Simultaneous analysis of ultrafast fluorescence decays of FMN binding protein and its mutated proteins by molecular dynamic simulation and electron transfer theory

Ultrafast fluorescence decays of FMN binding proteins (FBP) from Desulfovibrio vulgaris (Miyazaki F) were analyzed with an electron transfer (ET) theory by Kakitani and Mataga (KM theory). Time-dependent distances among isoalloxazine (Iso) and Trp-32, Tyr-35, and Trp-106 in wild-type FBP (WT), among Iso and Tyr-32, Tyr-35, and Trp-106 in W32Y (Trp-32 was replaced by Tyr-32), and among Iso and Tyr-35 and Trp-106 in W32A (Trp-32 was replaced by Ala-32) were determined by molecular dynamic simulation (MD). Electrostatic energies between Iso anion and all other ionic groups, between Trp-32 cation and all other ionic groups, and between Tyr-32 cation and all other ionic groups were calculated in WT, W32Y, and W32A, from the MD coordinates. ET parameters contained in KM theory, such as frequency (nu 0), a coefficient of the ET process (beta), a critical distance of the ET process ( R 0), standard free energy related to the electron affinity of the excited Iso ( G Iso (0)), and the static dielectric constant in FBP species (epsilon 0), were determined with and without inclusion of the electrostatic energy, so as to fit the calculated fluorescence decays with the observed decays of all FBP species, by a nonlinear least-squares method according to the Marquardt algorithm. In the analyses the parameters, nu 0, beta, and R 0 were determined separately between Trp residues and Tyr residues among all FBP species. Calculated fluorescence intensities with the inclusion of the electrostatic energy fit quite well with the observed ones of all WT, W32Y, and W32A.     

Donor-acceptor distance-dependence of photoinduced electron-transfer rate in flavoproteins

Ultrafast fluorescence quenching of flavin in flavodoxin from Megasphaera elsdenii was investigated by means of a fluorescence up-conversion method. Fluorescence lifetimes of flavodoxin from M. elsdenii were estimated to be tau(1) approximately 165 fs (0.97%) and tau(2) approximately 10 ps (0.03%). Correlation of photoinduced electron-transfer rates (k(ET)) with averaged distances (D(av)) between isoalloxazine and nearby tryptophan or tyrosine was examined and obtained an empirical equation of ln k(ET) vs D(av) by means of a nonlinear least-squares method using reported data together with flavodoxin from M. elsdenii. The values of D(av) were calculated from X-ray structures of the flavoproteins. The ln k(ET) was approximately linear at D(av) shorter than 7 A. The model free empirical equation was expressed as ln k(ET) = 29.7 + (-0.327 D(av) + 2.84 x 10(-5))/(0.698 - D(av)(2)). We also analyzed the observed values of ln k(ET) with Marcus theory, but could not obtain reasonable results. Our analysis suggests that the average distance, rather than the shortest (edge to edge) distance or interplanar angles between the aromatics rings, is the key factor in the process of the photoinduced electron transfer in these flavoproteins.     

Ultrafast fluorescence dynamics of FMN-binding protein from Desulfovibrio vulgaris (Miyazaki F) and its site-directed mutated proteins

Ultrafast fluorescence dynamics of FMN in FMN-binding protein (FMN-bp), and its mutated proteins, W32Y and W32A, were investigated by the fluorescence up-conversion method. Fluorescence lifetimes were 167 fs (96%) and 1.5 ps (4%) in wild-type FMN-bp (WT), and 3.4 ps (23%), 18.2 ps (74%), and 96 ps (3%) at 530 nm in W32Y, and 30.1 ps in W32A. The fluorescence lifetime of W32A, in which Trp-32 was absent, was about 140 times longer than that of WT. Tyr-32 in W32Y was not so effective quencher as Trp-32 in WT. This was explained in terms of different ionization potentials of quenchers and average donor–acceptor distances in the protein.     

