Giancarlo Agostini

EPR studies of the excited triplet states of C60O and C60C2H4N(CH3) fullerene derivatives and C70 in toluene and polymethylmethacrylate glasses and as films

Abstract The EPR spectra of excited triplet states of two fullerene derivatives, C60O and C60C2H4N(CH3), in the glassy matrices of toluene and of polymethylmethacrylate (PMMA) and as films were examined at low temperature and the magnetic parameters were measured. In films, species with zfs parameters smaller than those in toluene were found and they were assigned to triplet excitations visiting more than one molecule in the crystallites that form the film. When C60C2H4N(CH3) was excited in PMMA grown by in sity thermal polymerization a new triplet species was originated characterized by a dipolar splitting constant D about three times larger than that measured in toluene. Evidence was gained that such species participates to cross-linking in the polymer. The effect of PMMA matrix on the molecular dynamics of 3C70 was also investigated. It was found that a dynamical model holds based on the pseudorotation of the molecule around the axis of larger dipolar splitting. Motion is activated with activation energy of 210 ± 50 cm−1 a value comparable with that obtained for pseudorotation of 3C70 in cyclohexane.

Pulse ENDOR and density functional theory on the peridinin triplet state involved in the photo-protective mechanism in the peridinin–chlorophyll a–protein from Amphidinium carterae

The photoexcited triplet state of the carotenoid peridinin in the Peridinin-chlorophyll a-protein of the dinoflagellate Amphidinium carterae has been investigated by pulse EPR and pulse ENDOR spectroscopies at variable temperatures. This is the first time that the ENDOR spectra of a carotenoid triplet in a naturally occurring light-harvesting complex, populated by energy transfer from the chlorophyll a triplet state, have been reported. From the electron spin echo experiments we have obtained the information on the electron spin polarization dynamics and from Mims ENDOR experiments we have derived the triplet state hyperfine couplings of the alpha- and beta-protons of the peridinin conjugated chain. Assignments of beta-protons belonging to two different methyl groups, with aiso=7.0 MHz and aiso=10.6 MHz respectively, have been made by comparison with the values predicted from density functional theory. Calculations provide a complete picture of the triplet spin density on the peridinin molecule, showing that the triplet spins are delocalized over the whole pi-conjugated system with an alternate pattern, which is lost in the central region of the polyene chain. The ENDOR investigation strongly supports the hypothesis of localization of the triplet state on one peridinin in each subcluster of the PCP complex, as proposed in [Di Valentin et al. Biochim. Biophys. Acta 1777 (2008) 186-195]. High spin density has been found specifically at the carbon atom at position 12 (see Fig. 1B), which for the peridinin involved in the photo-protective mechanism is in close contact with the water ligand to the chlorophyll a pigment. We suggest that this ligated water molecule, placed at the interface between the chlorophyll-peridinin pair, is functioning as a bridge in the triplet-triplet energy transfer between the two pigments.

FDMR of Carotenoid and Chlorophyll triplets in light-harvesting complex LHCII of spinach

Fluorescence detected magnetic resonance (FDMR) of the light-harvesting complex LHCII of the spinach photosynthetic machinery revealed triplet contributions both from Carotenoids and Chlorophylls. All three carotenoids present in the complex (lutein, neoxanthin and violaxanthin) are evidenced as triplet states in the FDMR signals obtained as variation of the emission intensity of the Chlorophylls in the 680 nm region. The triplets show ⋎D⋎ values of 0.0401, 0.0388 and 0.0382 cm−1. A comparison with the results obtained by ADMR (Absorption Detected Magnetic Resonance) is made and discussed. An interesting concentration effect is discovered and discussed in terms of specific interactions between carotenoids and chlorophyll molecules. Signals are also obtained by microwave sweeping in the Chlorophyll regions and one triplet is detected (⋎D⋎=0.028−0.029 cm−1). The polarization of the carotenoid signals is discussed in terms of singlet-singlet and triplet-triplet energy transfer between carotenoids and chlorophylls, also with the help of double resonance experiments. Double resonance experiments involving both carotenoids and chlorophylls signals gave negative results. It is not possible as a consequence to assess that the chlorophyll whose triplets levels are scanned in the FDMR spectra are functionally connected to the carotenoids.

Metal-stabilized carbanions

Abstract The rate of deprotonation of fluorene and of π-(tricarbonylchromium)-flourene by an excess of KH in THF was measured by monitoring the hydrogen evolution. The pseudo-first order rate constant for the complex is ca. one order of magnitude higher than that for the free ligand. 1 H and 13 C NMR analysis showed that when the anion is produced at -20°C or below, the Cr(C0) 3 group is bonded to one of the phenyl rings (η 6 -anion), whereas ionization at room temperature produces solutions containing mainly the anion with the Cr(CO) 3 bonded to the cyclopentadienyl ring (η 5 -anion) in equilibrium with the η 6 -isomer. The effect of ionization on free and complexed systems, together with the effect of complexation of the free anion, is discussed on the basis of the NMR data. The η 6 π η 5 isomerization equilibrium was followed at various temperatures and different degrees of solvation are deduced for the two isomeric ion pairs from the kinetic and thermodynamic solvation parameters.

ODMR of carotenoid and chlorophyll triplets in CP43 and CP47 complexes of spinach

Abstract Optically detected magnetic resonance (ODMR) of the light-harvesting complexes CP43 and CP47 of the spinach photosynthetic machinery revealed triplet states both from carotenoids and chlorophylls. The triplet state of the only carotenoid present in the complexes (β-carotene) is observed by ODMR using fluorescence detection (FDMR) in the main chlorophyll emission band in the 680 nm region, and absorption detection in the visible region. Chlorophyll triplet signals are also obtained by microwave sweeping in the chlorophyll emission and absorption regions. MIA spectra are obtained and discussed for both complexes.

