Impact of Single-Point Mutations on the Excitonic Structure and Dynamics in a Fenna–Matthews–Olson Complex

Hole burning (HB) spectroscopy and modeling studies reveal significant changes in the excitonic structure and dynamics in several mutants of the FMO trimer from the <i>Chlorobaculum tepidum.</i> The excited-state decay times (<i>T</i>₁) of the high-energy excitons are significantly modified when mutation occurs near bacteriochlorophyll (BChl) 1 (V152N mutant) or BChl 6 (W184F). Longer (averaged) <i>T</i>₁ times of highest-energy excitons in V152N and W184F mutants suggest that site energies of BChls 1 and 6, believed to play an important role in receiving excitation from the baseplate BChls, likely play a critical role to ensure the femtosecond (fs) energy relaxation observed in wild-type FMO. HB spectroscopy reveals preferentially slower <i>T</i>₁ times (about 1 ps on average) because fs times prohibit HB due to an extremely low HB quantum yield. Uncorrelated (incoherent) excitation energy transfer times between monomers, the composition of exciton states, and average, frequency-dependent, excited-state decay times (<i>T</i>₁) are discussed

Dichotomous Disorder versus Excitonic Splitting of the B800 Band of Allochromatium vinosum

The LH2 antenna complex of the purple bacterium Allochromatium vinosum has a distinct double peak structure of the 800 nm band (B800). Several hypotheses were proposed to explain its origin. Recent 77 K two-dimensional electronic spectroscopy data suggested that excitonic coupling of dimerized bacteriochlorophylls (BChls) within the B800 ring is largely responsible for the B800 split [M. Schröter et al., <i>J. Phys. Chem. Lett.</i> <b>2018</b>, <i>9</i>, 1340]. Here we argue that the excitonic interactions between BChls in the B800 ring, though present, are weak and cannot explain the B800 band split. This conclusion is based on hole-burning data and modeling studies using an exciton model with dichotomous protein conformation disorder. Therefore, we uphold our earlier interpretation, first reported by Kell et al. [<i>J. Phys. Chem. B</i> <b>2017</b>, <i>121</i>, 9999], that the two B800 sub-bands are due to different site-energies (most likely due to weakly and strongly hydrogen-bonded B800 BChls)

Excitonic Energy Landscape of the Y16F Mutant of the <i>Chlorobium tepidum</i> Fenna–Matthews–Olson (FMO) Complex: High Resolution Spectroscopic and Modeling Studies

We report high-resolution (low-temperature) absorption, emission, and nonresonant/resonant hole-burned (HB) spectra and results of excitonic calculations using a non-Markovian reduced density matrix theory (with an improved algorithm for parameter optimization in heterogeneous samples) obtained for the Y16F mutant of the Fenna–Matthews–Olson (FMO) trimer from the green sulfur bacterium <i>Chlorobium tepidum</i>. We show that the Y16F mutant is a mixture of FMO complexes with three independent low-energy traps (located near 817, 821, and 826 nm), in agreement with measured composite emission and HB spectra. Two of these traps belong to mutated FMO subpopulations characterized by significantly modified low-energy excitonic states. Hamiltonians for the two major subpopulations (Sub₈₂₁ and Sub₈₁₇) provide new insight into extensive changes induced by the single-point mutation in the vicinity of BChl 3 (where tyrosine Y16 was replaced with phenylalanine F16). The average decay time(s) from the higher exciton state(s) in the Y16F mutant depends on frequency and occurs on a picosecond time scale

The structure of a red-shifted photosystem I reveals a red site in the core antenna

Cyanobacterial photosystem I has a highly conserved core antenna consisting of eleven subunits and more than 90 chlorophylls. Here via CryoEM and spectroscopy, the authors determine the location of a red-shifted low-energy chlorophyll that allows harvesting of longer wavelengths of light

