Nanoscale patterning in mixed fluorocarbon-hydrocarbon phospholipid bilayers

Conjugation of primary amine groups in targeted proteomics

Primary amines, in the form of unmodified N-terminus of peptide/protein and unmodified lysine residue, are perhaps the most important functional groups that can serve as the starting points in proteomic analysis, especially via mass spectrometry-based approaches. A variety of multifunctional probes that conjugate primary amine groups through covalent bonds have been developed and employed to facilitate protein/protein complex characterization, including identification, quantification, structure and localization elucidation, protein-protein interaction investigation, and so forth. As an integral part of more accurate peptide quantification in targeted proteomics, isobaric stable isotope-coded primary amine labeling approaches eventually facilitated protein/peptide characterization at the single-cell level, paving the way for single-cell proteomics. The development and advances in the field can be reviewed in terms of key components of a multifunctional probe: functional groups and chemistry for primary amine conjugation; hetero-bifunctional moiety for separation/enrichment of conjugated protein/protein complex; and functionalized linker/spacer. Perspectives are primarily focused on optimizing primary amine conjugation under physiological conditions to improve characterization of native proteins, especially those associated with the surface of living cells/microorganisms.

Thermodynamic study on hydrated bilayers of ether-linked phosphatidylcholines with terminal perfluorobutyl group

Novel terminally perfluorobutyl group-containing ether-linked phosphatidylcholines with different alkyl chain lengths (di-O-F4-Cn-PCs, n = 14,16 and 18) were developed as possible materials for stable liposomes aiming at applications of structural and functional analyses of membrane proteins. Differential scanning calorimetric investigations of the thermotropic transition of hydrated di-O-F4-Cn-PC bilayers demonstrated that the transition temperature of every di-O-F4-Cn-PC decreases by ~20 °C compared to their corresponding non-fluorinated PCs, di-O-Cn-PCs. With the elongation of the hydrophobic chain, on the other hand, the transition enthalpy (ΔH) and entropy (ΔS) increased in a linear manner. Comparison of ΔH and ΔS values against the net hydrocarbon chain length between di-O-F4-Cn-PCs and di-O-Cn-PCs strongly suggests that in the thermotropic transition of the di-O-F4-Cn-PC membrane, the perfluorobutyl segments undergo very limited structural changes; therefore, the hydrocarbon segments are mainly responsible for the phase transition.

Synthesis of Short Peptides with Perfluoroalkyl Side Chains and Evaluation of Their Cellular Uptake Efficiency

With an increasing demand for macromolecular biotherapeutics, the issue of their poor cell-penetrating abilities requires viable and relevant solutions. Herein, we report tripeptides bearing an amino acid with a perfluoroalkyl (RF ) group adjacent to the α-carbon. RF -containing tripeptides were synthesized and evaluated for their ability to transport a conjugated hydrophilic dye (Alexa Fluor 647) into the cells. RF -containing tripeptides with the fluorophore showed high cellular uptake efficiency and none of them were cytotoxic. Interestingly, we demonstrated that the absolute configuration of perfluoroalkylated amino acids (RF -AAs) affects not only nanoparticle formation but also the cell permeability of the tripeptides. These novel RF -containing tripeptides are potentially useful as short and noncationic cell-penetrating peptides (CPPs).

Towards Non-stick Silk: Tuning the Hydrophobicity of Silk Fibroin Protein

Silk fibroin protein is a biomaterial with excellent biocompatibility and low immunogenicity. These properties have catapulted the material as a leader for extensive use in stents, catheters, and wound dressings. Modulation of hydrophobicity of silk fibroin protein to further expand the scope and utility however has been elusive. We report that installing perfluorocarbon chains on the surface of silk fibroin transforms this water-soluble protein into a remarkably hydrophobic polymer that can be solvent-cast. A clear relationship emerged between fluorine content of the modified silk and film hydrophobicity. Water contact angles of the most decorated silk fibroin protein exceeded that of Teflon®. We further show that water uptake in prefabricated silk bars is dramatically reduced, extending their lifetimes, and maintaining mechanical integrity. These results highlight the power of chemistry under moderate conditions to install unnatural groups onto the silk fibroin surface and will enable further exploration into applications of this versatile biomaterial.

