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1. Synthesis of Polymer Nanoweb via a Lipid Template

   https://doi.org/10.1021/acsmacrolett.3c00255  

   Liu, Chung-Hao; Krueger, Susan; Nieh, Mu-Ping (July 2023, ACS Macro Letters)

   We report a generalized platform for synthesizing a polymer nanoweb with a high specific surface area via a bicellar template, composed of 1,2-dipalmitoyl phosphocholine (DPPC), 1,2-dihexanoyl phosphocholine (DHPC), and 1,2-dipalmitoyl phosphoglycerol (DPPG). The pristine bicelle (in the absence of monomer or polymer) yields a variety of well-defined structures, including disc, vesicle, and perforated lamella. The addition of styrene monomers in the mixture causes bicelles to transform into lamellae. Monomers are miscible with DPPC and DPPG initially, while polymerization drives polymers to the DHPC-rich domain, resulting in a polymer nanoweb supported by the outcomes of small angle neutron scattering, differential scanning calorimetry, and transmission electron microscopy. 

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2. Nanocomplex made up of antimicrobial metallo-supramolecules and model biomembranes – characterization and enhanced fluorescence

   https://doi.org/10.1039/d1nr04083a  

   Liu, Chung-Hao; Wang, Heng; Yang, Lin; Liu, Yun; Li, Xiaopeng; Nieh, Mu-Ping (September 2021, Nanoscale)

   null (Ed.)

   Antimicrobial pentatopic 2,2′:6′,2′′-terpyridines that form 3-D supramolecular hexagonal prisms with Cd 2+ through coordination driven self-assembly can be entrapped by lipid discoidal bicelles, composed of 1,2-dipalmitoyl- sn-glycero -3-phosphocholine, 1,2-dihexanoyl- sn-glycero -3-phosphocholine and 1,2-dipalmitoyl- sn-glycero -3-phospho-(1′-rac-glycerol) lipid, forming a well-defined nanocomplex. Structural characterization performed by very small angle neutron scattering, small angle X-ray scattering and transmission electron microscopy suggests that the hexagonal prisms are preferably located at the rim of bicellar discs with the hexagonal face in parallel with the bilayers, instead of face-to-face stacking. Such a configuration reduces the π−π interaction and consequently enhances the fluorescence emission. Since novel supramolecules were reported to have antibiotic functions, this study provides insight into the interactions of antimicrobial supermolecules with lipid membranes, leading to potential theranostic applications. 

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3. Issues with lipid probes in flip-flop measurements: A comparative study using sum-frequency vibrational spectroscopy and second-harmonic generation

   https://doi.org/10.1063/5.0226075  

   Taylor, Joshua M; Conboy, John C (August 2024, The Journal of Chemical Physics)

   Lian, Tianquan_Tim (Ed.)

   Fluorescent lipid probes such as 1-palmitoyl-2-(6-[7-nitro-2-1,3-benzoxadiazol-4-yl]amino-hexanoyl)-sn-glycero-3-phosphocholine (C6 NBD-PC) have been used extensively to study the kinetics of lipid flip-flop. However, the efficacy of these probes as reliable reporters of native lipid translocation has never been tested. In this study, sum-frequency vibrational spectroscopy (SFVS) was used to measure the kinetics of C6 NBD-PC lipid flip-flop and the flip-flop of native lipids in planar supported lipid bilayers. C6 NBD-PC was investigated at concentrations of 1 and 3 mol. % in both chain-matched 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and chain-mismatched 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) to assess the ability of C6 NBD-PC to mimic the behavior of the surrounding matrix lipids. It was observed that C6 NBD-PC exhibited faster flip-flop kinetics compared to the native lipids in both DPPC and DSPC matrices, with notably accelerated rates in the chain-mismatched DSPC system. SFVS was also used to measure the acyl chain orientation and gauche content of C6 NBD-PC in both DPPC and DSPC membranes. In the DSPC matrix (chain mismatched), C6 NBD-PC was more disordered in terms of both gauche content and acyl tilt, whereas it maintained an orientation similar to that of the native lipids in the DPPC matrix (chain matched). In addition, the flip-flop kinetics of C6 NBD-PC were also measured using second-harmonic generation (SHG) spectroscopy, by probing the motion of the NBD chromophore directly. The flip-flop kinetics measured by SHG were consistent with those obtained from SFVS. This study also marks the first instance of phospholipid flip-flop kinetics being measured via SHG. The results of this study clearly demonstrate that C6 NBD-PC does not adequately mimic the behavior of native lipids within a membrane. These findings also highlight the significant impact of the lipid matrix on the flip-flop behavior of the fluorescently labeled lipid, C6 NBD-PC. 

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4. Phase Transition in a Heterogeneous Membrane: Atomically Detailed Picture

   https://doi.org/10.1021/acs.jpclett.0c01255  

   Fathizadeh, Arman; Valentine, Mason; Baiz, Carlos R.; Elber, Ron (April 2020, The Journal of Physical Chemistry Letters)

   Membranes serve diverse functions in biological systems. Variations in their molecular compositions impact their physical properties and lead to rich phase behavior such as switching from the gel to fluid phase and/or separation to micro- and macrodomains with different molecular compositions. We present a combined computational and experimental study of the phase behavior of a mixed membrane of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) molecules. This heterogeneous membrane changes from gel to fluid and shows separate domains as a function of temperature. Atomically detailed simulations provide microscopic information about these molecular assemblies. However, these systems are challenging for computations since approaching equilibrium necessitates exceptionally long molecular dynamics trajectories. We use the simulation method of MDAS (Molecular Dynamics with Alchemical Steps) to generate adequate statistics. Isotope-edited IR spectroscopy of the lipids was used to benchmark the simulations. Together, simulations and experiments provide insight into the structural and dynamical features of the phase diagram. 

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5. Quantifying Solute Partitioning in Phosphatidylcholine Membranes

   Gobrogge, Christine A. Walker (October 2017, Analytical chemistry)

   Time-resolved fluorescence measurements were used to characterize and quantify solute partitioning into 1,2- dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid vesicles as a function of solute concentration and temperature. The solutes, coumarin 152 (C152) and coumarin 461 (C461), both belong to a family of 7-aminocoumarin dyes that have distinctive fluorescence lifetimes in different solvation environments. The two solutes differ in the 4-position where C152 has a trifluoromethyl group in place of C461’s -CH3 group. In vesicle containing solutions, multiexponential fluorescence decays imply separate solute populations in the aqueous buffer, solvated in the vesicle headgroup region and solvated in the acyl chain bilayer interior, respectively. Fluorescence amplitudes, corrected for differences in radiative rates, are used to calculate absolute partition coefficients and average number of solutes per vesicle as a function of coumarin:lipid ratio and average number of solutes per vesicle. Results show that C152 has an ∼10-fold greater affinity than C461 for lipid bilayers, despite both solutes having similar hydrophobicities as inferred from their log(P) values. Temperature-dependent partitioning data are used to calculate enthalpies and entropies of C152 partitioning as a function of concentration. These values are used to extrapolate to the infinitely dilute limit. Above and below the lipid gel−liquid crystalline temperature, partitioning is exothermic with negative changes in entropy. In the vicinity of the transition temperature, these quantities change sign with ΔHpart becoming endothermic (+70 kJ/mol) and entropically favored (ΔSpart = +240 J/(mol·K)). 

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