Surface chemistry of the frog sticky-tongue mechanism

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 N_1s and C_1s 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.