STUDIES OF CHLOROPHYLL-CHLOROPHYLL AND CHLOROPHYLL-LIGAND INTERACTIONS BY VISIBLE ABSORPTION AND INFRARED SPECTROSCOPY AT LOW TEMPERATURES

Abstract— A comparison of the visible absorption and infrared spectra of various chlorophyll-chlorophyll (Chl) and Chi-nucleophile aggregates at room temperature and at low temperatures has been made. The IR data provide structural information indispensable for the interpretation of the visible spectra. As a necessary preliminary, it is shown that Chl a solutions in nonpolar solvents can be prepared by appropriate drying techniques that contain at a conservative estimate ≤ 3 mol % of water (i.e. Chl a/H₂O > 30:1). Very dry solutions of Chl a or Pyrochl a(≥ 10 mM) in toluene or methylcyclohexane-isopentane solution show only slight changes in visible spectra on cooling to 77 K. From IR, additional Chl-Chl aggregation occurs on cooling in methylcyclohexane-isopentane but not to a significant extent in toluene. Dilute (10 μM) solutions of Chl a or Pyrochl a in nonpolar solvents form a new absorption peak near 700 nm at low temperatures, which we attribute to traces of water in the solvent or other residual nucleophiles not removed during the Chl purification. Addition of stoichiometric amounts of water increases the size of the ?700 nm peak even in dilute Chl solutions. Chlorophyll a, Pyrochl a, but not pheophytin a are shown to interact with nucleophiles of the general type RXH (where R= H or alkyl, and X = O, N, or S). Such nucleophiles can coordinate to the Mg atom of one Chl molecule by lone pairs on O, N, or S, and hydrogen bond to oxygen donor functions in another Chl molecule. A ?0.1 M solution of Chl a or Pyrochl a in toluene containing 1.5 equivalents of ethanol is converted almost entirely to a species absorbing at ?700 nm at 77 K. Infrared spectroscopy shows conclusively that it is the keto C=O function that is involved in the cross-linking by hydrogen bonding, a conclusion supported by the observation that Pyrochl a forms a very similar red-shifted species at low temperatures, despite the absence of a carbomethoxy C=O function. n-Butylamine and ethanethiol interact in much the same way as does ethanol to form species red shifted to ?700 nm. A variety of possible structures for the low temperature forms is discussed, and the use of these red shifted species as paradigms for photoreaction center Chl is described.     

Excited state dynamics in recombinant water-soluble chlorophyll proteins (WSCP) from cauliflower investigated by transient fluorescence spectroscopy

The present study describes the fluorescence emission properties of recombinant water-soluble chlorophyll (Chl) protein (WSCP) complexes reconstituted with either Chl a or Chl b alone (Chl a only or Chl b only WSCP, respectively) or mixtures of both pigments at different stoichiometrical ratios. Detailed investigations were performed with time and space correlated ps fluorescence spectroscopy within the temperature range from 10 to 295 K. The following points were found: (a) The emission spectra at room temperature (295 K) are well characterized by bands with a dominating Lorentzian profile broadened due to phonon scattering and peak positions located at 677, 684 and 693 nm in the case of Chl a only WSCP and at 665, 675 and 689 nm for Chl b only WSCP. In addition, all spectra contain minor bands in the longer wavelength region. (b) The emission spectra at 10 K of samples suspended in buffer containing 50% glycerol are dominated by bands peaking at 668 nm for Chl b only WSCP and at 685 nm for Chl a only WSCP and samples reconstituted with mixtures of Chl a and Chl b. (c) At 10 K and in buffer with 50% glycerol the decay kinetics of WSCP samples with Chl a only are dominated by a component with a time constant of 6.2 (+/-0.2) ns at 685 nm while those of WSCP containing mixtures of Chl a and Chl b are characterized by a slightly shorter value of 6.0 (+/-0.2) ns. WSCP containing Chl b only exhibits a distinctly longer value of 7.0 (+/-0.3) ns at an emission wavelength of 668 nm. (d) The decay associated emission spectra at 10 K of all samples exhibit at least 3 decay components with time constants of 80-120 ps, 2-4 ns and 6-7 ns in 50% glycerol. These results are consistently described within the framework of our previously presented model (J. Phys. Chem. B 2007, 111, No. 46, 13325; J. Phys. Chem. B 2007, 111, No. 35, 10487) , for the structural motifs of chlorophyll binding to the tetrameric protein matrix of WSCP. It is shown that formation of strongly coupled open sandwich dimers does not lead to quenching of 1Chl a* or 1Chl b*.     

