Energy and photoinduced electron transfer in a wheel-shaped artificial photosynthetic antenna-reaction center complex.

Functional mimics of a photosynthetic antenna-reaction center complex comprising five bis(phenylethynyl)anthracene antenna moieties and a porphyrin-fullerene dyad organized by a central hexaphenylbenzene core have been prepared and studied spectroscopically. The molecules successfully integrate singlet-singlet energy transfer and photoinduced electron transfer. Energy transfer from the five antennas to the porphyrin occurs on the picosecond time scale with a quantum yield of 1.0. Comparisons with model compounds and theory suggest that the Förster mechanism plays a major role in the extremely rapid energy transfer, which occurs at rates comparable to those seen in some photosynthetic antenna systems. A through-bond, electron exchange mechanism also contributes. The porphyrin first excited singlet state donates an electron to the attached fullerene to yield a P(*+)-C(60)(*-) charge-separated state, which has a lifetime of several nanoseconds. The quantum yield of charge separation based on light absorbed by the antenna chromophores is 80% for the free base molecule and 96% for the zinc analogue.

The design and synthesis of artificial photosynthetic antennas, reaction centres and membranes.

Artificial antenna systems and reaction centres synthesized in our laboratory are used to illustrate that structural and thermodynamic factors controlling energy and electron transfer in these constructs can be modified to optimize performance. Artificial reaction centres have been incorporated into liposomal membranes where they convert light energy to vectorial redox potential. This redox potential drives a Mitchellian, quinone-based, proton-transporting redox loop that generates a Deltamu H(+) of ca. 4.4 kcal mol(-1) comprising DeltapH ca. 2.1 and Deltapsi ca. 70 mV. In liposomes containing CF(0)F(1)-ATP synthase, this system drives ATP synthesis against an ATP chemical potential similar to that observed in natural systems.

Towards a synthetic model of the photosynthetic water oxidizing complex: [Mn3O4(O2CMe)4(bpy)2] containing the [MnIV3(mu-O)4]4+ core.

The 3 MnIV title compound has been prepared and characterized by X-ray crystallography and magnetochemistry; the complex contains a [Mn(mu-O)2Mn(mu-O)2Mn]4+ core and possesses an S = 3/2 ground state.

In vitro assembly of a beta2 cytochrome b559-like complex from the chemically synthesised beta-subunit encoded by the Synechocystis sp. 6803 psbF gene.

The alpha- and beta-subunits of cytochrome b559 encoded by the psbE and psbF gene, respectively, are essential components of photosystem II. The exact structure of this cytochrome is not yet known. The beta-subunit of the Synechocystis sp. 6803 cytochrome b559 complex was synthesised by means of solid-phase peptide synthesis. Under reducing conditions, two beta-peptide molecules could be assembled specifically with one haem to form a beta2 cytochrome b559-like complex. The spectral properties and the midpoint redox potential (48+/-5 mV) of the in vitro assembled beta2 cytochrome are nearly identical to those of the low potential form of the native cytochrome b559.

Reconstitution of core light-harvesting complexes of photosynthetic bacteria using chemically synthesized polypeptides. 1. Minimal requirements for subunit formation.

Described are the chemical synthesis, isolation and characterization of each of three polypeptides whose amino acid sequences reproduce portions of the amino acid sequence of the beta-polypeptides of the core light-harvesting complex (LH1) of Rhodobacter sphaeroides or Rhodospirillum rubrum. The native beta-polypeptides of LH1 of these organisms contain 48 and 54 amino acids, respectively. The smallest synthetic polypeptide had an amino acid sequence identical to that of the last 16 amino acids of the beta-polypeptide of Rb. sphaeroides (sph beta 16) but failed to form either a subunit- or LH1-type complex under reconstitution conditions. Also, this polypeptide, lengthened on the N terminus by adding the sequence Lys-Ile-Ser-Lys to enhance solubility, failed to form a subunit- or LH1-type complex. In contrast, polypeptides containing either the 31 amino acids at the C terminus of the beta-polypeptide of Rb. sphaeroides (sph beta 31) or the equivalent 31 amino acids of the beta-polypeptide of Rs. rubrum (rr beta 31) were fully competent in forming a subunit-type complex and exhibited association constants for complex formation comparable to or exceeding those of the native beta-polypeptides. The absorption and CD spectra of these subunit-type complexes were nearly identical to those of subunit complexes formed with native beta-polypeptides. It may be concluded that all structural features required to make the subunit complex are present in the well-defined, chemically synthesized polypeptides. Neither polypeptide appeared to interact with the native alpha-polypeptides to form a LH1-type complex. However, sph beta 31 formed a LH1-type complex absorbing at 849 nm without an alpha-polypeptide. Although chemical syntheses of polypeptides of this size are common, the purification of membrane-spanning segments is much more challenging because the polypeptides lack solubility in water. The chemical syntheses reported here represent the first such syntheses of membrane-spanning polypeptides which display native activity upon reconstitution.