Simulation of ultrafast non-exponential fluorescence decay induced by electron transfer in FMN binding protein

The ultrafast non-exponential fluorescence decay of FMN binding protein (FBP) was analyzed with three electron transfer (ET) theories, Marcus theory, Bixon and Jortner theory and Kakitani and Mataga theory. Center to center distances between electron acceptor, the excited isoalloxazine, and donors, Trp-32, Tyr-35 and Trp-106, in FBP were determined by molecular dynamic simulation. Electron transfer parameters containing in these theories were determined so as to fit the calculated decay with the observed decay, according to a non-linear least squares method. Introduction of electrostatic energies between isoalloxazine anion and other ionic groups and between the donor cations and other ionic groups in the protein into any ET theories improved the fitting. The non-exponential behavior in the fluorescence decay is considered to be ascribed to a fluctuation of the protein structure with long period.     

Structural basis for the temperature-induced transition of D-amino acid oxidase from pig kidney revealed by molecular dynamic simulation and photo-induced electron transfer

The structural basis for the temperature-induced transition in the D-amino acid oxidase (DAAO) monomer from pig kidney was studied by means of molecular dynamic simulations (MDS). The center to center (Rc) distances between the isoalloxazine ring (Iso) and all aromatic amino acids (Trp and Tyr) were calculated at 10 °C and 30 °C. Rc was shortest in Tyr224 (0.82 and 0.88 nm at 10 and 30 °C, respectively), and then in Tyr228. Hydrogen bonding (H-bond) formed between the Iso N1 and Gly315 N (peptide), between the Iso N3H and Leu51 O (peptide) and between the Iso N5 and Ala49 N (peptide) at 10 °C, whilst no H-bond was formed at the Iso N1 and Iso N3H at 30 °C. The H-bond of Iso O4 with Leu51 N (peptide) at 10 °C switched to that with Ala49 N (peptide) at 30 °C. The reported fluorescence lifetimes (228 and 182 ps at 10 and 30 °C, respectively) of DAAO were analyzed with Kakitani and Mataga (KM) ET theory. The calculated fluorescence lifetimes displayed an excellent agreement with the observed lifetimes. The ET rate was fastest from Tyr224 to the excited Iso (Iso*) at 10 °C and from Tyr314 at 30 °C, despite the fact that the Rc was shortest between Iso and Tyr224 at both temperatures. This was explained by the electrostatic energy in the protein. The differences in the observed fluorescence lifetimes at 10 and 30 °C were ascribed to the differences in electron affinity of the Iso* at both temperatures, in which the free energies of the electron affinity of Iso* at 10 and 30 °C were -8.69 eV and -8.51 eV respectively. The other physical quantities related to ET did not differ appreciably at both temperatures. The electron affinities at both temperatures were calculated with a semi-empirical molecular orbital method (MO) of PM6. Mean calculated electron affinities over 100 snapshots with 0.1 ps intervals were -7.69 eV at 10 °C and -7.59 eV at 30 °C. The difference in the calculated electron affinities, -0.11 eV, was close to the observed difference in the free energies, -0.18 eV. The present quantitative analysis predicts that the highest ET rate can occur from a donor with longer donor-acceptor distance, which was explained by differences in electrostatic energy.     

A femtosecond time-resolved investigation of dual fluorescence from N6,N6-dimethyladenine

The excited electronic state dynamics of N(6),N(6)-dimethyladenine (DMAde), a molecule known to emit dual fluorescence, has been studied in aqueous solution using femtosecond fluorescence up-conversion spectroscopy. Time profiles of the fluorescence of DMAde excited at lambda= 258 nm were measured at a series of wavelengths in the range 320 nm or= 500 nm), which appeared slightly delayed compared to the UV fluorescence, the long-lived fluorescence component (tau(3)) dominated, the second component (tau(2)) disappeared. The results are consistent with the assumption that DMAde is primarily excited to a short-lived local excited (LE) electronic state that fluoresces mostly in the UV and decays rapidly, on a approximately 0.5 ps timescale, to an intramolecular charge transfer (ICT) state that emits only at longer wavelengths in the visible spectrum. The fluorescence-time profiles and transient fluorescence spectra reconstructed from the time profiles provided further information on secondary relaxation processes within and between the excited states and their non-radiative relaxation to the electronic ground state.     

