Biomedical subjects

Steven L Regen

Publications and source records attributed to Steven L Regen.

At least 19 recordsLinked to original sources

PubMed2007

Cholesterol-phospholipid complexation in fluid bilayers as evidenced by nearest-neighbor recognition measurements.

Nearest-neighbor recognition experiments have been carried out using varying ratios of exchangeable dimer analogs of 1,2-dimyristoyl-sn-glycero-3-phosphatidylglycerol and 1,2-distearoyl-sn-glycero-3-phosphatidylglycerol in cholesterol-rich unilamellar vesicles at 60 degrees C. Equilibrium dimer distributions that were obtained support a structural model of the liquid-ordered bilayer in which free cholesterol and the longer-chain phospholipid homodimer are in equilibrium with a complex of unique stoichiometry, where one cholesterol molecule combines with two of the long-chain phospholipid homodimers. In this model, the mixing of the short-chain phospholipids with the uncomplexed long-chain phospholipids is ideal, and the complexed dimers are shielded from the disulfide exchange reaction.

Chemistry, Physical↗

PubMed2006

Covalent gluing and postgluing of Langmuir-Blodgett monolayers at hydrocarbon surfaces.

Langmuir-Blodgett (LB) monolayers of 5,11,17,23,29,35-hexaformyl-37,38,39,40,41,42-hexakis(1-n-octyloxy)calix[6]arene (2), deposited onto silylated silicon wafers, were cross-linked (i.e., "covalently glued") via Schiff base formation with poly(allylamine). Direct evidence for imine formation was obtained from X-ray photoelectron spectroscopy and from attenuated total reflection IR spectroscopy. These modified surfaces could be removed from the aqueous subphase into air with retention of the assembly and its orientation relative to the surface, as evidenced by atomic force microscopy, water contact angle measurements, and film thickness determinations by ellipsometry. Similar assemblies were also synthesized via a postgluing procedure, in which the substrate containing the LB monolayer was removed from the subphase and rapidly immersed into an aqueous solution containing poly(allylamine). The potential of combining postgluing methods with continuous LB film deposition as a surface modification technique is briefly discussed.

Journal Article↗

PubMed2006

Cholesterol-phospholipid association in fluid bilayers: a thermodynamic analysis from nearest-neighbor recognition measurements.

The mixing behavior of exchangeable, disulfide-based mimics of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol has been examined as a function of temperature in host membranes made from DPPC and cholesterol in the liquid-disordered phase (ld), in the liquid-ordered phase (lo), and in the liquid-disordered/liquid-ordered coexistence region (ld/lo). In the ld region, lipid mixing was found to be temperature insensitive, reflecting close to ideal behavior. In contrast, a significant temperature dependence was observed in the lo phase from 45 to 60 degrees C, when 35 or 40 mol % sterol was present. In this region, sterol-phospholipid association was characterized by DeltaHo = -2.06 +/- 0.14 kcal/mol of phospholipid and DeltaS degrees = -4.48 +/- 0.44 cal/K mol of phospholipid. From 60 to 65 degrees C, the mixing of these lipids was found to be insensitive to temperature, and sterol-phospholipid association was now entropy driven; that is, DeltaHo = -0.23 +/- 0.38 kcal/mol of phospholipid and DeltaS degrees = +1.68 +/- 1.12 cal/K mol of phospholipid. In the liquid-disordered/liquid-ordered coexistence region, changes in lipid mixing reflect changes in the phase composition of the membrane.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2006

Gluing Langmuir-Blodgett monolayers onto hydrocarbon surfaces.

This paper describes a new approach for modifying hydrophobic surfaces that is based on the use of ionically cross-linked (i.e., "glued") Langmuir-Blodgett monolayers. Specifically, this work shows how monolayers of 5,11,17,23,29,35-hexa(trimethylammonium methyl)-37,38,39,40,41,42-hexakis-n-hexadecyloxy-calix[6]arene (1a), which have been ionically cross-linked with poly(4-styrene-sulfonate) (PSS, MW 30,000-50,000), can be deposited onto silylated silicon wafers with their polar headgroups extending outward into air. This work also demonstrates the feasibility of using glued monolayers of 1a as "anchors" for attaching alternating layers of poly(diallyldimethylammonium) [PDADMA] and PSS ions onto these same hydrocarbon surfaces.

Journal Article↗

PubMed2006

Ethanol-induced reorganization of the liquid-ordered phase: enhancement of cholesterol-phospholipid association.

This paper records what is believed to be the first evidence for the reorganization of the liquid-ordered phase by ethanol. Specifically, ethanol has been found to significantly enhance sterol-phospholipid association in liquid-ordered bilayers derived from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) plus cholesterol and also 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) plus cholesterol. The evidence for such reorganization comes from a series of nearest-neighbor recognition (NNR) experiments that have been carried out, where low concentrations of equilibrating lipid dimers (i.e., "reporter molecules") have been used to detect changes in the phase composition of host membranes made from varying mixtures of DPPC/cholesterol, and also DSPC/cholesterol, in the presence and in the absence of ethanol. These findings have important biological implications, which are briefly discussed.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2005

Molecular umbrella-assisted transport of an oligonucleotide across cholesterol-rich phospholipid bilayers.