CHLOROPHYLL a FLUORESCENCE OF GONYAULAX POLYEDRA GROWN ON A LIGHT-DARK CYCLE AND AFTER TRANSFER TO CONSTANT LIGHT

Abstract— The chlorophyll a fluorescence properties of Gonyaulax polyedra cells before and after transfer from a lightdark cycle (LD) to constant dim light (LL) were investigated. The latter display a faster fluorescence transient from the level ‘I’ (intermediary peak) to ‘D’ (dip) to ‘P’ (peak) than the former (3 s as compared to 10 s), and a different pattern of decline in fluorescence from ‘I’ to ‘D’ and from ‘P’ to the steady state level with no clearly separable second wave of slow fluorescence change, referred to as ‘s' (quasi steady state)→‘M’ (maximum) →‘T’ (terminal steady state). The above differences are constant features of cells in LD and LL, and are not dependent on the time of day. They are interpreted as evidence for a greater ratio of photosystem II/photosystem I activity in cells in LL. After an initial photoadaptive response following transfer from LD to LL, the cell absorbance at room temperature and fluorescence emission spectra at 77 K for cells in LL and LD are comparable. The major emission peak is at 685–688 nm (from an antenna Chl a 680, perhaps Chl a-c complex), but, unlike higher plants and other algae, the emission bands at 696–698 nm (from Chl a_II complex, Chl a 685, close to reaction center II) and 710–720 nm (from Chl a₁, complexes, Chl a 695, close to reaction center I) are very minor and could be observed only in the fluorescence emission difference spectra of LL minus LD cells and in the ratio spectra of DCMU-treated to non-treated cells. Comparison of emission spectra of cells in LL and LD suggested that, in LL, there is a slightly greater net excitation energy transfer from the light-harvesting peridinin-Chl a (Chl a 670) complex, fluorescing at 675 nm, to the other antenna chlorophyll a complex fluorescing at 685–688 nm, and from the Chl a., complex to the reaction center II. Comparison of excitation spectra of fluorescence of LL and LD cells, in the presence of DCMU, confirmed that cells in LL transfer energy more extensively from the peridinin-Chl a complex to other Chl a complexes than do cells in LD.     

SPECTRAL PROPERTIES OF CHLOROPHYLL a MONOLAYERS: MONOLAYERS OF CHLOROPHYLL a AND PHEOPHYTIN AT A GAS-WATER INTERFACE*

Abstract— Surface and spectral properties of chlorophyll a monolayers were studied at a nitrogen-water interface. Direct spectral analysis of Chl monolayers indicated that compression results in a heterogenous mixture of Chl species. Fourth derivative and difference spectra showed the presence of minor bands at 692, 726 and 748 nm. The state of compression determines the quantity and type of spectral species formed. A Chl monolayer on an acid subphase results in the formation of a long wavelength absorbing species (705 nm) similar to that of pheophytin. The half-band width, optical density/monolayer, and extinction coefficients of Chl monolayers are given. It is concluded that in the monolayer the formation of various aggregated species of Chl can be induced.     

