The molecular structure of the light-harvesting complex 2 (LH2) bacteriochlorophyll-protein anten... more The molecular structure of the light-harvesting complex 2 (LH2) bacteriochlorophyll-protein antenna complex from the purple non-sulfur photosynthetic bacterium Rhodopseudomonas acidophilu, strain 10050 provides the positions and orientations of the 27 bacteriochlorophyll (BChl) molecules in the complex. Our structure-based model calculations of the distinctive optical properties (absorption, CD, polarization) of LH2 in the near-infrared region use a point-monopole approximation to represent the BChl Q, transition moment. The results of the calculations support the assignment of the ring of 18 closely coupled BChl to B850 (BChl absorbing at 850 nm) and the larger diameter, parallel ring of 9 weakly coupled BChl to BSOO. All of the significantly allowed transitions in the near infrared are calculated to be perpendicular to the C9 symmetry axis, in agreement with polarization studies of this membrane-associated complex. To match the absorption maxima of the B800 and B850 components using a relative permittivity (dielectric constant) of 2.1, we assign different site energies (12 500 and 12 260 cm-', respectively) for the Q, transitions of the respective BChl in their protein binding sites. Excitonic coupling is particularly strong among the set of B850 chromophores, with pairwise interaction energies nearly 300 cm-' between nearest neighbors, comparable with the experimental absorption bandwidths at room temperature. These strong interactions, for the full set of 18 B850 chromophores, result in an excitonic manifold that is 1200 cm-I wide. Some of the upper excitonic states should result in weak absorption and perhaps stronger CD features. These predictions from the calculations await experimental verification.
Abstract Different magnetic resonance spectroscopy techniques have been used to investigate the c... more Abstract Different magnetic resonance spectroscopy techniques have been used to investigate the carotenoid triplet state nature and the triplet-triplet energy transfer process among peridinin molecules in the peridinin-chlorophyll-protein (PCP) from the ...
ABSTRACT Fluorescence detected magnetic resonance (FDMR) of the light-harvesting complex LHCII of... more ABSTRACT 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.
Two histidines provide the axial ligands of the two chlorophyll a (Chl a) molecules which form th... more Two histidines provide the axial ligands of the two chlorophyll a (Chl a) molecules which form the primary electron donor (P700) of photosystem I (PSI). Histidine 676 in the protein subunit PsaA, His(A676), and histidine 656 in subunit PsaB, His(B656), were replaced in the green algae Chlamydomnas reinhardtii by site-directed mutagenesis with nonpolar, uncharged polar, acidic, and basic amino acid residues. Only the substitutions with uncharged polar residues led to a significant accumulation of PSI in the thylakoid membranes. These PSI complexes were isolated and the physical properties of the primary donor characterized. The midpoint potential of P700 +• /P700 was increased in all mutants (up to 140 mV) and showed a dependence on size and polarizability of the residues when His(B656) was substituted. In the light-minus-dark absorbance spectra, all mutations in PsaB exhibited an additional bleaching band at 665 nm at room temperature comparable with the published spectrum for the replacement of His(B656) with asparagine [
... The strong resonance in the center of the spectrum is characterized by an aaa eee polarizatio... more ... The strong resonance in the center of the spectrum is characterized by an aaa eee polarization for the six transitions arising from the three canonical orientations of a triplet state randomly oriented relative to the spectrometer magnetic field in a glassy medium. ...
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1997
. Ž . Optically detected magnetic resonance ODMR of P700 triplet state has been performed in larg... more . Ž . Optically detected magnetic resonance ODMR of P700 triplet state has been performed in large photosystem I PSI particles. The 'intactness' of the P700 environment in the PSI particles has been proved by the comparison of the spectra with those obtained in thylakoids and leaves. Effect of degradation of the samples on the spectra has also been tested. When Ž . detected in 'native' PSI centers the microwaves induced triplet-minus-singlet absorption spectra T y S show very sharp features which allow to estimate the minimum number of bands involved in the change of the electronic state of the primary Ž . donor singlet to triplet , and can be interpreted in terms of interactions among pigments in the reaction center. The analogy Ž with the reaction centers of purple bacteria is discussed. The previously published spectra H.J. den Blanken, A.J. Hoff, Ž . Ž . . Biochim. Biophys. Acta 724 1983 52-61; J. Vrieze, P. Gast, A.J. Hoff, J. Phys. Chem. 100 1996 9960-9967 obtained in particles having smaller antenna size, resemble the ones obtained in our degraded samples: it is suggested that the isolation procedure to small particles may produce an increase in the heterogeneity of the environment of the primary donor andror a change of the mutual orientations and interactions among pigments. The fluorescence detected magnetic resonance Ž . Ž FDMR in large PSI particles supports an excitation energy transfer model derived from time resolved optical studies R. Ž . . Van Grondelle, J.P. Dekker, T. Gillbro, V. Sundstrom, Biochim. Biophys. Acta 1187 1994 1-65 and adapted for helium temperature, characterized by the presence of antenna pools emitting at 690, 720 and 735 nm, respectively. The FDMR Ž . signal of P700 changes sign from positive to negative when detected at emission wavelength shorter than 700 nm. Comparison of FDMR of P700 triplet state in large PSI-200 particles, in thylakoids and, for the first time, in leaves has been done. q 1997 Elsevier Science B.V.
