Get to the point: Claw morphology impacts frictional interactions on rough substrates.

Claws are a common anatomical feature among limbed amniotes and contribute to a variety of functions including prey capture, locomotion, and attachment. Previous studies of both avian and non-avian reptiles have found correlations between habitat use and claw morphology, suggesting that variation in claw shape permits effective functioning in different microhabitats. How, or if, claw morphology influences attachment performance, particularly in isolation from the rest of the digit, has received little attention. To examine the effects of claw shape on frictional interactions, we isolated the claws of preserved specimens of Cuban knight anoles (Anolis equestris), quantified variation in claw morphology via geometric morphometrics, and measured friction on four different substrates that varied in surface roughness. We found that multiple aspects of claw shape influence frictional interactions, but only on substrates for which asperities are large enough to permit mechanical interlocking with the claw. On such substrates, the diameter of the claw's tip is the most important predictor of friction, with narrower claw tips inducing greater frictional interactions than wider ones. We also found that claw curvature, length, and depth influence friction, but that these relationships depend on the substrate's surface roughness. Our findings suggest that although claw shape plays a critical role in the effective clinging ability of lizards, its relative importance is dependent upon the substrate. Description of mechanical function, as well as ecological function, is critical for a holistic understanding of claw shape variation.

Selenomelanin: An Abiotic Selenium Analogue of Pheomelanin.

Melanins are a family of heterogeneous biopolymers found ubiquitously across plant, animal, bacterial, and fungal kingdoms where they act variously as pigments and as radiation protection agents. There exist five multifunctional yet structurally and biosynthetically incompletely understood varieties of melanin: eumelanin, neuromelanin, pyomelanin, allomelanin, and pheomelanin. Although eumelanin and allomelanin have been the focus of most radiation protection studies to date, some research suggests that pheomelanin has a better absorption coefficient for X-rays than eumelanin. We reasoned that if a selenium enriched melanin existed, it would be a better X-ray protector than the sulfur-containing pheomelanin because the X-ray absorption coefficient is proportional to the fourth power of the atomic number (Z). Notably, selenium is an essential micronutrient, with the amino acid selenocysteine being genetically encoded in 25 natural human proteins. Therefore, we hypothesize that selenomelanin exists in nature, where it provides superior ionizing radiation protection to organisms compared to known melanins. Here we introduce this novel selenium analogue of pheomelanin through chemical and biosynthetic routes using selenocystine as a feedstock. The resulting selenomelanin is a structural mimic of pheomelanin. We found selenomelanin effectively prevented neonatal human epidermal keratinocytes (NHEK) from G2/M phase arrest under high-dose X-ray irradiation. Provocatively, this beneficial role of selenomelanin points to it as a sixth variety of yet to be discovered natural melanin.

Freezing and melting of salt hydrates next to solid surfaces probed by infrared-visible sum frequency generation spectroscopy.

Understanding the freezing of salt solutions near solid surfaces is important in many scientific fields. Here we use sum frequency generation (SFG) spectroscopy to study the freezing of a NaCl solution next to a sapphire substrate. During cooling we observe two transitions. The first corresponds to segregation of concentrated brine next to the sapphire surface as we cool the system down to the region where ice and brine phases coexist. At this transition, the intensity of the ice-like peak decreases, suggesting the disruption of hydrogen-bonding by sodium ions. The second transition corresponds to the formation of NaCl hydrates with abrupt changes in both the SFG intensity and the sharpness of spectral peaks. The similarity in the position of the SFG peaks with those observed using IR and Raman spectroscopy indicates the formation of NaCl·2H2O crystals next to the sapphire substrate. The melting temperatures of the hydrates are very similar to those reported for bulk NaCl·2H2O. This study enhances our understanding of nucleation and freezing of salt solutions on solid surfaces and the effects of salt ions on the structure of interfacial ice.

Direct measurement of acid-base interaction energy at solid interfaces.

We have studied acid-base interactions at solid-liquid and solid-solid interfaces using interface-sensitive sum frequency generation (SFG) spectroscopy. The shift of the sapphire hydroxyl peak in contact with several polar and nonpolar liquids and polymers was used to determine the interaction energy. The trend in the interaction energies cannot be explained by measuring only water contact angles. Molecular rearrangements at the sapphire interface, to maximize the interaction of the acid-base groups, play a dominant role, and these effects are not accounted for in the current theoretical models. These results provide important insights into understanding adhesion, friction, and wetting on solid interfaces.

Interfacial structure and melting temperature of alcohol and alkane molecules in contact with polystyrene films.

Infrared-visible sum-frequency-generation spectroscopy (SFG) is used to investigate the interfacial structure of hexadecanol (C16H33OH) and heneicosane (C21H44) in contact with polystyrene films (PS) spin coated on a sapphire substrate. The interfacial structure of hexadecanol is very different from heneicosane in contact with PS. In the crystalline state, the hexadecanol molecules are oriented with the C-C-C axis parallel to the surface plane in contact with PS. For the crystalline heneicosane/PS interface, the SFG spectra are very similar to those observed for molecules oriented with the symmetry axis of the methyl groups parallel to the surface normal. The structure of both hexadecanol (or heneicosane) and the phenyl groups changes sharply at the melting temperature of hexadecanol (or heneicosane). Upon heating the hexadecanol/PS sample above the glass transition temperature (T(g)) of PS, the hexadecanol molecules penetrate through the PS film and adsorb on the sapphire substrate. The adsorbed hexadecanol molecules are oriented with the symmetry axis of the methyl groups parallel to the surface normal. The structure of the PS molecules at the sapphire interface is different because the PS phenyl groups are now in contact with the hydrophobic tails of the hexadecanol molecules, rather than the hydrophilic sapphire substrate. The adsorbed hexadecanol molecules do not disorder at the bulk melting temperature of hexadecanol. In comparison, no adsorption of heneicosane molecules next to sapphire interface upon annealing was observed. The differences between the adsorption of hexadecanol and heneicosane can be explained by the preferential interactions between the hydroxyl groups of the alcohol and hydrophilic sapphire substrate.