Temperature influence on the energy of nonspecific and specific interactions taking place between 4-aminophthalimide (4-AP) and homogeneous solvents.

Temperature induced spectral shifts of the 4-aminophthalimide (4-AP) emission spectra have been measured and compared to the predictions of the McRae solvent induced shift theory (J. Phys. Chem., 1957, 61, 562-572). Three moderately polar chloroalkanes selected as nonspecifically interacting media, and six hydrogen accepting or/and electron pair donating solvents have been used as the media in which the temperature influence on 4-AP-solvent interactions has been studied in the range of 180-320 K. Using the ab initio determined 4-AP ground state dipole moment and fitting appropriate expression originating from the mentioned theory to the shifts found in the chloroalkanes it has been possible to estimate the 4-AP excited state dipole moment, the probe excited state Onsager radius and its gas phase emission spectrum position. Using these values the thermochromic shifts of 4-AP emission spectra in hydrogen bond forming solvents have been predicted and compared to the experimental one. Temperature has been found to have different impact on the changes, upon excitation of the probe, in the mean values of the energies of different hydrogen bonds formed by 4-AP with solvents molecules.     

Photophysical studies of 3,3' dioctadecyloxacarbocyanine dye in model biological membranes and different solvents

The absorption and fluorescence spectra of 3,3'-dioctadecyloxacarbocyanine [DiOC18(3)], a cationic oxacarbocyanine dye have been studied in aqueous and nonaqueous media containing egg phosphatidylcholine (PC) as well as in different solvents of diverse nature. The results show the evidence of complex formation of the dye in the ground and in the excited states with PC. The excited state interaction of the dye with PC suggests the electron transfer from PC to dye and this is supported by photovoltage generation in a photoelectrochemical cell consisting of dye and PC in aqueous medium. An attempt has been made to determine the polarity of the microenvironment of the dye in PC liposome or PC reverse micelle from the spectral studies of the dye in different solvents of known polarity.     

Complexation with albumins of chiral aromatic substrates and their chemistry in ground and excited states. Catalytic and chirality recognition properties of the protein in the cases of binaphthol, its photoisomers, and ketoprofen.

The reactivity of organic molecules can be modified upon complexation with proteins: these changes can be different and more significant when the substrate is in an electronically excited state. Here we review UV, CD, and fluorescence spectroscopy studies on the photochemistry and on the chemistry of atropisomeric binaphthols and of ketoprofen, complexed to serum albumins. The chemical and photochemical properties of the organic substrates, complexed to the albumins or free in common solvents, are different. The role of the protein complexation is also evidenced in photoresolution processes of racemate-protein complexes. Catalytic effects due to serum albumins are also reported. In particular, the Arrhenius parameters for the rate of thermal isomerization of a metastable photoproduct of binaphthol in common solvents are compared with those of the bovine serum albumin catalyzed isomerization.     

The PsbU subunit of photosystem II stabilizes energy transfer and primary photochemistry in the phycobilisome-photosystem II assembly of Synechocystis sp. PCC 6803

The PsbU subunit of photosystem II (PSII) is one of three extrinsic polypeptides associated with stabilizing the oxygen evolving machinery of photosynthesis in cyanobacteria. We investigated the influence of PsbU on excitation energy transfer and primary photochemistry by spectroscopic analysis of a PsbU-less (or deltaPsbU) mutant. The absence of PsbU was found to have multiple effects on the excited state dynamics of the phycobilisome and PSII. DeltaPsbU cells exhibited decreased variable fluorescence when excited with light absorbed primarily by allophycocyanin but not when excited with light absorbed primarily by chlorophyll a. Fluorescence emission spectra at 77 K showed evidence for impaired energy transfer from the allophycocyanin terminal phycobilisome emitters to PSII. Picosecond fluorescence decay kinetics revealed changes in both allophycocyanin and PSII associated decay components. These changes were consistent with a decrease in the coupling of phycobilisomes to PSII and an increase in the number of closed PSII reaction centers in the dark-adapted deltaPsbU mutant. Our results are consistent with the assumption that PsbU stabilizes both energy transfer and electron transport in the PBS/PSII assembly.     

Switch between charge transfer and local excited states in 4-aminophenyl-substituted Hantzsch 1,4-dihydropyridine induced by pH change and transition metal ions.