A series of molecular umbrella conjugates, derived from cholic acid, deoxycholic acid, spermidine, lysine, and 5-mercapto-2-nitrobenzoic acid, have been synthesized and found capable of transporting an attached 16-mer oligonucleotide (S-dT16) across liposomal membranes made from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidyldglycerol (POPG), and cholesterol [POPC/POPG/cholesterol (65/5/30; mol/mol/mol, v/v/v)] at 37 degrees C. Those molecular umbrellas containing four choloyl (or deoxycholoyl) groups resulted in significantly faster rates of transport as compared to those containing only two such moieties. A model that accounts for these membrane transport processes is proposed.

Biological Transport↗

PubMed2005

Transbilayer complementarity of phospholipids. Proof of principle.

A series of nearest-neighbor recognition (NNR) experiments have been carried out, which provide a rigorous test of the existence of transbilayer complementarity of phospholipids, that is, the ability of phospholipids to select complementary phospholipids from an adjoining monolayer as nearest neighbors. The application of this test to membranes derived from exchangeable phospholipids bearing myristoyl groups (A), stearoyl groups (B), and one stearoyl and one n-dodecyl group (C) in the presence of analogous nonexchangeable templates made from A', B' and C' provides compelling evidence for such complementarity in the physiologically relevant fluid phase.

Chromatography, High Pressure Liquid↗

PubMed2005

Poly(choloyl)-based amphiphiles as pore-forming agents: transport-active monomers by design.

This paper describes the design and synthesis of a family of pore-forming amphiphiles. Two of these amphiphiles, which are derived from cholic acid, lysine, and p-phenylenediamine, can produce pores in lipid bilayers as individual molecules. In sharp contrast, analogous amphiphiles that do not contain a rigid 1,4-phenylenediamide moiety favor the formation of dimer-based pores. Kinetic evidence in support of monomer- and dimer-based pores has been obtained from Na+ transport measurements across bilayers made from 1-palmitoyl-2-oleoyl-2-sn-glycero-3-phosphocholine (POPC). Structure-activity studies that have been carried out with pore-forming, dimer-based amphiphiles have also revealed a significant activity dependence on their overall compactness. The practical potential of pore-forming amphiphiles with controllable supramolecular properties is briefly discussed.

Carbohydrate Sequence↗

PubMed2005

A chemical sensor for the liquid-ordered phase.

The mixing properties of exchangeable phospholipids, derived from 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine, with an exchangeable form of cholesterol have been used to monitor the transition from the liquid-disordered to the liquid-ordered phase in cholesterol-containing bilayers, made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-distearoyl-sn-glycero-3-phosphocholine, respectively.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2005

Thermally gated liposomes.

The combined use of a pore-forming amphiphile, 1 (derived from lysine, cholic acid, and spermine), and thermally sensitive liposomes (made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) to create "thermally gated liposomes" (TGLs) has been demonstrated. Thus, at temperatures that lie above the gel to liquid-crystalline phase transition temperature of DPPC (i.e., 41 degrees C), 1 creates pores within DPPC membranes through which entrapped aqueous solutes, such as carboxyfluorescein, can readily pass. Below this temperature, efflux rates are greatly reduced. The potential of TGLs as devices for the targeted delivery of therapeutic agents is briefly discussed.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2005

Transbilayer complementarity of phospholipids in cholesterol-rich membranes.

Lipid-lipid interactions across cholesterol-rich phospholipid bilayers were investigated by measuring nearest-neighbor preferences of exchangeable phospholipids derived from 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE), in the presence of nonexchangeable dimers (i.e., templates) made from DMPE or DSPE. When homotemplates were present, a significant preference for homophospholipid association was observed. In contrast, when the corresponding heterotemplate was present, heterodimer formation was favored. These results support a model in which the longer phospholipid in one monolayer preferentially associates with the shorter one in the adjoining monolayer. In the absence of cholesterol, transbilayer complementarity was also observed but to a lesser degree. Transbilayer complementarity of phospholipids is likely to play an important role in stabilizing biological membranes and in promoting a compositional interdependence of their two lipid leaflets.

Cholesterol↗

PubMed2005

Glued Langmuir-Blodgett bilayers having unusually high He/CO2 permeation selectivities.

Single Langmuir-Blodgett bilayers derived from 5,11,17,23,29,35-hexakis[(N,N,N-trimethylammonium)-N-methyl-37,38,39,40,41,42-hexakis-n-hexamedecyloxy-calix[6]arene hexachloride (1), which have been ionically cross-linked (i.e., "glued together") with poly(acrylic acid), have been found to exhibit He/CO2 permeation selectivities as high as 150. This degree of selectivity, for a membrane that is less than 6 nm in thickness, is without precedent. In principle, materials of this type could lead the way to improved membranes for hydrogen purification.

Journal Article↗

PubMed2005

A needle-and-thread approach to bilayer transport: permeation of a molecular umbrella-oligonucleotide conjugate across a phospholipid membrane.

