Spectroscopic properties and chromophore conformations of the photomorphogenic receptor: phytochrome

Fluorescence lifetimes of 'large (mol. wt. 120,000) and 'small' (mol. wt. 60,000) phytochromes isolated from oat and rye seedlings grown in the dark have been measured at 199 K and 298 K. Phytochrome model compounds have also been studied by phase modulation fluorometrically at 77 K for comparison with lifetime data for phytochrome. It was found that the fluorescence lifetime of 'large' phytochrome was significantly shorter than that of 'small' phytochrome and its chromophore models. The phytochrome chromophore of Pr form has been analyzed by fluorescence polarization, CD, and molecular orbital methods. The fluorescence excitation polarization of 'small' phytochrome and the chromophore model in buffer/glycerol mixture (3 : 1, v/v) at 77 K is very hight (0.4) at the main absorption band and is negative (--0.1) and close to 0 in the near ultraviolet band, respectively. Analyses of the spectroscopic data suggest that the chromophore conformation of Pr and Pfr forms of phytochrome are essentially identical. The induced ellipticity of 'large' rye phytochrome in the blue and near ultraviolet regions was found to be significantly higher than that of 'small' phytochrome, indicating that the binding interaction between the phytochrome chromophore and apoprotein is much tighter in the former than in the latter. In addition, the excitation energy transfer does occur from Trp residue(s) to the chromophore in 'large' phytochrome but not in 'small' Pr. This illustrates one feature of the role played by the large molecular weight apoprotein in the binding site interactions and primary photoprocesses of Pr. Finally, a plausible model for the primary photoprocesses and the mechanism of phytochrome interactions triggered by the Pr leads to Pfr phototransformation have been proposed on the basis of the above results.

Photobinding of 8-methoxypsoralen to transfer RNA and 5-fluorouracil-enriched transfer RNA

The photobinding of [3H]8MOP to tRNA upon irradiation at 365 nm in the absence of O2 was determined by gel filtration. The maximum photobinding was found to be ca. 4 mol of 8MOP er mol of tRNA and 5FU-tRNA, with an overall quantum yield of 2.3 X 10(-3). The photobinding kinetics for 8MOP-tRNA showed an apparent induction period or sigmoidal kinetic curve, indicating a specific initial photobinding site on tRNA which was identified as 4-thiouridine at position 8 from the 5'-end of Escherichia coli tRNA. Photobinding of 8MOP to 5FU-tRNA proceeded without an apparent induction period. 8MOP-tRNA and 8MOP-5FU-tRNA adducts were characterized by absorption, fluorescence, and CD spectroscopy. A modified procedure was also developed to analyze the nucleoside composition in modified 8MOP-tRNA and 8MOP-5FU-tRNA. The results showed that 8MOP photochemically added mainly to pyrimidine bases. The photobinding of 8MOP changed the conformation (secondary in particular) of tRNA and inhibited aminoacyl-tRNA synthetase activity.

The chromophore topography and binding environment of perididin.chlorophyll a.protein complexes from marine dinoflagellate algae

1. The peridinin.chlorophyll a.protein complex from Amphidinium carterae (Plymouth 450) shows spectroscopic characteristic (absorption, CD, fluorescence polarization, lifetime and energy transfer) essentially identical with peridinin.chlorophyll a.protein complexes from Glenodinium sp., Gonyaulax polyedra and Amphidinium rhyncocephaleum. 2. The apoprotein of peridinin.chlorophyll a.protein complexes is globular, with an isotropic rotational relaxation time (e.g. 33 ns for the A. caterae peridinin.chlorophyll a.protein), as deduced from the dynamic depolarization data. 3. The chromophores (4 peridinins and 1 chlorophyll a for peridinin.chlorophyll a.protein complexes from Glenodinium sp., G. polyedra and A. rhyncocephaleum and 9 and 2, respectively, for peridinin.chlorophyll a.protein of A. carterae) are accommodated in a hydrophobic crevice and not exposed to the solvent. The surface of the protein is highly hydrophilic. 4. No evidence for chlorophyll-chlorophyll interactions in the A. carterae peridinin.chlorophyll a.protein was obtained. This implies that binding crevices for two chlorophylls and half of peridinins (four to five) are located at some distance from each other. 5. The peridinin.chlorophyll a.protein complexes function as the photosynthetic antenna pigment. In addition, peridinins effectively protect chlorophyll a from photodecomposition.

Molecular topology of the photosynthetic light-harvesting pigment complex, peridinin-chlorophyll a-protein, from marine dinoflagellates

The photosynthetic light-harvesting complex, peridinin-chlorophyll a-protein, was isolated from several marine dinoflagellates including Glenodinium sp. by Sephadex and ion-exchange chromatography. The carotenoid (peridinin)-chlorophyll a ratio in the complex is estimated to be 4:1. The fluorescence excitation spectrum of the complex indicates that energy absorbed by the carotenoid is transferred to the chlorophyll a molecule with 100% efficiency. Fluorescence lifetime measurements indicate that the energy transfer is much faster than fluorescence emission from chlorophyll a. The four peridinin molecules within the complex appear to form two allowed exciton bands which split the main absorption band of the carotenoid into two circular dichronic bands (with negative ellipticity band at 538 nm and positive band at 463 nm in the case of peridinin-chlorophyl a-protein complex from Glenodinium sp.). The fluorescence polarization of chlorophyll a in the complex at 200 K is about 0.1 in both circular dichroic excitation bands of the carotenoid chromophore. From these circular dichroic and fluorescence polarization data, a possible molecular arrangement of the four peridinin and chlorophyll molecules has been deduced for the complex. The structure of the complex deduced is also consistent with the magnitude of the exciton spliting (ca. greater than 3000 cm-1) at the intermolecular distance in the dimer pair of peridinin (ca. 12 A). This structural feature accounts for the efficient light-harvesting process of dinoflagellates as the exciton interaction lengthens the lifetime of peridinin (radiative) and the complex topology increases the energy transfer probability. The complex is, therefore, a useful molecular model for elucidating the mechanism and efficiency of solar energy conversion in vivo as well as in vitro.

A spectroscopic analysis of vitamin B12 derivatives

1. Several vitamin B12 derivatives including descobalt B12 show unusual inversion of their CD signs upon rapid cooling to liquid N2 temperature, although the room temperature CD signs are conserved by a slower cooling to the same temperature. As a possible explanation for this puzzling observation, a micro-environmental birefringence around the chromophore imbedded in an organic glass is proposed. 2. Absorption, fluorescence, phosphorescence, and polarization spectra of descobalt B12 can be correlated with those of porphyrin free bases, as these two molecular systems share many similarites in their electronic structure. 3. Molecular orbital calculations of polarization directions further support the analoby between the spectroscopic characteristics of corrins and porphyrins, and are generally in good agreement with the fluorescence polarization data.