Conformational difference between two subunits in flavin mononucleotide binding protein dimers from Desulfovibrio vulgaris (MF): molecular dynamics simulation

The structural and dynamical properties of five FMN binding protein (FBP) dimers, WT (wild type), E13 K (Glu13 replaced by Lys), E13 R (Glu13 replaced by Arg), E13 T (Glu13 replaced by Thr) and E13Q (Glu13 replaced by Gln), were investigated using a method of molecular dynamics simulation (MDS). In crystal structures, subunit A (Sub A) and subunit B (Sub B) were almost completely equivalent in all of the five FBP dimers. However, the predicted MDS structures of the two subunits were not equivalent in solution, revealed by the distances and inter-planar angles between isoalloxazine (Iso) and aromatic amino acids (Trp32, Tyr35 and Trp106) as well as the hydrogen bonding pairs between Iso and nearby amino acids. Residue root of mean square fluctuations (RMSF) also displayed considerable differences between Sub A and Sub B and in the five FBP dimers. The dynamics of the whole protein structures were examined with the distance (RNN) between the peptide N atom of the N terminal (Met1) and the peptide N atom of the C terminal (Leu122). Water molecules were rarely accessible to Iso in all FBP dimers which are in contrast with other flavoenzymes.     

An ultrafast study of phenyl azide: the direct observation of phenylnitrenium ion

Ultrafast photolysis (lambda(ex) = 308 nm) of phenyl azide in 100% formic acid produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the azide. The azide excited state fragments within 300 fs to form singlet phenylnitrene. The decay of the nitrene (tau = 12.0 ps) produces a new species with absorption centered at 500 nm, which is assigned to phenylnitrenium ion. The lifetime of phenylnitrenium ion is 110 ps in 100% formic acid. This is the first spectroscopic observation of phenylnitrenium ion.     

Multiple time scales in solvation dynamics of DNA in aqueous solution: the role of water, counterions, and cross-correlations

Recent time domain experiments have explored solvation dynamics of a probe located inside a DNA duplex, in an effort to gain information, e.g., on the dynamics of water molecules in the DNA major and minor grooves and their environment. Multiple time constants in the range of a few picoseconds to several nanoseconds were obtained. We have carried out 15 ns long atomistic molecular dynamics simulations to study the solvation dynamics of bases of a 38 base-pair long DNA duplex in an aqueous solution containing counterions. We have computed the energy-energy time correlation function (TCF) of the four individual bases (A, T, G, and C) to characterize the solvation dynamics. All the TCFs display highly nonexponential decay with time. When the trajectories are analyzed with 100 fs time resolution, the TCF of each base shows initial ultrafast decay (with tau1 approximately equal 60-80 fs) followed by two intermediate components (tau2 approximately equal 1 ps, tau3 approximately equal 20-30 ps), in near complete agreement with a recent time domain experiment on DNA solvation. Interestingly, the solvation dynamics of each of the four different nucleotide bases exhibit rather similar time scales. To explore the existence of slow relaxation at longer times reported recently in a series of experiments, we also analyzed the solvation TCFs calculated with longer time trajectories and with a larger time resolution of 1 ps. In this case, an additional slow component with a time constant of the order of 250 ps is observed. Through an analysis of partial solvation TCFs, we find that the slow decay originates mainly from the interaction of the nucleotides with the dipolar water molecules and the counterions. An interesting negative cross-correlation between water and counterions is observed, which makes an important contribution to relaxation at intermediate to longer times.     

