Surface pressure (pi)-, surface potential (deltaV)-, and dipole moment (mu(perpendicular))-area (A) isotherms and morphological behavior were examined for monolayers of a newly designed 18-mer amphiphilic alpha-helical peptide (Hel 13-5), DPPC, and DPPC/egg-PC (1:1) and their combinations by the Wilhelmy method, ionizing electrode method, fluorescence microscopy (FM), and atomic force microscopy (AFM). The newly designed Hel 13-5 showed rapid adsorption into the air-liquid interface to form interfacial films such as a SP-B function. Regardless of the composition and constituents in their multicomponent system of DPPC/egg-PC, the collapse pressure (pi(c); approximately 42 mN m(-1)) was constant, implying that Hel 13-5 with the fluid composition of egg-PC is squeezed out of Hel 13-5/DPPC/egg-PC monolayers accompanying a two- to three-dimensional phase transformation. FM showed that adding a small amount of Hel 13-5 to DPPC induced a dispersed pattern of ordered domains with a "moth-eaten" appearance, whereas shrinkage of ordered domains in size occurred for the DPPC/egg-PC mixture with Hel 13-5. Furthermore, AFM indicated that (i) the intermediate phase was formed in pure Hel 13-5 systems between monolayer states and excluded nanoparticles, (ii) protrusions necessarily located on DPPC monolayers, and (iii) beyond the collapse pressure of Hel 13-5, Hel 13-5 was squeezed out of the system into the aqueous subphase. Furthermore, hysteresis curves of these systems nicely resemble those of the DPPC/SP-B and DPPC/SP-C mixtures reported before.

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The surface pressure (pi)-area (A) and surface potential (DeltaV)-A isotherms were measured for two-component monolayers made of dipalmitoylphosphatidylcholine (DPPC)/single-chain (perfluorooctyl)pentanol (F8C5OH) and DPPC/single-chain (perfluorooctyl)pentylphosphocholine (F8C5PC) on a substrate solution of 0.15 M NaCl at 293.2 K as a function of the composition of the two components. The Langmuir method and the ionizing electrode method were used. The data for these systems were analyzed using an additivity rule. Assuming a regular surface mixture, the Joos equation, which allows description of the collapse pressure of a monolayer made of two miscible components, was used to establish the miscibility within the monolayer. An interaction parameter and an interaction energy were calculated. The two-component DPPC/F8C5OH and DPPC/F8C5PC monolayers were miscible. Furthermore, the mean molecular area, surface potential, and phase diagrams enabled us to determine the molecular orientation of DPPC/F8C5OH and DPPC/F8C5PC in the monolayer. Two types of phase diagrams were obtained and classified into the positive azeotropic and negative azeotropic types. Fluorescence microscopy (FM) and Brewster angle microscopy (BAM) for the DPPC/F8C5OH and DPPC/F8C5PC systems show that both systems can dissolve the ordered micrometer-size solid DPPC domains. However, morphological analyses using atomic force microscopy (AFM) suggest partial miscibility or phase separation for DPPC and the partially fluorinated compounds on the nanometer scale. In particular, triskelion-shaped domains were evidenced for F8C5OH. These results indicate that the partially fluorinated amphiphiles investigated here fluidize the DPPC monolayer upon lateral compression and that these chemicals may be useful to develop their innovative applications in the biomedical field.