The photochemistry of a molecular triad consisting of a porphyrin (P) covalently linked to a carotenoid polyene (C) and a fullerene derivative (C 60 ) has been studied at 20 K by time-resolved EPR spectroscopy following laser excitation. Excitation of the porphyrin moiety yields C-1 P-C 60 , which decays by photoinduced electron transfer to yield C-P •+ -C 60 •-. This state rapidly evolves into a final charge-separated state C •+ -P-C 60 •-, whose spin-polarized EPR signal was observed and simulated. There is a weak exchange interaction between the electrons in the radical pair (J ) 1.2 G). The C •+ -P-C 60 •state decays to give the carotenoid triplet in high yield with a time constant of 1.2 µs. The spin polarization of 3 C-P-C 60 is characteristic of a triplet formed by charge recombination of a singlet-derived radical pair. The kinetics of the decay of 3 C-P-C 60 to the ground state were also determined. The photoinduced electron transfer from an excited singlet state at low temperature and the high yield of charge recombination to a spin-polarized triplet mimic similar processes observed in photosynthetic reaction centers.

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 [Webber, A. N., Su Hui, Bingham, S. E., Käss, H., Krabben, L., Kuhn, M., Jordan, R., Schlodder, E., and Lubitz, W. (1996) Biochemistry 35, 12857-12863]. Substitutions of His(A676) showed an additional shoulder around 680 nm. In the low-temperature absorbance difference spectra of P700(+)(*)/P700, a blue shift of the main bleaching band by 2 nm and some changes in the spectral features around 660 nm were observed for mutations of His(B656) in PsaB. The analogous substitution in PsaA showed only a shift of the main bleaching band. Similar effects of the mutations were found in the (3)P700/P700 absorbance difference spectra at low temperatures (T = 2 K). The zero-field splitting parameters of (3)P700 were not significantly changed in the mutated PSI complexes. The electron spin density distribution of P700(+)(*), determined by ENDOR spectroscopy, was only changed when His(B656) was replaced. In all measurements, two general observations were made. (i) The replacement of His(B656) had a much stronger impact on the physical properties of P700 than the mutation of His(A676). (ii) The exchange of His(B656) with glutamine induces the smallest changes in the spectra or the midpoint potential, whereas the other replacements exhibited a stronger but very similar influence on the spectroscopic features of P700. The data provide convincing evidence that the unpaired electron in the cation radical and the triplet state of P700 are mainly localized on the Chl a of the dimer which is axially coordinated by His(B656).

The molecular structure of the light-harvesting complex peridinin−chlorophyll−protein from the dinoflagellate Amphidinium carterae (A−PCP) provides the positions and orientations of the eight peridinin (Per) and two chlorophyll a (Chl) molecules in the complex whose apoprotein is 32 kD. We made structure-based calculations of the distinctive optical properties (absorption and CD spectra) of A−PCP and of the complex containing a ratio of four peridinin and one chlorophyll per complex (apoprotein ∼ 15 kD) obtained from the related species Heterocapsa pygmaea (H−PCP). The latter structure has not been determined but can be inferred from that of A−PCP. A point−monopole approximation was used to represent the low-energy transition of peridinin in the blue region of the spectrum and that of chlorophyll in the Soret region. Vibronic interactions are taken into account for peridinin because of the strong vibrational progression exhibited by the spectrum of the latter. From the calculations, we are able to simulate the absorption and CD spectra for H−PCP and A−PCP by using, in addition to the atomic coordinates taken from the A−PCP structure, one and only one set of parameters, adjusted for the small unit of four Per and one Chl common to both systems. In particular, the four peridinin site energies were assigned values in the range 18 500−19 500 cm-1, and those for the B x and B y transitions of chlorophyll a were given the common value 23 100 cm-1. The transition moments for peridinin were in the range 10.6−12.4 D, and those of the chlorophyll B x and B y transitions were 9.0 and 1.0 D, respectively. Each resolved vibronic transition was given the same Gaussian line width of 550 cm-1. Excitonic coupling among the different chromophores of the small cluster unit of the complex is not sufficient to describe the A−PCP optical properties. Intercluster interactions are necessary in order to reproduce the CD spectrum. The H−PCP spectrum, being practically identical to the former, is reproduced only if such interactions are maintained, meaning that the solution unit is a dimer of the monomeric polypeptide as previously inferred from the biochemical properties.