Poezijos. Antologija

Poesia lituana

A theoretical investigation of the photophysical consequences of major plant light-harvesting complex aggregation within the photosynthetic membrane.

Spectroscopic measurements of Arabidopsis leaves have shown that the energy-dependent component of non-photochemical quenching (NPQ), known as qE, is associated with an absorption change at 535 nm (ΔA(535)). Identical measurements on the zeaxanthin-deficient mutant npq1 reveal a similar spectroscopic signature at 525 nm (ΔA(525)). We investigated whether these red-shifts may arise from excitonic interactions among homodimers of xanthophylls, zeaxanthin, and violaxanthin, bound at the peripheral V1 binding site on adjacent light-harvesting complex II (LHCII) trimers. Estimates of the relative geometries of these pigment pairs were obtained from the structure of LHCII. The excitonic couplings of zeaxanthin and violaxanthin dimers were probed using the time-dependent density functional theory method (TD-DFT). Calculations indicated that dimers formed between zeaxanthin or violaxanthin molecules using the published LHCII structure resulted in absorption blue shifts, typical of an H-type (parallel) geometry. In contrast, if the volume of the LHCII structure was modified to reflect the change in membrane thickness that occurs upon ΔpH formation, then both zeaxanthin and violaxanthin dimers adopted a J-type (collinear) geometry, and the resulting spectral shift was to the red region. The magnitudes of these predicted red-shifts are in good agreement with the experimental magnitudes. We therefore conclude that the observed xanthophyll red-shift results from the combination of both LHCII aggregation and changes in membrane thickness during qE. ΔA(535) may therefore be considered a "marker of aggregation" between LHCII trimers upon qE formation

Electronic and Vibrational Properties of Allene Carotenoids

International audienceCarotenoids are conjugated linear molecules built from the repetition of terpene units, which display a large structural diversity in nature. They may, in particular, contain several types of side or end groups, which tune their functional properties, such as absorption position and photochemistry. We report here a detailed experimental study of the absorption and vibrational properties of allene-containing carotenoids, together with an extensive modeling of these experimental data. Our calculations can satisfactorily explain the electronic properties of vaucheriaxanthin, where the allene group introduces the equivalent of one CC double bond into the conjugated CC chain. The position of the electronic absorption of fucoxanthin and butanoyloxyfucoxanthin requires long-range corrections to be found correctly on the red side of that of vaucheriaxanthin; however, these corrections tend to overestimate the effect of the conjugated and nonconjugated CO groups in these molecules. We show that the resonance Raman spectra of these carotenoids are largely perturbed by the presence of the allene group, with the two major Raman contributions split into two components. These perturbations are satisfactorily explained by modeling, through a gain in the Raman intensity of the CC antisymmetric stretching mode, induced by the presence of the allene group in the carotenoid CC chain

Resonance Raman spectra of carotenoid molecules: influence of methyl substitutions

We report here the resonance Raman spectra and the quantum chemical calculations of the Raman spectra for β-carotene and 13,13'-diphenyl-β-carotene. The first aim of this approach was to test the robustness of the method used for modeling β-carotene, and assess whether it could accurately predict the vibrational properties of derivatives in which conjugated substituents had been introduced. DFT calculations, using the B3LYP functional in combination with the 6-311G(d,p) basis set, were able to accurately predict the influence of two phenyl substituents connected to the β-carotene molecule, although these deeply perturb the vibrational modes. This experimentally validated modeling technique leads to a fine understanding of the origin of the carotenoid resonance Raman bands, which are widely used for assessing the properties of these molecules, and in particular in complex media, such as binding sites provided by biological macromolecules

Modelling Dynamic Conformations of Organic Molecules: Alkyne Carotenoids in Solution

International audienceCalculating the spectroscopic properties of complex conjugated organic molecules in their relaxed state is far from simple. An additional complexity arises for flexible molecules in solution, where the rotational energy barriers are low enough so that non-minimum conformations may become dynamically populated. These metastable conformations quickly relax during the minimization procedures preliminary to DFT calculations, and so accounting for their contribution to the experimentally-observed properties is problematic. We describe a strategy for stabilising these non-minimum conformations in silico, allowing their properties to be calculated. Diadinoxanthin and alloxanthin present atypical vibrational properties in solution, indicating the presence of several conformations. Performing energy calculations in vacuo and polarizable continuum model calculations in different solvents, we found three different conformations with values for the δ dihedral angle of the end-ring ca. 0°, 180° and 90° with respect to the plane of the conjugated chain. The latter conformation, a non-global minimum, is not stable during the minimization necessary for modelling its spectroscopic properties. To circumvent this classical problem we used a Car-Parinello MD supermolecular approach, in which diadinoxanthin was solvated by water molecules so that metastable conformations were stabilized by hydrogen bonding interactions. We progressively removed the number of solvating waters to find the minimum required for this stabilisation. This strategy represents the first modelling of a carotenoid in a distorted conformation, and provides an accurate interpretation of the experimental data