Observation of the diffusion-free intermolecular excimer of 9-methyladenine in aqueous solution by picosecond time-resolved spectroscopy

Diffusion-free intermolecular excimer formation in liquid solution is observed for an aqueous solution of 9-methyladenine using the synchroscan streak camera technique. The excimer fluorescence (330 ps lifetime) appears at 380 nm, at longer wavelength than the monomer fluorescence (5 ps lifetime). The excimer is formed via a weakly coupled stacking dimer composed of ground-state monomers.     

The direct detection of an aryl azide excited state: an ultrafast study of the photochemistry of para- and ortho-biphenyl azide

Ultrafast laser flash photolysis (266 nm) of para- and ortho-biphenyl azide in acetonitrile produces azide excited states that have broad absorption bands centered at 480 nm. The para-biphenyl azide excited singlet state has a lifetime of 100 fs. The excited-state lifetime of the ortho-azide isomer is 450 +/- 150 fs. Decay of the azide excited states is accompanied by the formation of the corresponding known singlet nitrenes (para, lambdamax = 350 nm, ortho, lambdamax = 400 nm). Singlet para-biphenylnitrene is born with excess energy and undergoes vibrational cooling with a time constant of 11 ps to form the long-lived (tau approximately 9 ns) relaxed singlet nitrene. Singlet ortho-biphenylnitrene decays with a lifetime of 16 ps in acetonitrile at ambient temperature.     

Ultrafast relaxation of zinc protoporphyrin encapsulated within apomyoglobin in buffer solutions

The relaxation dynamics of a zinc protoporphyrin (ZnPP) in THF, KPi buffer, and encapsulated within apomyoglobin (apoMb) was investigated in its excited state using femtosecond fluorescence up-conversion spectroscopy with S2 excitation (lambda(ex) = 430 nm). The S2 --> S1 internal conversion of ZnPP is ultrafast (tau < 100 fs), and the hot S1 ZnPP species are produced promptly after excitation. The relaxation dynamics of ZnPP in THF solution showed a dominant offset component (tau = 2.0 ns), but it disappeared completely when ZnPP formed aggregates in KPi buffer solution. When ZnPP was reconstituted into the heme pocket of apoMb to form a complex in KPi buffer solution, the fluorescence transients exhibited a biphasic decay feature with the signal approaching an asymptotic offset: at lambda(em) = 600 nm, the rapid component decayed in 710 fs and the slow one in 27 ps; at lambda(em) = 680 nm, the two time constants were 950 fs and 40 ps. We conclude that (1) the fast-decay component pertains to an efficient transfer of energy from the hot S1 ZnPP species to apoMb through a dative bond between zinc and proximal histidine of the protein; (2) the slow-decay component arises from the water-induced vibrational relaxation of the hot S1 ZnPP species; and (3) the offset component is due to S1 --> T1 intersystem crossing of the surviving cold S1 ZnPP species. The transfer of energy through bonds might lead the dative bond to break, which explains our observation of the degradation of ZnPP-Mb samples in UV-vis and CD spectra upon protracted excitation.     

Lifetime determination of fluorescence and phosphorescence of a series of oligofluorenes

The photoluminescence (PL) properties of oligofluorenes with 2-ethylhexyl group in 9, 9' position in solution and as thin films were investigated by time-resolved techniques at both room temperature and 77 K. The fluorescence lifetimes of the oligomers decrease with chain length. The lifetimes tau follow the relation tau=386+808(1/n) (ps) where n is the number of fluorene units in the oligomer. Concentration and laser excitation energy dependences of PL spectra of the oligofluorenes are also given. Phosphorescence was observed for oligofluorenes in the frozen matrix of MTHF at 77 K. The lifetime of phosphorescence increases with increasing molecular length. Similar emission bands were observed for oligofluorenes with a central ketogroup. A lifetime analysis clearly reveals that the "green emission" of the oligomers free of ketogroups results from a phosphorescence with lifetime tau of 3 ms while the green emission from the keto-oligomer is a fluorescence from a charge transfer pi-pi* level of tau=8 ns.     

