Phase behavior of a designed cyclopropyl analogue of monoolein: implications for low-temperature membrane protein crystallization.

Lipidic cubic phases (LCPs) are used in areas ranging from membrane biology to biodevices. Because some membrane proteins are notoriously unstable at room temperature, and available LCPs undergo transformation to lamellar phases at low temperatures, development of stable low-temperature LCPs for biophysical studies of membrane proteins is called for. Monodihydrosterculin (MDS) is a designer lipid based on monoolein (MO) with a configurationally restricted cyclopropyl ring replacing the olefin. Small-angle X-ray scattering (SAXS) analyses revealed a phase diagram for MDS lacking the high-temperature, highly curved reverse hexagonal phase typical for MO, and extending the cubic phase boundary to lower temperature, thereby establishing the relationship between lipid molecular structure and mesophase behavior. The use of MDS as a new material for LCP-based membrane protein crystallization at low temperature was demonstrated by crystallizing bacteriorhodopsin at 20 °C as well as 4 °C.

Design and assembly of pH-sensitive lipidic cubic phase matrices for drug release.

Bicontinuous lipidic cubic phases (LCPs) exhibit a combination of material properties that make them highly interesting for various biomaterial applications: they are nontoxic, biodegradable, optically transparent, thermodynamically stable in excess water, and can incorporate active molecules of virtually any polarity. Here we present a molecular system comprising host lipid, water, and designed lipidic additive, which form a structured, pH-sensitive lipidic matrix for hydrophilic as well as hydrophobic drug incorporation and release. The model drug doxorubicin (Dox) was loaded into the LCP. Tunable interactions with the lipidic matrix led to the observed pH-dependent drug release from the phase. The rate of Dox release from the cubic phase at pH 7.4 was low but increased significantly at more acidic pH. A small amount of a tailored diacidic lipid (lipid 1) added to the monoolein LCP modified the release rate of the drug. Phase identity and structural parameters of pure and doped mesophases were characterized by small-angle X-ray scattering (SAXS), and release profiles from the matrix were monitored electrochemically. Analysis of the release kinetics revealed that the total amount of drug released from the LCP matrix is linearly dependent on the square root of time, implying that the release mechanism proceeds according to the Higuchi model.

Dependence of interfacial film organization on lipid molecular structure.

Combination of surface analytical techniques was employed to investigate the interfacial behavior of the two designed lipids-N-stearoylglycine (1) and its bulky neutral headgroup-containing derivative N-stearoylvaline ethyl ester (2)-at the air-solution interface and as transferred layers on different substrates. Formation of monolayers at the air-water interface was monitored on pure water and on aqueous solutions of different pH. Crystallization effects were visualized at pure water by recording the hystereses in the Langmuir-Blodgett (LB) isotherms and by transferring the layers onto mica, gold (111), and ITO (indium-tin oxide on glass) electrodes. Subphase pH affects the morphology and patch formation in monolayers of 1, as evidenced by BAM measurements. At pH 8.2, formation of well-ordered crystallites is observed, which upon compression elongate according to predominantly 1-D growth mechanism to form a dense layer of crystallites. This effect is not observed in monolayers of 2, whose headgroup is not protonated. The orientation of layers of 1 transferred to the solid supports is also pH dependent, and their stability can be related to formation of a hydrogen-bonded networks. AFM images of 1 exhibited platelets of multilayer phase. The IR spectra of the ITO substrates covered by 1 indicated formation of hydrogen bonds between the amide groups. The nature of the adsorption layer and its organization as a function of potential were studied in-depth by EC STM using Au(111) as the substrate. A model showing the arrangement of hydrogen bonds between adsorbed molecules is presented and related to the observed organization of the layer.

Tailored host-guest lipidic cubic phases: a protocell model exhibiting nucleic acid recognition.

A classical conundrum in origin-of-life studies relates to the nature of the first chemical system: was it a carrier of genetic information or a facilitator of cellular compartmentalization? Here we present a system composed of tailor-made nucleolipids and hydrated monoolein, which assemble at ambient temperatures to form host-guest lipidic cubic phase (LCP) materials that are stable in bulk water and can perform both functions. As such, they may represent a molecular model for a protocell in origin-of-life studies. Nucleolipids within the lipidic material sequester and bind selectively complementary oligonucleotide sequences from solution by virtue of base-pairing; noncomplementary sequences diffuse freely between the LCP material and the bulk aqueous environment. Sequence specific enrichment of nucleic acids within the LCP material demonstrates an effective mechanism for selection of genetic material in these cell-mimetic systems.

