ON THE ROLE OF FORBIDDEN LOW-LYING EXCITED STATES OF LIGHT-HARVESTING CAROTENOIDS IN ENERGY TRANSFER IN PHOTOSYNTHESIS

Spectroscopic Studies of the Low-Lying Singlet Excited Electronic States and Photochemical Properties of Carotenoids

Investigations of the singlet excited state properties of carotenoids using steady-state fluorescence, transient absorption pump-probe, two-photon excitation, and resonance Raman excitation spectroscopies are described. The application of these experimental techniques to the specific problem of determining the S1 excited energies of carotenoids is discussed in detail, and the recent literature pertaining to the assignment of charge transfer states in carotenoids and states described as having particular pseudoparity elements is reviewed. Hypothetical schemes for how these states may account for some of the dynamic and photochemical behavior of carotenoids are presented.

Energy transfer reactions involving carotenoids: quenching of chlorophyll fluorescence

Carotenoids have a key role in photosynthesis in photosynthetic systems, transferring excitation energy to chlorophyll (Chl) during light harvesting. These pigments also protect the photosynthetic apparatus from photodamage by quenching the Chl triplet state and singlet oxygen. In addition, in higher plants and some algae, a number of xanthophylls also have the ability to deactivate excited Chl under conditions of excess excitation via the operation of the xanthophyll cycle (violaxanthin<-->antheraxanthin<-->zeaxanthin or diadinoxanthin<-->diatoxanthin). The formation of zexanthin (or diatoxanthin) can be clearly correlated with the non-photochemical quenching of Chl fluorescence, and is now recognized as a major photoprotective process in higher plants and a number of algal genera. The interconversion of these xanthophylls in response to a changing light environment alters the extent of their carbon-carbon double bond conjugation, which, in turn, affects the excited state energies and lifetimes of the carotenoids and may also alter their structure/conformation and hydrophobicity. The possible roles of these photophysical and physicochemical changes in the mechanism(s) of xanthophyll-mediated energy dissipation via quenching of Chl fluorescence are discussed.

Reconstitution of the bacterial core light-harvesting complexes of Rhodobacter sphaeroides and Rhodospirillum rubrum with isolated alpha- and beta-polypeptides, bacteriochlorophyll alpha, and carotenoid

Methodology has been developed to reconstitute carotenoids and bacteriochlorophyll alpha with isolated light-harvesting complex I (LHI) polypeptides of both Rhodobacter sphaeroides and Rhodospirillum rubrum. Reconstitution techniques first developed in this laboratory using the LHI polypeptides of R. rubrum, R. sphaeroides, and Rhodobacter capsulatus reproduced bacteriochlorophyll alpha spectral properties characteristic of LHI complexes lacking carotenoids. In this study, carotenoids are supplied either as organic-solvent extracts of chromatophores or as thin-layer chromatography or high performance liquid chromatography-purified species. The resulting LHI complexes exhibit carotenoid and bacteriochlorophyll a spectral properties characteristic of native LHI complexes of carotenoid-containing bacteria. Absorption and circular dichroism spectra support the attainment of a native-like carotenoid environment in the reconstituted LHI complexes. For both R. sphaeroides- and R. rubrum-reconstituted systems, fluorescence excitation spectra reveal appropriate carotenoid to bacteriochlorophyll alpha energy-transfer efficiencies based on comparisons with the in vivo systems. In the case of R. rubrum reconstitutions, carotenoids afford protection from photodynamic degradation. Thus, carotenoids reconstituted into LHI exhibit spectral and functional characteristics associated with native pigments. Heterologous reconstitutions demonstrate the applicability of the developed assay in dissecting the molecular environment of carotenoids in light-harvesting complexes.