The absorption and fluorescence spectra of a Hantzsch 1,4-dihydropyridine derivative bearing a N,N-dimethylaminophenyl group at 4-position (H(2)Py-PhN(CH(3))(2)) in aprotic solvents have been examined and compared to model compounds 4-phenyl- and 4-methyl-substituted Hantzsch 1,4-dihydropyridines (H(2)Py-Ph and H(2)Py-Me). While H(2)Py-Ph and H(2)Py-Me show fluorescence around 420 nm from the local excited state of the dihydropyridine chromophore, H(2)Py-PhN(CH(3))(2) exhibits fluorescence around 520 nm from the intramolecular charge transfer (ICT) state involving the aniline and dihydropyridine groups as donor and acceptor, respectively. Upon addition of an acid to the solution of H(2)Py-PhN(CH(3))(2), the amino group in the aniline is protonated. Thus, the photoinduced intramolecular charge transfer is prevented, and only the fluorescence from the local excited state of the dihydropyridine chromophore can be detected. These changes in the fluorescence behavior are fully reversible: subsequent addition of a base to the acidic solution leads to the recovery of the ICT fluorescence and the quenching of the local fluorescence. Transition metal ions also can switch the fluorescence of H(2)Py-PhN(CH(3))(2). Evidence for the interaction between transition metal ions and the amino group in the dimethylaniline have been provided by absorption and emission spectrum as well as NMR studies.     

Role of beta-carotene in the reaction centres of photosystems I and II of spinach chloroplasts prepared in non-polar solvents.

Spinach chloroplasts have been prepared nonaqueously using non-polar solvents (n-hexane, CCl4, n-hepatane) and the beta-carotene content extracted in a controlled manner. This procedure is reproducible and does not result in large structural or spectral changes of the chloroplasts. The organisation of the chlorophyll-proteins is unaltered, as fragmentation with digitonin results in the appearance of the same fractions as found previously for aqueously-prepared chloroplasts, including the pink zone containing cytochromes f and b6 in the ratio 1 : 2. The chloroplasts possess both Photosystem I activity (P-700 photo-bleaching, and NADP+ photoreduction) and Photosystem II activity (parabenzoquinone reduction with Mn2+ as electron donor, and chlorophyll fluorescence induction). Use of moderate intensity red illumination has allowed a study of the role of beta-carotene in photochemistry separate from its roles in energy transfer and photoprotection. Removal of the fraction of beta-carotene closely associated with the Photosystem I reaction centre caused the rate of NADP+ photoreduction to fall to a low, but significantly non-zero level. Thus, in the complete absence of beta-carotene, photochemistry can still be observed, however the specific association of beta-carotene with the reaction centre is required for maximal rates. We propose that beta-carotene bound at the reaction centre decreases the rate of transfer of excitation energy away from the reaction centre, and increases the rate of photochemistry. It is possible that this occurs via formation of an exciplex between ground state beta-carotene and chlorophyll in the first excited state.     

Laurdan solvatochromism: solvent dielectric relaxation and intramolecular excited-state reaction.

Absorption, steady-state, and time-resolved fluorescence measurements have been performed on laurdan dissolved either in white viscous apolar solvents or in ethanol as a function of temperature. The heterogeneity of the absorption spectra in white oils or in ethanol is consistent with semiempirical calculations performed previously on Prodan. From steady-state and time-resolved fluorescence measurements in apolar media, an excited state reaction is evidenced. The bimodal lifetime distribution determined from the maximum entropy method (MEM) analysis is attributed to the radiative deexcitation of a "locally excited" (LE) state and of a "charge transfer" (CT) state, whereas a very short component (20 ps), the sign and the amplitude of which depend on the emission wavelength, is attributed to the kinetics of the interconvertion reaction. The observation of an isoemissive point in the temperature range from -50 degrees C to -110 degrees C in ethanol suggests an interconvertion between two average excited-state populations: unrelaxed and solvent-relaxed CT states. A further decrease in temperature (-190 degrees C), leading to frozen ethanol, induces an additional and important blue shift. This low temperature spectrum is partly attributed to the radiative deexcitation of the LE state. Time-resolved emission spectra (TRES) measurements at -80 degrees C in the ethanol liquid phase show a large spectral shift of approximately 2500 cm(-1) (stabilization energy of the excited state: 7.1 kcal x M(-1)). The time-dependent fluorescence shift (TDFS) is described for its major part by a nanosecond time constant. The initial part of the spectral shift reveals, however, a subnanosecond process that can be due to fast internal solvent reorientation and/or to intramolecular excited-state reactions. These two relaxation times are also detected in the analysis of the fluorescence decays in the middle range of emission energy. The activation energy of the longest process is approximately 3 kcal x M(-1). At -190 degrees C, one subnanosecond and one nanosecond excited-state reactions are also evidenced. They are likely due to intramolecular rearrangements after the excitation, leading to the CT state and not to solvent relaxation, which is severely hindered in these temperature conditions. Therefore, both intramolecular and solvent relaxations are responsible for the large Stokes shift displayed by this probe as a function of solvent polarity. A possible scheme is proposed for the deexcitation pathway, taking into account the kinetics observed in these different solvents.     

