Adsorption of Frog Foam Nest Proteins at the Air-Water Interface

The surfactant properties of aqueous protein mixtures (ranaspumins) from the foam nests of the tropical frog Physalaemus pustulosus have been investigated by surface tension, two-photon excitation fluorescence microscopy, specular neutron reflection, and related biophysical techniques. Ranaspumins lower the surface tension of water more rapidly and more effectively than standard globular proteins under similar conditions. Two-photon excitation fluorescence microscopy of nest foams treated with fluorescent marker (anilinonaphthalene sulfonic acid) shows partitioning of hydrophobic proteins into the air-water interface and allows imaging of the foam structure. The surface excess of the adsorbed protein layers, determined from measurements of neutron reflection from the surface of water utilizing H2O/D2O mixtures, shows a persistent increase of surface excess and layer thickness with bulk concentration. At the highest concentration studied (0.5 mg ml−1), the adsorbed layer is characterized by three distinct regions: a protruding top layer of ∼20 Å, a middle layer of ∼30 Å, and a more diffuse submerged layer projecting some 25 Å into bulk solution. This suggests a model involving self-assembly of protein aggregates at the air-water interface in which initial foam formation is facilitated by specific surfactant proteins in the mixture, further stabilized by subsequent aggregation and cross-linking into a multilayer surface complex.