ASSOCIATION OF CHLOROPHYLL WITH AMIDES ON PLASTICIZED POLYETHYLENE PARTICLES–III. UNUSUAL SPECTRA OF CHLOROPHYLL a WITH N-METHYLMYRISTAMIDE*

—The absorption spectrum of chlorophyll a, adsorbed with the amphiphilic amide, N-methylmyristamide. to particles of polyethylene swollen with tetradecane, is unusual in that the red band apparently consists of three main components, which by Gaussian deconvolution are located at 664.5, 678 and 687 nm. The last is very narrow, with a band width of only 5-6 nm at half maximum. At low amide concentration, the 744 nm band of chlorophyll hydrate is also observed. Room temperature fluorescence is weak, and indistinctly resolved into bands. However, on gradual cooling to 80 K, the fluorescence intensifies greatly and becomes resolvable into at least eight bands. The circular dichroism spectrum of the red band region shows optical rotatory strength in two narrow bands at 677 and 686 nm which is enormous, compared to that of monomeric chlorophyll or even the 744 nm hydrate. It is suggested that cyclic oligomer structures, in which adjacent chlorophylls are linked through the amide group of N-methylmyristamide, might be responsible for the spectral phenomena. Unsubstituted myristamide and N.N-dimethylmyristamide do not produce these narrow-banded phenomena with chlorophyll at all.     

NEW SPECTRAL COMPONENTS IN HIGH RESOLUTION ABSORPTION SPECTRA OF GREEN BACTERIAL REACTION CENTER COMPLEXES AT 5K*

Abstract—absorption spcctra of reaction center Complexes I and II from Chlorobium limicola f. thiosul-fatophilum were taken from 760 and 860 nm at 5 K. Fourth and eighth derivatives of the spectra were calculated from the digital data. Light-minus-dark difference spectra were taken, also at 5 K, with 590 nm actinic light. A shoulder not visible at 77 K appears on the long wavelength side of the 834 nm peak in Complex I. In Complex II, which is derived by guanidine HCI treatment of I, the shoulder is much more pronounced; derivative peaks appear at 834 and 838 nm. In the difference spectra, there are troughs at 832 and 838 nm. The latter trough is the first instance in green bacteria of a wavelength coincidence between a light-induced bleaching and a peak in (derivative) absorbance. There is also a nearly symmetrical pair of features, a trough at 814 nm and a peak at 818 nm, that appear to represent a light-induced bathochromic shift of the absorbance at 816 nm, a peak which occurs in both complexes as well as the photochemically inert bacteriochlorophyll a (Bchl a) protein. Other features in the absorption spcctra of both Complexes occur at virtually the same wavelengths as the peaks in purified Bchl a-protein trimer. We conclude that a large fraction of the Bchl a in Complex II is in a conformation similar to that of a single subunit of Bchl a-protein.     

Changes in the structure of chlorophyll-protein complexes and excitation energy transfer during photoinhibitory treatment of isolated photosystem I submembrane particles

The activity of light-induced oxygen consumption, absorption spectra, low temperature (77 K) chlorophyll fluorescence emission and excitation spectra were studied in suspensions of photosystem (PS) I submembrane particles illuminated by 2000 microE m(-2) s(-1) strong white light (WL) at 4 degrees C. A significant stimulation of oxygen uptake was observed during the first 1-4 h of photoinhibitory treatment, which rapidly decreased during further light exposure. Chlorophyll (Chl) content gradually declined during the exposure of isolated PSI particles to strong light. In addition to the Chl photobleaching, pronounced changes were found in Chl absorption and fluorescence spectra. The position of the major peak in the red part of the absorption spectrum shifted from 680 nm towards shorter wavelengths in the course of strong light exposure. A 6-nm blue shift of that peak was observed after 5-h illumination. Even more pronounced changes were found in the characteristics of Chl fluorescence. The magnitude of the dominating long-wavelength emission band at 736 nm located in untreated particles was five times reduced after 2-h exposure, whereas the loss in absolute Chl contents did not exceed 10% of its initial value. The major peak in low-temperature Chl fluorescence emission spectra shifted from 736 to 721 nm after 6-h WL treatment. Individual Chl-protein complexes differed in the response of their absorption spectra to strong WL. Unlike light-harvesting complexes (LHC), LHCI-680 and LHC-730, which did not exhibit changes in the major peak position, its maximum was shifted from 678 to 671 nm in CPIa complex after PSI submembrane particles were irradiated with strong light for 6 h. The results demonstrated that excitation energy transfer represents the stage of photosynthetic utilization of absorbed quanta which is most sensitive to strong light in isolated PSI particles.     

