recombination of the radical pair P(+)H(A)(-). This indicates the occurrence of considerable dielectric relaxations. Our data and data from the literature were analyzed by means of a reaction scheme consisting of three states, namely, A, P, and P(+)H(A)(-). A time-dependent DeltaG(t) was introduced by assuming a time-dependent rate constant of the back-reaction, k(-1)(t). With the exception of the latter rate constant, all other parameters of the model are reliably known within narrow limits. This allowed us to distinguish between the three cases assumed for DeltaG degrees (t): (1)DeltaG degrees (t) = constant; (2)DeltaG degrees (t) as published by Peloquin et al. [Peloquin, J. M., Williams, J. C., Lin, X. M., Alden, R. G., Taguchi, A. K. W., Allen, J. P., and Woodbury, N. W. (1994) Biochemistry 33, 8089-8100]; and a (3)DeltaG degrees (t) that fits the present data. The assumption that (1)DeltaG degrees (t) = constant is incompatible with our photovoltage data, and (2)DeltaG degrees (t) is incompatible with the constraint that the ratio of fluorescence yields in the closed and open state is F(m)/F(o) approximately 2. We specify a (3)DeltaG degrees (t) that should be valid for photosynthetic reaction centers in vivo. Furthermore, the overall kinetics of the electric relaxation, e(t), in response to the primary charge separation were determined.

Long-lived charge-separated states in bacterial reaction centers isolated from Rhodobacter sphaeroides

We studied the accumulation of long-lived charge-separated states in reaction centers isolated from Rhodobacter sphaeroides, using continuous illumination, or trains of single-turnover flashes. We found that under both conditions a long-lived state was produced with a quantum yield of about 1%. This long-lived species resembles the normal P(+)Q(-) state in all respects, but has a lifetime of several minutes. Under continuous illumination the long-lived state can be accumulated, leading to close to full conversion of the reaction centers into this state. The lifetime of this accumulated state varies from a few minutes up to more than 20 min, and depends on the illumination history. Surprisingly, the lifetime and quantum yield do not depend on the presence of the secondary quinone, Q(B). Under oxygen-free conditions the accumulation was reversible, no changes in the normal recombination times were observed due to the intense illumination. The long-lived state is responsible for most of the dark adaptation and hysteresis effects observed in room temperature experiments. A simple method for quinone extraction and reconstitution was developed.

Time-resolved absorption and photothermal measurements with recombinant sensory rhodopsin II from Natronobacterium pharaonis

Purified wild-type sensory rhodopsin II from Natronobacterium pharaonis (pSRII-WT) and its histidine-tagged analog (pSRII-His) were studied by laser-induced optoacoustic spectroscopy (LIOAS) and flash photolysis with optical detection. The samples were either dissolved in detergent or reconstituted into polar lipids from purple membrane (PML). The quantum yield for the formation of the long-lived state M(400) was determined as Phi(M) = 0.5 +/- 0.06 for both proteins. The structural volume change accompanying the production of K(510) as determined with LIOAS was DeltaV(R,1) </= 10 ml for both proteins, assuming Phi(K) >/= Phi(M), indicating that the His tag does not influence this early step of the photocycle. The medium has no influence on DeltaV(R,1), which is the largest so far measured for a retinal protein in this time range (<10 ns). This confirms the occurrence of conformational movements in pSRII for this step, as previously suggested by Fourier transform infrared spectroscopy. On the contrary, the decay of K(510) is an expansion in the detergent-dissolved sample and a contraction in PML. Assuming an efficiency of 1.0, DeltaV(R,2) = -3 ml/mol for pSRII-WT and -4.6 ml/mol for pSRII-His were calculated in PML, indicative of a small structural difference between the two proteins. The energy content of K(510) is also affected by the tag. It is E(K) = (88 +/- 13) for pSRII-WT and (134 +/- 11) kJ/mol for pSRII-His. A slight difference in the activation parameters for K(510) decay confirms an influence of the C-terminal His on this step. At variance with DeltaV(R,1), the opposite sign of DeltaV(R,2) in detergent and PML suggests the occurrence of solvation effects on the decay of K(510), which are probably due to a different interaction of the active site with the two dissolving media.

