Search

Conventional linear and time-resolved spectroscopic techniques are often not appropriate to elucidate specific pigment-pigment interactions in light-harvesting pigment-protein complexes (LHCs). Nonlinear (laser-) spectroscopic techniques, including nonlinear polarization spectroscopy in the frequency domain (NLPF) as well as step-wise (resonant) and simultaneous (non-resonant) two-photon excitation spectroscopies may be advantageous in this regard. Nonlinear spectroscopies have been used to elucidate substructure(s) of very complex spectra, including analyses of strong excitonic couplings between chlorophylls and of interactions between (bacterio)chlorophylls and "optically dark" states of carotenoids in LHCs, including the major antenna complex of higher plants, LHC II. This article shortly reviews our previous study and outlines perspectives regarding the application of selected nonlinear laser-spectroscopic techniques to disentangle structure-function relationships in LHCs and other pigment-protein complexes.

The present study shows that small admixtures of one chlorophyll a (Chla) molecule per several hundred lipid molecules have strong destabilizing effect on lipid bilayers. This effect is clearly displayed in the properties of the L(alpha)-H(II) transformations and results from a Chla preference for the H(II) relative to the L(alpha) phase. Chla disfavors the lamellar liquid crystalline phase L(alpha) and induces its replacement with inverted hexagonal phase H(II), as is consistently demonstrated by DSC and X-ray diffraction measurements on phosphatidylethanolamine (PE) dispersions. Chla lowers the L(alpha)-H(II) transition temperature (42 degrees C) of the fully hydrated dipalmitoleoyl PE (DPoPE) by approximately 8 degrees C and approximately 17 degrees C at Chla/DPoPE molar ratios of 1:500 and 1:100, respectively. Similar Chla effect was recorded also for dielaidoyl PE dispersions. The lowering of the transition temperature and the accompanying significant loss of transition cooperativity reflect the Chla repartitioning and preference for the H(II) phase. The reduction of the H(II) phase lattice constant in the presence of Chla is an indication that Chla favors H(II) phase formation by decreasing the radius of spontaneous monolayer curvature, and not by filling up the interstitial spaces between the H(II) phase cylinders. The observed Chla preference for H(II) phase and the substantial bilayer destabilization in the vicinity of a bilayer-to-nonbilayer phase transformation caused by low Chla concentrations can be of interest as a potential regulatory or membrane-damaging factor.

The primary excited state processes of phytochrome, the plant chromoprotein responsible for most photomorphogenetic responses, are investigated by picosecond intensity-dependent transmission measurements. At sufficient high excitation intensity (starting at photon flux densities of about 1′1025 cm−2 s−l) a moderate bleaching of the absorption is observed. For the quantitative analysis of the experimental data a rate equation system approach for the energy levels responsible for the ultrafast transformation Pr lumi-R and the photon transport through the sample was utilized. The following was found for the excited state parameters of the phytochrome Pr: (1) an excited state absorption of the same order of magnitude as the ground state absorption (cross section: Sexc= 4′10−16 cm−2 at 650 nm), (2) a fast relaxation of the higher excited singlet state on the femtosecond time scale.

Symmetrically ( 1 – 4 ) and unsymmetrically ( 5 – 7 ) benzoannelated metal-free porphyrazines bearing tert -butyl substituents have been investigated in regard to their photophysical properties and their splitting behavior of the Q -band. Q -band splitting in the series of symmetrical tetraazaporphyrine ( 1 ), tetrabenzoporphyrazine ( 2 ), tetranaphthoporphyrazine ( 3 ), is decreased from ΔQ ∼2000 to ∼0 cm −1 while the tetraanthraporphyrazine system ( 4 ) shows a low splitting of about Δ Q =265 cm −1 again. In the unsymmetrically substituted series in going from tribenzomononaphthoporphyrazine ( 6 ) (Δ Q =903 cm −1 ) to tribenzomonoanthraporphyrazine ( 7 ) (Δ Q =875 cm −1 ) Q -band splitting is very similar while tribenzoporphyrazine ( 5 ) (Δ Q =2073 cm −1 ) exhibits a drastic increase. In the series 1 – 4 , fluorescence lifetimes 2.6, 5.5, 3.0, 0.5 ns, fluorescence quantum yields 0.17, 0.44, 0.19, 0.01 and singlet oxygen quantum yields 0.53, 0.21, 0.03, −4 were determined. Fluorescence lifetimes of 5 – 7 are in the range 3.1–4.7 ns. The absorption- and fluorescence spectra as well as the fluorescence lifetimes of the separated isomers of 5 do not differ.

