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Fish skin makes fish shiny |
| TheAllINeed.com |
(NC&T/ESRF) Cosmetics and fish have something in common. Many of the beauty products on the market today have a pearly iridescent effect thanks to a molecule called guanine. This is a compound that fish naturally grow on their skin in the form of tiny crystals and that, combined with cytoplasm, creates the characteristic metallic lustre of these animals.
Fish have some control over the crystallisation process, and this is expressed in the morphology of the crystals, their size and orientation. Until today scientists knew that the crystals have the morphology of elongated thin plates in the range of 50-100 nanometre thick (nearly a thousand times thinner than a hair), with a length of 20 micrometres and width of 3 micrometres.
The team of Israeli scientists has just proved that the form of the guanine on the fish's skin is anhydrous, i.e. it contains no water. However incredible it may sound, fish form guanine crystals in almost dry crystal chambers (one per crystal). Levy-Lior and her colleagues studied in vitro grown guanine crystals as well as biogenic crystals produced by the Japanese Koi fish (carp).
The team used the technique of X-ray powder diffraction at the ESRF ID31 beamline to study the structure of in vitro grown crystals and to compare it with that of biogenic crystals. The need for using powder diffraction was due to the small size of the guanine single crystals.
The results showed that the carp's crystals are thin plates orientated in such a way that the face of the crystal, which is also the one containing hydrogen-bonded network of guanine molecules, is exposed to the light. It is known that the in-plane molecular polarisability - and, with it, the light reflectivity - of guanine molecules is higher in the plane of the molecule than it is perpendicular to it. On the other hand, in vitro crystals are much thicker and irregular in shape. These results were unexpected and differ from theoretical predictions.
 | | A Japanese Koi fish. (Photo: M.C. Stevens, www.organicpixel.com) |
The outcome of this research shows that natural phenomena like the growth of crystals on the skin of fish have a reason behind them. "Fish invest energy in inhibiting crystal growth in the fast growing direction. This allows them to have the best possible optical properties for reflection", explains Levy-Lior.
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