Steady-state and ultrafast time-resolved optical spectroscopic investigations have been carried out at 293 and 10 K on LH2 pigment-protein complexes isolated from three different strains of photosynthetic bacteria: Rhodobacter (Rb.) sphaeroides G1C, Rb. sphaeroides 2.4.1 (anaerobically and aerobically grown), and Rps. acidophila 10050. The LH2 complexes obtained from these strains contain the carotenoids, neurosporene, spheroidene, spheroidenone, and rhodopin glucoside, respectively. These molecules have a systematically increasing number of π-electron conjugated carbon-carbon double bonds. Steady-state absorption and fluorescence excitation experiments have revealed that the total efficiency of energy transfer from the carotenoids to bacteriochlorophyll is independent of temperature and nearly constant at ~90% for the LH2 complexes containing neurosporene, spheroidene, spheroidenone, but drops to ~53% for the complex containing rhodopin glucoside. Ultrafast transient absorption spectra in the near-infrared (NIR) region of the purified carotenoids in solution have revealed the energies of the S1 (21Ag−) → S2 (11Bu+) excited-state transitions which, when subtracted from the energies of the S0 (11Ag−) → S2 (11Bu+) transitions determined by steady-state absorption measurements, give precise values for the positions of the S1 (21Ag−) states of the carotenoids. Global fitting of the ultrafast spectral and temporal data sets have revealed the dynamics of the pathways of de-excitation of the carotenoid excited states. The pathways include energy transfer to bacteriochlorophyll, population of the so-called S* state of the carotenoids, and formation of carotenoid radical cations (Car•+). The investigation has found that excitation energy transfer to bacteriochlorophyll is partitioned through the S1 (11Ag−), S2 (11Bu+), and S* states of the different carotenoids to varying degrees. This is understood through a consideration of the energies of the states and the spectral profiles of the molecules. A significant finding is that, due to the low S1 (21Ag−) energy of rhodopin glucoside, energy transfer from this state to the bacteriochlorophylls is significantly less probable compared to the other complexes. This work resolves a long-standing question regarding the cause of the precipitous drop in energy transfer efficiency when the extent of π-electron conjugation of the carotenoid is extended from ten to eleven conjugated carbon–carbon double bonds in LH2 complexes from purple photosynthetic bacteria.

Numerous femtosecond time-resolved optical spectroscopic experiments have reported that the lifetime of the low-lying S1 state of carbonyl-containing polyenes and carotenoids decreases with increasing solvent polarity. The effect becomes even more pronounced as the number of double bonds in the conjugated π-electron system decreases. The effect has been attributed to an intramolecular charge transfer (ICT) state coupled to S1, but it is still not clear what the precise molecular nature of this state is, and how it is able to modulate the spectral and dynamic properties of polyenes and carotenoids. In this work we examine the nature of the ICT state in three substituted polyenes: crocetindial, which contains two terminal, symmetrically-substituted carbonyl groups in conjugation with the π-electron system, 8,8'-diapocarotene-8'-ol-8-al, which has one terminal conjugated carbonyl group and one hydroxyl group, and 8,8'-diapocarotene-8,8'-diol, which has two terminal, symmetrically-positioned, hydroxyl groups but no carbonyls. Femtosecond time-resolved optical spectroscopic experiments on these molecules reveal that only the asymmetrically substituted 8,8'-diapocarotene-8'-ol-8-al exhibits any substantial effect of solvent on the excited state spectra and dynamics. The data are interpreted using molecular orbital theory which shows that the ICT state develops via mixing of the low-lying S1 (21Ag-like) and S2 (11Bu-like) excited singlet states to form a resultant state that preferentially evolves in polar solvent and exhibits a very large (~25D) dipole moment. Molecular dynamics calculations demonstrate that the features of the ICT state are present in ~20 fs.