Photo-deactivation pathways of a double H-bonded photochromic Schiff base investigated by combined theoretical calculations and experimental time-resolved studies

The photophysics of N,N'-bis(salicylidene)-p-phenylenediamine (BSP) is analyzed both theoretically and experimentally. The alternative intramolecular proton-transfer reactions lead to three different tautomers. We performed DFT and TDDFT calculations to analyze the topography of the reactions connecting the three tautomers. Deactivation paths through a Conical Intersection (CI) region are also analyzed to explain the low fluorescence quantum yield of the phototautomers. The complex molecular structure of BSP provides a large number of deactivation paths, almost all of them energetically available following the initial photoexcitation. Femtosecond (fs) time-resolved emission studies in solution and flash photolysis experiments (nano to millisecond regime) were performed to get detailed information on the time domain of the full photocycle. The picture that emerges by combining theoretical and experimental results shows a very fast (less than 100 fs) photoinduced single proton transfer process leading to a phototautomer where a single proton has moved. This species may deactivate through a low-energy CI leading in about 20 ps to a rotameric form in the ground state that has a lifetime of several tens of microseconds in solution. This process competes with another deactivation path taking place prior to the proton-transfer reaction which involves a low-energy CI leading to a rotamer of the enol structure. In the flash photolysis studies, the rotamer of the enol structure was directly identified by the positive transient absorption band in the 250-260 nm and its lifetime in n-hexane (10 ms) is almost 3 orders of magnitude longer than the lifetime of the photochrome (around 40 μs). Our findings do not exclude a double proton transfer reaction in the excited enol form to give a tautomer in less than 100 fs during the first (impulsive) phase of the reaction which reverts back to the photoproducts of the simple proton transfer in 1-3 ps.     

Femtosecond/picosecond time-resolved fluorescence study of hydrophilic polymer fine particles

Femtosecond/picosecond time-resolved fluorescence study of hydrophilic polymer fine particles (polyacrylamide, PAAm) was reported. Ultrafast fluorescence dynamics of polymer/water solution was monitored using a fluorescent probe molecule (C153). In the femtosecond time-resolved fluorescence measurement at 480 nm, slowly decay components having lifetimes of tau(1) approximately 53 ps and tau(2) approximately 5 ns were observed in addition to rapid fluorescence decay. Picosecond time-resolved fluorescence spectra of C153/PAAm/H2O solution were also measured. In the time-resolved fluorescence spectra of C153/PAAm/H2O, a peak shift from 490 to 515 nm was measured, which can be assigned to the solvation dynamics of polymer fine particles. The fluorescence peak shift was related to the solvation response function and two time constants were determined (tau(3) approximately 50 ps and tau(4) approximately 467 ps). Therefore, the tau(1) component observed in the femtosecond time-resolved fluorescence measurement was assigned to the solvation dynamics that was observed only in the presence of polymer fine particles. Rotational diffusion measurements were also carried out on the basis of the picosecond time-resolved fluorescence spectra. In the C153/PAAm/H2O solution, anisotropy decay having two different time constants was also derived (tau(6) approximately 76 ps and tau(7) approximately 676 ps), indicating the presence of two different microscopic molecular environments around the polymer surface. Using the Stokes-Einstein-Debye (SED) equation, microscopic viscosity around the polymer surface was evaluated. For the area that gave a rotational diffusion time of tau(6) approximately 76 ps, the calculated viscosity is approximately 1.1 cP and for tau(7) approximately 676 ps, it is approximately 10 cP. The calculated viscosity values clearly revealed that there are two different molecular environments around the polyacrylamide fine particles.     