Excited state properties of bilirubin and its photoproducts using picosecond flourescence and ciruclarly polarized luminescence spectroscopy

We have used picosecond time-correlated photon counting to investigate the excited state decay of bilirubin in dioctadecyldimethylammonium (DODAC) vesicles, human and bovine serum albumin, organic solvents and chloroform/water mixtures. In each case a biexponential fluorescence decay τ₁ of 20–90 ps and 99% initial intensity and τ₂ of 1–2 ns, depending on the system, were observed. The fraction of the longer decay increased with observation wavelength. On irradiation of bilirubin in DODAC vesicles or CHCl₃/H₂O solutions the percentage of the longer fluorescence decay in the total fluorescence increased, on prolonged irradiation this percentage decreased as it did when solutions were left in the dark. These observations are interpreted in terms of bilirubin photoisomerisation from the first singlet state, the longer decays being due to the photoproduct isomers, E-Z, Z-E and E-E. Circularly polarised fluorescence spectra are also suggestive of more than one species contributing to the measured fluorescence. One species has the same conformation as bilirubin, the other, which emits at longer wavelengths, has a different excited state conformation.     

The fluorescence from the chromophore of the purple membrane protein.

The fluorescence from the purple membrane protein (PM) of Halobacterium halobium and its relation to the primary photochemical events have been studied. The emission spectrum at 77 degrees K has structure, with peaks at 680, 710-715, and 730-735 nm. The excitation spectrum shows a single peak centered at 580 nm. This and a comparison of the fluorescence intensity at 77 degrees K under a variety of conditions with the amounts of the bathoproduct (or K, the only photoproduct seen at this temperature) formed suggest that the source of the fluorescence is the purple membrane itself, not the photoproduct. From the difference in several of their properties, we suggest that the fluorescing state of the pigment is different from the excited state which leads to photoconversion.     

Characterisation of the lowest singlet and triplet excited states of S-flurbiprofen.

The photophysical properties of S-flurbiprofen [S-2-fluoro-alpha-methyl-4-biphenylacetic acid], a nonsteroidal anti-inflammatory drug, have been examined using steady-state and time-resolved spectroscopic techniques. The energy of its first singlet excited state is 99 kcal mol(-1). The fluorescence quantum yields and lifetimes (at 300 nm) have been determined in acetonitrile, methanol, hexane and PBS; they are in the range 0.15相似文献   

Calorimetric and Fourier transform infrared spectroscopic study of solid proteins immersed in low water organic solvents.

Calorimetric heat effects and structural rearrangements assessed by means of Fourier transform infrared (FTIR) amide I spectra were followed by immersing dry human serum albumin and bovine pancreatic alpha-chymotrypsin in low water organic solvents and in pure water at 298 K. Enthalpy changes upon immersion of the proteins in different media are in a good linear correlation with the corresponding IR absorbance changes. Based on calorimetric and FTIR data the solvents were divided into two groups. The first group includes carbon tetrachloride, benzene, nitromethane, acetonitrile, 1,4-dioxane, n-butanol, n-propanol and pyridine where no significant heat evolution and structural changes were found during protein immersion. Due to kinetic reasons no significant protein-solvent interactions are expected in such systems. The second group of solvents includes dimethyl sulfoxide, methanol, ethanol, and water. Immersion of proteins in these media results in protein swelling and involves significant exothermic heat evolution and structural changes in the protein. Dividing of different media in the two groups is in a qualitative correlation with the solvent hydrophilicity defined as partial excess molar Gibbs free energy of water at infinite dilution in a given solvent. The first group includes the solvents with hydrophilicity exceeding 2.7 kJ/mol. More hydrophilic second group solvents have this energy values less than 2.3 kJ/mol. The hydrogen bond donating ability of the solvents also assists in protein swelling. Hydrogen bonding between protein and solvent is assumed to be a main factor controlling the swelling of dry solid proteins in the studied solvents.     