Reconstitution of core light-harvesting complexes of photosynthetic bacteria using chemically synthesized polypeptides. 2. Determination of structural features that stabilize complex formation and their implications for the structure of the subunit complex.

Chemically synthesized polypeptides have been utilized with a reconstitution assay to determine the role of specific amino acid side chains in stabilizing the core light-harvesting complex (LH1) of photosynthetic bacteria and its subunit complex. In the preceding paper [Meadows, K. A., Parkes-Loach, P. S., Kehoe, J. W., and Loach, P. A. (1998) Biochemistry 37, 3411-3417], it was demonstrated that 31-residue polypeptides (compared to 48 and 54 amino acids in the native polypeptides) having the same sequence as the core region of the beta-polypeptide of Rhodobacter sphaeroides (sph beta 31) or Rhodospirillum rubrum (rr beta 31) could form subunit-type complexes. However, neither polypeptide interacted with the native alpha-polypeptides to form a native LH1 complex. In this paper, it is demonstrated that larger segments of the native Rb. sphaeroides beta-polypeptide possess native behavior in LH1 formation. Polypeptides were synthesized that were six (sph beta 37) and ten amino acids (sph beta 41) longer than sph beta 31. Although sph beta 37 exhibited behavior nearly identical to that of sph beta 31, sph beta 41 behaved more like the native polypeptide. In the case of rr beta 31, a polypeptide with four additional amino acids toward the C terminus was synthesized (rr beta 35). Because LH1-forming behavior was not recovered with this longer polypeptide, one or more of the three remaining amino acids at the C-terminal end of the native beta-polypeptide seem to play an important role in LH1 stabilization in Rs. rubrum. Three analogues of the core region of the Rb. sphaeroides beta-polypeptide were synthesized, in each of which one highly conserved amino acid was changed. Evidence was obtained that the penultimate amino acid, a Trp residue, is especially important for subunit formation. When it was changed to Phe, the lambda Max of the subunit shifted from 823 to 811 nm and the association constant decreased about 500-fold. Changing each of two other amino acids had smaller effects on subunit formation. Changing Trp to Phe at the location six amino acid residues toward the C terminus from the His coordinated to Bchl resulted in an approximately 10-fold decrease in the association constant for subunit formation but did not affect the formation of a LH1-type complex compared to sph beta 31. Finally, changing Arg to Leu at the location seven amino acid residues toward the C terminus from the His coordinated to Bchl decreased the association constant for subunit formation by about 30-fold. In this case, no LH1-type complex could be formed. On the basis of these results, in comparison with the crystal structure of the LH2 beta-polypeptide of Rhodospirillum molischianum, two possible structures for the subunit complex are suggested.

Synthetic S-(2,3-dihydroxypropyl)-cysteinyl peptides derived from the N-terminus of the cytochrome subunit of the photoreaction centre of Rhodopseudomonas viridis enhance murine splenocyte proliferation.

Various lipopeptides representing the N-terminal part of the cytochrome subunit of the photosynthetic reaction centre from the purple bacterium Rhodopseudomonas virdis were prepared by solid-phase peptide synthesis. These lipopeptides consisted of a S-[2,3-dihydroxypropyl]-cysteinyl (Dhc) residue N-terminally coupled to the nonapeptide FEPPPATTT. Different numbers of palmitoyl (Pam) chains were attached to Dhc via ester and/or amide bonds. The lipopeptide Dhc(Pam)2-FEPPPATTT containing two ester-bonded palmitoyl residues and a free N-terminus was a potent polyclonal activator of murine (BALB/c) spleen cells at subnanomolar concentrations. The lipopeptide Pam-Dhc(Pam)2-FEPPPATTT containing three palmitoyl residues, the two-chain lipopeptide Pam-Dhc(Pam)-FEPPPATTT containing one amide- and one ester-bonded palmitoyl residue, and the N-terminally elongated lipopeptide SLVAG-Dhc(Pam)2-FEPPPATTT were less active. The nonapeptide FEPPPATTT and the decapeptide Dhc-FEPPPATTT were only marginal splenocyte activators, even at concentrations as high as 1 microM. Thus, lipopeptide Dhc(Pam)2-FEPPPATTT constitutes the first potent splenocyte stimulation Dhc-lipopeptide described so far that contains only two fatty acid residues.