Fluorescence, the absorption of short-wavelength electromagnetic radiation reemitted at longer wavelengths, has been suggested to play several biological roles in metazoans. This phenomenon is uncommon in tetrapods, being restricted mostly to parrots and marine turtles. We report fluorescence in amphibians, in the tree frog Hypsiboas punctatus, showing that fluorescence in living frogs is produced by a combination of lymph and glandular emission, with pigmentary cell filtering in the skin. The chemical origin of fluorescence was traced to a class of fluorescent compounds derived from dihydroisoquinolinone, here named hyloins. We show that fluorescence contributes 18−29% of the total emerging light under twilight and nocturnal scenarios, largely enhancing brightness of the individuals and matching the sensitivity of night vision in amphibians. These results introduce an unprecedented source of pigmentation in amphibians and highlight the potential relevance of fluorescence in visual perception in terrestrial environments.Amphibia | Anura | Hylidae | visual ecology | fluorophore F luorescence occurs when short-wavelength electromagnetic radiation is absorbed and then reemitted at longer wavelength. This phenomenon is broadly distributed in marine and terrestrial environments and is found in distantly related organisms (1). Among aquatic vertebrates, fluorescence is widespread phylogenetically within cartilaginous and ray-finned fishes (2) and has been documented in sea turtles (3), whereas among terrestrial vertebrates, it is only known to occur in parrots (4). With few exceptions (5-7), the molecular basis of most of those reports remains unstudied. Many roles have been suggested for fluorescence in animals, such as photoprotection (8), antioxidation (9), and visual communication (10)(11)(12)(13)(14).Amphibians (frogs, toads, salamanders, newts, and caecilians) have a wide range of skin coloration (15) caused by an integumental pigmentary system in which the combination of different types of chromatophore cells create coloration through the integration of chemical and structural features (16). Although the chemical nature and distribution of chromophores has been studied (16), fluorescence has not been reported in any of the 7,600 species of amphibians (17). Here we report a case of fluorescence in this highly diverse group, introduce a class of fluorescent compounds, and assess its importance by quantifying its contribution to overall coloration under natural light conditions. Results and DiscussionFluorescence in Hypsiboas punctatus. The South American tree frog H. punctatus (Family Hylidae) is unusual among amphibians in possessing a translucent skin, a crystal-containing layer in the peritonea and bladder, and a high concentration of biliverdin in lymph and tissues. We observed that living adults and juveniles illuminated with UV-A blue light produced a bright blue/green fluorescent emission (Fig. 1 A-C) that was clearly discernible from the body surface of the specimens. To characterize the fluorescence of f...

Cysteine is a common amino acid with a thiol group that plays a pivotal role in a variety of scenarios in redox biochemistry. In contrast, selenocysteine, the 21st amino acid, is only present in 25 human proteins. Classical force-field parameters for cysteine and selenocysteine are still scarce. In this context, we present a methodology to obtain Lennard-Jones parameters for cysteine and selenocysteine in different physiologically relevant oxidation and protonation states. The new force field parameters obtained in this work are available at . The parameters were adjusted to reproduce water radial distribution functions obtained by density functional theory ab initio molecular dynamics. We validated the results by evaluating the impact of the choice of parameters on the structure and dynamics in classical molecular dynamics simulations of representative proteins containing catalytic cysteine/selenocysteine residues. There are significant changes in protein structure and dynamics depending on the parameters choice, specifically affecting the residues close to the catalytic sites.