Pigment-Protein Interactions in the Antenna-Reaction Center Complex of Heliobacillus mobilis

Membrane fragments of Heliobacillus (Hc.) mobilis were characterized using resonance Raman (RR) spectroscopy in order to determine the configuration of the neurosporene carotenoid, the pigment-protein interactions of the bacteriochlorophyll (BChl) g molecules, and the Chl a-like chlorin pigments present in the antenna-reaction center complex constituting the photosynthetic apparatus. Using 363.8 nm excitation, the Raman contributions of the BChl g molecules were selectively resonantly enhanced over those of the carotenoid and the Chl a-like chlorin pigments. The RR spectrum of BChl g in these membranes excited at 363.8 nm exhibits bands at 1614 and 1688 cm^?1, which correspond to a C_aC_m methine bridge stretching mode and a keto carbonyl group stretching mode, respectively. Both of these bands are 16 cm^?1 wide (full width at half maximum, FWHM), indicating that a sole population of BChl g molecules is being enhanced at this excitation wavelength. The observed frequency of the C_aC_m stretching mode (1614 cm^?1) indicates that the bulk of BChl g molecules is pentacoordinated with only one axial ligand to the central Mg atom while that of the keto carbonyl stretching mode (1668 cm^?1) indicates that these groups are engaged in a hydrogen bond. This homogeneous population of BChl g molecules bound to the heliobacterial core polypeptides is in contrast to the heterogeneous population of Chl a molecules bound to the core polypeptides of the reaction center of photosystem I of Synechocystis 6803 as observed by the inhomogeneously broadened C₉ keto carbonyl band in its RR spectrum. The RR spectrum of the Chl a-like chlorin pigments in Hc. mobilis excited at 441.6 nm exhibits a broad keto carbonyl band (43 cm^?1 FWHM) with components at 1665, 1683 and 1695 cm^?1, indicating several populations of these pigments differing in their protein interactions at the level of the keto carbonyl group. Fourier transform (FT) pre-RR spectroscopic measurements of intact whole cells and membrane fragments at room temperature using 1064 nm excitation indicate that high quality vibrational spectra of the BChl g molecules can be obtained with no photodegradation. Low-temperature FT Raman spectra excited at 1064 nm reveals an inhomogeneously broadened 1665 cm^?1 band corresponding to the C₉ keto carbonyl stretching mode. Spectral deconvolution and second derivative analysis of this band reveal that it is comprised of components at 1665, 1682 and 1695 cm^?1, the latter two most likely arising from BChl g photoconversion products. Excitation using 885 nm to enhance the preresonance effect of the BChl g molecules yields an FT Raman spectrum where the keto carbonyl band at 1665 cm^?1 is narrow, as is the case in the Soret RR spectra, reflecting a sole population of BChl g molecules, which are engaged in an H bond. The RR spectrum of the neurosporene molecule in Hc. mobilis membranes excited at 496.5 nm is compared to that of 1,2-dihydroneurosporene bound in a cis configuration in reaction centers of Rhodopseudomona viridis and to that of the same carotenoid in its all-trans configuration extracted from these reaction centers in the presence of light. The similarity of this latter RR spectrum with that of neurosporene in the Hc. mobilis membranes indicates that it is bound in an all-trans configuration.     

Spectral and photochemical properties of aromatic nitrenes

Absorption and emission spectra of para-substituted phenyl nitrenes have been studied in glassy ethanol at 77 K. Absorption spectra of nitrenes with neutral and donor substituents are similar to benzyl radical absorption and show three bands at 400–450, 320 and 250 nm. The emission is assigned to T₁-T₀ fluorescence. Nitrenes could be photoreduced to substituted anilines at 77 K.     

