Volume Changes Correlate with Enthalpy Changes during the Photoinduced Formation of the 3MLCT State of Ruthenium(II) Bipyridine Cyano Complexes in the Presence of Salts. A Case of the Entropy−Enthalpy Compensation Effect

Deciphering Solvation Effects in Aqueous Binary Mixtures by Fluorescence Behavior of 4-Aminophthalimide: The Comparison Between Ionic Liquids and Alcohols as Cosolvents

Ionic liquids (ILs) have received attention for many years due to them being very promising as green solvent substitutes, but they are not fully understood, especially their behavior dissolved in other solvents, for example, water. Thus, the goal of this contribution is to show insight into the different IL-water mixtures interaction. In this way, two protic ILs (PILs), ethylammonium nitrate (EAN) and 1-methylimidazolium acetate (MIA), mixed with water were investigated. To study the PILs-water interaction, the unique spectroscopical behavior in water of the molecular probe 4-aminophthalimide (4-AP) was used. 4-AP emission spectra show hypsochromic shifting by changing the excitation wavelength and, using time-resolved spectroscopy, can be detected by a blue shifting with time. Also, the water mixture of an aprotic IL, 1-methyl-3-butylimidazolium tetrafluoroborate (bmimBF₄), and three alcohols, methanol (MeOH), 2-propanol (2-PrOH), and t-butanol (t-BOH), were investigated for comparison. Our results show that the water-ILs interaction is dominated by the size of the IL components, in particular, the cation size. Thus, in MIA-water and bmimBF₄-water mixtures, 4-AP is mostly solvated by the IL, even at a low IL molar fraction, as in the t-BOH-water mixture. This finding is especially interesting when ILs-water mixtures are used as a solvent in an organic reaction, where it may call attention to water probably not being the solvent that is interacting with the reactants.

Photosynthetic energy storage efficiency in Chlamydomonas reinhardtii, based on microsecond photoacoustics

Using a novel, pulsed micro-second time-resolved photoacoustic (PA) instrument, we measured thermal dissipation and energy storage (ES) in the intact cells of wild type (WT) Chlamydomonas reinhardtii, and mutants lacking either PSI or PSII reaction centers (RCs). On this time scale, the kinetic contributions of the thermal expansion component due to heat dissipation of absorbed energy and the negative volume change due to electrostriction induced by charge separation in each of the photosystems could be readily distinguished. Kinetic analysis revealed that PSI and PSII RCs exhibit strikingly different PA signals where PSI is characterized by a strong electrostriction signal and a weak thermal expansion component while PSII has a small electrostriction component and large thermal expansion. The calculated ES efficiencies at ~10 μs were estimated to be 80 ± 5 and 50 ± 13% for PSII-deficient mutants and PSI-deficient mutants, respectively, and 67 ± 2% for WT. The overall ES efficiency was positively correlated with the ratio of PSI to PSI + PSII. Our results suggest that the shallow excitonic trap in PSII limits the efficiency of ES as a result of an evolutionary frozen metabolic framework of two photosystems in all oxygenic photoautotrophs.

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 Large Entropic Term Due to Water Rearrangement is Concomitant with the Photoproduction of Anionic Free-Base Porphyrin Triplet States in Aqueous Solutions†

The enthalpy change, DeltaTH, and volume change, DeltaTV, associated with triplet state formation upon excitation of free-base meso-tetra-(4-sulfonatophenyl)porphyrin, TSPP4-, its Zn derivative, ZnTSPP4-, and meso-tetra-(4-carboxyphenyl)porphyrin, TCPP4-, were obtained in aqueous solutions by the application of laser-induced optoacoustics spectroscopy in the presence of phosphate salts of various monovalent cations (Li+, Na+, K+, NH4+ and Cs+). A linear correlation was found between DeltaTH and DeltaTV at different phosphate concentrations for the free-base porphyrins. The intercepts (132 +/- 8 kJ mol(-1) for TSPP4- and 164 +/- 23 kJ mol(-1) for TCPP4-) of these plots correspond to the respective value of the triplet energy content obtained from phosphorescence at 77 K (140 and 149 kJ mol(-1)). This suggests that DeltaTG for the triplet state formation is independent of the medium and an enthalpy-entropy compensation is responsible for the much smaller and salt-dependent DeltaTH values obtained at room temperature. The Gibbs energy for triplet state formation of the free-base porphyrins at room temperature is thus mainly determined by the entropic term due to solvent rearrangement. The DeltaTH values for 3ZnTSPP4- at different buffer concentrations and different cations are all between 130 and 150 kJ mol(-1), close to the triplet energy obtained from phosphorescence (E(T) = 155 kJ mol(-1)). The solvent structure and the nature of the counterion have a negligible influence on the 3ZnTSPP4-formation due to the blockage of the electron pairs on the central N atoms. Thus, the small DeltaTV value should be due to intrinsic bond changes upon 3ZnTSPP4- formation and no correlation between DeltaTH and DeltaTV should be expected in this case. The enthalpy change determines the Gibbs energy for 3ZnTSPP4-formation at room temperature.

