Among the biological phenomena that fall within the emerging field of “quantum biology” is the suggestion that magnetically sensitive chemical reactions are responsible for the magnetic compass of migratory birds. It has been proposed that transient radical pairs are formed by photo-induced electron transfer reactions in cryptochrome proteins and that their coherent spin dynamics are influenced by the geomagnetic field leading to changes in the quantum yield of the signaling state of the protein. Despite a variety of supporting evidence, it is still not clear whether cryptochromes have the properties required to respond to magnetic interactions orders of magnitude weaker than the thermal energy,
k
B
T
. Here we demonstrate that the kinetics and quantum yields of photo-induced flavin—tryptophan radical pairs in cryptochrome are indeed magnetically sensitive. The mechanistic origin of the magnetic field effect is clarified, its dependence on the strength of the magnetic field measured, and the rates of relevant spin-dependent, spin-independent, and spin-decoherence processes determined. We argue that cryptochrome is fit for purpose as a chemical magnetoreceptor.
N-doping
of conjugated polymers either requires a high dopant fraction
or yields a low electrical conductivity because of their poor compatibility
with molecular dopants. We explore n-doping of the polar naphthalenediimide–bithiophene
copolymer p(gNDI-gT2) that carries oligoethylene glycol-based side
chains and show that the polymer displays superior miscibility with
the benzimidazole–dimethylbenzenamine-based n-dopant N-DMBI.
The good compatibility of p(gNDI-gT2) and N-DMBI results in a relatively
high doping efficiency of 13% for n-dopants, which leads to a high
electrical conductivity of more than 10–1 S cm–1 for a dopant concentration of only 10 mol % when
measured in an inert atmosphere. We find that the doped polymer is
able to maintain its electrical conductivity for about 20 min when
exposed to air and recovers rapidly when returned to a nitrogen atmosphere.
Overall, solution coprocessing of p(gNDI-gT2) and N-DMBI results in
a larger thermoelectric power factor of up to 0.4 μW K–2 m–1 compared to other NDI-based polymers.
There is growing evidence that the remarkable ability of animals, in particular birds, to sense the direction of the Earth's magnetic field relies on magnetically sensitive photochemical reactions of the protein cryptochrome. It is generally assumed that the magnetic field acts on the radical pair [formed by the transfer of an electron from a group of three tryptophan residues to the photo-excited flavin adenine dinucleotide cofactor within the protein. ] arise from the asymmetric distribution of hyperfine interactions among the two radicals and the near-optimal magnetic properties of the flavin radical. We close by discussing the identity of Z † and possible routes for its formation as part of a spin-correlated radical pair with an FAD radical in cryptochrome.
Thermoelectricity has gained considerable interest in the last decade due to the advent of organic thermoelectric materials. Crystallinity and doping level crucially determine the thermoelectric figure of merit of semi-conducting polymers. Hence, detailed insight into these factors is prerequisite for developing efficient devices. Here we show that the semicrystalline structure of aligned P3HT films shows superior thermoelectric efficiencies as compared to the smectic-like phase because of both a higher in-plane orientation and a higher doping level. Conductivities up to 160 S/cm and power factors of 56 W m-1 K-2 along the rubbing direction are obtained versus a few W m-1 K-2 for non-oriented films. Different intercalation mechanisms of F 4 TCNQ in the layers of alkyl side chains are evidenced by electron diffraction in doped oriented films of the smectic-like and the semi-crystalline phases. We provide compelling evidence that doping of the smectic-like phase promotes ordering of P3HT backbones along the chain direction within individual -stacks whereas for the semi-crystalline phase, dopant intercalation reorganizes the arrangement of successive stacks and perturbs the packing of alkyl side chains. Insight in the orientation of F 4 TCNQanions in the layers of alkyl side chains of P3HT crystals was further retrieved from a detailed polarized UV-vis-NIR spectroscopic analysis. Our results demonstrate that both orientation of the polymer chains and crystallinity enhance the thermoelectric properties as well as the doping level. We anticipate that detailed control of polymer morphology in films further improves the thermoelectric figure of merit of semiconducting polymers.
One of the principal models of magnetic sensing in migratory birds rests on the quantum spindynamics of transient radical pairs created photochemically in ocular cryptochrome proteins. We consider here the role of electron spin entanglement and coherence in determining the sensitivity of a radical pair-based geomagnetic compass and the origins of the directional response. It emerges that the anisotropy of radical pairs formed from spin-polarized molecular triplets could form the basis of a more sensitive compass sensor than one founded on the conventional hyperfine-anisotropy model. This property offers new and more flexible opportunities for the design of biologically inspired magnetic compass sensors.
Phenothiazine
(PTZ)–anthracene (An) compact electron donor/acceptor
dyads were synthesized. The molecular conformation was constrained
by rotation restriction to achieve an orthogonal geometry between
the electron donor (PTZ) and the electron acceptor (An), with the
aim to enhance the spin–orbit charge-transfer intersystem crossing
(SOCT–ISC). The substitution positions on the PTZ and An moieties
were varied to attain dyads with different mutual orientations of
the donor/acceptor as well as different rotation-steric hindrances.
The electronic coupling strengths between the electron donor and the
acceptor were quantified with the matrix elements (V
DA, 0.04–0.18 eV); the smallest value was observed
for the dyad with orthogonal geometry. Charge-transfer absorption
and fluorescence emission bands were observed for the dyads, for which
the intensity varied, manifested by the V
DA values. The fluorescence of the An moiety was significantly quenched
in the dyads, efficient ISC, and the formation of the triplet state
were confirmed with nanosecond transient absorption spectroscopy (ΦΔ = 65%, τT = 209 μs). The rotation-steric
hindrance was analyzed with potential energy curves, and PTZ was found
to be an ideal electron donor to attain SOCT–ISC. Time-resolved
electron paramagnetic resonance spectra revealed the electron-spin
polarization (ESP) of the triplets of the dyads, which is drastically
different from that of An, thus confirming the SOCT–ISC mechanism.
Moreover, we found that the ESP patterns of the dyads strongly depend
on the topological features of the molecules and the structure of
the electron donor, thus indicating that the relationship between
the molecular conformation and the ESP parameters of the triplet state
of the dyads cannot be described solely by the orthogonal geometry,
as was previously observed.
In chemical synthesis, the widely used Birch reduction of aromatic compounds to cyclic dienes requires alkali metals in ammonia as extremely low-potential electron donors. An analogous reaction is catalyzed by benzoyl-coenzyme A reductases (BCRs) that have a key role in the globally important bacterial degradation of aromatic compounds at anoxic sites. Because of the lack of structural information, the catalytic mechanism of enzymatic benzene ring reduction remained obscure. Here, we present the structural characterization of a dearomatizing BCR containing an unprecedented tungsten cofactor that transfers electrons to the benzene ring in an aprotic cavity. Substrate binding induces proton transfer from the bulk solvent to the active site by expelling a Zn(2+) that is crucial for active site encapsulation. Our results shed light on the structural basis of an electron transfer process at the negative redox potential limit in biology. They open the door for biological or biomimetic alternatives to a basic chemical synthetic tool.
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