A comparative electron paramagnetic resonance (EPR) study has been performed on a series of struc... more A comparative electron paramagnetic resonance (EPR) study has been performed on a series of structurally related molecular triads which undergo photoinduced electron transfer and differ one from the other in terms of the acceptor or donor moieties. The molecular triads, C-P-C6o , TTF-P-C6o and C-P-PF, share the same free-base tetraarylporphyrin (P) as the primary electron donor, which after light excitation initiates the electron transfer process, but differ either in terms of the electron acceptor (fullerene derivative, C6o , versus fluorinated free-base porphyrin, PF), or in terms of the final electron donor (carotenoid polyene, C, versus tetrathiafulvalene, TTF). AII these molecular triads can be considered artificial photosynthetic reaction centers in their ability to mimic several key properties of the reaction center primary photochemistry. Photoinduced charge separation and recombination have been followed by time-resolved EPR in a glass of 2-methyltetrahydrofuran and in the nematic phase of the uniaxial liquid crystal E-7. All the triads undergo photoinduced electron transfer, with the generation of charge-separated states in both the low-dielectric environment of the 2-methyltetrahydrofuran glass and in anisotropic E-7 medium. Different photochemical pathways have been recognized depending on the specific donor and acceptor moieties constituting the molecular triads. In the presence of the tetrathiafulvalene electron donor singlet-and triplet-initiated electron transfer routes are concurrently active. Recombination to the low-lying carotenoid triplet state occurs in the carotene-based triads, while singlet recombination is the only active route for the TTF-P-C6o triad, where a low-lying triplet state is lacking. Long-lived charge separation has been observed in the case of TTF-P-C6o: about 8 p.s for the singlet-born radical pair in the glassy isotropic matrix and about 7 ~ts for the triplet-born radical pair in the nematic phase of E-7. For all the molecular triads, a weak exchange interaction (J ~ I G) between the electrons in the final spin-correlated radical pair has been evaluated by simulation of the EPR spectra, providing evidence for superexchange electronic interactions mediated by the tetraarylporphyrin bridge.
The molecular structure of the light-harvesting complex 2 (LH2) bacteriochlorophyll-protein anten... more The molecular structure of the light-harvesting complex 2 (LH2) bacteriochlorophyll-protein antenna complex from the purple non-sulfur photosynthetic bacterium Rhodopseudomonas acidophilu, strain 10050 provides the positions and orientations of the 27 bacteriochlorophyll (BChl) molecules in the complex. Our structure-based model calculations of the distinctive optical properties (absorption, CD, polarization) of LH2 in the near-infrared region use a point-monopole approximation to represent the BChl Q, transition moment. The results of the calculations support the assignment of the ring of 18 closely coupled BChl to B850 (BChl absorbing at 850 nm) and the larger diameter, parallel ring of 9 weakly coupled BChl to BSOO. All of the significantly allowed transitions in the near infrared are calculated to be perpendicular to the C9 symmetry axis, in agreement with polarization studies of this membrane-associated complex. To match the absorption maxima of the B800 and B850 components using a relative permittivity (dielectric constant) of 2.1, we assign different site energies (12 500 and 12 260 cm-', respectively) for the Q, transitions of the respective BChl in their protein binding sites. Excitonic coupling is particularly strong among the set of B850 chromophores, with pairwise interaction energies nearly 300 cm-' between nearest neighbors, comparable with the experimental absorption bandwidths at room temperature. These strong interactions, for the full set of 18 B850 chromophores, result in an excitonic manifold that is 1200 cm-I wide. Some of the upper excitonic states should result in weak absorption and perhaps stronger CD features. These predictions from the calculations await experimental verification.
Abstract Different magnetic resonance spectroscopy techniques have been used to investigate the c... more Abstract Different magnetic resonance spectroscopy techniques have been used to investigate the carotenoid triplet state nature and the triplet-triplet energy transfer process among peridinin molecules in the peridinin-chlorophyll-protein (PCP) from the ...
ABSTRACT Fluorescence detected magnetic resonance (FDMR) of the light-harvesting complex LHCII of... more ABSTRACT 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.