Interquinone electron transfer in photosystem I as evidenced by altering the hydrogen bond strength to the phylloquinone(s).

The kinetics of electron transfer from phyllosemiquinone (PhQ(*-)) to the iron sulfur cluster F(X) in Photosystem I (PS I) are described by lifetimes of approximately 20 and approximately 250 ns. These two rates are attributed to reactions involving the quinones bound primarily by the PsaB (PhQ(B)) and PsaA (PhQ(A)) subunits, respectively. The factors leading to a approximately 10-fold difference between the observed lifetimes are not yet clear. The peptide nitrogen of conserved residues PsaA-Leu722 and PsaB-Leu706 is involved in asymmetric hydrogen-bonding to PhQ(A) and PhQ(B), respectively. Upon mutation of these residues in PS I of the green alga, Chlamydomonas reinhardtii , we observe an acceleration of the oxidation kinetics of the PhQ(*-) interacting with the targeted residue: from approximately 255 to approximately 180 ns in PsaA-L722Y/T and from approximately 24 to approximately 10 ns in PsaB-L706Y. The acceleration of the kinetics in the mutants is consistent with a perturbation of the H-bond, destabilizing the PhQ(*-) state, and increasing the driving force of its oxidation. Surprisingly, the relative amplitudes of the phases reflecting PhQ(A)(*-) and PhQ(B)(*-) oxidation were also affected by these mutations: the apparent PhQ(A)(*-)/PhQ(B)(*-) ratio is shifted from 0.65:0.35 in wild-type reaction centers to 0.5:0.5 in PsaA-L722Y/T and to 0.8:0.2 in PsaB-L706Y. The most consistent account for all these observations involves considering reversibility of oxidation of PhQ(A)(*-) and PhQ(B)(*-) by F(X), and asymmetry in the driving forces for these electron transfer reactions, which in turn leads to F(x)-mediated interquinone electron transfer.

Triplet–triplet energy transfer in the major intrinsic light-harvesting complex of Amphidinium carterae as revealed by ODMR and EPR spectroscopies

We present an optically detected magnetic resonance (ODMR) and electron paramagnetic resonance (EPR) spectroscopic study on the quenching of photo-induced chlorophyll triplet states by carotenoids, in the intrinsic light-harvesting complex (LHC) from the dinoflagellate Amphidinium carterae. Two carotenoid triplet states, differing in terms of optical and magnetic spectroscopic properties, have been identified and assigned to peridinins located in different protein environment. The results reveal a parallelism with the triplet-triplet energy transfer (TTET) process involving chlorophyll a and luteins observed in the LHC-II complex of higher plants. Starting from the hypothesis of a conserved alignment of the amino acid sequences at the cores of the LHC and LHC-II proteins, the spin-polarized time-resolved EPR spectra of the carotenoid triplet states of LHC have been calculated by a method which exploits the conservation of the spin momentum during the TTET process. The analysis of the spectra shows that the data are compatible with a structural model of the core of LHC which assigns the photo-protective function to two central carotenoids surrounded by the majority of Chl a molecules present in the protein, as found in LHC-II. However, the lack of structural data, and the uncertainty in the pigment composition of LHC, leaves open the possibility that this complex posses a different arrangement of the pigments with specific centers of Chl triplet quenching.

Carotenoid triplet detection by time-resolved EPR spectroscopy in carotenopyropheophorbide dyads

Carotenoid triplets play a photoprotective role in natural photosynthesis. The main process of carotenoid triplet formation is known to be triplet-triplet energy transfer from chlorophyll triplets. The structural requirements for high transfer yields are still a matter of discussion and the presence of competitive triplet formation pathways has not been excluded. Transient EPR measurements of triplet states formed by photoexcitation allow detection of the initial spin polarization. This pattern derives from the mechanism of triplet formation. In the case of triplet-triplet energy transfer, if the condition of spin angular momentum conservation is fulfilled, simulation of the EPR spectra gives information about the donor-acceptor mutual orientation. We describe transient EPR experiments on two artificial photosynthetic dyads, consisting of a carotenoid covalently-linked to a free-base or zinc substituted pyropheophorbide moiety and we discuss the results in terms of possible dyad conformations.

Energy transfer and spin polarization of the carotenoid triplet state in synthetic carotenoporphyrin dyads and in natural antenna complexes

A series of carotenoporphyrin dyads, in which the carotenoid is covalently linked to a tetraarylporphyrin at the ortho, meta or para position of one of the meso aromatic rings, has been studied using Time-Resolved Electron Paramagnetic Resonance (TREPR) spectroscopy. In parallel, an investigation has been carried, on two different photosynthetic antenna systems, the B800–B850 complex ofR. acidophila and the LHCII complex of higher organisms. The initial spin polarization of the carotenoid triplet-state, populated indirectly by laser excitation, has been detected. It has been demonstrated that the initial polarization is not a characteristic property of the carotenoid triplet-state, as previously stated, but depends on the donor-acceptor mutual orientation. The triplet energy transfer to the carotenoid from a chlorophyll or porphyrin triplet state is discussed on the basis of the observed spin polarization.

ON THE PHOTOREDUCTION OF URANYL COMPLEXES WITH ALKYLPHOSPHATES IN NONAQUEOUS MEDIA

Abstract Uranyl complexes with triethylphosphate and trimethylphosphate dissolved in the corresponding alkyl phosphates are photoreduced by irradiation in their UV-VIS absorption band. The photoreaction proceeds through the formation of an intermediate species which was identified as U(V). The latter is quite stable in the dark, but undergoes a photochemical disproportionation giving U(IV) as the final product.