Excitation quenching in chlorophyll-carotenoid antenna systems: 'coherent' or 'incoherent'

Plants possess an essential ability to rapidly down-regulate light-harvesting in response to high light. This photoprotective process involves the formation of energy-quenching interactions between the chlorophyll and carotenoid pigments within the antenna of Photosystem II (PSII). The nature of these interactions is currently debated, with, among others, ‘incoherent’ or ‘coherent’ quenching models (or a combination of the two) suggested by a range of time-resolved spectroscopic measurements. In ‘incoherent quenching’, energy is transferred from a chlorophyll to a carotenoid and is dissipated due to the intrinsically short excitation lifetime of the latter. ‘Coherent quenching’ would arise from the quantum mechanical mixing of chlorophyll and carotenoid excited state properties, leading to a reduction in chlorophyll excitation lifetime. The key parameters are the energy gap, Δ=Car−Chl, Δ ε = ε C a r − ε C h l , and the resonance coupling, J, between the two excited states. Coherent quenching will be the dominant process when −<Δ<, − J < Δ ε < J , i.e., when the two molecules are resonant, while the quenching will be largely incoherent when Chl>(Car+). ε C h l > ( ε C a r + J ) . One would expect quenching to be energetically unfavorable for Chl<(Car−). ε C h l < ( ε C a r − J ) . The actual dynamics of quenching lie somewhere between these limiting regimes and have non-trivial dependencies of both J and Δ. Δ ε . Using the Hierarchical Equation of Motion (HEOM) formalism we present a detailed theoretical examination of these excitation dynamics and their dependence on slow variations in J and Δ. Δ ε . We first consider an isolated chlorophyll–carotenoid dimer before embedding it within a PSII antenna sub-unit (LHCII). We show that neither energy transfer, nor the mixing of excited state lifetimes represent unique or necessary pathways for quenching and in fact discussing them as distinct quenching mechanisms is misleading. However, we do show that quenching cannot be switched ‘on’ and ‘off’ by fine tuning of Δ Δ ε around the resonance point, Δ=0. Δ ε = 0. Due to the large reorganization energy of the carotenoid excited state, we find that the presence (or absence) of coherent interactions have almost no impact of the dynamics of quenching. Counter-intuitively significant quenching is present even when the carotenoid excited state lies above that of the chlorophyll. We also show that, above a rather small threshold value of >10cm−1 J > 10 c m − 1 quenching becomes less and less sensitive to J (since in the window −<Δ< − J < Δ ε < J the overall lifetime is independent of it). The requirement for quenching appear to be only that >0. J > 0. Although the coherent/incoherent character of the quenching can vary, the overall kinetics are likely robust with respect to fluctuations in J and Δ. Δ ε . This may be the basis for previous observations of NPQ with both coherent and incoherent features

FTIR/PCA study of propanol in argon matrix: the initial stage of clustering and conformational transitions

FTIR spectra of 1-propanol in an argon matrix were studied in the range 11 to 30 K. Principal component analysis of dynamic FTIR spectra and nonlinear band shape fitting has been carried out. The peaks of monomer, open dimer, mixed propanol-water dimer and those of higher H-bond clusters have been resolved and analyzed. The attribution of certain FTIR peaks has been supported by proper density functional theory calculations. Analyzing dependences of the integral band intensities of various aggregates on temperature it has been deduced that in the initial stage of clustering monomers and dimers are the basic building blocks forming higher H-bond clusters. The peaks assigned to two conformers of monomers and mixed propanol-water dimers were investigated processing the temperature dependences of their integral intensities in Arrhenius plot. The obtained values of 0.18 kJ⋅mol⁻¹ for propanol monomer and 0.26 kJ⋅mol⁻¹ for mixed dimer are well comparable with the energy differences between the global minimum conformation of 1-propanol (Gt) and some other energetically higher structures (Tt or Tg)

Ising Universality in Three Dimensions: A Monte Carlo Study

We investigate three Ising models on the simple cubic lattice by means of Monte Carlo methods and finite-size scaling. These models are the spin-1/2 Ising model with nearest-neighbor interactions, a spin-1/2 model with nearest-neighbor and third-neighbor interactions, and a spin-1 model with nearest-neighbor interactions. The results are in accurate agreement with the hypothesis of universality. Analysis of the finite-size scaling behavior reveals corrections beyond those caused by the leading irrelevant scaling field. We find that the correction-to-scaling amplitudes are strongly dependent on the introduction of further-neighbor interactions or a third spin state. In a spin-1 Ising model, these corrections appear to be very small. This is very helpful for the determination of the universal constants of the Ising model. The renormalization exponents of the Ising model are determined as y_t = 1.587 (2), y_h = 2.4815 (15) and y_i = -0.82 (6). The universal ratio Q = ^2/ is equal to 0.6233 (4) for periodic systems with cubic symmetry. The critical point of the nearest-neighbor spin-1/2 model is K_c=0.2216546 (10).Comment: 25 pages, uuencoded compressed PostScript file (to appear in Journal of Physics A

Improved high-temperature expansion and critical equation of state of three-dimensional Ising-like systems

High-temperature series are computed for a generalized $3d$ Ising model with arbitrary potential. Two specific ``improved'' potentials (suppressing leading scaling corrections) are selected by Monte Carlo computation. Critical exponents are extracted from high-temperature series specialized to improved potentials, achieving high accuracy; our best estimates are: $\gamma=1.2371(4)$, $\nu=0.63002(23)$, $\alpha=0.1099(7)$, $\eta=0.0364(4)$, $\beta=0.32648(18)$. By the same technique, the coefficients of the small-field expansion for the effective potential (Helmholtz free energy) are computed. These results are applied to the construction of parametric representations of the critical equation of state. A systematic approximation scheme, based on a global stationarity condition, is introduced (the lowest-order approximation reproduces the linear parametric model). This scheme is used for an accurate determination of universal ratios of amplitudes. A comparison with other theoretical and experimental determinations of universal quantities is presented.Comment: 65 pages, 1 figure, revtex. New Monte Carlo data by Hasenbusch enabled us to improve the determination of the critical exponents and of the equation of state. The discussion of several topics was improved and the bibliography was update

Magnetic relaxation in La0.250Pr0.375Ca0.375MnO3 with varying phase separation

We have studied the magnetic relaxation properties of the phase-separated manganite compound La0.250Pr0.375Ca0.375MnO3 . A series of polycrystalline samples was prepared with different sintering temperatures, resulting in a continuous variation of phase fraction between metallic (ferromagnetic) and charge-ordered phases at low temperatures. Measurements of the magnetic viscosity show a temperature and field dependence which can be correlated to the static properties. Common to all the samples, there appears to be two types of relaxation processes - at low fields associated with the reorientation of ferromagnetic domains and at higher fields associated with the transformation between ferromagnetic and non-ferromagnetic phases.Comment: 30 pages with figures, PDF, accepted to be published in Physical Review