Incorporation of a high potential quinone reveals that electron transfer in Photosystem I becomes highly asymmetric at low temperature

Photosystem I (PS I) has two nearly identical branches of electron-transfer co-factors. Based on point mutation studies, there is general agreement that both branches are active at ambient temperature but that the majority of electron-transfer events occur in the A-branch. At low temperature, reversible electron transfer between P700 and A1A occurs in the A-branch. However, it has been postulated that irreversible electron transfer from P700 through A1B to the terminal iron-sulfur clusters FA and FB occurs via the B-branch. Thus, to study the directionality of electron transfer at low temperature, electron transfer to the iron-sulfur clusters must be blocked. Because the geometries of the donor–acceptor radical pairs formed by electron transfer in the A- and B-branch differ, they have different spin-polarized EPR spectra and echo- modulation decay curves. Hence, time-resolved, multiple-frequency EPR spectroscopy, both in the direct-detection and pulse mode, can be used to probe the use of the two branches if electron transfer to the iron-sulfur clusters is blocked. Here, we use the PS I variant from the menB deletion mutant strain of Synechocyctis sp. PCC 6803, which is unable to synthesize phylloquinone, to incorporate 2,3-dichloro-1,4-naphthoquinone (Cl2NQ) into the A1A and A1B binding sites. The reduction midpoint potential of Cl2NQ is approximately 400 mV more positive than that of phylloquinone and is unable to transfer electrons to the iron-sulfur clusters. In contrast to previous studies, in which the iron-sulfur clusters were chemically reduced and/or point mutations were used to prevent electron transfer past the quinones, we find no evidence for radical-pair formation in the B-branch. The implications of this result for the directionality of electron transfer in PS I are discussed

Characterization of chlorophyll f synthase heterologously produced in Synechococcus sp. PCC 7002.

In diverse terrestrial cyanobacteria, Far-Red Light Photoacclimation (FaRLiP) promotes extensive remodeling of the photosynthetic apparatus, including photosystems (PS)I and PSII and the cores of phycobilisomes, and is accompanied by the concomitant biosynthesis of chlorophyll (Chl) d and Chl f. Chl f synthase, encoded by chlF, is a highly divergent paralog of psbA; heterologous expression of chlF from Chlorogloeopsis fritscii PCC 9212 led to the light-dependent production of Chl f in Synechococcus sp. PCC 7002 (Ho et al., Science 353, aaf9178 (2016)). In the studies reported here, expression of the chlF gene from Fischerella thermalis PCC 7521 in the heterologous system led to enhanced synthesis of Chl f. N-terminally [His]10-tagged ChlF7521 was purified and identified by immunoblotting and tryptic-peptide mass fingerprinting. As predicted from its sequence similarity to PsbA, ChlF bound Chl a and pheophytin a at a ratio of ~ 3-4:1, bound β-carotene and zeaxanthin, and was inhibited in vivo by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Cross-linking studies and the absence of copurifying proteins indicated that ChlF forms homodimers. Flash photolysis of ChlF produced a Chl a triplet that decayed with a lifetime (1/e) of ~ 817 µs and that could be attributed to intersystem crossing by EPR spectroscopy at 90 K. When the chlF7521 gene was expressed in a strain in which the psbD1 and psbD2 genes had been deleted, significantly more Chl f was produced, and Chl f levels could be further enhanced by specific growth-light conditions. Chl f synthesized in Synechococcus sp. PCC 7002 was inserted into trimeric PSI complexes

Mechanism and Stereocontrol in Isotactic <i>rac</i>-Lactide Polymerization with Copper(II) Complexes

Reaction of <i>N</i>-R,<i>N</i>′-R′-2,5-diiminopyrroles (R = R′ = <i>S</i>-CH­(Me)­Ph; R = R′ = CH₂Ph; R = <i>S</i>-CH­(Me)­Ph, R′ = H) with Cu­(OMe)₂ in the presence of chelating alcohols, ROH (R1 = C₂H₄NMe₂, R2 = C₂H₄Py, R3 = CH₂Py, R4 = CMe₂Py) yielded the dinuclear, alkoxide-bridged complexes L₂Cu₂(OR)₂. The complexes catalyze the polymerization of <i>rac</i>-lactide at room temperature with catalyst concentrations of 1–3 mM in 4–24 h (<i>v</i> = <i>k</i>[cat]­[monomer] with <i>k</i> = [2.3(5)] × 10² – [6.5(6)] × 10² M^–1 h^–1). EPR and mechanistic studies indicate that the complexes remain dinuclear during the polymerization reaction. In complexes with OR1, both alkoxides of the dimer initiate polymerization, with OR2 or OR3 only one alkoxide initiates polymerization, and OR4 is inactive in polymerization. The nature of the bridging ligand in the dinuclear complex determines stereocontrol. Independent of the spectator ligand L, complexes which retain an OR3 or OR4 bridging ligand in the active species show preference for isotactic polymerizations (<i>P</i>_m = 0.60–0.75), while those with only polymeryloxo bridges or OR2 as the bridging ligand provide atactic polymer. Stereocontrol follows a chain-end control mechanism, with the catalytic site likely adapting to the configuration of the chain end

Mechanism and Stereocontrol in Isotactic <i>rac</i>-Lactide Polymerization with Copper(II) Complexes

Reaction of <i>N</i>-R,<i>N</i>′-R′-2,5-diiminopyrroles (R = R′ = <i>S</i>-CH­(Me)­Ph; R = R′ = CH₂Ph; R = <i>S</i>-CH­(Me)­Ph, R′ = H) with Cu­(OMe)₂ in the presence of chelating alcohols, ROH (R1 = C₂H₄NMe₂, R2 = C₂H₄Py, R3 = CH₂Py, R4 = CMe₂Py) yielded the dinuclear, alkoxide-bridged complexes L₂Cu₂(OR)₂. The complexes catalyze the polymerization of <i>rac</i>-lactide at room temperature with catalyst concentrations of 1–3 mM in 4–24 h (<i>v</i> = <i>k</i>[cat]­[monomer] with <i>k</i> = [2.3(5)] × 10² – [6.5(6)] × 10² M^–1 h^–1). EPR and mechanistic studies indicate that the complexes remain dinuclear during the polymerization reaction. In complexes with OR1, both alkoxides of the dimer initiate polymerization, with OR2 or OR3 only one alkoxide initiates polymerization, and OR4 is inactive in polymerization. The nature of the bridging ligand in the dinuclear complex determines stereocontrol. Independent of the spectator ligand L, complexes which retain an OR3 or OR4 bridging ligand in the active species show preference for isotactic polymerizations (<i>P</i>_m = 0.60–0.75), while those with only polymeryloxo bridges or OR2 as the bridging ligand provide atactic polymer. Stereocontrol follows a chain-end control mechanism, with the catalytic site likely adapting to the configuration of the chain end