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. II. Electron dynamics and outer ionization of the nanoplasma

We explore electron dynamics in molecular (CD4)(1061) clusters and elemental Xen (n=249-2171) clusters, responding to ultraintense (intensity I=10(16)-10(19) W cm(-2)) laser fields. Molecular dynamics simulations (including magnetic field and relativistic effects) and analyses of high-energy electron dynamics and nuclear ion dynamics in a cluster interacting with a Gaussian shaped laser field (frequency 0.35 fs(-1), photon energy 1.44 eV, phase 0, temporal width 25 fs) elucidated the time dependence of inner ionization, the formation of a nanoplasma of unbound electrons within the cluster or its vicinity, and of outer ionization. We determined the cluster size and the laser intensity dependence of these three sequential-parallel electronic processes. The characteristic times for cluster inner ionization (tau(ii)) and for outer ionization (tau(oi)) fall in the femtosecond time domain, i.e., tau(ii)=2-9 fs and tau(oi)=4-15 fs for (CD4)(1061), tau(ii)=7-30 fs and tau(oi)=5-13 fs for Xe(n) (n=479,1061), with both tau(ii) and tau(oi) decreasing with increasing I, in accord with the barrier suppression ionization mechanism for inner ionization of the constituents and the cluster barrier suppression ionization mechanism for outer ionization. The positive delay times Deltatau(OI) between outer and inner ionization (e.g., Deltatau(OI)=6.5 fs for Xen at I=10(16) W cm(-2) and Deltatau(OI)=0.2 fs for (CD4)(1061) at I=10(19) W cm(-2)) demonstrate that the outer/inner ionization processes are sequential. For (CD4)(1061), tau(ii)tau(oi), reflecting on the energetic hierarchy in the ionization of the Xe atoms. Quasiresonance contributions to the outer ionization of the nanoplasma were established, as manifested in the temporal oscillations in the inner/outer ionization levels, and in the center of mass of the nanoplasma electrons. The formation characteristics, dynamics, and response of the nanoplasma in molecular or elemental clusters were addressed. The nanoplasma is positively charged, with a high-average electron density [rho(P)=(2-3)10(22) cm(-3)], being characterized by high-average electron energies epsilon(av) (e.g., in Xe(1061) clusters epsilon(av)=54 eV at I=10(16) W cm(-2) and epsilon(av)=0.56-0.37 keV at I=10(18) W cm(-2), with epsilon(av) proportional, variant I(1/2)). Beyond the cluster boundary the average electron energy markedly increases, reaching electron energies in the range of 1.2-40 keV for outer ionization of Xe(n) (n=249-2171) clusters. The nanoplasma exhibits spatial inhomogeneity and angular anisotropy induced by the laser field. Femtosecond time scales are predicted for the nanoplasma production (rise times 7-3 fs), for the decay (decay times approximately 5 fs), and for the persistence time (30-10 fs) of a transient nanoplasma at I=10(17)-10(18) W cm(-2). At lower intensities of I=10(16) W cm(-2) a persistent nanoplasma with a "long" lifetime of > 50 fs will prevail.     

Ultrafast fluorescence depolarisation in the yellow fluorescent protein due to its dimerisation.

Transient absorption spectroscopy with sub-100 fs time resolution was performed to investigate the oligomerisation behaviour of eYFP in solution. A single time constant tau(AD)=2.2+/-0.15 ps is sufficient to describe the time-resolved anisotropy decay up to at least 200 ps. The close contact of two protein barrels is deduced as the exclusive aggregation state in solution. From the final anisotropy r(infinity)=0.28+/-0.02, the underlying quaternary structure can be traced back to the somewhat distorted structure of the dimers of wt-GFP. The use of autofluorescent proteins as rulers in F?rster resonance energy transfer (FRET) measurements may demand polarisation-sensitive detection of the fluorescence with high time resolution.     

