Showing papers by "Gregory S. Engel published in 2016"

Mutations to R. sphaeroides Reaction Center Perturb Energy Levels and Vibronic Coupling but Not Observed Energy Transfer Rates.

Abstract: The bacterial reaction center is capable of both efficiently collecting and quickly transferring energy within the complex; therefore, the reaction center serves as a convenient model for both energy transfer and charge separation. To spectroscopically probe the interactions between the electronic excited states on the chromophores and their intricate relationship with vibrational motions in their environment, we examine coherences between the excited states. Here, we investigate this question by introducing a series of point mutations within 12 A of the special pair of bacteriochlorophylls in the Rhodobacter sphaeroides reaction center. Using two-dimensional spectroscopy, we find that the time scales of energy transfer dynamics remain unperturbed by these mutations. However, within these spectra, we detect changes in the mixed vibrational-electronic coherences in these reaction centers. Our results indicate that resonance between bacteriochlorophyll vibrational modes and excitonic energy gaps promote electronic coherences and support current vibronic models of photosynthetic energy transfer.

Dark states enhance the photocell power via phononic dissipation

Abstract: The high efficiency of the photon-to-charge conversion process found in photosynthetic complexes has inspired researchers to explore a new route for designing artificial photovoltaic materials. Quantum coherence can provide a mean to surpass the Shockley–Quiesser device concept limit by reducing the radiative recombination. Taking inspiration from these new discoveries, we consider a linearly-aligned system as a light-harvesting antennae composed of two-level optical emitters coupled with each other by dipole–dipole interactions. Our simulations show that the certain dark states can enhance the power with the aid of intra-band phononic dissipation. Due to cooperative effects, the output power will be improved when incorporating more emitters in the linear system.

Cysteine-mediated mechanism disrupts energy transfer to prevent photooxidation

Abstract: Unlike higher plants, which are often awash in sunlight, green sulfur bacteria survive in some of the darkest and most inhospitable environments for photosynthetic organisms. From the bottom of the Black Sea to the underside of thick microbial mats growing atop hot springs, these anaerobic archaea subsist on the rare photons that penetrate into their dark environments. For this reason, it is surprising that they would evolve a photoprotective mechanism. Indeed, until the work by Orf et al. (1), no photoprotection mechanisms have been well established in green sulfur bacteria; photoprotection was primarily the domain of higher plants and photosynthetic organisms that had the luxury (curse?) to exist in light so bright that it could damage their photosynthetic apparatus. Although green sulfur bacteria need not face bright sunlight, they still combat photo-induced oxidative damage when the redox potential of their environment rises. Orf et al. (1) demonstrate that an operative photoprotection mechanism exists in green sulfur bacteria and that this mechanism is activated by oxidation of two cysteine residues. This new photoprotection mechanism identified by Orf et al. (1) differs from more familiar motifs; the new mechanism employs amino acid residues instead of isomerization of dedicated photoprotective chromophores, such as carotenoids. It also seems to protect against damage from a single excitation (rather than multiple excitations). That is, the mechanism depends on redox potential, not light intensity. In contrast, photoprotective mechanisms are normally used by plants, algae, and bacteria to withstand conditions of excess sunlight (2). Typical strategies include nonradiative relaxation, adjustments of the proton gradient in the system to control the rate of redox reactivity, and physical detachment of the involved photosynthetic machinery (2⇓–4). These approaches limit the risk of multiple simultaneous excitations inducing triplet formation and leading to generation of reactive oxygen species. In this … [↵][1]1To whom correspondence should be addressed. Email: gsengel{at}uchicago.edu. [1]: #xref-corresp-1-1

Electronic Structure and Dynamics of Higher-Lying Excited States in Light Harvesting Complex 1 from Rhodobacter sphaeroides

Abstract: Light harvesting in photosynthetic organisms involves efficient transfer of energy from peripheral antenna complexes to core antenna complexes, and ultimately to the reaction center where charge separation drives downstream photosynthetic processes. Antenna complexes contain many strongly coupled chromophores, which complicates analysis of their electronic structure. Two-dimensional electronic spectroscopy (2DES) provides information on energetic coupling and ultrafast energy transfer dynamics, making the technique well suited for the study of photosynthetic antennae. Here, we present 2DES results on excited state properties and dynamics of a core antenna complex, light harvesting complex 1 (LH1), embedded in the photosynthetic membrane of Rhodobacter sphaeroides. The experiment reveals weakly allowed higher-lying excited states in LH1 at 770 nm, which transfer energy to the strongly allowed states at 875 nm with a lifetime of 40 fs. The presence of higher-lying excited states is in agreement with effecti...

Optical resonance imaging: An optical analog to MRI with sub-diffraction-limited capabilities.

Abstract: We propose here optical resonance imaging (ORI), a direct optical analog to magnetic resonance imaging (MRI). The proposed pulse sequence for ORI maps space to time and recovers an image from a heterodyne-detected third-order nonlinear photon echo measurement. As opposed to traditional photon echo measurements, the third pulse in the ORI pulse sequence has significant pulse-front tilt that acts as a temporal gradient. This gradient couples space to time by stimulating the emission of a photon echo signal from different lateral spatial locations of a sample at different times, providing a widefield ultrafast microscopy. We circumvent the diffraction limit of the optics by mapping the lateral spatial coordinate of the sample with the emission time of the signal, which can be measured to high precision using interferometric heterodyne detection. This technique is thus an optical analog of MRI, where magnetic-field gradients are used to localize the spin-echo emission to a point below the diffraction limit of the radio-frequency wave used. We calculate the expected ORI signal using 15 fs pulses and 87° of pulse-front tilt, collected using f/2 optics and find a two-point resolution 275 nm using 800 nm light that satisfies the Rayleigh criterion. We also derive a general equation for resolution in optical resonance imaging that indicates that there is a possibility of superresolution imaging using this technique. The photon echo sequence also enables spectroscopic determination of the input and output energy. The technique thus correlates the input energy with the final position and energy of the exciton.