Stepwise two-photon excitation of chlorophyll a and b in the higher plant main light-harvesting complex (LHC II) and the minor complex CP29 (as well as in organic solution) with 100-fs pulses in the Q(y) region results in a weak blue fluorescence. The dependence of the spectral shape of the blue fluorescence on excitation wavelength offers a new approach to elucidate the long-standing problem of the origin of spectral "chlorophyll forms" in pigment-protein complexes, in particular the characterization of chlorophyll a/b-heterodimers. As a first result we present evidence for the existence of strong chlorophyll a/b-interactions (excitonically coupled transitions at 650 and 680 nm) in LHC II at ambient temperature. In comparison with LHC II, the experiments with CP29 provide further evidence that the lowest energy chlorophyll a transition (at approximately 680 nm) is not excitonically coupled to chlorophyll b.

Replacement of the central Mg in chlorophylls by Ni opens an ultrafast (tens of femtoseconds time range) radiationless de-excitation path, while the principal ground-state absorption and coordination properties of the pigment are retained. A method has been developed for substituting the native bacteriochlorophyll a by Ni-bacteriochlorophyll a ([Ni]-BChl) in the light harvesting antenna of the core complex (LH1) from the purple bacterium, Rhodobacter (Rb.) sphaeroides, to investigate its unit size and excited state properties. The components of the complex have been extracted with an organic solvent from freeze-dried membranes of an LH1-only strain of Rb. sphaeroides and transferred into the micelles of n-octyl-beta-glucopyranoside (OG). Reconstitution was achieved by solubilization in 3.4% OG, followed by dilution, yielding a complex nearly identical to the native one, in terms of absorption, fluorescence, and circular dichroism spectra as well as energy transfer efficiency from carotenoid to bacteriochlorophyll. By adding increasing amounts of [Ni]-BChl to the reconstitution mixture, a series of LH1 complexes was obtained that contain increasing levels of this efficient excitation trap. In contrast to the nearly unchanged absorption, the presence of [Ni]-BChl in LH1 markedly affects the emission properties. Incorporation of only 3.2 and 20% [Ni]-BChl reduces the emission by 50% and nearly 100%, respectively. The subnanosecond fluorescence kinetics of the complexes were monoexponential, with the lifetime identical to that of the native complex, and its amplitude decreasing in parallel with the steady-state fluorescence yield. Quantitative analysis of the data, based on a Poisson distribution of the modified pigment in the reconstituted complex, suggests that the presence of a single excitation trap per LH1 unit suffices for efficient emission quenching and that this unit contains 20 +/- 1 BChl molecules.

A software package, named PRES, is presented for the numerical solution of the coupled system of rate equations and photon transport equation(s) which is useful in nonlinear UV/VIS/NIR laser spectroscopy investigating multiphoton processes of excitation and deactivation in the manifold of electronic excited states. By semidiscretization the system of partial differential equations is transformed into two systems of ordinary differential equations. Because rate equations are often stiff and nonlinear they are solved with a modification of the A-stable trapezoidal rule. For the solution of the photon transport equations a special Runge–Kutta method is developed.

Phytochrome Pr has been investigated by means of picosecond intensity-dependent transmission measurements at several excitation wavelengths inside the S0→S1 absorption band. The experimental data exhibit relatively strong excited-state absorption (ESA) within the whole spectral range of the S0→S1 absorption, which is followed by fast relaxations on the femtosecond time scale (

From the isolated LH2 antenna of Rhodobacter sphaeroides , called B800-850, the bacteriochlorophyll a pigment complement BChl-B800, mainly responsible for the 800 nm absorption band, has been extracted. The remaining complex, with the main NIR absorption peaking at 850 nm (termed pure (p) B850), has a small but distinct sideband in the 800 nm region and contributes there about 20% to the total absorption of the integral B800-850 antenna. In the latter the energy transfer 800→850 nm is per se (at least) biphasic with the pB850 intra-aggregate relaxation as the fastest component known so far ( k ≈10 13 s −1 ).

