Piotr Wawrzyniak

Alternating syn-anti bacteriochlorophylls form concentric helical nanotubes in chlorosomes

Chlorosomes are the largest and most efficient light-harvesting antennae found in nature, and they are constructed from hundreds of thousands of self-assembled bacteriochlorophyll (BChl) c, d, or e pigments. Because they form very large and compositionally heterogeneous organelles, they had been the only photosynthetic antenna system for which no detailed structural information was available. In our approach, the structure of a member of the chlorosome class was determined and compared with the wild type (WT) to resolve how the biological light-harvesting function of the chlorosome is established. By constructing a triple mutant, the heterogeneous BChl c pigment composition of chlorosomes of the green sulfur bacteria Chlorobaculum tepidum was simplified to nearly homogeneous BChl d. Computational integration of two different bioimaging techniques, solid-state NMR and cryoEM, revealed an undescribed syn-anti stacking mode and showed how ligated BChl c and d self-assemble into coaxial cylinders to form tubular-shaped elements. A close packing of BChls via π–π stacking and helical H-bonding networks present in both the mutant and in the WT forms the basis for ultrafast, long-distance transmission of excitation energy. The structural framework is robust and can accommodate extensive chemical heterogeneity in the BChl side chains for adaptive optimization of the light-harvesting functionality in low-light environments. In addition, syn-anti BChl stacks form sheets that allow for strong exciton overlap in two dimensions enabling triplet exciton formation for efficient photoprotection.

Zinc chlorins for artificial light-harvesting self-assemble into antiparallel stacks forming a microcrystalline solid-state material

We introduce a concept to solve the structure of a microcrystalline material in the solid-state at natural abundance without access to distance constraints, using magic angle spinning (MAS) NMR spectroscopy in conjunction with X-ray powder diffraction and DFT calculations. The method is applied to a novel class of materials that form (semi)conductive 1D wires for supramolecular electronics and artificial light-harvesting. The zinc chlorins 3-devinyl-31-hydroxymethyl-132-demethoxycarbonylpheophorbide a (3′,5′-bis-dodecyloxy)benzyl ester zinc complex 1 and 3-devinyl-31-methoxymethyl-132-demethoxycarbonylpheophorbide a (3′,5′-bis-dodecyloxy)benzyl ester zinc complex 2, self-assemble into extended excitonically coupled chromophore stacks. 1H-13C heteronuclear dipolar correlation MAS NMR experiments provided the 1H resonance assignment of the chlorin rings that allowed accurate probing of ring currents related to the stacking of macrocycles. DFT ring-current shift calculations revealed that both chlorins self-assemble in antiparallel π-stacks in planar layers in the solid-state. Concomitantly, X-ray powder diffraction measurements for chlorin 2 at 80 °C revealed a 3D lattice for the mesoscale packing that matches molecular mechanics optimized aggregate models. For chlorin 2 the stacks alternate with a periodicity of 0.68 nm and a 3D unit cell with an approximate volume of 6.28 nm3 containing 4 molecules, which is consistent with space group P21221.

Intramolecular phosphine-phosphine donor-acceptor complexes.

The reaction of 5-diphenoxyphosphanyl-6-diisopropylphosphinoacenaphthene 12 with chlorotrimethylsilane unexpectedly gave a phosphonium-phosphine compound 13, containing the structural motif of four phosphorus atoms connected in a chain. To explain the mechanism of this complex transformation, a proposed intermediate 5-dichlorophosphino-6-diisopropylphosphinoacenaphthene 14 was synthesized by an alternative method. The two (formally) phosphine environments in 14 form an intramolecular donor-acceptor (phosphonium-phosphoranide) complex, stable at room temperature in the solid state and as a solution in certain solvents. A (31)P NMR mechanistic study showed that, despite the presence of a rigid acenaphthene backbone, 14 is unstable in the presence of nucleophiles and disproportionates into 13 and other phosphorus containing products. Both 13 and 14 have been crystallographically characterized.