Anthocyanins play a variety of adaptive roles in both vegetative tissues and reproductive organs of plants. The broad functionality of these compounds requires sophisticated regulation of the anthocyanin biosynthesis pathway to allow proper localization, timing, and optimal intensity of pigment deposition. While it is well-established that the committed steps of anthocyanin biosynthesis are activated by a highly conserved MYB-bHLH-WDR (MBW) protein complex in virtually all flowering plants, anthocyanin repression seems to be achieved by a wide variety of protein and small RNA families that function in different tissue types and in response to different developmental, environmental, and hormonal cues. In this review, we survey recent progress in the identification of anthocyanin repressors and the characterization of their molecular mechanisms. We find that these seemingly very different repression modules act through a remarkably similar logic, the so-called "double negative logic". Much of the double-negative regulation of anthocyanin production involves signal-induced degradation or sequestration of the repressors from the MBW protein complex. We discuss the functional and evolutionary advantages of this logic design compared with simple or sequential positive regulation. These advantages provide a plausible explanation to why plants have evolved so many anthocyanin repressors.

SummaryCarotenoids are yellow, orange, and red pigments that contribute to the beautiful colors and nutritive value of many flowers and fruits. The structural genes in the highly conserved carotenoid biosynthetic pathway have been well characterized in multiple plant systems, but little is known about the transcription factors that control the expression of these structural genes.By analyzing a chemically induced mutant of Mimulus lewisii through bulk segregant analysis and transgenic experiments, we have identified an R2R3-MYB, Reduced Carotenoid Pigmentation 1 (RCP1), as the first transcription factor that positively regulates carotenoid biosynthesis during flower development.Loss-of-function mutations in RCP1 lead to down-regulation of all carotenoid biosynthetic genes and reduced carotenoid content in M. lewisii flowers, a phenotype recapitulated by RNA interference in the wild-type background. Overexpression of this gene in the rcp1 mutant background restores carotenoid production and, unexpectedly, results in simultaneous decrease of anthocyanin production in some transgenic lines by down-regulating the expression of an activator of anthocyanin biosynthesis.Identification of transcriptional regulators of carotenoid biosynthesis provides the 'toolbox' genes for understanding the molecular basis of flower color diversification in nature and for potential enhancement of carotenoid production in crop plants via genetic engineering.

Femtosecond transient-grating spectroscopy with heterodyne detection was employed to characterize the nonradiative decay pathway in β-carotene from the S2 (1(1)Bu(+)) state to the S1 (2(1)Ag(-)) state in benzonitrile solution. The results indicate definitively that the S2 state populates an intermediate state, Sx, on an ultrafast time scale prior to nonradiative decay to the S1 state. Numerical simulations using the response function formalism and the multimode Brownian oscillator model were used to fit the absorption and dispersion components of the transient-grating signal with a common set of parameters for all of the relevant Feynman pathways, including double-quantum coherences. The requirement for inclusion of the Sx state in the nonradiative decay pathway is the observed fast rise time of the dispersion component, which is predominantly controlled by the decay of the stimulated emission signal from the optically prepared S2 state. The finding that the excited-state absorption spectrum from the Sx state is significantly red-shifted from that of S2 and S1 leads to a new assignment for the spectroscopic origin of the Sx state. Rather than assigning Sx to a discrete electronic state, such as the (1)Bu(-) state suggested in previous work, it is proposed that the Sx state corresponds to a transition-state-like structure on the S2 potential surface. In this hypothesis, the 12 fs time constant for the decay of the S2 state corresponds to a vibrational displacement of the C-C and C═C bond-length alternation coordinates of the conjugated polyene backbone from the optically prepared Franck-Condon structure to a potential energy barrier on the S2 surface that divides planar and torsionally displaced structures. The lifetime of the Sx state would be associated with a subsequent relaxation along torsional coordinates over a steep potential energy gradient toward a conical intersection with the S1 state. This hypothesis leads to the idea that twisted structures with intramolecular charge-transfer character along the S2 torsional gradient are active in excitation energy-transfer mechanisms to (bacterio)chlorophyll acceptors.