Photoinduced Electron Transfer from Tetrasulfonated Porphyrin to Benzoquinone Revisited. The Structural Volume-Normalized Entropy Change Correlates with Marcus Reorganization Energy

Time-resolved laser-induced optoacoustic spectroscopy was used for the determination of the enthalpy, DeltaTH, and structural volume changes, DeltaTV, concomitant with triplet state formation upon excitation of meso-tetrakis(4-sulfonatophenyl)porphyrin, TSPP(4-), as well as with the triplet state electron-transfer (ET) quenching by benzoquinone, BQ (DeltaRH and DeltaRV). The values of DeltaTH and DeltaTV for (3)TSPP(4-) formation in the presence of different cations (Li+, Na+, K+, NH4+, and Cs+) correlated with each other and afforded a value of DeltaTG = 140 +/- 20 kJ mol(-1), equal to the value of E+ at 77 K, but much larger than the DeltaTH values in solution at room temperature, due to the large entropic factor in solution. The influence of the cations on DeltaTH and DeltaTV (a contraction ranging from 5.4 to 3.8 cm3 mol(-1)) is attributed to changing chromophore-water interactions in the ground and triplet states. Upon quenching of 3TSPP(4-) by BQ, the quantum yield of free radical formation, PhiR = 0.66 +/- 0.04, is the same in the solutions of the five cations. The values of DeltaRH and DeltaRV are small and have a large error. The energy level of the free radicals formed is thus very similar to that of 3TSPP(4-). TDeltaRS and X = TDeltaRS/DeltaRV, i.e., the structural volume change-normalized entropy change for free radicals formation, were derived using average values of DeltaRH and DeltaRV together with the calculated DeltaRG degrees . The measured Marcus reorganization energy, lambda, and X fall into the lambda vs X linear dependence we previously found for the radical formation upon ET quenching of triplet flavins (3FMN and 3FAD) by amines and amino acids. Thus, X = TDeltaRS/DeltaRV in aqueous solutions is a property of the particular donor-acceptor pair linearly correlated to the corresponding Marcus reorganization energy. The value of X is much larger than the predicted value applying the electrostriction concepts in view of the noncontinuum nature of the aqueous solutions.

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.

Synthesis, Characterization, Spectroscopic, and Electrochemiluminescence Properties of a Solvatochromic Azacrown-Containing Cyanoruthenate(II): Potential Applications in Separation and Indirect Photometric Detection of Cations and Amino Acids in HPLC

A new anionic ruthenium(II) complex, (Et4N)[Ru(tpyA18C6)(CN)3] (tpyA18C6=N-[4'-(2,2':6',2''-terpyridyl)]-1,4,7,10,13-pentaoxa-16-azacyclohexadodecane), has been synthesized and characterized. The complex was found to show pronounced solvatochromic behavior and, when dissolved in solution, changed its color from purple to yellow when the solvent system was varied from pure acetonitrile to pure water. Its absorption and emission energies in various solvents showed a linear dependence of the Gutman's acceptor number. The characteristic photoluminescence and electrochemiluminescence (ECL) of the complex were also found to be progressively quenched as the proportion of water in a water/acetonitrile mixture increased. Large changes in the chemical shifts of the 1H NMR and 13C NMR signals of [Ru(tpyA18C6)(CN)3]- in different solvents were observed. The complex has also been demonstrated to serve as a mobile-phase additive in high-performance liquid chromatography for separation of metal cations and amino acids. Comparison studies with the crown-free analogue, (Et4N)[Ru(tpy)(CN)3] (tpy = 2,2':6',2''-terpyridine), showed that other than the ion-pair effect, the allosteric host-guest interaction provided by the presence of the pendant crown was essential to the separation performance of the complex. Indirect detection of nonabsorbing analytes has been achieved by monitoring the absorbance changes of the eluent at the metal-to-ligand charge-transfer (MLCT) absorption band maximum of the complex at 445 nm. The effects of pH, ionic strength, and polarity of the mobile phase as well as the complex concentration on the selectivity and resolution have also been studied.

