Triplet-driven chemical reactivity of β-carotene and its biological implications.

The endoperoxides of β-carotene (βCar-EPOs) are regarded as main products of the chemical deactivation of O by β-carotene, one of the most important antioxidants, following a concerted singlet-singlet reaction. Here we challenge this view by showing that βCar-EPOs are formed in the absence of O in a non-concerted triplet-triplet reaction: O + β-carotene → βCar-EPOs, in which β-carotene manifests a strong biradical character. Thus, the reactivity of β-carotene towards oxygen is governed by its excited triplet state.

The origin of the dark S state in carotenoids: a comprehensive model.

In carotenoids, by analogy to polyenes, the symmetry of the π-electron system is often invoked to explain their peculiar electronic features, in particular the inactivity of the S → S transition in one-photon excitation. In this review, we verify whether the molecular symmetry of carotenoids and symmetry of their π-electron system are supported in experimental and computational studies. We focus on spectroscopic techniques which are sensitive to the electron density distribution, including the X-ray crystallography, electronic absorption, two-photon techniques, circular dichroism, nuclear magnetic resonance, Stark and vibrational spectroscopies, and on this basis we seek for the origin of inactivity of the S state.

Side Methyl Groups Control the Conformation and Contribute to Symmetry Breaking of Isoprenoid Chromophores.

Ab initio DFT computations reveal that the essential structural and photophysical features of the conjugated π-electron system of retinal and carotenoids are dictated by "innocent" methyl substituents. These methyl groups shape the conformation and symmetry of the isoprenoid chromophores by causing a sigmoidal distortion of the polyene skeleton and increasing its flexibility, which facilitates fitting to their binding pockets in proteins. Comparison of in vacuo conformations of the chromophores with their native (protein-bound) conformations showed, surprisingly, that the peripheral groups and interactions with the protein environment are much less significant than the methyl side groups in tuning their structural features.

Effects of Molecular Symmetry on the Electronic Transitions in Carotenoids.

The aim of this work is the verification of symmetry effects on the electronic absorption spectra of carotenoids. The symmetry breaking in cis-β-carotenes and in carotenoids with nonlinear π-electron system is of virtually no effect on the dark transitions in these pigments, in spite of the loss of the inversion center and evident changes in their electronic structure. In the cis isomers, the S2 state couples with the higher excited states and the extent of this coupling depends on the position of the cis bend.

High-pressure and theoretical studies reveal significant differences in the electronic structure and bonding of magnesium, zinc, and nickel ions in metalloporphyrinoids.

High pressure in combination with optical spectroscopy was used to gain insights into the interactions between Mg(2+), Zn(2+), and Ni(2+) ions and macrocyclic ligands of porphyrinoid type. In parallel, the central metal ion-macrocycle bonding was investigated using theoretical approaches. The symmetry properties of the orbitals participating in this bonding were analyzed, and pigment geometries and pressure/ligation effects were computed within DFT.

Molecular symmetry determines the mechanism of a very efficient ultrafast excitation-to-heat conversion in Ni-substituted chlorophylls.

In the Ni-substituted chlorophylls, an ultrafast (<60 fs) deactivation channel is created, which is not present in Ni-porphyrins. This observation prompted us to investigate in detail the mechanism of excitation-to-heat conversion in Ni-substituted chlorophylls, experimentally, using time-resolved laser-induced optoacoustic spectroscopy, and theoretically, using group theory approach. The Ni-substituted chlorophylls show exceptional photostability and the optoacoustic measurements confirm the prompt and very efficient (100%) excitation-into-heat conversion in these complexes.

Tuning the thermodynamics of association of transmembrane helices.

Modular photosynthetic LH1 complex is applied as a model system to investigate the thermodynamics of a self-assembling membrane protein and the effects of cosolvents and cofactor (carotenoid) on the process. Native chromophores of LH1, bacteriochlorophyll, and carotenoid are excellent intrinsic spectroscopic reporter molecules. Their presence allows us to follow the association of transmembrane helices of LH1, without the use of any external markers, by electronic absorption/emission and circular dichroism.

Excited-state double proton transfer in 1H-pyrazolo[3,4-b]quinoline dimers.

Pyrazoloquinolines are highly fluorescent, both in liquid solutions and in the solid state, which makes them good candidates for various optical devices. The aim of the current work is to understand the photochemical behavior of pyrazolo[3,4-b]quinoline (PQ), which is quite complicated since in n-alkane solvents PQ tends to form strong complexes with protic solvent constituents (often present as minor impurities), as well as dimers. Both types of H-bond complexes were studied systematically by temperature-dependent conventional absorption and fluorescence spectroscopy; the effect of protic solvent constituents was mimicked by varying the ethanol concentration in n-octane in the range from 0.

Circular dichroism and absorption studies of (-)-2,2'-dimethyl-4,5-(1-naphthyl)-1,3-dioxolane in terms of vibronic coupling theory.

The absorption and circular dichroism (CD) spectra of (-)-2,2'-dimethyl-4,5-(1-naphthyl)-1,3-dioxolane (DND) were studied in the energy region 30,000 cm(-1) to 50,000 cm(-1). The DND ketal is treated as a naphthalene dimer and its spectra are interpreted in terms of a vibronic dimer model which includes the (1)L(a) and (1)B(b) states of the naphthalene chromophore. To fix the most stable conformation of DND molecule, the MNDO/AM1, RHF/6-31G, and SVWN5, BPW91 methods are employed with 6-31G and 6-31G(d',p') basis sets.