Albert Gossauer

Synthesis of a macrocyclic porphyrin hexamer with a nanometer-sized cavity as a model for the light-harvesting arrays of purple bacteria

Abstract The synthesis of a cyclic hexaporphyrin array with an intergral acceptor chromophore, mimicking the light-harvesting complex of photosynthetic purple bacteria, is described. The cavity of this hexagonal macrocycle, which is circumscribed by five porphyrin rings, as Zn(II) chelates, and one porphyrin ring, as free base, has a diameter of about 4.6 nm. The quantum efficiency of intramolecular transfer of singlet excited-state energy determined by comparison with a solution of a mixture of the corresponding porphyrin monomers in a 5:1 ratio amounts to 40 %.

Chlorophyll catabolism. Isolation and structure elucidation of chlorophyll b catabolites in Chlorella protothecoides

Abstract When grown in a medium rich in glucose but poor in a nitrogen source, Chlorella protothecoides cells excrete different pigments which have been characterized as products of chlorophyll a catabolism. Now, degradation products of chlorophyll b have been isolated and characterized for the first time.

Formation of oxoniachlorins on photooxidation of 20-trifluoroacetoxy- and 20-chloro-chlorophyll derivatives

Zinc complexes of both 20‐trifiuoroacetoxy‐ and 20‐chloro‐pheophorbide‐a methyl ester yield the same isolable 20‐oxonia‐pheophorbide (2b) on irradiation with visible light in methylene chloride for 90 and 10 min, respectively. In the case of the 20‐chloro derivative, the yield amounts to40–50% along with 30% of starting material. Zinc complexes of 20‐trifluoroacetyl‐ and 20‐chloro‐pyropheophorbide‐a methyl ester behave likewise. In aqueous solution, the expected dihydro‐biliverdin derivatives 2 are obtained among other products, some of them display a strong red fluorescence. In the presence of nucleophiles the oxoniachlorin macrocycle is readily cleaved, yielding the corresponding linear tetrapyrroles. Particularly, reaction with cyanide ions leads to biline derivatives, which could serve as intermediates for the synthesis of natural occurring luciferins from bioluminescent plankton. The possible role of some of the above mentioned reactions in connection with chlorophyll catabolism is discussed.

A biomimetic partial synthesis of the red chlorophyll-a catabolite from Chlorella protothecoides.

Abstract The red bilin derivative which is excreted in the culture medium by Chlorella protothecoides when this microalgae is grown in a medium rich in glucose but poor in nitrogen has been synthesized in two steps from one of the products of photo-oxidation of the Cd(II) complex of pyropheophorbide-a methyl ester. Under the same conditions, the Zn(II) complex of the latter is transformed into a blue bilin derivative which is an isomer of the foregoing chlorophyll-a catabolite.

CHLOROPHYLL CATABOLISM. PART 3. STRUCTURE ELUCIDATION AND PARTIAL SYNTHESIS OF A NEW RED BILIN DERIVATIVE FROM Chlorella protothecoides

The structure of a new red pigment produced during the “bleaching” of Chlorella protorhecoides cells has been elucidated by analytical methods as well as by in vitro isomerization into the new compound of a synthetic sample of a previously characteriLed chlorophyll a catabolite.

MECHANISM OF PHOTOOXYGENATION OF THE CD (II) COMPLEX OF PYROPHEOPHORBIDE A METHYL ESTER

Photooxygenation of (pyropheophorbidato a methyl ester)cadmium (II) was studied using 18,18O2 labeling of the molecular oxygen required for cleavage of the macrocycle. After reductive demetallation of the primary oxidation product (4,5‐dioxo‐4,5‐secopyropheophorbidato a methyl ester)cadmium (II), the isotope content of formylbilinone 4a was analyzed by repeated‐scan fast atom bombardment mass spectrometry. Comparison of the spectroscopic data of the labeled pigment 4a with the statistical probabilities of18 O isotope incorporation calculated for four possible reaction mechanisms clearly proves that photooxidative ring cleavage occurred by the one‐molecule mechanism, i.e. the terminal oxygen atoms of 4a were derived from one oxygen molecule. Furthermore, a study of the exchange of the18 O‐labeled atoms revealed that no exchange occurs within the pH 4.5–9.5 range. In stronger alkaline or acidic solutions, only the oxygen atom of the formyl group is exchanged. Hydrolysis of the methyl ester group of 4a was achieved, without loss of the18 O label on the formyl group, at pH 7.2 in the presence of pig liver esterase.