The Characterization of the High-Frequency Vibronic Contributions to the 77 K Emission Spectra of Ruthenium−Am(m)ine−Bipyridyl Complexes, Their Attenuation with Decreasing Energy Gaps, and the Implications of Strong Electronic Coupling for Inverted-Region Electron Transfer

The 77 K emission spectra of a series of [Ru(Am)6-2n(bpy)n]2+ complexes (n = 1-3) have been determined in order to evaluate the effects of appreciable excited state (e)/ground state (g) configurational mixing on the properties of simple electron-transfer systems. The principal focus is on the vibronic contributions, and the correlated distortions of the bipyridine ligand in the emitting MLCT excited state. To address the issues that are involved, the emission band shape at 77 K is interpreted as the sum of a fundamental component, corresponding to the {e,0'} --> {g,0} transition, and progressions in the ground-state vibrational modes that correlate with the excited-state distortion. Literature values of the vibrational parameters determined from the resonance-Raman (rR) for [Ru(NH3)4bpy]2+ and [Ru(bpy)3]2+ are used to model the emission spectra and to evaluate the spectral analysis. The Gaussian fundamental component with an energy Ef and bandwidth Deltanu1/2 is deconvoluted from the observed emission spectrum. The first-, second-, and third-order terms in the progressions of the vibrational modes that contribute to the band shape are evaluated as the sums of Gaussian-shaped contributions of width Deltanu1/2. The fundamental and the rR parameters give an excellent fit of the observed emission spectrum of [Ru(NH3)4bpy]2+, but not as good for the [Ru(bpy)3]2+ emission spectrum probably because the Franck-Condon excited state probed by the rR is different in symmetry from the emitting MLCT excited state. Variations in vibronic contributions for the series of complexes are evaluated in terms of reorganizational energy profiles (emreps, Lambdax) derived from the observed spectra, and modeled using the rR parameters. This modeling demonstrates that most of the intensity of the vibronic envelopes obtained from the frozen solution emission spectra arises from the overlapping of first-order vibronic contributions of significant bandwidth with additional convoluted contributions of higher order vibronic terms. The emrep amplitudes of these complexes have their maxima at about 1500 cm(-1) in frozen solution, and Lambdax(max) decreases systematically by approximately 2-fold as Ef decreases from 17,220 for [Ru(bpy)3]2+ to 12,040 cm(-1) for [Ru(NH3)4bpy]2+ through the series of complexes. Corrections for higher order contributions and bandwidth differences based on the modeling with rR parameters indicate that the variations in Lambdax(max) imply somewhat larger decreases in first-order bpy vibrational reorganizational energies. The large attenuation of vibrational reorganizational energies of the [Ru(Am)6-2n(bpy)n]2+ complexes contrasts with the apparent similarity of reorganizational energy amplitudes for the absorption and emission of [Ru(NH3)4bpy]2+. These observations are consistent with increasing and very substantial excited-state/ground-state configurational mixing and decreasing excited-state distortion as Ef decreases, but more severe attenuation for singlet/singlet than triplet/singlet mixing (alphage > alphaeg for the configurational mixing coefficients at the ground-state and excited-state potential energy minima, respectively); it is inferred that 0.18 > or = alphage2 > or = 0.09 for [Ru(bpy)3]2+ and 0.37 > or = alphage2 > or = 0.18 for [Ru(NH3)4bpy]2+ in DMSO/water glasses, where the ranges are based on models that there is or is not a spin restriction on configurational mixing (alphage > alphaeg and alphage = alphaeg), respectively, for these complexes.