The instantaneous fluorescence quantum yield φins—a new quantity for fluorescence studies defined as the ratio of the fluorescence intensity to the optical density, both measured at the moment of the maximum of the exciting pulse—proves to be a very sensitive function for excited-state processes. Dependent on the excitation intensity φins exhibits characteristic features (maxima/minima) indicating, for example, excited-state absorptions and annihilation processes. φins is therefore more informative as the intensity dependence of the usually utilized fluorescence yield, the information content of which is restricted because this function is hardly structured. In the paper the influences of specific molecular parameters (excited-state absorption cross section, annihilation constant) on φins are given, problems of the experimental accessibility of φins are discussed, and an experimental setup for determination of this new quantity is presented. The application of the method is demonstrated for identification of excited-state absorptions of organic molecules in solution.

Summary Polyvinylpyrrolidone (PVP) is used for the final purification of phytochrome. If not all traces of PVP are carefully removed the photophysical behaviour of phytochrome can be influenced. The maximum of fluorescence emission at 153 K is shifted by PVP from 686 nm to 680–682 nm. Absorption measurements show that PVP completely blocks the normal phototransformation Pr→lumi-R at this temperature. The same behaviour has been described for ‹membrane›-bound phytochrome in plants. We conclude that PVP can simulate «membrane»-binding of phytochrome.

— Octaphenyltetraanthraporphyrazinato palladium undergoes a self-sensitized photoreaction in the presence of oxygen to form a substituted palladium phthalocyanine with four endoperoxide bridges. This compound exhibits photophysical behavior similiar to palladium tetra-tert-butyl-phthalocyanine. The phthalocyanine-palladium complex with four endoperoxide bridges ejects molecular oxygen when excited by consecutive two-photon absorption in the Q-band region at 662 nm. This photocyclo-reversion, which produces palladium porphyrazines bearing a diminished number of endoperoxide bridges, can occur up to four times per initial molecule. Irradiation of these photoproducts in the presence of oxygen produces substituted palladium phthalocyanine containing four endoperoxide groups.

The properties of one- and two-exciton states of the circular aggregate responsible for the B850 band in the peripheral light harvesting antenna (LH2), extracted from the purple bacterium Rhodopseudomonas acidophila/strain 10050, have been studied at room temperature by non-linear absorption, differential optical density spectroscopy as well as time-resolved fluorescence spectroscopy. As a main result, giant dipole moments for the transition between ground state and the one-exciton band as well as for the transition between the one-exciton band and the two-exciton band have been estimated, corresponding to a delocalization of the excitation energy over most if not all of the 18 BChl a molecules of the aggregate.

The primary relaxation rate of excited phytochrome is measured by time-resolved laser fluorescence techniques at temperatures between 77 and 300 K in a buffer glycerol mixture. The known multiexponential fluorescence decay of the red absorbing form P r at physiological temperatures reduces to a single exponential below 150 K with a lifetime τ of (1.3 ± 0.2) ns at 77 K. This is unambiguously consistent with the dynamics of two excited states which are in thermal equilibrium with each other and represent the initially excited P r and an excited intermediate of the chromophore during the ZE photoisomerization.

In addition to chlorophylls a and b light-harvesting complex (LHC II) binds xanthophylls. Nonlinear polarization spectroscopy in the frequency domain (NLPF) was used to investigate the changes in the interactions between xanthophylls and chlorophylls in LHC II upon alteration of its aggregation state. Additionally, two-photon excitation profiles in the xanthophylls presumed 11Ag − → 21Ag − transition region were measured of LHC II samples containing different xanthophyll complements, as well as of chlorophylls a and b in solution. Implications of the results for recently proposed mechanism(s) of qE/NPQ will be discussed.