Hydrophobic acridine dyes for fluorescence staining of mitochondria in living cells. 1.Thermodynamic and spectroscopic properties of 10-n-alkylacridine orange chlorides

10-n-Alkyl-acridine-orange-chlorides (alkyl-AOs) are excellent dyes for fluorescence staining of mitochondria in living cells. The thermodynamic and spectroscopic properties of the series alkyl = methyl to nonyl have been investigated. The dyes form dimers in aqueous solution. The dimerisation is mainly a consequence of the hydrophobic interaction. The dissociation constant K respectively association constant K-1 of the dimers describes the hydrophobic interaction and therefore the hydrophobic properties of the dye cations. The dissociation constant K = K0 at the standard temperature T = 298 K has been determined spectroscopically in aqueous solution. It depends on the length of the alkyl residue n-CmH2m + 1 (m = 1 - 9) (Table 2). In addition the standard dissociation enthalpies (energies) delta H0 and dissociation entropies delta S0 have been determined from the temperature dependence of K (Table 2). With increasing chain length m the thermodynamic parameters K0, delta H0, delta S0 decrease. Therefore with growing m the dimers are stabilized. This stabilization is an entropic effect which is diminished by the energetic effect. The change of the thermodynamic parameters with m is in agreement with the concept of hydrophobic interaction and the stabilization of water structure in the surroundings of hydrophobic residues. As one would expect nonyl-AO is the most hydrophobic dye of the series. As an example the spectroscopic properties of nonyl-AO have been determined. We measured the absorption, luminescence and polarization spectra in rigid ethanol at 77 K. Under these conditions alkyl-AOs associate like dyes in Water at room temperature. The spectra depend on the concentration of the solution. In very dilute solution we observe mainly the spectra of the monomers M, in concentrated solution the spectra of the dimers D. The spectra of M and D are characteristically different. The monomers have one long wave length absorption M1 = 20.000 cm-1 with resonance fluorescence. In addition there is a long living phosphorescence at 16.600 cm-1. Its polarization is nearly perpendicular to the plane of the AO residue. The dimers have two long wave length absorption bands D1 = 18.700 and D2 = 21.200 cm-1 with very different intensities. D1 has very low intensity and is forbitten, D2 is allowed. D1 shows fluorescence. Phosphorescence has not been observed. D1, D2 and also M1 are polarized in the plane of the AO residue. At short wave length absorption and polarization spectra are very similar. From the spectra we constructed the energy level diagram of M and D (Fig. 9). The first excited state of M splits in D in two levels. The level splitting and the transition i     

Picosecond kinetics of excited-state charge separation in 4-(dimethylamino)pyridine.

4-(Dimethylamino)pyridine (DMAP) shows solvent-dependent dual fluorescence from the initially excited state B* and a highly polar TICT state A*. Room-temperature time-resolved picosecond fluorescence investigations prove the bimodal kinetics of the excited-state electron transfer reaction B*-->A* in polar aprotic media. In medium polarity solvents (such as ethyl acetate) two emitting states of DMAP are shown to reach equilibrium within 50 ps. Both emitting states originate from the same ground state. The rate of excited-state charge separation depends on polarity and proton donating ability of the surrounding medium. The effects of temperature on the quantum yields of both fluorescences of DMAP in polar aprotic media indicate the transition from the kinetic regime (at low temperatures) to the equilibrium regime (at high temperatures). The kinetic behaviour of the dual luminescence of DMAP in protic solvents is more complex than in aprotic ones. In alcohols an efficient nonradiative channel competes with excited-state charge separation.     

A photophysical model for diphenylhexatriene fluorescence decay in solvents and in phospholipid vesicles.