Potential-Independent Intracellular Drug Delivery and Mitochondrial Targeting

In this study, two types of the fluoroamphiphile analogs were synthesized and self-assembled into the "core-shell" micellar nanocarriers for intracellular delivery and organelle targeting. Using the fluorescent dyes or vitamin E succinate as the cargo, the drug delivery and targeting capabilities of the fluoroamphiphiles and their micelles were evaluated in the cell lines, tumor cell spheroids, and tumor-bearing mice. The "core-fluorinated" micelles exhibited favorable physicochemical properties and improved the cellular uptake of the cargo by around 20 times compared to their "shell-fluorinated" counterparts. The results also indicated that the core-fluorinated micelles underwent an efficient clathrin-mediated endocytosis and a rapid endosomal escape thereafter. Interestingly, the internalized fluoroamphiphile micelles preferentially accumulated in mitochondria, by which the efficacy of the loaded vitamin E succinate was boosted both in vitro and in vivo. Unlike the popularly used cationic mitochondrial targeting ligands, as a charge-neutral nanocarrier, the fluoroamphiphiles' mitochondrial targeting was potential independent. The mechanism study suggested that the strong binding affinity with the phospholipids, particularly the cardiolipin, played an important role in the fluoroamphiphiles' mitochondrial targeting. These charge-neutral fluoroamphiphiles might have great potential to be a simple and reliable tool for intracellular drug delivery and mitochondrial targeting.

Fluoropolymers in biomedical applications: state-of-the-art and future perspectives

Biomedical applications of fluoropolymers in gene delivery, protein delivery, drug delivery, ¹⁹F MRI, PDT, anti-fouling, anti-bacterial, cell culture, and tissue engineering.

Perfluorocarbons in Chemical Biology

Perfluorocarbons, saturated carbon chains in which all the hydrogen atoms are replaced with fluorine, form a separate phase from both organic and aqueous solutions. Though perfluorinated compounds are not found in living systems, they can be used to modify biomolecules to confer orthogonal behavior within natural systems, such as improved stability, engineered assembly, and cell-permeability. Perfluorinated groups also provide handles for purification, mass spectrometry, and 19 F NMR studies in complex environments. Herein, we describe how the unique properties of perfluorocarbons have been employed to understand and manipulate biological systems.

Langmuir Films of Perfluorinated Fatty Alcohols: Evidence of Spontaneous Formation of Solid Aggregates at Zero Surface Pressure and Very Low Surface Density

In this work, Langmuir films of two highly fluorinated fatty alcohols, CF3(CF2)12CH2OH (F14OH) and CF3(CF2)16CH2OH (F18OH), were studied. Atomic Force Microscopy (AFM) images of the films transferred at zero surface pressure and low surface density onto the surface of silicon wafers by the Langmuir-Blodgett technique revealed, for the first time, the existence of solid-like domains with well-defined mostly hexagonal (starry) shapes in the case of F18OH, and with an entangled structure of threads in the case of F14OH. A (20:80) molar mixture of the two alcohols displayed a surprising combination of the two patterns: hexagonal domains surrounded by zigzagging threads, clearly demonstrating that the two alcohols segregate during the 2D crystallization process. Grazing Incidence X-Ray Diffraction (GIXD) measurements confirmed that the molecules of both alcohols organize in 2D hexagonal lattices. Atomistic Molecular Dynamics (MD) simulations provide a visualization of the structure of the domains and allow a molecular-level interpretation of the experimental observations. The simulation results clearly showed that perfluorinated alcohols have an intrinsic tendency to aggregate, even at very low surface density. The formed domains are highly organized compared to those of hydrogenated alcohols with similar chain length. Very probably, this tendency is a consequence of the characteristic stiffness of the perfluorinated chains. The diffraction spectrum calculated from the simulation trajectories compares favorably with the experimental spectra, fully validating the simulations and the proposed interpretation. The present results highlight for the first time an inherent tendency of perfluorinated chains to aggregate, even at very low surface density, forming highly organized 2D structures. We believe these findings are important to fully understand related phenomena, such as the formation of hemi-micelles of semifluorinated alkanes at the surface of water and the 2D segregation in mixed Langmuir films of hydrogenated and fluorinated fatty acids.

Aromatic Fluorination of Multiblock Amphiphile Enhances Its Incorporation into Lipid Bilayer Membranes

We designed multiblock amphiphiles AmF and AmH, which consist of perfluorinated and non-fluorinated hydrophobic units, respectively. Absorption spectroscopy revealed that both amphiphiles are molecularly dispersed in organic solvent, while they form aggregates under aqueous conditions. Furthermore, we investigated whether AmF and AmH can be incorporated into DOPC lipid bilayer membranes, and found that the maximum concentration of AmF that can be incorporated into DOPC lipid bilayer membranes is 43 times higher than that of AmH.