The photophysical properties of tetra-(tert-butyl)-phthalocyanato-magnesium (t 4 -PcMg) in solution and microheterogeneous systems (liposomes and micelles) were investigated. Radical cation formation occurs in chloroform during UV excitation in the presence of an electron acceptor (CBr 4 ). The same result is achieved by two-step absorption in the singlet manifold using pulsed excitation at λ exc =670 nm, which is of interest from the viewpoint of photon delivery through the therapeutic window of tissues. To obtain a deeper insight into the photophysics leading to radical cation formation via the higher excited singlet state, the transient spectra and singlet—singlet absorption cross-sections were determined. In addition to strong excited state absorption within the spectral range of the Q x -band, relatively large absorption cross-sections were also found in regions with low ground state absorption. The importance of these transitions for an effective two-colour excitation regime is discussed with regard to new start mechanisms for photodynamic laser tumour theraphy.

Linearly benzo-annulated copper porphyrazines, such as octaphenyltetraanthraporphyrazinato-copper and octapentoxytetranaphthoporphyrazinato-copper, with absorption maxima at about 880 nm, were synthesized. These compounds exhibit very low-lying triplets with an energy below the activation energy of singlet oxygen. Therefore they do not generate singlet oxygen. The transient absorption spectra of the T 1 —T x transition were measured. The solvent dependence of the photophysical parameters in the triplet manifold is discussed. On irradiation in the presence of CBr 4 , π-cation radicals are formed.

By numerical investigations it is shown that the content of information of the instantaneous fluorescence quantum yield, recently defined at the end of a rectangular excitation pulse, is conserved if this quantity is related to the maximum of a proper chosen Gaussian pulse.

Summary Photosynthetically active pigments are usually organized into pigment–protein complexes. These include light-harvesting antenna complexes (LHCs) and reaction centers. Site energies of the bound pigments are determined by interactions with their environment, i.e., by pigment–protein as well as pigment–pigment interactions. Thus, resolution of spectral substructures of the pigment–protein complexes may provide valuable insight into structure–function relationships. By means of conventional (linear) and time-resolved spectroscopic techniques, however, it is often difficult to resolve the spectral substructures of complex pigment–protein assemblies. Nonlinear polarization spectroscopy in the frequency domain (NLPF) is shown to be a valuable technique in this regard. Based on initial experimental work with purple bacterial antenna complexes as well as model systems NLPF has been extended to analyse the substructure(s) of very complex spectra, including analyses of interactions between chlorophylls and “optically dark” states of carotenoids in LHCs. The paper reviews previous work and outlines perspectives regarding the application of NLPF spectroscopy to disentangle structure–function relationships in pigment–protein complexes.

The fluorescence lifetime τF of chlorophyll b (chl b) was measured in various solvents. The excitation wavelength was varied and the fluorescence was measured at various wavelengths in the spectral range 650–710 nm. For freshly prepared pure chl b solutions, τF, does not depend on the excitation and emission wavelengths. However, for contaminated chl b solutions, a significant dependence of τF on the emission wavelength is observed. In this case there are also clear changes in the molar absorption coefficients of the main absorption bands which correlate with the formation of degradation products of chl b.

The excited-state and lasing properties of 20-chloro-chlorophyll a in ether solution were compared to those of chlorophyll a. Desactivation parameters and cross-sections were obtained from non-linear absorption spectroscopy in combination with a physico-mathematical methods package. The Cl substituent at C-20 (1) increases both intersystem crossing and internal conversion, (2) produces a blue-shift of the S1 absorption spectrum, and (3) leads to pronounced photochemistry.

Stepwise two-photon excited fluorescence (TPEF) spectra of the photosynthetic antenna complexes PCP, CP47, CP29, and light-harvesting complex II (LHC II) were measured. TPEF emitted from higher excited states of chlorophyll (Chl) a and b was elicited via consecutive absorption of two photons in the Chl a/b Qy range induced by tunable 100-fs laser pulses. Global analyses of the TPEF line shapes with a model function for monomeric Chl a in a proteinaceous environment allow distinction between contributions from monomeric Chls a and b, strongly excitonically coupled Chls a, and Chl a/b heterodimers/-oligomers. The analyses indicate that the longest wavelength-absorbing Chl species in the Qy region of LHC II is a Chl a homodimer with additional contributions from adjacent Chl b. Likewise, in CP47 a spectral form at approximately 680 nm (that is, however, not the red-most species) is also due to strongly coupled Chls a. In contrast to LHC II, the red-most Chl subband of CP29 is due to a monomeric Chl a. The two Chls b in CP29 exhibit marked differences: a Chl b absorbing at approximately 650 nm is not excitonically coupled to other Chls. Based on this finding, the refractive index of its microenvironment can be determined to be 1.48. The second Chl b in CP29 (absorbing at approximately 640 nm) is strongly coupled to Chl a. Implications of the findings with respect to excitation energy transfer pathways and rates are discussed. Moreover, the results will be related to most recent structural analyses.