The fluorescence decay of 1,6-diphenyl-1,3,5-hexatriene (DPH) in pure solvents and in phospholipid vesicles has been measured using frequency domain fluorometry. Data analysis uses a model with two energetically close excited states. The model explains the high quantum yield and the double exponential decay of DPH observed in some pure solvents and in phospholipid vesicles. This model assumes that after excitation to a first excited state, there is a rapid interconversion to a lower excited state and that most of the emission occurs from this state. The interconversion rates between the two excited states determine the average lifetime. For DPH in solvents, we find that the interconversion rates are solvent and temperature dependent. For DPH in phospholipid vesicles, we find that the back reaction rate from excited state 2 to excited state 1 (R12) is what determines the fluorescence properties. The phospholipid phase transition affects only this back reaction rate. The model was analyzed globally for a range of solvents, temperatures and vesicle composition. Of the six parameters of the model, only two, the interconversion rates between the two excited states, varied in all different samples examined. For DPH in phospholipid vesicles, there is an additional feature of the model, which is related to the apparent distribution of the rate R12. Significantly better fits were obtained using a continuous lorentzian distribution of interconversion rates. The resulting lifetime distribution was asymmetric and showed a definite narrowing above the phase transition.     

Interpreting the visible absorption bands of 1,4-(dihydroxy)-9,10-anthraquinone and its metal chelates

The visible absorption spectra of 1,4-(dihydroxy)-9,10-anthraquinone and of Co(II), Ni(II), Cu(II) and Zn(II) chelates have been studied in different organic solvents. This system provides a model for the anthracycline antibiotics and their metal chelates. The band structure of the spectrum has been determined using the second and fourth derivatives of the spectrum. The visible absorption band of the parent molecule can be assigned to a single electronic state with a reduced dipole moment in the excited state; structure in this band is ascribed to two overlapping vibrational progressions. In contrast, the dianion (hydroxy protons removed) shows a single electronic state with an increased dipole moment in the excited state; structure in this band can be assigned to a single vibrational progression. All of the metal chelates show spectra which are similar in appearance to that of the dianion although the identity of the metal determines the bathochromic shift of the absorption band. Titration of 1,4-dihydroxyanthraquinone with Cu(ClO4)2.6H2O demonstrates that three chelates with metal-to-ligand ratios of 1:2, 1:1 and 2:1 can form depending on the identity of the metal, ratio of metal to ligand, and donor character of the solvent.     

Photophysics of thionine dye in aqueous and liposome media in presence of different reducing agents.

The photoelectrochemical and spectral (both absorption and fluorescence) studies of thionine, a cationic phenothiazine dye, have been carried out in aqueous and phosphatidylcholine liposome media in the presence of different reducing agents, such as I(-), Br(-), Cl(-) and Fe(2+). The results show that the photovoltage generation from photoelectrochemical studies and Stern-Volmer quenching constant studied by fluorescence quenching support the photoinduced electron transfer from the reducing agent to the singlet excited thionine dye. Moreover, a good correlation between photovoltages/Stern-Volmer quenching constants vs. reduction potentials of the reducing agents also confirms the above electron transfer in the photoexcited state.     

Intramolecular charge transfer processes in donor-acceptor substituted vinyltetrahydropyrenes.

Two novel donor-acceptor-substituted vinyltetrahydropyrene derivatives, 2-N,N-dimethylamino-7-(1-carbethoxyvinyl)-4,5,9,10-tetrahydropyrene, , and 2-N,N-dimethylamino-7-(1,1-dicyanovinyl)-4,5,9,10-tetrahydropyrene, , were synthesized and their photophysical properties investigated in solvents of different polarities. Our studies revealed the existence of intramolecular charge transfer excited states in these molecules. For both compounds the fluorescence maxima exhibited solvent polarity-dependent red shifts. These were quantitatively analysed by the Lippert-Mataga and Liptay equations to obtain the excited state dipole moments. Our results indicated that in the case of , emission takes place from a planar (1)CT state in all non-protic solvents. In the case of , the nature of the excited state depends on the solvent. A fast relaxation to a triplet state is proposed in cyclohexane. The emitting state in medium polar solvents is a planar (1)CT state. In highly polar solvents a twisted (1)CT state is invoked to explain the low fluorescence quantum yield. For both compounds CT nature of the emitting states were further confirmed by studies in acidic medium. The ground and excited state pK(a) values for and were determined using absorption and emission spectral changes observed in the presence of protic acids.     