Conformational and photophysical studies on porphyrin-containing donor–bridge–acceptor compounds. Charge separation in micellar nanoreactors

Photophysical studies with semi-rigid, 1, and flexible, 2, donor-bridge-acceptor (D-b-A2+) molecules with D a porphyrin and A2+ a methyl viologen moiety, were performed in neat polar solvents as well as included in surfactant (DTAB) aqueous and in reverse AOT/n-alkane micelles. The micelles acted as nanoreactors for the photoinduced electron transfer reaction upon laser excitation. In spite of the longer lifetime of the charge separated (CS) state in the semi-rigid tetrad 1(ca. 200 ns vs. ca. 100 ns for the flexible dyad 2), the CS formation quantum yield, for example in acetonitrile, was lower for the former (phi(CS) = 0.13) than for the latter (0.58). Comparison of the time-resolved fluorescence data in neat solvent and in the micelles yielded the phi(CS) values in the dilute micellar solutions. Application of laser-induced optoacoustic spectroscopy at various temperatures to 1 dissolved in a polar organic solvent (benzonitrile, BZN) included in aqueous DTAB nanoreactors afforded structural volume changes for the production in hundreds of ps of the CS state upon excitation of a polar molecule. The contraction during CS formation upon excitation of the collapsed conformer in BZN is attributed to the entering of solvent into the open molecular cavity. The opening upon formation of the CS state due to photoinduced electron transfer in the 1 collapsed conformation arises from the repulsion of the two positively charged ends in this state, as previously calculated. Inclusion of 1 in reverse AOT micelles in various n-alkanes also led to a contraction upon excitation, but the data had much more error due to the limited range of variability of the ratio of thermoelastic parameters. The data obtained with the more flexible "supermolecule" 2 showed the predicted large conformation flexibility of these molecules.

Time-Resolved Photoacoustics Study of the Ruthenium(II) Bis(2,2‘-bipyridine)(4,4‘-dicarboxy-2,2‘-bipyridine) Complex

The formation and the decay of the triplet metal to ligand charge transfer state ((3)MLCT) of ruthenium(II) bis(2,2'-bipyridine)(4,4'-dicarboxy-2,2'-bipyridine) (Ru(bpy)(2)(dcbpy)) were characterized using photoacoustic calorimetry. At pH 6 and 2, the (3)MLCT state formation leads to a volume change of -8 mL mol(-1) and enthalpy changes of 17 kcal mol(-1) and 13 kcal mol(-1), respectively. We attribute the volume contraction to structural changes and to solvent electrostriction. At pH 4, the photoexcitation of the complex leads to an expansion of 14 mL mol(-1) and an enthalpy change of approximately 119 kcal mol(-1) due to protonation of the carboxyl group in the excited state.

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.

Thermodynamic changes associated with the formation of the hydrated electron after photoionization of inorganic anions: a time-resolved photoacoustic study

The enthalpy and volume changes, deltaH and deltaV, associated with the 266 nm laser-induced photoionization reactions of aqueous ferrocyanide and iodide ions, to yield the hydrated electron, e(-)aq, and oxidized products were determined by temperature-dependent time-resolved photoacoustics. The photoionization quantum yield as function of temperature (9-30 degrees C) was determined by laser flash photolysis actinometry. The obtained values were used for the calculation of thermodynamic parameters associated with the formation of e(-)aq, such as the apparent partial molar volume, V(o)e = 26 cm3 mol(-1), and the standard formation enthalpy and entropy changes, deltaH(o)f,e = 31 kJ mol(-1) and TdeltaS(o)f,e = 338 kJ mol(-1). These results indicate that the formation of the aqueous excess electron solution is governed by the increase in entropy in the three-dimensional hydrogen-bonding network of water.

Using extrathermodynamic relationships to model the temperature dependence of Henry's law constants of 209 PCB congeners

Our previous measurements of the temperature dependencies of Henry's law constants of 26 polychlorinated biphenyls (PCBs) showed a well-defined linear relationship between the enthalpy and the entropy of phase change. Within a homologue group, the Henry's law constants converged to a common value at a specific isoequilibrium temperature. We use this relationship to model the temperature dependencies of the Henry's law constants of the remaining PCB congeners. By using experimentally measured Henry's law constants at 11 degrees C for 61 PCB congeners described in this paper combined with the isoequilibrium temperatures from our previous measurements of Henry's law constants of 26 PCB congeners, we have derived an empirical relationship between the enthalpies and the entropies of phase change for these additional PCB congeners. A systematic variation in the enthalpies and entropies of phase change was found to be partially dependent on the chlorine number and substitution patterns on the biphenyl rings, allowing further estimation of the temperature dependence of Henry's law constants for the remaining 122 PCB congeners. The enthalpies of phase change for all 209 PCB congeners ranged between 10 and 169 kJ mol(-1), where the enthalpies of phase change decreased as the number of ortho chlorine substitutions on the biphenyl rings increased within homologue groups. These data are used to predict the temperature dependence of Henry's law constants for all 209 PCB congeners.

