Surface chemistry of the frog sticky-tongue mechanism

Abstract

Frogs capture their prey with a highly specialized tongue. Recent studies indicate this tongue is covered with fibril-forming mucus that acts as a pressure sensitive adhesive. However, no analysis of the interfacial chemistry of frog tongue mucus has been performed. The goal of this study is to examine the chemical structure of the surface of mucus after a tongue strike. Previous studies of mucus from other animals suggest that mucus from a frog's tongue consists of mucins—serine-, threonine-, and proline-rich glycoproteins. Therefore, the authors expect to observe chemical bonds associated with glycoproteins, as well as fibrils formed at the mucus–tongue interface. To test this hypothesis, they collected both near-edge x-ray absorption fine structure (NEXAFS) microscopy images and sum frequency generation (SFG) vibrational spectra from layers of mucus left after frog tongue strikes on cleaned glass slides. NEXAFS imaging demonstrates a uniform distribution of amide, hydroxyl, and carbon–carbon bonds across the mucus surface. Difference spectra of individual N1s and C1s K-edge spectra pulled from these images indicate a structure consistent with fibril formation as well as disorder of oligosaccharide groups near the mucus surface. C—H region SFG spectra reveal surface active modes which likely stem from serine and threonine within the mucin protein. Combined, this work suggests that glycoproteins are well-ordered at the mucus–tongue interface.

NEXAFS imaging to characterize the physio-chemical composition of cuticle from African Flower Scarab Eudicella gralli

“…NEXAFS has been developed to provide a detailed analysis of biological surfaces and has been used to track the binding, orientation and structure of proteins 18,19 and DNA 20 when attached to surfaces, but also biominerals 21 , hard and soft biological tissue 22,23 , as well as the surface chemistry of snake scales 24 and frog tongue mucus 25 . STXM and XPEEM both can record NEXAFS spectra with high spatial resolution up to several tens of nanometers and have been used to study protein assembly at biomaterial surfaces and the mineralization of biological tissue 26,27 .…”

Comparative and functional analysis of the digital mucus glands and secretions of tree frogs

“…The presence of predominantly methyl vibrations at the interface in both PPP and SSP polarization may be associated to the presence of a well-ordered monolayer formed by long chain fatty acids or lipids present in the mucus [39, 40]. Additionally, a small peak at ∼3050 cm -1 assigned to the C-H stretch of aromatic amino acids groups [38, 41, 42] indicates hydrophobic amino acid residues. The hydroxyl region (3100–3800 cm -1 ) shows a broad peak around ∼3550 cm -1 in both PPP and SSP polarizations.…”

“…Interestingly, indications of methyl and methylene groups as found in the amino acid side chains in mucosubstances were also observed in the SFG spectrum of the tongue mucus of the frog Ceratophrys sp. [42]. A future analysis of IR spectra below 1500 cm -1 and of SFG spectra in the amide I/II region would allow a direct comparison between the two spectroscopic methods and hence an investigation of the proteinaceous components of tree frog mucus (e.g.…”

Tesafilm auf der Zunge? Mechanochemie beim Beutefang des Froschs

“…Was passiert darin beim Beutefang von Fröschen? Für alle Arten von von Klebemechanismen im Tierreich interessiert sich Prof. Stanislav Gorb vom zoologischen Institut der Universität Kiel, einer der Autoren der Studie . „Die Flüssigkeit selbst haftet sehr gut auf hydrophilen, aber auch hydrophoben Oberflächen“, so seine Beobachtung.…”