Ionic EAP Actuators with Electrodes Based on Carbon Nanomaterials

Flexible polymer-based actuators, often also called artificial muscles, are an essential part of biomimetic systems that mimic the movement principles of animal world creatures. The most used electrode material to force the actuator move is an ensemble of noble metal nanoparticles in the electroactive polymer surface. Noble metal electrodes have enough electrical conductivity and elasticity and are not subjected to oxidation. However, high cost of such electrodes and their tendency to cracking dictate the need for searching other materials, primarily carbon ones. The review considers several options for this search. For example, carbon nanotubes and graphene have excellent properties at the level of a single individually taken nanotube or graphene sheet. However, conservation of these properties in structurally imperfect film electrodes requires a separate study. In addition, there are problems of compatibility of such electrodes with the polymers that requires cumbersome technologies, e.g., hot pressing, which complicates the production of the actuator as a whole. The review concerns the technology options of manufacturing actuators and the results obtained on their basis, both including hot pressing and avoiding this procedure. In particular, the required level of the graphene oxide reduction in hydrazine provides sufficient adhesion at rather high electrical conductivity of the graphene film. The ability to simultaneous achieving these properties is a nontrivial result, providing the same level of actuation as with expensive noble metal electrodes. Actuators that additionally require greater lifetime resource should be obtained in other ways. Among them are using the graphdiyne electrodes and laser processing of the graphene electrodes

Temperature- and Pressure-Reducing Regimes in the Growth Cell of HPHT Diamonds, Optimal for Preserving Crystal Integrity after Growth Completion

With its exceptional strength characteristics, diamond has some mechanical drawbacks, significant brittleness being among them. In particular, some HPHT-grown diamonds crack when the extreme parameters inherent to the diamond growth process gradually decrease. The cracking is caused by excessive stress due to the poor plastic properties of the diamond growth catalytic medium at certain stages of reducing the pressure and the temperature. An insulating container with the growth cell and heating circuit fragment inside can also make a significant contribution to the probability of cracking. This paper considers the possibility of minimizing the mechanical stress in the growth cell and, consequently, in the diamond crystal by choosing the optimal trajectory for the decrease in the pressure and temperature from diamond growth conditions to normal conditions

Structure-Based Exciton Hamiltonian and Dynamics for the Reconstituted Wild-type CP29 Protein Antenna Complex of the Photosystem II

We provide an analysis of the pigment composition of reconstituted wild type CP29 complexes. The obtained stoichiometry of 9 ± 0.6 Chls <i>a</i> and 3 ± 0.6 Chls <i>b</i> per complex, with some possible heterogeneity in the carotenoid binding, is in agreement with 9 Chls <i>a</i> and 3.5 Chls <i>b</i> revealed by the modeling of low-temperature optical spectra. We find that ∼50% of Chl <i>b</i>614 is lost during the reconstitution/purification procedure, whereas Chls <i>a</i> are almost fully retained. The excitonic structure and the nature of the low-energy (low-E) state(s) are addressed via simulations (using Redfield theory) of 5 K absorption and fluorescence/nonresonant hole-burned (NRHB) spectra obtained at different excitation/burning conditions. We show that, depending on laser excitation frequency, reconstituted complexes display two (independent) low-E states (i.e., the A and B traps) with different NRHB and emission spectra. The red-shifted state A near 682.4 nm is assigned to a minor (∼10%) subpopulation (sub. II) that most likely originates from an imperfect local folding occurring during protein reconstitution. Its lowest energy state A (localized on Chl <i>a</i>604) is easily burned with λ_B = 488.0 nm and has a red-shifted fluorescence origin band near 683.7 nm that is not observed in native (isolated) complexes. Prolonged burning by 488.0 nm light reveals a second low-E trap at 680.2 nm (state B) with a fluorescence origin band at ∼681 nm, which is also observed when using a direct low-fluence excitation near 650 nm. The latter state is mostly delocalized over the <i>a</i>611, <i>a</i>612, <i>a</i>615 Chl trimer and corresponds to the lowest energy state of the major (∼90%) subpopulation (sub. I) that exhibits a lower hole-burning quantum yield. Thus, we suggest that major sub. I correspond to the native folding of CP29, whereas the red shift of the Chl <i>a</i>604 site energy observed in the minor sub. II occurs only in reconstituted complexes