Thermodynamic changes in the photoinduced proton-transfer reaction of the triplet state of safranine-T

The enthalpy and volume changes occurring in the triplet excited state proton-transfer reactions of safranine-T (SH+) in aqueous solutions at pH 4.8, 8.3, and 10.4 were investigated using time-resolved photoacoustics (TRP). The transient triplet state species were also studied using laser-flash photolysis (LFP). The LFP experiments showed the prompt formation of 3SH+ with a triplet quantum yield phiT = 0.28 between pH 4.8 and 10.4. At pH 8.3 3SH+ decays directly to the ground state. However, at pH 4.8 and 10.4, 3SH+ reacts with protons or hydroxy ions to form the dication 3SH2(2+) or the neutral 3S species, with diffusion-controlled rate constants of kH+ = 1.6 x 10(10) M(-1) s(-1), and kHO- = 2.6 x 10(10) M(-1) s(-1), respectively. Under the same experimental conditions, the TRP measurements allowed the accurate determination of the energy content of the rapidly formed triplet state 3SH+ i.e. E(T) = 175 kJ mol(-1). The slow component (0.1-3 micros) of the TRP signal at pH 4.8 and 10.4 was attributed to the formation of the species 3SH2(2+) and 3S, respectively. The enthalpy changes associated with the proton-transfer reactions of 3SH+, calculated from the values of the heat released as obtained by TRP, were in remarkable agreement with the values estimated from the thermodynamic data of the acid-base equilibria of the triplet states of the dye. The formation of 3SH+ was accompanied by a volume expansion of 1.8 cm3 mol(-1), which was explained by changes in the hydrogen-bonding interaction of the dye with its solvation sphere. Instead, the volume changes observed upon the formation of 3SH2(2-) and 3S accounted for the electrostrictive effect produced by the change in the charge distribution on the dye after the proton-transfer reaction.

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.

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.

Kinetics of local helix formation in poly-L-glutamic acid studied by time-resolved photoacoustics: neutralization reactions of carboxylates in aqueous solutions and their relevance to the problem of protein folding

Photoactivatable caged protons have been used to trigger proton transfer reactions in aqueous solutions of acetate, glutamate, and poly-L-glutamic acid, and the volumetric and enthalpic changes have been detected and characterized by means of time-resolved photoacoustics. Neutralization of carboxylates in aqueous solutions invariably results in an expansion of the solution due to the disappearance of two charges and is accompanied by little enthalpic change. The reactions occur with thermally activated, apparent bimolecular rates on the order of 10(10) M(-1)s(-1). In the case of aqueous solutions of poly-L-glutamic acid at pH around the pK(a) of the coil-to-helix transition, diffusional binding of a proton by carboxylates is followed by a sequential reaction with rate 1.06 (+/- 0.05) x 10(7)s(-1). This step is not thermally activated in the temperature range we have investigated and is likely related to local formation of hydrogen bonds near the protonation site. This structural event may constitute a rate-limiting step in helix propagation.

Photodecarboxylation of ketoprofen in aqueous solution. A time-resolved laser-induced optoacoustic study

The photodecarboxylation reaction of 2-(3-benzoylphenyl)propionate (ketoprofen anion, KP-) was studied in water and in 0.1 M phosphate buffer solutions in the pH range 5.7-11.0 by laser-induced optoacoustic spectroscopy (LIOAS, T range 9.5-31.6 degrees C). Upon exciting KP- with 355 nm laser pulses under anaerobic conditions, two components in the LIOAS signals with well-separated lifetimes were found (tau 1 < 20 ns; 250 < tau 2 < 500 ns) in the whole pH range, whereas a long-lived third component (4 < tau 3 < 10 microseconds) was only detected at pH < or = 6.1. The heat and structural volume changes accompanying the first step did not depend on pH or on the presence of buffer. The carbanion resulting from prompt decarboxylation within the nanosecond pulse (< 10 ns) drastically reduces its molar volume ([-18.9 +/- 2.0] cm3/mol) with respect to KP- and its enthalpy content is (256 +/- 10) kJ/mol. At acid pH (ca 6), a species is formed with a lifetime in the hundreds of ns. The enthalpy and structural volume change for this species with respect to KP- are (181 +/- 15) kJ/mol and (+0.6 +/- 2.0) cm3/mol, respectively. This species is most likely a neutral biradical formed by protonation of the decarboxylated carbanion, and decays to the final product 3-ethylbenzophenone in several microsecond. At basic pH (ca 11), direct formation of 3-ethylbenzophenone occurs in hundreds of ns involving a reaction with the solvent. The global decarboxylation reaction is endothermic ([45 +/- 15] kJ/mol) and shows an expansion of (+14.5 +/- 0.5) cm3/mol with respect to KP-. At low pH, the presence of buffer strongly affects the magnitude of the structural volume changes associated with intermolecular proton-transfer processes of the long-lived species due to reactions of the buffer anion with the decarboxylated ketoprofen anion.