Differential Charge Polarization of Axial Histidines in Bacterial Reaction Centers Balances the Asymmetry of the Special Pair

In photosynthesis, light energy is transformed into chemical energy that sustains most forms of life on earth. Solid-state NMR spectroscopy in conjunction with density functional theory modeling can resolve electronic structure down to the atomic level in large membrane proteins. In this work, we have used this technique to address the mechanisms underlying the photochemical reactivity of the special pair in the bacterial reaction center. For charge separation, the electrostatics is important, as the Coulomb barrier must be overcome. On the basis of (15)N NMR data, we resolve a subtle charge-balancing mechanism in the ground state by an axial histidine that is connected to the central Mg(2+) on one side and hydrogen-bonded on the other side. Formation of the hydrogen bond between BChl-a-His and H(2)O leads to a difference in electron density relative to the separate BChl-a-His and H(2)O fragments, with excess positive charge on the imidazole ring. This can lower the kinetic barrier for accommodating the different length scales of electron and proton transfer for separation of spin and charge in a bidirectional proton-coupled electron-transfer mechanism.

Nuclear Magnetic Resonance Secondary Shifts of a Light-Harvesting 2 Complex Reveal Local Backbone Perturbations Induced by Its Higher-Order Interactions

Protein nuclear magnetic resonance (NMR) secondary chemical shifts are widely used to predict the secondary structure, and in solid-state NMR, they are often the only unambiguous structural parameters available. However, the employed prediction methods are empirical in nature, relying on the assumption that secondary shifts are only affected by shielding effects of neighboring atoms. We analyzed the secondary shifts of a photosynthetic membrane protein with a high density of chromophores and very tight packing, the light-harvesting 2 (LH2) complex of Rhodopseudomonas acidophila. A relation was found between secondary shift anomalies and protein-protein or pigment-protein tertiary and quaternary contacts. For several residues, including the bacteriochlorophyll-coordinating histidines (alphaH31 and betaH30) and the phenylalanine alphaF41 that has strongly twisted C(b)-C(a)-C and C(a)-C-N conformations in the LH2 crystal structure, the perturbing effects on the backbone chemical shifts were tested by density functional theory (DFT) calculations. We propose that higher-order interactions in the tightly packed complex can induce localized perturbations of the backbone conformation and electronic structure, related to functional pigment-protein or protein-protein interactions.

On the rate-determining step and the ligand electronic effects in rhodium catalysed hydrogenation of enamines and the hydroaminomethylation of alkenes

In the course of studies on the tandem hydroformylation–reductive amination (hydroaminomethylation), fluorinated mono-phosphines were found to be more active than their more electron-donating counterparts in the enamine hydrogenation step of the reaction; this is in contrast to the widely held view that alkene hydrogenation activity increases with ligand donor strength. DFT calculations comparing the reaction pathways for a simple alkene and a representative enamine show that the rate-determining step changes from the first insertion into a Rh–H bond for but-2-ene to the final reductive elimination step from the Rh–hydride–alkyl species in the enamine hydrogenations.

2-Hydroxy- and 2-Amino-Functional Arylphosphines—Syntheses, Reactivity, and Use in Catalysis

We present the synthesis, resolution, structures and catalytic activity of selected representatives of 2-(1S)-camphanoyloxy-phenyl- and biphenylphosphines and the access to novel asymmetric P,N-heterocyclic ethylenebis(phosphine) ligands.

Dealkanative Main Group Couplings across the peri-Gap

Here, we highlight the ability of peri-substitution chemistry to promote a series of unique P-P/P-As coupling reactions, which proceed with concomitant C-H bond formation. This dealkanative reactivity represents an interesting and unexpected expansion to the established family of main-group dehydrocoupling reactions. These transformations are exceptionally clean, proceeding essentially quantitatively at relatively low temperatures (70-140 °C), with 100% diastereoselectivity in the products. The reaction appears to be radical in nature, with the addition of small quantities of a radical initiator (azobis(isobutyronitrile)) increasing the rate dramatically, as well as altering the apparent order of reaction. DFT calculations suggest that the reaction involves dissociation of a phosphorus centered radical (stabilized by the peri-backbone) to the P-P coupled product and a free propyl radical, which carries the chain. This unusual reaction demonstrates the powerful effect that geometric constraints, in this case a rigid scaffold, can have on the reactivity of main group species, an area of research that is gaining increasing prominence in recent years.