The fluorination effect of fluoroamphiphiles in cytosolic protein delivery

Direct delivery of proteins into cells avoids many drawbacks of gene delivery, and thus has emerging applications in biotherapy. However, it remains a challenging task owing to limited charges and relatively large size of proteins. Here, we report an efficient protein delivery system via the co-assembly of fluoroamphiphiles and proteins into nanoparticles. Fluorous substituents on the amphiphiles play essential roles in the formation of uniform nanoparticles, avoiding protein denaturation, efficient endocytosis, and maintaining low cytotoxicity. Structure-activity relationship studies reveal that longer fluorous chain length and higher fluorination degree contribute to more efficient protein delivery, but excess fluorophilicity on the polymer leads to the pre-assembly of fluoroamphiphiles into stable vesicles, and thus failed protein encapsulation and cytosolic delivery. This study highlights the advantage of fluoroamphiphiles over other existing strategies for intracellular protein delivery.

Proteins can serve as means of medical treatment, but their efficient delivery to cells is difficult. Here, the authors present a type of polymers, fluoroamphiphiles, acting as chemical chaperones that can facilitate the import of proteins into the inner compartment, i.e. cytosol, of cells.

Progress in the synthesis of fluorinated phosphatidylcholines for biological applications

Fluorinated phospholipids have attracted a lot of interest over the past 40 years. While mono- and polyfluorinated analogs are mostly designed to be used as ¹⁹F NMR probes, highly fluorinated phospholipids are mainly developed as drug delivery devices and oxygen carriers. This review describes their synthetic pathways, their properties and potential applications.

Routes to the preparation of mixed monolayers of fluorinated and hydrogenated alkanethiolates grafted on the surface of gold nanoparticles

The use of binary blends of hydrogenated and fluorinated alkanethiolates represents an interesting approach to the construction of anisotropic hybrid organic-inorganic nanoparticles since the fluorinated and hydrogenated components are expected to self-sort on the nanoparticle surface because of their reciprocal phobicity. These mixed monolayers are therefore strongly non-ideal binary systems. The synthetic routes we explored to achieve mixed monolayer gold nanoparticles displaying hydrogenated and fluorinated ligands clearly show that the final monolayer composition is a non-linear function of the initial reaction mixture. Our data suggest that, under certain geometrical constraints, nucleation and growth of fluorinated domains could be the initial event in the formation of these mixed monolayers. The onset of domain formation depends on the structure of the fluorinated and hydrogenated species. The solubility of the mixed monolayer nanoparticles displayed a marked discontinuity as a function of the monolayer composition. When the fluorinated component content is small, the nanoparticle systems are fully soluble in chloroform, at intermediate content the nanoparticles become soluble in hexane and eventually they become soluble in fluorinated solvents only. The ranges of monolayer compositions in which the solubility transitions are observed depend on the nature of the thiols composing the monolayer.

Self-organization of mixtures of fluorocarbon and hydrocarbon amphiphilic thiolates on the surface of gold nanoparticles

Self-assembled monolayers composed of a mixture of thiolate molecules, featuring hydrocarbon or perfluorocarbon chains (H- and F-chains) terminating with a short poly(oxoethylene) (PEG) moiety, are the most extreme example of surfactant immiscibility on gold nanoparticles reported so far. The phase segregation between H-chains and F-chains and the consequent, peculiar folding of PEG chains are responsible for the increased affinity of a selected radical probe for the fluorinated region, which increases as the size of the fluorinated domains decrease, independently of the shape of such domains. This feature has been revealed by ESR measurements and an in silico innovative multiscale molecular simulations approach in explicit water. Our results reveal an underlying mechanism of a transmission of the organization of the monolayer from the inner region close to the gold surface toward the external hydrophilic PEG region. Moreover, this study definitively proves that a mixed monolayer is a complex system with properties markedly different from those characterizing the parent homoligand monolayers.

Hydrogenated/fluorinated catanionic surfactants as potential templates for nanostructure design

The structure and physicochemical properties of the nanoparticles spontaneously formed within aqueous mixtures of the hydrogenated/fluorinated catanionic surfactant cetyltrimetylammonium perfluorooctanoate in the absence of counterions as a function of its concentration are investigated by a combined experimental/computational study at room temperature. Apparent molar volumes, isentropic apparent molar compressibilities, and dynamic light scattering measurements together with transmission and cryo-scanning electron as well as confocal laser microscopy images, and computational molecular dynamics simulations indicate that a variety of structures of different sizes coexist in solution with vesicles of ∼160 nm diameter. Interestingly, the obtained nanostructures were observed to self-assemble from a random distribution of monomers in a time scale easily accessible by atomistic classical molecular dynamics simulations, allowing to provide a comprehensive structural and dynamic characterization of the surfactant molecules at atomic level within the different aggregates. Overall, it is demonstrated that the use of mixed fluorinated hydrogenated surfactant systems represents an easy strategy for the design of specific nanoscale structures. The detailed structural analysis provided in the present work is expected to be useful as a reference to guide the design of new nanoparticles based on different hydrogenated/fluorinated catanionic surfactants.

