Molecular packing of functionalized fluorinated lipids in Langmuir monolayers.

Fluorinated amphipaths are a fascinating class of compounds, which, despite significant challenges associated with their syntheses, have found use across a number of areas of biotechnology. Applications range from the in vitro stabilization of membrane proteins to the development of enhanced stability intravenous drug and gene delivery systems. More recently, monolayer-forming fluorinated lipids have found use in the 2D crystallization of detergent-solubilized hydrophobic or partially hydrophobic proteins at the air-water interface. In this study, we investigate the surface properties of a novel suite of monolayer forming, partially fluorinated lipids. These modular lipid structures contain a densely fluorinated insertion in the hydrocarbon tail and a synthetically modifiable headgroup. Analyses of surface-pressure area isotherms and X-ray reflectometry profiles reveal that the lipids spread into fluid monolayers and are more compressible than their non-fluorinated counterparts. Furthermore, the data support a model whereby the partially fluorinated chains of the lipid tails form a film which is fundamentally incompatible with detergents and other destabilizing amphipaths.

Gaseous adsorption in melanins: hydrophilic biomacromolecules with high electrical conductivities.

The melanins are an important class of multifunctional biomacromolecules that possess a number of intriguing physical and chemical properties including electrical and photoconductivity. Unusually for a conducting organic material, eumelanin is hydrophilic and its electrical properties are strongly dependent on its hydration state. We have therefore measured adsorption isotherms for two polar adsorbates, water and ethanol, in the pressed powder pellets of synthetic eumelanin typically used in electrical studies. We show that a simple kinetic monolayer Langmuir model describes the adsorption and find that there are strong adsorbate-eumelanin interactions in both cases. These isotherms allow the proper scaling of electrical conductivity data and in doing so make progress toward a better understanding of eumelanin electrical properties, which is a critical prerequisite to the design of new eumelanin-like bioelectronic materials.

Multilayer nanostructured porphyrin arrays constructed by layer-by-layer self-assembly.

UV-vis absorption, atomic force microscopy (AFM), contact angle, and X-ray reflectivity experiments were performed on thin films deposited on crystalline silicon substrates as alternating layers of a porphyrin with anionic functionality, tetra-5,10,15,20-(4-sulfonatophenyl)porphine (TSPP) or the metalated version, Cu(II)TSPP, and the cationic polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA). The films were made by dipping in alternating aqueous solutions containing film components (layer-by-layer deposition). Modeling of the X-ray reflectivity data revealed differences in the films' thickness depending on the method of film deposition. An unusual decrease in film thickness after each polyelectrolyte dip was also observed for films using TSSP. UV-vis measurements revealed that a similar amount of TSSP was included within films despite the method of formation. UV-vis measurements also revealed the presence of free-base, H-aggregate, and J-aggregate forms of the porphyrin after TSPP dipping, and the subsequent disappearance of the J-aggregate after dipping in the PDDA solution. A model of film formation was proposed on the basis of the concept of two different types of porphyrin aggregates being present after dipping in porphyrin solution. A layer of porphyrin molecules initially attach to the Si surface such that the planar molecules are arranged side by side as H-aggregates with an excess of J-aggregated material on top. The J-aggregate is then removed and replaced by a layer of PDDA. A change in contact angle of 14 degrees was observed between porphyrin and polyelectrolyte layers due to the more hydrophobic nature of the polymer. The presence of the J-aggregate was confirmed in AFM images obtained from the porphyrin layer. Exposure of the films to solutions of alternating pHs of 10 and 1.8 resulted in reproducible switching of the UV-vis spectra, indicating a possible sensing application.

Rigid films of an anionic porphyrin and a dialkyl chain surfactant.

The 2D complex formed at the air-water interface between the dialkyl chain cationic surfactant, dihexadecyldimethylammonium bromide, and the anionic porphyrin, tetrakis-(4-sulfonatophenyl) porphine, was studied using surface pressure-area isotherms as well as X-ray and neutron reflection measurements. The surface structure of these films was determined by the use of simultaneously constrained analysis of the neutron and X-ray reflectometry data and BAM images. Isotopic contrast variation methods were employed to enhance the information content of the neutron reflection data. The rigid complex forms at the interface due to the electrostatic interaction between the cationic headgroups of the surfactant and the anionic functional groups at the meso position of the porphyrin. The surface pressure-area isotherms show three distinct regions on compression: an initial condensed phase that ends with a pressure peak at 36 mN m-1, a second plateau region of high compressibility, and a final condensed phase. BAM images show that at the beginning of the plateau region in the isotherm there is complete surface coverage by a monolayer. The constrained simultaneous fitting of neutron and X-ray data measured just prior to and after the pressure peak shows a structurally similar 2D complex at the interface. Modeling of X-ray reflectometry data also reveals that in the final high-pressure phase the film has folded to form a trilayer. The conclusion is that the plateau region of the isotherm is due to the formation of trilayer surface coverage through localized buckling or folding, and that after this is complete there is some condensation before final film collapse.

Interfacial behavior of tetrapyridylporphyrin monolayer arrays.

The behavior of monolayer films of free base 5,10,15,20-tetrapyridylporphinato (TPyP) and 5,10,15,20-tetrapyridylporphinato zinc(II) (ZnTPyP) on pure water, 0.1 M CdCl2, and 0.1 M CuCl2 subphases was investigated by surface pressure-area isotherms, specular X-ray reflectometry, and polarized total reflection X-ray absorption spectroscopy (PTRXAS). Surface pressure-area isotherms showed significant differences in the area per molecule on pure water compared to that on salt subphases, with a marked increase in the area observed on the salt solutions. This behavior was noted for both forms of the porphyrin and both salts investigated. Modeling of specular X-ray reflectometry data indicated that thinner and more electron dense layers on salt subphases best fit the observed profiles. These data suggest that the porphyrin macrocycle is oriented parallel to the interface on salt subphases and takes on a tilted conformation on pure water. In the case of ZnTPyP, PTRXAS was used to determine the orientation of the porphyrin moiety relative to the surface and to probe the coordination of the central Zn ion. In agreement with the pressure-area isotherms and reflectometry, the PTRXAS data indicate a change in orientation on the salt subphases.

Mixed thin films of a cationic amphiphilic porphyrin and n-alkanes.

Langmuir and Langmuir-Blodgett (LB) films of a cationic amphiphilic porphyrin mixed with n-alkanes octadecane and hexatriacontane were prepared and characterized, to examine the influence of the alkanes on film structure and stability. While the structure present in these films was controlled primarily by the porphyrin, the addition of the alkanes resulted in significant changes to both the phase behavior of the Langmuir films and the molecular arrangement of the LB films. These changes, as well as the observed chain length effects, are explained in terms of the intermolecular interactions present in the films.