The fluorescence of melanin in human skin tissue reflects the structural changes in the process of skin cancer progression towards malignant melanoma. A selectively excitation of melanin in skin tissue, however, is only possible by stepwise two-photon absorption (TPA) in the NIR spectral range (λ exc ≈ 800 nm). Due to the very short lifetime of the energy level populated by the absorption of the first photon (< 5 ps) an effective TPA process can be only achieved with laser pulses on the femtosecond time scale. To use the two-photon excited fluorescence (TPF) of melanin for early diagnosis of malignant melanoma a small, mobile TPF spectrometer was developed. It consists (i) of a fiber laser the radiation of which was amplified in a short pulse nitrogen laser pumped dye cell, (ii) an articulated arm with special mirrors for nearly lossless direction of the fs laser beam to the skin position of investigation, (iii) a camera objective which include visualization and excitation as well as acquisition of fluorescence and (iv) a monochromator with a multichannel photomultiplier and a PC with special software for spectrometer control and data processing. First investigations of the equipment concerning the early diagnosis of malignant melanoma were carried out.

Spectral and time-resolved fluorescence studies of different eumelanins (natural, synthetic, enzymatic) in solution have been carried out by two-photon excitation at 800 nm, using 80 fs pulses with photon flux densities ≤ 1027 cm-2.s-1. Whereas all samples show monotonously decreasing absorption between near UV and near IR, their fluorescence behavior indicates strong heterogeneity. With respect to the also measured one-photon excited fluorescence (OPF) of melanin at 400 nm, the overall spectral shape of the two-photon excited fluorescence (TPF) is red-shifted. Both OPF and TPF exhibit three-exponential decay with a shortest component £ 200 ps. As is also shown, the fluorescence properties of melanin are dependent on the micro-environment. This allows the hypothesis, that the process of malignant transformation in skin tissue could be reflected in the fluorescence, provided the melanin in skin is selectively excited. The latter is realized by the described stepwise absorption of two 800 nm photons. In this way, indeed characteristic differences between the TPF spectra of healthy tissue, nevus cell nevi and malignant melanoma have been found.

By means of fluorescence spectroscopy and nonlinear absorption experiments, excited-state processes of the modified pigments [3-acetyl]-chlorophyll a, [31-OH]-bacteriochlorophyll a and [3-vinyl]-bacteriochlorophyll a were investigated and compared with those of chlorophyll a and bacteriochlorophyll a.

An intrinsic steady-state fluorescent system for bovine adrenodoxin has been developed to study the protein structure in solution and the processes involved in protein unfolding. Since mature Adx contains no natural Trp residue as internal probe, all of the aromatic amino acids, tyrosine at position 82 and four phenylalanines at positions 11, 43, 59 and 64, were at each case replaced by tryptophan. The resulting single tryptophan containing mutants kept their biological function compared with the wild type. Molecular modeling studies verify thermal unfolding experiments which point to a dramatically reduced stability caused by steric hindrance only for mutant F59W. Fluorescence spectra, Stern-Volmer quenching constants, and fluorescence energy transfer calculations indicated the analyzed positions to be situated in solution in the same immediate environment as in the crystal structure. Unfolding experiments with Gdn-HCl and time-resolved stopped-flow measurements provide evidence for differential stability and a chronologically ordered unfolding mechanism of the different fluorescence probe positions in the protein.

A quantitative (multistep) excitation-deactivation model of rose bengal (RB) has been developed which includes energy and electron transfer to oxygen and the possibility of photoproduct formation via higher excited triplet-singlet states. The model is based on previous measurements of non-linear absorption (NLA) and emission of RB with picosecond pulses at 532 nm and on NLA measurements with nanosecond pulses. A coupled rate equation and photon transport equation approach for non-linear light-matter interaction is used. The resulting term scheme with all relevant excited state parameters confirms that (i) in the first excited state of RB, relevant absorption at 532 nm takes place only in the triplet, and (ii) the previously reported intensity dependence of RB-sensitized enzyme inhibition is well modelled by the intensity-dependent RB-T1 population and (as the main process) subsequent energy transfer to form singlet oxygen.