A New Paradigm for Protein Design and Biological Self-Assembly

Very few molecules with biological origins contain the element fluorine. Nature's inability to incorporate fluorine into biomolecules is related to the low concentration of free fluoride in sea and surface water. However, judicious introduction of fluorine into proteins, nucleic acids, lipids and carbohydrates has allowed mechanistic scrutiny of enzyme catalysis, control of protein oligomerization in membranes, clustered display of ligands on surfaces of living cells, and in increasing the protease stability of protein and peptide therapeutics.

Fluorinated lipid constructs permit facile passage of molecular cargo into living cells

Fluorinated lipids get rapidly internalized into living cells and are also displayed on the cell surface. The uptake of lipids is energy dependent and is likely via the clathrin-mediated endocytic pathway. Fluorinated lipids are 3-5-fold more efficient in acting as molecular transporters of noncovalently bound proteins than their hydrocarbon counterparts. These materials could serve as efficient molecular transporters for molecules that function in the cytoplasm such as short interfering RNAs (siRNAs).

Biomembrane sensitivity to structural changes in bound polymers

Anionic liposomes containing a 4:1 molar ratio of neutral to anionic phospholipids were treated with an excess of five zwitterionic polymers differing only in the spacer length separating their cationic and anionic moieties. Although the polymers do not disrupt the structural integrity of the liposomes, they can induce spacer-dependent molecular rearrangements within the liposomes. Thus, the following were observed: spacer length = 1, no binding to the liposomes; spacer length = 2, adsorption to the liposomes, but no molecular rearrangement; spacer length = 3, lateral lipid segregation but little or no flip-flop; spacer length = 4 or 5, lateral lipid segregation and flip-flop. These diverse behaviors are relevant to the use of biomedical formulations where polyelectrolytes play a role.

Structure and thermotropic phase behavior of fluorinated phospholipid bilayers: a combined attenuated total reflection FTIR spectroscopy and imaging ellipsometry study

Lipid bilayers consisting of lipids with terminally perfluoroalkylated chains have remarkable properties. They exhibit increased stability and phase-separated nanoscale patterns in mixtures with nonfluorinated lipids. In order to understand the bilayer properties that are responsible for this behavior, we have analyzed the structure of solid-supported bilayers composed of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) and of a DPPC analogue with 6 terminal perfluorinated methylene units (F6-DPPC). Polarized attenuated total reflection Fourier-transform infrared spectroscopy indicates that for F6-DPPC, the tilt of the lipid acyl chains to the bilayer normal is increased to 39 degrees as compared to 21 degrees for native DPPC, for both lipids in the gel phase. This substantial increase of the tilt angle is responsible for a decrease of the bilayer thickness from 5.4 nm for DPPC to 4.5 nm for F6-DPPC, as revealed by temperature-controlled imaging ellipsometry on microstructured lipid bilayers and solution atomic force microscopy. During the main phase transition from the gel to the fluid phase, both the relative bilayer thickness change and the relative area change are substantially smaller for F6-DPPC than for DPPC. In light of these structural and thermotropic data, we propose a model in which the higher acyl-chain tilt angle in F6-DPPC is the result of a conformational rearrangement to minimize unfavorable fluorocarbon-hydrocarbon interactions in the center of the bilayer due to chain staggering.

Nanoscale patterning in mixed fluorocarbon-hydrocarbon phospholipid bilayers

A growing body of literature suggests that fluorocarbons can direct self-assembly within hydrocarbon environments. We report here the fabrication and characterization of supported lipid bilayers (SLBs) composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a synthetic, fluorocarbon-functionalized analogue, 1. AFM investigation of these model membranes reveals an intricate, composition-dependent domain structure consisting of approximately 50 nm stripes interspersed between approximately 1 microm sized domains. Although DSC of 1 showed a phase transition near room temperature, DSC of DPPC:1 mixtures exhibited complex phase behavior suggesting domain segregation. Finally, temperature-dependent AFM of DPPC:1 bilayers shows that, while the stripe structures can be melted above the Tm of 1, the stripes and domains result from immiscibility of the hydrocarbon and fluorocarbon lipid gel phases. Fluorination appears to be a promising strategy for chemical self-assembly in two dimensions. In particular, because no modification is made to the lipid headgroups, it may be useful for nanopatterning biologically relevant ligands on bilayers in vitro or in living cells.