|
Hypericin is a widely occurring natural perylene quinone pigment, best known from flowering plants in the genus Hypericum. It has a broad spectrum of light-induced biological activities. |
|
Hypericin also exhibits photodynamic properties. In order to understand this behavior, it is important to study hypericin in complex with biologically significant macromolecules. There is a strong influence of the environment on the photophysics of hypericin and its analogs. Dramatic changes in excited-state behavior are observed as hypericin is moved from the homogeneous environment of organic solvents to the much more structured surroundings provided by the complexes it forms with proteins. It is clear from our studies that the interaction with a protein modifies the photophysics of hypericin. Understanding the molecular basis of this interaction is one of the outstanding problems in elucidating the function of hypericin and hypericin-like chromophores. |
|
Structure Elucidation of a Hypericin-Like Pigment from Marine Ciliate Maristentor dinoferus |
|
Blepharismins |
|
Oxyblepharismins |
|
Stentorin |
|
Several protozoa in a group of heterotrich ciliate families known collectively as stentorids have photoreceptor pigments that are structurally related to hypericin. Extensive efforts have been devoted towards structure elucidation of the pigments associated in these ciliates and structures have been determined for pigments from only two heterotrichs to date. Stentor coeruleus contains stentorin, which is very similar to hypericin, while Blepharisma japonicum is colored by several blepharismins and oxyblepharismins. The pigments in both these species are involved in the organisms’ abrupt photophobic responses. They are also involved in defense reactions, in which a cloud of pigment is released from a patch of cortical granules to deter raptorial ciliate predators. |
|
Although other heterotrichs are known to have spectrally similar pigments, but not much effort has been devoted towards this end. Some other Stentor spp. have both pigment and symbiotic green algae (zoochlorellae) and are positively phototactic, but neither their photobiology nor their pigment structure has been studied. Recently Chris S. Lobban and coworkers, University of Guam, discovered a marine ciliate, Maristentor dinoferus, which is prominently pigmented and also contains some 600 endosymbiotic golden algal cells (zooxanthellae) (above). |
|
It resembles Stentor but was placed in its own family within the stentorid group. It is positively phototactic. Recently in collaboration with Chris Lobban, we have elucidated the structure of the chromophore from Maristentor dinoferus and reported the spectral properties of the pigment. It is remarkable that while the pigments are structurally similar in S. coeruleus and M. dinoferus, in the former there is an abrupt photophobic response, whereas in the latter there is a slow response toward light. Clearly the immediate environment of the pigment modulates the photo behavior in these species and it would be interesting to study the photophysics of these pigments in complex with their respective proteins. Maristentorin is interesting both as a novel analogue of hypericin and as a new member of the multifunctional pigments from stentorids. Recent efforts have been devoted to extend similar effort towards two folliculinids in which we have demonstrated the existence of hypericin like chromophores. This work has been performed in collaboration with Steven J. Hallam, University of British Columbia and Chris S. Lobban, University of Guam. |