Die photophysikalischen Grundlagen der bisher bekannten beiden Startmechanismen der Photodynamischen Tumortherapie (PDT) beruhen auf einer Ein-Photonen Anregung des Sensibilisators: Nach Besetzung des ersten angeregten Singulettzustandes (S1) wird durch Intersystem Crossing (ISC) der energetisch niedrigste Triplettzustand (T1) des Sensibilisators bevolkert. Von hier aus erfolgt durch Energie- oder Elektronentransfer zum umgebenden Gewebe die Bildung toxischer Photoprodukte. Je nach Art der entstehenden Photoprodukte unterscheidet man zwischen Typ I (Radikalbildung durch Wechselwirkung mit dem Substrat) und Typ II Mechanismen (Bildung von Singulettsauerstoff nach Energieubertragung). Aus photophysikalischer Sicht sollte sich ein optimaler Sensibilisator durch eine moglichst intensive S0 – S1 Absorption im Transmissionsfenster von Gewebe (λ > 650 nm) sowie durch hohe Quantenausbeuten fur die ISC- und Energie- bzw. Elektronentransferprozesse auszeichnen. In der klinischen Anwendung kommt zu diesen Eigenschaften noch die Forderung nach moglichst selektiver Anlagerung im Tumorgewebe hinzu.

Three substituted tetraazaporphyrins, octa-(4-tert-butylphenyl)-tetrapyrazinoporphyrazine, tetra-(4-tert-butyl)phthalocyanine and tetra-(4-tert-butyl)phthalocyanatomagnesium(t4-PcMg), were spectroscopically checked in solutions and liposomes with respect to suitability as potential sensitizers of a possible new start mechanism for photodynamic therapy (PDT) from a stepwise excited higher singlet state. This PDT start mechanism was recently proposed to overcome the problem of O2 (1delta2)-caused cutaneous phototoxicity in PDT. By means of absorption and fluorescence measurements as well as nonlinear absorption investigation, transient spectroscopy and lasing experiments, compound t4-PcMg was found to have the most appropriate properties: Based on a high fluorescence quantum yield (phiF1 = 0.84) and a very low crossing to the triplet (phiISC = 0.05), two higher excited states can be effectively populated both by two stepwise absorption transitions at 674nm consecutive absorption transitions at 684 and and 710 nm. Moreover, t4-PcMg incorporates into liposomes very well with spectroscopic properties similar to those in solution.

The relaxation rate of primary excited native (124kDa) phytochrome from oat, is measured by time-resolved fluorescence techniques at temperatures between 77 and 300 K in a buffer/glycerol mixture. The known multiexponential fluorescence decay of the red absorbing form P r is found at physiological temperatures while it can be characterized by a single exponential below 160 K with a lifetime of (1.3 ± 0.2) ns at 77 K. This is consistent with the dynamics of the initially excited P r and an excited intermediate of the chromophore during the Z-E photoisomerization.

Fluorescence of synthetic melanin in the solvents H2O, KOH, ethylene glycol monomethyl ether, and dimethyl sulfoxide has been excited by two-photon absorption at 800 nm, using 120-fs pulses with photon flux densities of ≥1027 cm−2.S−1. Compared to the one-photon (400-nm)-induced fluorescence of melanin, the overall spectral shape is red-shifted and shows a strong environment sensitivity. The decay of the two-photon-induced fluorescence (TPF) of melanin is three-exponential, with a shortest main component of about 200 ps. The results of the TPF studies in line with the unique light absorption property of melanin of a monotonously decreasing absorption spectrum between the near UV-region and the near infrared region indicate that the TPF is realized via stepwise absorption of two 800-nm photons. In comparison to the simultaneous absorption of two photons, the stepwise process needs lower photon flux densities to get a sufficient population of the fluorescent level. This stepwise process offers new possibilities of selective excitation of melanin in skin tissue in a spectral region where there is no overlap with any absorption of another fluorescent tissue component. The first results with different samples of excised human skin tissue (healthy, nevus cell nevi, malignant melanoma) suggest that fluorescence excited in this way yields information on malignant transformation.