DELAYED EMISSION OF CHLOROPHYLL a AGGREGATES AND RHODAMINE 6G EMBEDDED IN POLYMER MATRIX*

Delayed luminescence (in the microsecond time range) of the chlorophyll (Chl) a“dry” form as well as hydrated dimers located in a polyvinylalcohol film was measured from room temperature down to 8 K. In the same matrix the delayed luminescence of rhodamine 6G (Rhod) was investigated. The delayed emission both of Chl a and Rhod is probably due to the formation and delayed recombination of a radical pair. It seems that this process occurs without participation of triplet states, as it does not reflect their well-known sensitivity to oxygen. The temperature dependence of the delayed luminescence of vanous Chl forms is different. In the region around 678 nm (dry monomer) delayed luminescence needs a thermal activation energy of about 0.03 eV, whereas at 740 nm (wet aggregates) delayed luminescence intensity increases linearly with decreasing temperature. Its assignment as a-type delayed luminescence from the low-lying triplet state can consistently be excluded from both the weak temperature dependence of the delayed fluorescence and its large intensity as compared to the prompt fluorescence. Delayed luminescence of Rhod is almost independent of temperature between 8 K and 300 K. The dependence of delayed luminescence intensity on exciting light intensity is linear at lower intensities and tends to saturation at higher. Therefore the delayed luminescence is not related to exciton annihilation. Positions and intensities of the Chl delayed luminescence bands show that it is not phosphorescence (β-type delayed luminescence). The aggregation of both Chl and Rhod molecules strongly influences delayed luminescence since it differs in several properties if excited in the monomer or in the aggregate absorption range. Every aggregational form of dye emits its characteristic delayed luminescence band.     

Efficient energy transfer from the long-wavelength antenna chlorophylls to P700 in photosystem I complexes from Spirulina platensis

To study the role of the long-wavelength chlorophylls (Chl) in photosystem I (PSI), the action spectra of P700 photooxidation at 293 and 77 K have been measured for PSI trimeric and monomeric complexes isolated from Spirulina platensis. The long-wavelength Chls which absorb in the region 710dash740 nm transfer excitation energy to the reduced P700 with the same efficiency as bulk antenna Chls, causing the oxidation of P700. The relative quantum yield of P700 photooxidation is about unity (293-77 K) even under the direct excitation of Chl absorbing at 735 nm (Chl735). At 77 K Chl735 exhibits a fluorescence band at 760 nm (F760) whose intensity is quenched under illumination of the PSI trimeric complexes from Spirulina. The relative quantum yield of F760 quenching is not dependent on the wavelength of excitation in the region 620–750 nm. Since the value of the overlap integral between the band of F760 and the absorption band of the cation radical of P700 (P700⁺) is higher than that of the P700 band, it is suggested that Chl735 transfers energy to P700⁺ more efficiently than to reduced P700; energy transfer to P700⁺ causes the quenching of F760. A linear relationship between the photooxidation rate of P700 and the fraction of P700⁺ at 293 K indicates that the energy exchange between PSI subunits of the trimer is negligible. Thus, the antenna of PSI trimers of Spirulina is organized in separate photosynthetic units.     

Spectroscopic evidence for triplet excitation energy transfer among carotenoids in the LH2 complex from photosynthetic bacterium <Emphasis Type="Italic">Rhodopseudomonas palustris</Emphasis>