A di-walled molecular umbrella, composed of two choloyl groups, one spermidine moiety, and a 5-thiol(2-nitrobenzoyl) "handle", was covalently attached to a 16-mer oligonucleotide (S-dT16) through a disulfide bond. Incubation of this conjugate (1) with vesicles made from 1-palmitoyl-2-oleyol-sn-glycero-3-phosphocholine and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylglycerol (95/5, mol/mol) (200 nm diameter, extrusion) that contained entrapped glutathione (GSH) at 37 degrees C resulted in the liberation of the oligonucleotide and the umbrella-bound 5-mercapto(2-nitrobenzoyl) moiety (USH) via thiolate-disulfide interchange. The appearance of USH, together with the absence of leakage of entrapped GSH and a vesicular capture of the oligonucleotide that matches the extent of USH formation, provides compelling evidence for delivery of S-dT16 into the aqueous compartment of these vesicles. In a sense, the molecular umbrella functions like a "needle" in providing a pathway for the oligonucleotide (the "thread") to cross the membrane.

Cell Membrane Permeability↗

PubMed2004

Persulfated molecular umbrellas as anti-HIV and anti-HSV agents.

A series of persulfated molecular umbrellas have been synthesized from putrescine, spermidine, spermine, lysine, and cholic acid (1a, 2a, 3a, 4a, and 5a) and their anti-HIV and anti-HSV activities determined. Despite it size, the most active of these conjugates (5a) was able to cross phospholipid bilayers made from 1-palmitoyl-2-oleyol-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylglycerol (POPG). The unique combination of anti-viral activity, an ability to cross hydrophobic barriers, a lack of cytotoxicity, and a simple three-step synthesis from biogenic starting material suggests that 5a and related conjugates may be exploitable as a novel class of anti-viral agents for systemic and topical applications.

Anti-HIV Agents↗

PubMed2004

A quantitative assessment of the influence of permanent kinks on the mixing behavior of phospholipids in cholesterol-rich bilayers.

The mixing behavior of a phospholipid containing a cis-cyclopropyl moiety (1) with one that contains two myristoyl groups (3a) has been investigated in fluid bilayers via the nearest-neighbor recognition (NNR) method. In the absence of cholesterol, these lipids mix ideally. In the presence of cholesterol, they show a modest preference for homo-phospholipid association. A trans-form of 1 (i.e., 2) was found to have similar behavior, except that the influence of cholesterol in promoting homo-phospholipid association was greater. Similar results have been found in membranes in which 3a is replaced with a phospholipid bearing two palmitoyol chains (3b). In this case, the effect of the kink is approximately twice as great. The implications of these findings, with respect to the "trans-fatty acid debate", are briefly discussed

Cholesterol↗

PubMed2004

Don't forget Langmuir-Blodgett films.

Langmuir-Blodgett (LB) films have been investigated, extensively, over the past 70 years. Despite considerable efforts aimed at exploiting their nonlinear optical, piezoelectric, pyroelectric, semiconducting, sensing and barrier properties, problems associated with film quality and stability have hampered their practical development. This feature article highlights one recent advance in the LB area (i.e., the ionic cross-linking of LB films or "gluing"), which has resulted in single bilayer membranes possessing extraordinary quality and stability. The gluing of LB films provides new opportunities for an old technology that has been waning in recent years.

Journal Article↗

PubMed2004

Probing the hydration of lipid bilayers using a solvent isotope effect on phospholipid mixing.

Nearest-neighbor recognition experiments, which have been carried out using exchangeable dimers derived from 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine,and 1-palmitoyl-2-oleoyl-sn-glycerophosphoethanolamine, indicate that replacement of H2O by D2O can significantly influence phospholipid mixing, but only in bilayers that are saturated and devoid of cholesterol. These findings, together with those of previous electron spin resonance spin-labeling studies,indicate that mammalian membranes, which are rich in cholesterol and unsaturated phospholipids, are ideal hydrophobic barriers.

Chemical Phenomena↗

PubMed2004

Exceptional gas permeation selectivity of a glued Langmuir-Blodgett bilayer by pH control.

The monolayer properties of 5,11,17,23,29,35-hexakis[(N,N,N-trimethylamonium)-N-methyl-37,38,39,40,41,42-hexakis-n-hexadecyloxy-calix[6]arene hexachloride (1) have been characterized over aqueous solutions of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) as a function of pH. At high pH values (e.g., pH 10), such monolayers show relatively low surface viscosities. At low pH (e.g., pH 4.4), these monolayers exhibit relatively high surface viscosities. The barrier properties of single Langmuir-Blodgett bilayers of 1, which have been ionically cross-linked (i.e., "glued together") with PAA were found to correlate with changes in surface viscosity. Thus, bilayers that were fabricated under low pH conditions exhibited high permeation barriers and high permeation selectivity with respect to He and N(2). Given the extreme thinness of these glued bilayers (ca. 6 nm), the optimized He/N(2) selectivity of ca. 1000 is extraordinary. These results, taken together, demonstrate the feasibility of fine tuning the surface viscosity of monolayers of 1, and also the barrier properties of corresponding glued bilayers, by adjusting the pH of an aqueous subphase that contains a weak polyacid.

Acrylic Resins↗