The photophysics of nitrocarbazoles studied by using spectroscopic,photoacoustic and luminescence techniques

The photophysical behaviour of 1-nitrocarbazol and 3-nitrocarbazol in different organic solvents has been studied by using spectroscopic, luminescence and photoacoustic techniques. Absorption spectra and triplet state transient spectra were recorded and formation quantum yields and lifetimes determined. Phosphorescence emission at 77 K and laser induced luminescence in acetonitrile solution at 298 K for the nitrocarbazoles were studied. Results are discussed in terms of the optimized structure of 1-nitrocarbazole and 3-nitrocarbazole and compared with that of carbazole. The nitro group attached to the carbazole moiety induces important changes in the photophysical behavior of those compounds.     

Protic solvent effects on the photophysical properties of O=Ti(IV)TSPP: photoinduced electron transfer.

The photophysical properties of oxotitanium(IV)meso-tetra(4-sulfonatophenyl) porphyrin (O=Ti(IV)TSPP) have been investigated in water and methanol by laser spectroscopic techniques. The fluorescence emission spectrum of O=Ti(IV)TSPP in methanol exhibits two strong emission bands at 610 and 670 nm at room temperature with the decay time of ca. 310 +/- 10 ps and the rise time shorter than 30 ps, in contrast to the extremely weak emission with the decay time of ca. 27 +/- 4 ps in water, indicating that the fluorescence emissive states are different in the two solvents as supported by the solvent dependences of the excitation spectrum. The transient Raman spectra of O=Ti(IV)TSPP in water has been observed to exhibit a remarkable enhancement of phenyl-related mode at 1599 cm(-1), while in methanol, the Raman frequencies of the porphyrin skeletal modes (upsilon2 and upsilon4) are down-shifted without any apparent enhancement of the phenyl-related mode, indicating different interactions of the two solvents with the excited O=Ti(IV)TSPP. These Raman studies reveal that methanol molecule interacts with the photoexcited O=Ti(IV)TSPP more strongly than water, forming the exciplex, O=Ti(IV)TSPP(MeOH)*, suggesting that the two different emissive states are the singlet Franck-Condon state and the exciplex state in methanol and water, respectively. A broad triplet transient absorption of O=Ti(IV)TSPP has been also observed at 480 nm in water as well as in methanol, which is decreased upon addition of methyl viologen (MV2+) with appearance of a new absorption band at 620 nm. This indicates that the photoinduced electron transfer (PET) takes place from the porphyrin to MV2+ in both solvents. The kinetic analysis of the transient absorption band exhibits the PET rate constants of 4.76 x 10(5) s(-1) and 3,03 x 10(4) s(-1) in methanol and water, respectively. All these results infer that the PET takes place from the (d,pi) CT state and the triplet state of the excited porphyrin in methanol and water, respectively.     

Excited state intramolecular proton transfer (ESIPT) in dihydroxyphenyl anthracenes

The photochemistry of three 9-(dihydroxyphenyl)anthracenes 6-8 was studied in neat CH(3)CN and selected organic solvents, to investigate excited state intramolecular proton transfer (ESIPT) from the phenol to the anthracene moiety. In D(2)O-CH(3)CN mixtures, the observed deuterium exchange of 6-8 is consistent with water-mediated (formal) ESIPT process from the ortho phenolic OH to the 10'-position of the anthracene ring, giving rise to quinone methide (QM) intermediates 12-14. There is no ESIPT for the corresponding methoxy-substituted compounds. Introduction of an extra hydroxyl group onto the phenol ring at different positions led to a range of deuterium exchange quantum yields (Φ = 0.03 to 0.15). In addition to the anticipated ESIPT process to the 10'-position, in neat CH(3)CN and other organic solvents, 6 (but not 7 or 8) undergoes a clean photocyclization to give bridged product 19 in quantitative yield. The mechanism of this unique photocyclization may involve a direct ESIPT or a 1,4-hydrogen transfer from the ortho phenolic OH to the 9'-position of the anthracene ring, generating a zwitterion (20) or diradical (21) intermediate, respectively, followed by ring closure. Fluorescence studies of 6 in various solvents show the existence of both local excited and intramolecular charge transfer states whereas only the former was present for 7 and 8, offering a possible rationalization for the photocyclization pathway.