Time-resolved absorption and photothermal measurements with sensory rhodopsin I from Halobacterium salinarum

An expansion accompanying the formation of the first intermediate in the photocycle of transducer-free sensory rhodopsin I (SRI) was determined by means of time-resolved laser-induced optoacoustic spectroscopy. For the native protein (SRI-WT), the absolute value of the expansion is approximately 5.5 mL and for the mutant SRI-D76N, approximately 1.5 mL per mol of phototransformed species (in 0.5 M NaCl), calculated by using the formation quantum yield for the first intermediate (S610) of Phi610 = 0.4 +/- 0.05 for SRI-WT and 0.5 +/- 0.05 for SRI-D76N, measured by laser-induced optoacoustic spectroscopy and by laser flash photolysis. The similarity in Phi610 and in the determined value of the energy level of S610, E610 = (142 +/- 12) kJ/mol for SRI-WT and SRI-D76N indicates that Asp76 is not directly involved in the first step of the phototransformation. The increase with pH of the magnitude of the structural volume change for the formation of S610 in SRI-WT and in SRI-D76N upon excitation with 580 nm indicates also that amino acids other than Asp76, and other than those related to the Schiff base, are involved in the process. The difference in structural volume changes as well as differences in the activation parameters for the S610 decay should be attributed to differences in the rigidity of the cavity surrounding the chromophore. Except for the decay of the first intermediate, which is faster than in the SRI-transducer complex, the rate constants of the photocycle for transducer-free SRI in detergent suspension are strongly retarded with respect to wild-type membranes (this comparison should be done with great care because the preparation of both samples is very different).

Conformation-activated protonation in reaction centers of the photosynthetic bacterium Rhodobacter sphaeroides

Kinetics and stoichiometry of proton binding/unbinding induced by intense (1 W cm-2) and continuous illumination were measured in the isolated reaction center (RC) protein from photosynthetic purple bacterium Rhodobacter sphaeroides in the absence of an external electron donor. At high ionic strength (100 mM), large proton release (approximately 6 H+ per RC) was observed at pH 6 and substoichiometric H+-ion binding (approximately 0.3 H+ per RC) at pH 8. These observations together with optical spectroscopy on the oxidized dimer indicate that, at room temperature, two distinct conformations of the RC can be obtained depending on the pH, Eh, and illumination. Acidic pH, a large redox gap between the actual Eh of the solution and the midpoint potential of the acceptor quinone, and strong illumination favor the conversion of the RC from the dark-adapted state to the light-adapted state. These conformations differ greatly in the rates of primary photochemistry, the reoxidation of semiquinone and the rereduction of the oxidized dimer, and the protonation states of the amino acids of the protein. Whereas substoichiometric proton unbinding is observed in the P+Q redox state of the protein in the dark-adapted conformation, much larger H+-ion release is detected in the light-adapted conformation. From the pH dependence of the key processes in the conformational change and reoxidation of semiquinone, we concluded that they are controlled by protonatable groups available in the protein. A simple phenomenological model is presented that relates the rates and equilibrium constants of the electron transfer reactions and the conformational change of the RC.

Atomic Force Sensing of Light-Induced Protein Dynamics with Microsecond Time Resolution in Bacteriorhodopsin and Photosynthetic Reaction Centers

This paper reports on experiments that have monitored protein microsecond dynamics with a cantilevered near-field optical glass fiber. In these experiments two photoactive proteins, bacteriorhodopsin (bR) and the photosynthetic reaction center (PS I), are used to demonstrate that such probes can measure light-induced microsecond protein dynamics even though the resonance frequencies of the glass cantilevers used are on the order of a few hundred kilohertz. In the case of the light-driven proton pump, bR, the light-induced atomic force sensing (AFS) signal is negative (indicating contraction) in the microsecond time domain of the L photointermediate and becomes positive (corresponding to expansion) in the subsequent M intermediate that lives for milliseconds. Double pulse experiments from M to bR show that the latter process reverses the AFS signal. Thus, the AFS structural changes are coupled with the (optical) photocycle intermediates. Light-induced contraction and expansion phenomena are also observed in the case of PS I. In both systems the time regime of the dynamic phenomena that have been measured with AFS is five orders of magnitude faster than the fastest previously recorded atomic force detection of dynamic phenomena. This advance portends a new era in dynamic imaging of protein conformational changes.

A photophysical study of purines and theophylline by using laser-induced optoacoustic spectroscopy

The photophysics of purinic compounds (purine, 6-methylpurine, 6-aminopurine [adenine], 6-chloropurine, 6-methoxypurine) and theophylline in acetonitrile solution were studied by pulsed laser-induced optoacoustic spectroscopy (LIOAS) exciting at 266 nm. The effect of O2, Xe and MnCl2 on the photophysical behavior of these compounds was studied; as well, the formation quantum yield of purine and 6-methylpurine triplet states were determined, with phi T = 0.88 +/- 0.03 for both compounds. Multiphotonic and depletion processes were observed at high laser fluences. In order to explain this behavior, theoretical UV-visible absorption electronic spectra from both the S0 and S1 state have been calculated for purines and theophylline by using the semiempirical PM3 and ZINDO/S methods.

