Abstract
Chlorophyll derivatives are known to enhance vision in vertebrates. They are thought to bind visual pigments (i.e., opsins apoproteins bound to retinal chromophores) directly within the retina. Consistent with previous findings in vertebrates, here we show that chlorin e6 — a chlorophyll derivative — enhances photophobicity in a flatworm (Dugesia japonica), specifically when exposed to UV radiation (λ = 405 nm) or red light (λ = 660 nm). This is the first report of chlorophyll derivatives acting as modulators of invertebrate phototaxis, and in general the first account demonstrating that they can artificially alter animal response to light at a behavioral level. Our findings show that the interaction between chlorophyll derivatives and opsins virtually concerns the vast majority of bilaterian animals, and also occurs in visual systems based on rhabdomeric (rather than ciliary) opsins.
Highlights
In order to sense light, animals rely on specialized macromolecules, namely photoreceptor proteins; these are required for the initial sensory transduction event that eventually leads to vision
We confirmed emission and absorption spectra for Ce6 dissolved in 1% dimethyl sulfoxide (DMSO)
Our spectroscopic analyses were substantially aimed at confirming that a water solution containing 1% DMSO is an acceptable solvent for Ce6
Summary
In order to sense light, animals rely on specialized macromolecules, namely photoreceptor proteins; these are required for the initial sensory transduction event that eventually leads to vision. 2. Opsins bind retinal, a chromophore that captures light and transfers energy to the apoprotein; together they form a visual pigment. The meso-bathypelagic fish Malacosteus niger is capable of seeing far-red light in spite of the absorption spectrum of its opsins, which are green-tuned and display negligible absorption already at λ = 650 nm. M. niger accumulates demetallated and defarnesylated derivatives of bacteriochlorophylls C and D4–8. Following this natural example, some studies explored the possibility of CDs being modulators of vertebrate vision. A handful of CDs were tested as candidate photosensitizers of purified bovine rhodopsin (an opsin): one of them, a substituted chlorin named chlorin e6 (Ce6, Fig. 1), proved to be a potent bleaching agent for the protein upon exposure to red light[9].
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