The LH2 complex from Rhodopsudomonas (Rps.) palustris is unique in the heterogeneous carotenoid compositions. The dynamics of triplet excited state Carotenoids (³Car* has been investigated by means of sub-microsecond time-resolved absorption spectroscopy both at physiological temperature (295 K) and at cryogenic temperature (77K). Broad and asymmetric T _n ←T ₁ transient absorption was observed at room temperature following the photo-excitation of Car at 532 nm, which suggests the contribution from various carotenoid compositions having different numbers of conjugated C=C double bonds (N_c=c). The triplet absorption bands of different carotenoids, which superimposed at room temperature, could be clearly distinguished upon decreasing the temperature down to 77 K. At room temperature the shorter-wavelength side of the main T_n04T₁ absorption band decayed rapidly to reach a spectral equilibration with a characteristic time constant of ∽1 μs, the same spectral dynamics, however, was not observed at 77 K. The aforementioned spectral dynamics can be explained in terms of the triplet-excitation transfer among heterogeneous carotenoid compositions. Global spectral analysis was applied to the time-resolved spectra at room temperature, which revealed two spectral components peaked at 545 and 565 nm, and assignable to the T_n04 T₁ absorption of Cars with Nc=c=11 and Nc=c=13, respectively. Surprisingly, the decay time constant of a shorter-conjugated Car, i.e. 0.72 ώs (aerobic) and 1.36 ώs (anaerobic), is smaller than that of a longer-conjugated Car, i.e. 2.12 us (aerobic) and 3.75 ώs (anaerobic), which is contradictory to the general rule of carotenoids and relative polyenes. The results are explained in terms of triplet-excitation transfer among different types of Cars. It is postulated that two Cars with different conjugation lengths coexist in an α, β-subunit in the LH2 complex.     

Circular and Linear Dichroism of Aggregates of Chlorophyll a and Chlorophyll b in 3-Methylpentane and Paraffin Oil

A circular (CD) and linear dichroism (LD) study of the water adducts of the green plant chlorophylls a (Chl a) and b (Chl b) in hydrocarbon solvents 3-methylpentane and paraffin oil is presented. A strong red shift of the Q_y-absorption band from 663 to 746 nm (1678 cm^?1) is observed as the water adduct of Chl a is formed. The Chl a-water adduct shows a strong, nonconservative CD signal, which is characterized by a positive peak at 748 nm and two negative peaks at 720 and 771 nm. The maximum CD (A_L - A_R) is only one order of magnitude smaller than the isotropic absorption maximum. We propose that this exceptionally strong signal is the so-called psi-type CD. The LD spectrum was measured in a flow of paraffin oil. The isotropic absorption maximum peaks at 742 nm in paraffin oil, whereas the maximum of the LD signal is at 743 nm. The LD signal is positive over the whole water-adduct absorption band indicating that the transition dipole of the 742 nm transition is preferentially oriented along the long axis of the aggregate. The structure of the Chl b-water adduct is less well defined. The preparations of the Chl b-water adduct are unstable. The Chl b-water adduct absorption band maximum is at 683 nm. The CD signal of the Chl a-water adduct is about 200-fold the CD of the Chl b-water adduct. We could not orient the Chl b-water adducts by flow, which suggests that the adducts are small or disordered.     

Electronic spectra and transitions of the fullerene C60

Absorption spectra of C₆₀ have been measured in the ranges (a) 190–700 nm in n-hexane solutions at 300 K, (b) 390–700 nm in n-hexane and in 3-methylpentane solutions at 77 K. 40 vibronic bands were observed. They exhibit a large range of bandwidths and intensities, whose significance is discussed. Assignment of electronic transitions has been carried out using the results of theoretical calculations. Vibronic structures have been analyzed within the framework of theories of electronic transitions of polyatomic molecules applied to the I_h symmetry group. Nine allowed ¹T_1u−¹A_g transitions have been assigned in the 190–410 nm region. Observed and calculated allowed transition energies and oscillator strengths are compared. Detailed vibronic analyses of the 1 ¹T_1u−1 ¹A_g and 2 ¹T_1u−1 ¹A_g transitions illustrate the role of Jahn-Teller couplings. Orbitally forbidden singlet-singlet transitions are observed between 410 and 620 nm. Their vibronic structures were analyzed in terms of concurrent Herzberg-Teller and Jahn-Teller vibronic interactions. The 77 K spectra provided useful information on hot bands and on other aspects of the analyses. Vibronic bands belonging to triplet←singlet transitions were detected between 620 and 700 nm.     

