Structural Basis for the Brilliant Colors of the Sapphirinid Copepods

Males of sapphirinid copepods use regularly alternating layers of hexagonal-shaped guanine crystals and cytoplasm to produce spectacular structural colors. In order to understand the mechanism by which the different colors are produced, we measured the reflectance of live individuals and then characterized the organization of the crystals and the cytoplasm layers in the same individuals using cryo-SEM. On the basis of these measurements, we calculated the expected reflectance spectra and found that they are strikingly similar to the measured ones. We show that variations in the cytoplasm layer thickness are mainly responsible for the different reflected colors and also that the copepod color strongly depends on the angular orientation relative to the incident light, which can account for its appearance and disappearance during spiral swimming in the natural habitat

Guanine Crystallization in Aqueous Solutions Enables Control over Crystal Size and Polymorphism

Anhydrous guanine crystals are among the most widespread organic crystals used by organisms to produce structural colors. The main advantage of guanine is its exceptionally high refractive index in the reflecting direction (∼1.8). For the same reason, guanine is a promising candidate material for a variety of different optical applications. Crystallization of guanine is challenging and usually involves using polar aprotic organic solvents such as dimethyl sulfoxide (DMSO). Here, we show that the crystallization of guanine from aqueous solutions is possible under conditions that provide control over crystal polymorphism and size. Using this approach we were able produce large crystals of the elusive guanine monohydrate phase. We were also able to rationalize the formation of the different phases obtained as a function of which tautomer of guanine is stable in solutions of varying pH