Low-lying electronic states of carotenoids

Four all-trans carotenoids, spheroidene, 3,4-dihydrospheroidene, 3,4,5,6-tetrahydrospheroidene, and 3,4,7,8-tetrahydrospheroidene, have been purified using HPLC techniques and analyzed using absorption, fluorescence and fluorescence excitation spectroscopy of room temperature solutions. This series of molecules, for which the extent of pi-electron conjugation decreases from 10 to seven carbon-carbon double bonds, exhibits a systematic crossover from S2----S0 (1(1)Bu----1(1)Ag) to S1----S0 (2(1)Ag----1(1)Ag) emission with decreasing chain length. Extrapolation of the S1----S0 transition energies indicates that the 2(1)Ag states of longer carotenoids have considerably lower energies than previously thought. The energies of the S1 states of spheroidenes and other long carotenoids are correlated with the S1 energies of their chlorophyll partners in antenna complexes of photosynthetic systems. Implications for energy transfer in photosynthetic antenna are discussed.

Fluorescence properties of the allenic carotenoid fucoxanthin: Implication for energy transfer in photosynthetic pigment systems

The fluorescence spectrum of an allenic carotenoid, all-trans-fucoxanthin isolated from a brown alga, has been reported for the first time. This carotenoid is known to function efficiently as a primary photosynthetic antenna pigment in marine algae. The emission bands were located around 630, 685 and 750 nm in CS2 at 20°C, absorption bands being located at 448, 476 and 505 nm. The energy difference between the 0-0 bands of absorption and emission spectra was about 3900 cm(-1) and location of the emission maximum was less sensitive to the polarizability of solvents than that of the absorption maximum. These clearly indicate that the emission originates from the optically forbidden singlet state (2Ag). This is in contrast to other carotenoids whose emission is assigned to 1Bu state, probably due to the symmetric structure of the conjugated double bond responsible for the absorption in the visible region. A rapid internal conversion from 1Bu to 2Ag state might be facilitated by distorted structure of the conjugated double bond of fucoxanthin. The energy level responsible for the emission is almost identical to the Qy level of the acceptor molecule (Chl a), thus we propose an energy transfer pathway from the optically forbidden 2Ag state of the carotenoid to the Qy transition of Chl a in algal pigment systems.

Relaxin modulates synthesis and secretion of procollagenase and collagen by human dermal fibroblasts

Relaxin is believed to play a role in connective tissue remodeling during pregnancy (Bell, R.J., Eddie, L. W., Lester, A. R., Wood, E. C., Johnston, P.D., and Niall, H. D. (1987) Obstet. Gynecol. 69, 585-589; MacLennan, A. H. (1983) Clin. Reprod. Fertil. 2, 77-95). In the present study, normal human fibroblasts exposed to concentrations of a synthetic bioactive relaxin peptide from 0.1 to 10 ng/ml synthesized and secreted the metalloproteinase procollagenase, which was immunoprecipitable as a doublet of 52 and 57 kDa by a monoclonal antibody to human collagenase. The stimulation in procollagenase protein expression was reflected in an elevation in procollagenase mRNA levels. Media conditioned for 48 h by relaxin-treated fibroblasts (0.1 ng/ml) contained 1.7 units/ml activatable collagenase compared with 0.2 units/ml by untreated fibroblasts. In addition, relaxin caused a modest decrease in the levels of tissue inhibitor of metalloproteinases, as detected by reverse zymography and Northern analysis. Relaxin was also a potent modulator of the collagen secretory phenotype of these fibroblasts. Relaxin at 100 ng/ml down-regulated collagen secretion by 40%. When fibroblasts were treated simultaneously with cytokines such as transforming growth factor beta or interleukin 1 beta, which stimulated collagen synthesis to at least 9-fold of basal levels, relaxin at 100 ng/ml was able to down-regulate collagen expression by up to 88%. This decrease was reflected by changes at the mRNA level. These results indicate that relaxin can cause significant collagen turnover both by stimulating collagenase expression and by down-modulating collagen synthesis and secretion.

Excitation energy transfer between β-carotene and chlorophyll-a in various systems

The excitation energy transfer from β-carotene to chlorophyll-a in several seminatural systems such as liposomes, lipid layers and PSI complex has been studied at room and liquid nitrogen temperature. Only in a case of PSI complex an efficient energy transfer (about 30%) from β-carotene to chlorophyll-a has been observed. The results of energy transfer were discussed on the ground of Dexter's mechanism by taking into account the recently discovered energy level ((1)Ag) of β-carotene.