Fluorescence of synthetic melanin in dimethyl sulfoxide has been excited by two-photon absorption at 800 nm, using 120 fs pulses with photon flux densities > or = 10(27) cm-2 s-1. The shortest main component of the three-exponential decay of fluorescence is 200 +/- 2 ps. The overall spectral shape is red-shifted with respect to the 400 nm excited fluorescence. Two-photon excited melanin fluorescence also has been measured from excised samples of healthy human skin tissue. Because of the selectivity of melanin excitation via resonant two-photon absorption, it is hypothesized that fluorescence excited in this way may yield information on malignant transformation.

Using a nitrogen laser pumped dye laser (pulse duration τ ∼ 300 ps in conjunction with a boxcar integrator BCI 280 the fluorescence decay time of the J-aggregates of pseudoisocyanine chloride in aqueous solution has been measured. A strong dependence of the fluorescence lifetime on temperature (6 °C

The photophysical behavior of six 1-(N-aryl-N-alkyl)-amino-3-anthryl-(9)-propanes was studied by steady-state and time-resolved fluorescence measurements in methylcyclohexane/ isopentane (MP). Structurally, these compounds are most suitable for the formation of intramolecular exciples as shown by the high exciplex quantum yields. The rate constants and the activation energies show that the rate of the exciplex formation depends on steric effects of the N-alkyl group and on the donor effect of the N-aryl group. From the rise curves of the exciplex emission it follows that also in nonpolar solvents exciplexes with a conformation deviating from the favored sandwich structure can be formed. The solvent effect on the fluorescence properties was investigated.

Two molecules, bis-2,5-(2-benzoxazolyl)hydroquinone (II) and bis-3,6-(2-benzoxazolyl)pyrocatechol (III), undergo a very effective excited-state intramolecular proton transfer. The single proton transfer in II involves an energy barrier on the potential energy curve. The proton-transfer reaction is described as a fully reversible process, Δ H 0 = −0.5 kcal/mole. MoIecule III shows a barrierless, double proton transfer; the activation energy of the radiationless decay rate of the excited tautomer, Δ E , is 4.1 kcal/mole.

The photophysical processes of 1-(N- p -anisyl-N-methyl)-amino-3-anthryl-(9)-propane, I, in methylcyclohexane/isopentane at different temperatures were investigated by steady state and time resolved fluorescence measurements. Structurally, I has a higher tendency to intramolecular exciplex formation than 1- p -dimethylaminophenyl-3-anthryl-(9)-propane, a compound of an isomeric type studied by Mataga and Eisenthal.

Singlet-singlet-annihilation of the exciton domains of pseudoisocyanine (PIC) J-aggregates could be proved by nonlinear spectroscopy. The relative fluorescence efficiency drastically decreases for excitation intensities >1019 photons cm-2 s-1. The fluorescence lifetime at low signal conditions (Iexc

Prochlorococcus marinus SS120 is an ecologically important and biochemically intriguing marine cyanobacterium. In addition to divinyl chlorophylls (DV-Chls) a and b it possesses a particular form of phycoerythrin (PE), but no other phycobilins and therefore no complete phycobilisomes. Here, a spectroscopic characterisation of these DV-Chls and PE is provided. Comparison of fluorescence quantum yields, excited state lifetimes and absorption characteristics indicate similar light-harvesting properties of the DV-Chls as their monovinyl counterparts. PE, which is present only in tiny amounts, was purified and considerably enriched. A phycourobilin to phycoerythrobilin ratio of 3:1 chromophores per (alphabeta) PE monomer is suggested. The in vitro fluorescence lifetime of PE is 1.74 ns. In vivo time-resolved fluorescence measurements with synchrotron radiation were used to investigate the possible role of PE in light-harvesting. The fluorescence decay time for PE is about 550 ps, indicating an unusually slow excitation energy transfer. The decay time slowed to 1 ns after addition of glycerol to cell cultures. The contribution of PE to total light-harvesting capacity was estimated to be about one (alphabeta) PE monomer per 330 DV-Chl b molecules. Thus, the capacity of PE to function primarily as a photosynthetic light-harvesting pigment in P. marinus SS120 is low.