Energetics and kinetics of radical pairs in reaction centers from Rhodobacter sphaeroides. A femtosecond transient absorption study

Femtosecond transient absorption spectra on reaction centers from Rhodobacter sphaeroides wild type have been recorded with high time and wavelength resolution and a very high S/N ratio in the 500-940 nm range with a diode array system. The data have been analyzed by global analysis. Five lifetime components of 1.5, 3.1, 10.8, and 148 ps and long-lived (several nanoseconds) were required to fit the entire three-dimensional data surface adequately with a single set of lifetimes and decay-associated difference spectra (DADS). Up to 30 ps, there is little dispersion in the lifetimes, but in the longer time range (50-250 ps), a substantial variation in lifetime was observed, depending on detection wavelength. The data from the global analysis have been subjected to kinetic modeling comparing sequential kinetic schemes either including (reversible model) or excluding (forward model) back-reactions in the early electron transfer process(es). Thus, the molecular rate constants for the model(s) and the difference spectra of the pure intermediates [species-associated difference spectra (SADS)] were obtained. The data unequivocally confirm the necessity of an electron transfer intermediate with spectral characteristics of P+B-H prior to the formation of the P+BH- state (P is special pair, B is accessory chlorophyll, and H is pheophytin), irrespective of the model chosen. Besides being in much better agreement with the observation of long-lived fluorescence kinetics components, the reversible model results in SADS, in particular for the P+BH- state, that are in somewhat better agreement with expectations than for the pure forward model. For these and other reasons, the reversible model is preferred over the pure forward model. The electrochromic shifts of the H bands in the P+B- state and of the B bands in the P+H- state are revealed clearly in the spectra, thus supporting the assignments. Within the reversible model, the rate constant for the forward reaction in the first step P*-->P+B-H is slightly larger [k12 approximately (2.48 ps)-1] than for the second step P+B-H-->P+BH- [k23 approximately equal to (2.53 ps)-1], in contrast to the pure forward model. From the rate constants for the respective back-reactions, the free energy differences delta G relative to P* for the states P+B-H and P+BH- have been determined to be -41 and -91 meV, respectively. Thus, the free energy difference for the P+BH- state at early times after electron transfer is by a factor of 2-3 smaller than assumed so far. This has the important consequence that a quasi-equilibrium exists from about 10 ps until further electron transfer on the 200 ps time scale with a substantial percentage (approximately 16%) of the P+B-H state present. These results present the first direct evidence from transient absorption data, where the nature of the intermediate can be assigned, for the validity of the slow radical pair relaxation concept. The results have various consequences for understanding the mechanism of the overall electron transfer reaction and imply a much more active role of the protein in the early charge separation processes of the reaction center than assumed so far. The data are discussed in terms of current electron transfer theory. It is suggested that the two first-electron steps operate at a rate very close to the maximal possible rate.

Photoacoustic diagnostics of fast photochemical and photobiological processes. Analysis of inverse problem solution

Photoacoustic (PA) diagnostics is a time-resolved experimental technique that measures transient photoinduced volume changes on the nano- and microsecond time-scales. The technique has been used to study the energetics and dynamics of chemical and biochemical reactions initiated by absorption of light. The dynamics of the volume changes and associated relaxation processes can be restored from PA-waveforms by solving an ill-posed problem of deconvolution. For the systems with known relaxation kinetics scheme this problem is usually solved by an iterative approximation algorithm. In complex photoactive systems (e.g. photosynthetic samples), where information about kinetics of fast photoinduced volume changes is not available, an algorithm of direct deconvolution must be used. The implementation of one of the linear deconvolution algorithms (Tikhonov's alpha-regularization) for the PA-diagnostics is therefore considered. The problem of the optimal choice of experimental set-up, and restoration algorithm is analyzed by a numerical simulation. It is shown that the quality of PA-diagnostic experiment is mainly determined by a transfer function converting the relaxation spectrum to the spectrum of output electric signal. The analytical expressions to calculate this function (so called PA-filter) are presented. The performance of two widely used schemes of PA-diagnostics are then directly compared. The time-resolution of the PA-diagnostics in analysis of simultaneous bi-exponential decay is evaluated, and the relationship between the resolving power and parameters of the experimental set-up (signal-to-noise ratio, sampling rate, shape of the PA-filter) is found. The advantage of front face irradiation scheme with piezopolymer film detector for time-resolved measurements is demonstrated.