Design and synthesis of lipids for the fabrication of functional lipidic cubic-phase biomaterials.

A series of novel lipids with designed functionalities were synthesized. These lipids are based on conjugation of α-amino acids and their esters, cationic, anionic, neutral, and photochromic moieties to the lipophilic 9-cis octadecenyl chains by amide, ester, thioester, or amine bonds. Because of the plasticity of lipidic cubic phases, it is envisaged that when mixed with monooleoyl-rac-glycerol (monoolein, MO) and water at appropriate proportions, they would assemble to form bicontinuous lipidic cubic phases (LCPs) that exhibit the well-known material properties of LCPs such as phase stability, optical transparency, and chemical permeability. Moreover, due to the nature and position of the functionality at the headgroup region, we envision them to perform as functional materials by design.

Synthesis of (±)-desethylrhazinal using a tandem radical addition-cyclization process.

The indolizidine ring system present in (±)-rhazinal, was assembled using a xanthate-based sequential intermolecular radical addition-cyclization process. The novel (±)-desethylrhazinal was prepared in seven steps in approximately 12% overall yield from 2-formylpyrrole, using this strategy.

Synthesis of 5,6-dihydropyrrolo[2,1-a]isoquinolines featuring an intramolecular radical-oxidative cyclization of polysubstituted pyrroles, and evaluation of their cytotoxic activity.

A three-step protocol for the synthesis of 1,2,3,8,9-pentasubstituted-5,6-dihydropyrrolo[2,1-a]isoquinolines is described, using van Leusen's polysubstituted pyrrole construction followed by intramolecular radical-oxidative cyclization of the isoquinoline system. The cytotoxic activities of the dihydropyrroloisoquinolines were tested on six tumor cell lines. Preliminary structure-activity studies revealed the importance of the identity of the aromatic substituent at the C-2 position, particularly a phenyl, m-(amino) phenyl or m-(cyclohexylmethylpiperazinamide) phenyl substituent, for cytotoxic activity.

Efficient, intermolecular, oxidative radical alkylation of heteroaromatic systems under "tin-free" conditions.

Novel and efficient radical alkylation of several heterocyclic systems including pyrroles, indoles, furan and thiophenes is described using xanthate based radical chemistry. The present methodology could be used to provide rapid access to various nonsteroidal antiinflammatory drugs.

Mapping the landscape of potentially primordial informational oligomers: (3'→2')-D-phosphoglyceric acid linked acyclic oligonucleotides tagged with 2,4-disubstituted 5-aminopyrimidines as recognition elements.

The (3'→2')-phosphodiester glyceric acid backbone containing an acyclic oligomer tagged with 2,4-disubstituted pyrimidines as alternative recognition elements have been synthesized. Strong cross-pairing of a 2,4-dioxo-5-aminopyrimidine hexamer, rivaling locked nucleic acid (LNA) and peptide nucleic acid (PNA), with complementary adenine-containing DNA and RNA sequences was observed. The corresponding 2,4-diamino- and 2-amino-4-oxo-5-aminopyrimidine-tagged oligomers were synthesized, but difficulties in deprotection, purification, and isolation thwarted further investigations. The acyclic phosphate backbone structure of the protected oligomer seems to be prone to an eliminative degradation owing to the acidic hydrogen at the 2'-position--an arrangement that renders the oligomer vulnerable to the conditions used for the removal of the protecting groups on the heterocyclic recognition element. However, the free oligomers seem to be stable under the conditions investigated.

Efficient, "tin-free" radical cyclization to aromatic systems. synthesis of 5,6,8,9,10,11-hexahydroindolo[2,1-a]isoquinolines.

Efficient radical cyclization of alkyl iodides to various aromatic systems including pyrrole, indole, isoquinolone, pyridone, and benzene, mediated by dicumyl peroxide, is described. The methodology was used to provide access to 5,6,8,9,10,11-hexahydroindolo[2,1-a]isoquinoline derivatives.