Electronic and vibrational spectroscopy of cis-beta-[CrCl(2)(1,4,7,11-tetrazaundecane)chromium(III) perchlorate

The sharp-line absorption spectrum of cis-beta-[CrCl(2)(2,2,3-tet)]ClO(4) (2,2,3-tet: 1,4,7,11-tetraazaundecane) has been measured between 13,000 and 16,000 cm(-1) at 5K. The 77K emission, 298 K infrared and visible spectra have also been measured. The zero-phonon band in the highly resolved absorption spectrum splits into four components. The vibrational intervals of the electronic ground state were determined from the far-infrared and emission spectra. The 10 electronic bands due to spin-allowed and spin-forbidden transitions were assigned. Using the observed transitions, a ligand field analysis has been performed to probe the ligand field properties of coordinated atoms in the title chromium(III) complex. The X-ray crystal structure at low-temperature was determined to help in assigning the sharp-line electronic transitions.     

THE CHLOROPHYLL a AGGREGATE ABSORBING NEAR 685 nm IS SELECTIVELY FORMED IN AQUEOUS TETRAHYDROFURAN*

An aqueous solution of 2–12% (vol/vol) tetrahydrofuran (THF) induced the selective aggregation of chlorophyll a (Chl a) to form a novel species, A-685, absorbing near 685 nm. The formation of A-685 was closely correlated with a decrease in water activity of the solution. A Raman spectrum of the Chl a species formed in the presence of 6% THF suggests a unique interaction among Chl a, solvent THF and water molecules to give a stacked aggregate (Chl a.THF.H₂O.THF.Chl a). The circular dichroic spectrum of the Chl a species formed in the 6% THF aqueous solution showed an intense signal that had negative and positive wings with about 100-fold larger molar ellipticity for the A-685 than for monomer. However, Chl a', the C10 epimer of Chl a, and chlorophyllide, with a phytyl chain replaced by an ethyl group, did not form A-685 in 6% THF. These clearly indicate that 10-methylcarboxylate and the phytyl chain have a significant role in stabilizing A-685. A possible structure for A-685 is proposed as a novel in vitro model for the P-680 Chl a dimer.     

Stability of electrons—monomers and dimers—in gamma-irradiated hydroxide glasses

The increase of temperature from 77 K brings about the retrapping of electrons and/or recombination processes manifested by the changes of absorption spectra and ESR signals. The new absorption bands at 700–840 nm, which belong to diamagnetic electron dimers, have been observed at 215–225 K. These species are connected with NaOH·X H₂O hydrates as it has been shown by near-IR spectra. The dispersive kinetic parameters of photobleaching of electrons—monomers and dimers—at 80 K have been measured.     

Examination of the photophysical processes of chlorophyll d leading to a clarification of proposed uphill energy transfer processes in cells of Acaryochloris marinas

A comprehensive study of the photophysical properties of chlorophyll (Chl) d in 1:40 acetonitrile-methanol solution is performed over the temperature range 170-295 K. From comparison of absorption and emission spectra, time-dependent density-functional calculations and homologies with those of Chl a, we assign the key features of the absorption and fluorescence spectra. Possible photophysical energy relaxation mechanisms are summarized, and thermal equilibration processes are studied in detail by monitoring the observed emission profiles and quantum yields as a function of excitation energy. In particular, we concentrate on emission subsequent to excitation in the extreme far-red tail of the Qy absorption spectrum, with this emission partitioned into contributions from hot-band absorptions as well as uphill energy transfer processes that occur subsequent to absorption. No unusual photophysical processes are detected for Chl d; it appears that all intramolecular relaxation processes reach thermal equilibration on shorter timescales than the fluorescence lifetime even at 170 K. The results from these studies are used to reinterpret a previous study of photochemical processes observed in intact cells and their acetone extracts of the photosynthetic system of Acaryochloris marina. In the study of Mimuro et al., light absorbed by Chl d at 736 nm is found to give rise to emission by another species, believed to also be Chl d, at 703 nm; this uphill energy transfer process is easily rationalized in terms of the thermal equilibration processes that we deduced for Chl d. However, no evidence is found in the experimental results of Mimuro et al. to support claims that (nonequilibrium) uphill energy transfer is additionally observed to Chl a species that emit at 670-680 nm. This finding is relevant to broader issues concerning the nature of the special pair in photosystem II of A. marina because suggestions that it is comprised of Chl a can only be correct if nonthermal uphill energy transfer processes from Chl d are operative.     