Two histidines provide the axial ligands of the two chlorophyll a (Chl a) molecules which form th... more Two histidines provide the axial ligands of the two chlorophyll a (Chl a) molecules which form the primary electron donor (P700) of photosystem I (PSI). Histidine 676 in the protein subunit PsaA, His(A676), and histidine 656 in subunit PsaB, His(B656), were replaced in the green algae Chlamydomnas reinhardtii by site-directed mutagenesis with nonpolar, uncharged polar, acidic, and basic amino acid residues. Only the substitutions with uncharged polar residues led to a significant accumulation of PSI in the thylakoid membranes. These PSI complexes were isolated and the physical properties of the primary donor characterized. The midpoint potential of P700 +• /P700 was increased in all mutants (up to 140 mV) and showed a dependence on size and polarizability of the residues when His(B656) was substituted. In the light-minus-dark absorbance spectra, all mutations in PsaB exhibited an additional bleaching band at 665 nm at room temperature comparable with the published spectrum for the replacement of His(B656) with asparagine [
... The strong resonance in the center of the spectrum is characterized by an aaa eee polarizatio... more ... The strong resonance in the center of the spectrum is characterized by an aaa eee polarization for the six transitions arising from the three canonical orientations of a triplet state randomly oriented relative to the spectrometer magnetic field in a glassy medium. ...
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1997
. Ž . Optically detected magnetic resonance ODMR of P700 triplet state has been performed in larg... more . Ž . Optically detected magnetic resonance ODMR of P700 triplet state has been performed in large photosystem I PSI particles. The 'intactness' of the P700 environment in the PSI particles has been proved by the comparison of the spectra with those obtained in thylakoids and leaves. Effect of degradation of the samples on the spectra has also been tested. When Ž . detected in 'native' PSI centers the microwaves induced triplet-minus-singlet absorption spectra T y S show very sharp features which allow to estimate the minimum number of bands involved in the change of the electronic state of the primary Ž . donor singlet to triplet , and can be interpreted in terms of interactions among pigments in the reaction center. The analogy Ž with the reaction centers of purple bacteria is discussed. The previously published spectra H.J. den Blanken, A.J. Hoff, Ž . Ž . . Biochim. Biophys. Acta 724 1983 52-61; J. Vrieze, P. Gast, A.J. Hoff, J. Phys. Chem. 100 1996 9960-9967 obtained in particles having smaller antenna size, resemble the ones obtained in our degraded samples: it is suggested that the isolation procedure to small particles may produce an increase in the heterogeneity of the environment of the primary donor andror a change of the mutual orientations and interactions among pigments. The fluorescence detected magnetic resonance Ž . Ž FDMR in large PSI particles supports an excitation energy transfer model derived from time resolved optical studies R. Ž . . Van Grondelle, J.P. Dekker, T. Gillbro, V. Sundstrom, Biochim. Biophys. Acta 1187 1994 1-65 and adapted for helium temperature, characterized by the presence of antenna pools emitting at 690, 720 and 735 nm, respectively. The FDMR Ž . signal of P700 changes sign from positive to negative when detected at emission wavelength shorter than 700 nm. Comparison of FDMR of P700 triplet state in large PSI-200 particles, in thylakoids and, for the first time, in leaves has been done. q 1997 Elsevier Science B.V.
A comparative electron paramagnetic resonance (EPR) study has been performed on a series of struc... more A comparative electron paramagnetic resonance (EPR) study has been performed on a series of structurally related molecular triads which undergo photoinduced electron transfer and differ one from the other in terms of the acceptor or donor moieties. The molecular triads, C-P-C6o , TTF-P-C6o and C-P-PF, share the same free-base tetraarylporphyrin (P) as the primary electron donor, which after light excitation initiates the electron transfer process, but differ either in terms of the electron acceptor (fullerene derivative, C6o , versus fluorinated free-base porphyrin, PF), or in terms of the final electron donor (carotenoid polyene, C, versus tetrathiafulvalene, TTF). AII these molecular triads can be considered artificial photosynthetic reaction centers in their ability to mimic several key properties of the reaction center primary photochemistry. Photoinduced charge separation and recombination have been followed by time-resolved EPR in a glass of 2-methyltetrahydrofuran and in the nematic phase of the uniaxial liquid crystal E-7. All the triads undergo photoinduced electron transfer, with the generation of charge-separated states in both the low-dielectric environment of the 2-methyltetrahydrofuran glass and in anisotropic E-7 medium. Different photochemical pathways have been recognized depending on the specific donor and acceptor moieties constituting the molecular triads. In the presence of the tetrathiafulvalene electron donor singlet-and triplet-initiated electron transfer routes are concurrently active. Recombination to the low-lying carotenoid triplet state occurs in the carotene-based triads, while singlet recombination is the only active route for the TTF-P-C6o triad, where a low-lying triplet state is lacking. Long-lived charge separation has been observed in the case of TTF-P-C6o: about 8 p.s for the singlet-born radical pair in the glassy isotropic matrix and about 7 ~ts for the triplet-born radical pair in the nematic phase of E-7. For all the molecular triads, a weak exchange interaction (J ~ I G) between the electrons in the final spin-correlated radical pair has been evaluated by simulation of the EPR spectra, providing evidence for superexchange electronic interactions mediated by the tetraarylporphyrin bridge.
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Papers by Giovanni Giacometti