Photonic crystals are some of the more spectacular realizations that periodic arrays can change the behavior of electromagnetic waves. In nature, so-called structural colors appear in insects and even plants. Some species create beautiful color patterns as part of biological behavior such as reproduction or defense mechanisms as a form of biomimetics. The interaction between light and matter occurs at the surface, producing diffraction, interference and reflectance, and light transmission is possible under suitable conditions. In particular, there are two Colombian butterflies, Morpho cypris and Greta oto, that exhibit iridescence phenomena on their wings, and in this work, we relate these phenomena to the photonic effect. The experimental and theoretical approaches of the optical response visible region were studied to understand the underlying mechanism behind the light-matter interaction on the wings of these Colombian butterflies. Our results can guide the design of novel devices that use iridescence as angular filters or even for cosmetic purposes. Nature is colored and attracts our attention due to its beauty and complex colors. Colors can be produced by pigmentation or by the arrangement of nanostructures. Recent studies showed that some species use this latter option to create beautiful color patterns as part of biological behavior such as reproduction or defense mechanisms as a form of mimicry 1-8 , and these tools have been perfected over millions of years in nature. In all cases, the interaction between light and matter occurs at the surface, producing diffraction, interference and reflectance phenomena. Additionally, light transmission is possible under suitable conditions. In the last decade, another optical phenomenon that has generated great interest among researchers is iridescence, which is related to the angular dependence of the observed color; there are many biological species and minerals that have this feature 9-11. In particular, there are two Colombian butterflies, Morpho cypris and Greta oto, that exhibit an iridescent effect on their wings. Iridescence has many feasible applications 12 , and a key advantage of structural colors over pigmented colors is the more intense coloration, especially under high light conditions. However, the physical interpretation of this phenomenon remains unclear and deserves more study. Iridescence is an effect that takes place in the visible spectrum 13,14 ; thus, the interaction of light with the surfaces of these systems implies the existence of nanostructures with sizes comparable to the wavelength in the visible region. On the other hand, there are man-made structures that exhibit iridescent effects, such as TiO 2 traces 15-17 , but total control over this property in man-made systems is not yet available. In this way, comprehension of the proper working conditions when light interacts with a butterfly's wings is a very important step toward man-made systems that will use this property. The Morpho cypris and Greta oto butterfly samples were provided, wit...

The predicted structure has been calculated for a protein-based biosensor for inorganic phosphate (Pi), previously developed by some of us (Okoh et al., Biochemistry, 2006, 45, 14764). This is the phosphate binding protein from Escherichia coli labelled with two rhodamine fluorophores. Classical molecular dynamics and hybrid Car-Parrinello/molecular mechanics simulations allow us to provide molecular models of the biosensor both in the presence and in the absence of Pi. In the latter case, the rhodamine fluorophores maintain a stacked conformation in a 'face A to face B' orientation, which is different from the 'face A to face A' stacked orientation of free fluorophores in aqueous solution (Ilich et al., Spectrochim. Acta, Part A, 1996, 52, 1323). A protein conformation change upon binding Pi prevents significant stacking of the two rhodamines. In both states, the rhodamine fluorophores form hydrophobic contact with LEU291, without establishing significant hydrogen bonds with the protein. The accuracy of the models is established by a comparison between calculated and experimental absorption and circular dichroism spectra.

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