Photophysical Characterization of the Naturally Occurring Dioxobacteriochlorin Tolyporphin A and Synthetic Oxobacteriochlorin Analogues

Tolyporphins are tetrapyrrole macrocycles produced by a cyanobacterium‐containing culture known as HT‐58‐2. Tolyporphins A–J are free base dioxobacteriochlorins, whereas tolyporphin K is an oxochlorin. Here, the photophysical characterization is reported of tolyporphin A and two synthetic analogues, an oxobacteriochlorin and a dioxobacteriochlorin. The characterization (in toluene, diethyl ether, ethyl acetate, dichloromethane, 1‐pentanol, 2‐butanone, ethanol, methanol, N,N‐dimethylformamide and dimethylsulfoxide) includes static absorption and fluorescence spectra, fluorescence quantum yields and time‐resolved data. The data afford the lifetime of the lowest singlet excited state and the yields of the nonradiative decay pathways (intersystem crossing and internal conversion). The three macrocycles exhibit only modest variation in spectroscopic and excited‐state photophysical parameters across the solvents. The long‐wavelength (Q_y) absorption band of tolyporphin A appears at ~680 nm and is remarkably narrow (full‐width‐at‐half‐maximum ~7 nm). The position of the long‐wavelength (Q_y) absorption band of tolyporphin A (~680 nm) more closely resembles that of chlorophyll a (662 nm) than bacteriochlorophyll a (772 nm). The absorption spectra of tolyporphins B–I, K (which were available in minute quantities) are also reported in methanol; the spectra of B–I closely resemble that of tolyporphin A. Taken together, tolyporphin A generally exhibits spectral and photophysical features resembling those of chlorophyll a.     

Assignment of the lowest Qy-state and spectral dynamics of the CP29 chlorophyll a/b antenna complex of green plants: a hole-burning study

Low-temperature absorption, fluorescence and persistent non-photochemical hole-burned spectra are reported for the CP29 chlorophyll (Chl) a/b antenna complex of photosystem II of green plants. The absorption-origin band of the lowest Qy-state lies at 678.2 nm and carries a width of approximately 130 cm-1 that is dominated by inhomogeneous broadening at low temperatures. Its absorption intensity is equivalent to that of one of the six Chl a molecules of CP29. The absence of a significant satellite hole structure produced by hole burning, within the absorption band of the lowest state, indicates that the associated Chl a molecule is weakly coupled to the other Chl and, therefore, that the lowest-energy state is highly localized on a single Chl a molecule. The electron-phonon coupling of the 678.2 nm state is weak with a Huang-Rhys factor S of 0.5 and a peak phonon frequency (omega m) of approximately 20 cm-1. These values give a Stokes shift (2S omega m) in good agreement with the measured positions of the absorption band at 678.2 nm and a fluorescence-origin band at 679.1 nm. Zero-phonon holes associated with the lowest state have a width of approximately 0.05 cm-1 at 4.2 K, corresponding to a total effective dephasing time of approximately 400 ps. The temperature dependence of the zero-phonon holewidth indicates that this time constant is dominated at temperatures below 8 K by pure dephasing/spectral diffusion due to coupling of the optical transition to the glass-like two-level systems of the protein. Zero-phonon hole-widths obtained for the Chl b bands at 638.5 and 650.0 nm, at 4.2 K, lead to lower limits of 900 +/- 150 fs and 4.2 +/- 0.3 ps, respectively, for the Chl b-->Chl a energy-transfer times. Downward energy transfer from the Chl a state(s) at 665.0 nm occurs in 5.3 +/- 0.6 ps at 4.2 K.