Fast events in protein folding: structural volume changes accompanying the early events in the N-->I transition of apomyoglobin induced by ultrafast pH jump

Ultrafast, laser-induced pH jump with time-resolved photoacoustic detection has been used to investigate the early protonation steps leading to the formation of the compact acid intermediate (I) of apomyoglobin (ApoMb). When ApoMb is in its native state (N) at pH 7.0, rapid acidification induced by a laser pulse leads to two parallel protonation processes. One reaction can be attributed to the binding of protons to the imidazole rings of His24 and His119. Reaction with imidazole leads to an unusually large contraction of -82 +/- 3 ml/mol, an enthalpy change of 8 +/- 1 kcal/mol, and an apparent bimolecular rate constant of (0.77 +/- 0.03) x 10(10) M(-1) s(-1). Our experiments evidence a rate-limiting step for this process at high ApoMb concentrations, characterized by a value of (0. 60 +/- 0.07) x 10(6) s(-1). The second protonation reaction at pH 7. 0 can be attributed to neutralization of carboxylate groups and is accompanied by an apparent expansion of 3.4 +/- 0.2 ml/mol, occurring with an apparent bimolecular rate constant of (1.25 +/- 0.02) x 10(11) M(-1) s(-1), and a reaction enthalpy of about 2 kcal/mol. The activation energy for the processes associated with the protonation of His24 and His119 is 16.2 +/- 0.9 kcal/mol, whereas that for the neutralization of carboxylates is 9.2 +/- 0.9 kcal/mol. At pH 4.5 ApoMb is in a partially unfolded state (I) and rapid acidification experiments evidence only the process assigned to carboxylate protonation. The unusually large contraction and the high energetic barrier observed at pH 7.0 for the protonation of the His residues suggests that the formation of the compact acid intermediate involves a rate-limiting step after protonation.

Ground-State Properties and Excited-State Reactivity of 8-Quinolate Complexes of Ruthenium(II)

In an effort to explore new systems with highly reducing excited states, we prepared a series of Ru(II) complexes of the type Ru(L)(2)quo(+) (L = bpy (2,2'-bipyridine), phen (1,10-phenanthroline), dmphen (4,7-dimethyl-1,10-phenanthroline), tmphen (3,4,7,8-tetramethyl-1,10-phenanthroline); quo(-) = 8-quinolate) and investigated their photophysical and redox properties. The absorption and emission spectra of the Ru(L)(2)quo(+) are significantly red-shifted relative to those of the parent complexes Ru(L)(3)(2+), with emission maxima in the 757-783 nm range in water. The Ru(L)(2)quo(+) systems are easily oxidized with E(1/2)(Ru(III/II)) values ranging from +0.62 to +0.70 V vs NHE, making the emissive Ru --> phen MLCT (metal-to-ligand charge transfer) excited states (E(00) approximately 1.95 eV in CH(3)CN) of the Ru(L)(2)quo(+) complexes significantly better reducing agents than the MLCT states of the parent Ru(L)(3)(2+) complexes. Emission lifetimes of 17.0 and 32.2 ns were measured for Ru(phen)(2)quo(+) in water and acetonitrile, respectively, and 11.4 ns for Ru(bpy)(2)quo(+) in water. Transient absorption results are consistent with the formation of reduced methyl viologen upon Ru(phen)(2)quo(+) excitation with visible light in water. The possibility of observing the Marcus inverted region in the forward bimolecular electron transfer reaction from the highly reducing Ru(phen)(2)quo(+) excited state was explored with neutral electron acceptors with reduction potentials ranging from +0.25 to -1.15 V vs NHE.