Transient lens spectroscopy of ultrafast internal conversion processes in citranaxanthin

The ultrafast internal conversion (IC) dynamics of the apocarotenoid citranaxanthin have been studied for the first time by means of two-color transient lens (TL) pump-probe spectroscopy. After excitation into the high-energy edge of the S2 band by a pump pulse at 400 nm, the subsequent intramolecular processes were probed at 800 nm. Experiments were performed in a variety of solvents at room temperature. Upper limits for the S2 lifetime tau2 on the order of 100-200 fs are estimated. The S1 lifetime tau1 varies only slightly between solvents (10-12 ps), and the only clear decrease is observed for methanol (8.5 ps). The findings are consistent with earlier results from transient absorption studies of other apocarotenoids and carotenoid ketones and transient lens experiments of C40 carbonyl carotenoids. Possible reasons for the observed weak solvent dependence of tau1 for citranaxanthin are discussed.     

Dynamics of photoinduced processes in adenine and thymine base pairs

The excited-state dynamics of adenine and thymine dimers and the adenine-thymine base pair were investigated by femtosecond pump-probe ionization spectroscopy with excitation wavelengths of 250-272 nm. The base pairs showed a characteristic ultrafast decay of the initially excited pi pi* state to an n pi* state (lifetime tau(pi pi*) approximately 100 fs) followed by a slower decay of the latter with tau(n pi*) approximately 0.9 ps for (adenine)2, tau(n pi*) = 6-9 ps for (thymine)2, and tau(n pi*) approximately 2.4 ps for the adenine-thymine base pair. In the adenine dimer, a competing decay of the pi pi* state via the pi sigma* state greatly suppressed the n pi* state signals. Similarities of the excited-state decay parameters in the isolated bases and the base pairs suggest an intramonomer relaxation mechanism in the base pairs.     

Modified windmill porphyrin arrays: coupled light-harvesting and charge separattion, conformational relaxation in the S1 state, and S2-S2 energy transfer

The architecture of windmill hexameric zinc(II) -porphyrin array 1 is attractive as a light-harvesting functional unit in view of its three-dimensionally extended geometry that is favorable for a large cross-section of incident light as well as for a suitable energy gradient from the peripheral porphyrins to the meso-meso-linked diporphyrin core. Three core-modified windmill porphyrin arrays 2-4 were prepared for the purpose of enhancing the intramolecular energy-transfer rate and coupling these arrays with a charge-separation functional unit. Bisphenylethynylation at the meso and meso' positions of the diporphyrin core indeed resulted in a remarkable enhancement in the intramolecular S1-S1 energy transfer in 2 with tau=2 approximately 3 ps, as revealed by femtosecond time-resolved transient absorption spectroscopy. The fluorescence lifetime of the S2 state of the peripheral porphyrin energy donor determined by the fluorescence up-conversion method was 68 fs, and thus considerably shorter than that of the reference monomer (150 fs), suggesting the presence of the intramolecular energy-transfer channel in the S2 state manifold. Such a rapid energy transfer can be understood in terms of large Coulombic interactions associated with the strong Soret transitions of the donor and acceptor. Picosecond time-resolved fluorescence spectra and transient absorption spectra revealed conformational relaxation of the S1 state of the diporphyrin core with tau = 25 ps. Upon photoexcitation of models 3 and 4, which bear a naphthalenetetracarboxylic diimide or a meso-nitrated free-base porphyrin attached to the modified diporphyrin core as an electron acceptor, a series of photochemical processes proceeded, such as the collection of the excitation energy at the diporphyrin core, the electron transfer from the S1 state of the diporphyrin to the electron acceptor, and the electron transfer from the peripheral porphyrins to the diporphyrin cation radical, which are coupled to provide a fully charge-separated state such as that in the natural photosynthetic reaction center. The overall quantum yield for the full charge separation is better in 4 than in 3 owing to the slower charge recombination associated with smaller reorganization energy of the porphyrin acceptor.