Multicomponent synthesis of pyrano[2,3-c]coumarins.

A base-catalyzed, pseudo-four-component reaction of 4-hydroxycoumarin, two molecules of acetone, and amine towards the synthesis of pyrano[2,3-c]coumarins is reported. The mechanism of this multicomponent reaction is proposed. The reaction is further extended to the preparation of coumarin-substituted pyrano[2,3-c]coumarins by a base-catalyzed, pseudo four-component reaction of two molecules of 4-hydroxycoumarin and two molecules of acetone.

Shear Stress Modulates Resistin-Induced CC Chemokine Ligand 19 Expression in Human Aortic Endothelial Cells.

Resistin may be an important link between obesity and diabetes. Recent studies have suggested an association between resistin and atherogenic processes. In addition, CCL19 is highly expressed in human atherosclerotic lesions. The interplays among resistin, CCL19, and shear stress in regulating vascular endothelial function are not clearly understood. In the present study, resistin stimulation induced dose- and time-dependent CCL19 expression in human aortic endothelial cells (HAECs). By using neutralizing antibody and small interfering (si)RNA, we demonstrated that toll-like receptor 4 (TLR4) is critical for resistin-induced CCL19 expression. Transcription factor ELISA and chromatin immunoprecipitation assays showed that resistin increased NF-κB-DNA binding activities in ECs. Inhibition of NF-κB activation by specific siRNA blocked the resistin-induced CCL19 promoter activity and expression. Preshearing of ECs for 12 h at 20 dyn/cm(2) inhibited the resistin-induced NF-κB activation and CCL19 expression. Our findings serve to elucidate the molecular mechanisms underlying the resistin induction of CCL19 expression in ECs and the shear-stress protection against this induction.

Synthesis of coumarin/pyrrole-fused heterocycles and their photochemical and redox-switching properties.

Two coumarin/pyrrole-fused heterocycles were synthesized to investigate their photochemical and redox-switching properties. Photooxidation of the colorless diphenyl-substituted pyrrolocoumarin resulted in a distinct change to red and a sharp decrease in fluorescence intensity. The photooxidized product can be swiftly reverted to the original form by NaCNBH3 reduction or hydrogenation.

Stability of gp41 hairpin and helix bundle assembly probed by combined stacking and circular dichroic approaches.

The envelope glycoprotein gp41 of HIV-1 undergoes structural rearrangement to form a helix hairpin during the virus-mediated fusion. Previous studies to investigate the folding and stability of hairpin did not monitor the end-to-end distance of the molecule. To directly probe the distance change, rhodamine dye was conjugated to the gp41 recombinant near the N- and C-terminal regions to detect the UV absorption and fluorescence intensity changes induced by the chemical denaturant guanidinium chloride (GdmCl). Using the singly- and doubly-labeled constructs allowed us to distinguish between the hairpin formation and protein oligomerization. A biphasic transition of helical structure for the wild type protein was revealed by circular dichroism measurements while unfolding of the hairpin occurred at 6M GdmCl. The relevance of our study to the fusion inhibitor for HIV-1 was borne out by results on the mutants at the positions within the N-terminal heptad repeat (NHR) and the C-terminal heptad repeat (CHR) regions. A monophasic transition at lower denaturant concentration was detected for the NHR mutant supporting the concept of differential stability of NHR and CHR helical structure. The conclusion that the observed unstacking of doubly-labeled variant arises principally from the intra-molecular dimers was drawn from the unstacking of the protein labeled in the loop. Remarkably, it is deduced that the hairpin is more stable than the CHR helical structure. A model for denaturation of the helix hairpin bundle was proposed from these results. The biological implications of the findings and further applications of the distance-based approach were discussed.

An efficient production and characterization of HIV-1 gp41 ectodomain with fusion peptide in Escherichia coli system.

We demonstrated a high level expression and purification of recombinant human immunodeficiency virus type 1 gp41 ectodomain (gp41e-FP) using glass bead approach with a final yield of 12±2mg/L bacterial culture. The proper folding of gp41e-FP encompassing the fusion peptide (FP) was ascertained by circular dichroism (CD) measurement and recognition by NC-1 antibody. The latter assay revealed stabilization of the gp41 coiled coil structure in the presence of liposome dispersion. The differential affinity of gp41e-FP and gp41e (devoid of FP) by NC-1 suggested an aggregated state for gp41e-FP and/or possible proximity of the fusion peptide domain to the coiled coil structure of gp41 ectodomain. Perfluorooctanoate (PFO)-PAGE electrophoresis experiment revealed the trimeric propensity of the recombinant gp41e-FP. In comparison to gp41e, the lipid mixing activity of gp41e-FP was two-fold higher suggesting a role of FP in promoting membrane fusion. The present approach to efficiently and quantitatively preparing the functional full-length recombinant gp41 ectodomain protein can be employed for structural and biomedical investigations and the extraction of other inclusion body-embedded recombinant proteins.

The fusion peptide domain is the primary membrane-inserted region and enhances membrane interaction of the ectodomain of HIV-1 gp41.

To execute the membrane fusion function, it is necessary for the fusion protein of the virus to penetrate into the hydrophobic milieu of membrane bilayer. Hence identification of the region(s) of the ectodomain of viral fusion proteins involved in the membrane insertion and their interaction with the rest of the fusion protein in the membrane would be important for the mechanistic study of membrane fusion. To this end, we examined membrane activity of the fusion peptide, and the ectodomain protein with or without the fusion peptide domain of HIV-1 gp41 by several biophysical measurements. The results revealed that the ectodomain protein containing the fusion peptide domain had higher membrane-perturbing activity and deeper membrane insertion, while the construct lacking the fusion peptide domain had much lower membrane activity. Strikingly, the N-terminal heptad repeat region was found to be induced deeper into the membrane by the fusion peptide, consistent with the role of the latter in the membrane penetration. We concluded that the fusion peptide is the only stretch of gp41 ectodomain that embeds deeply in the membrane interior in the prefusion stage. The function of fusion peptide in terms of membrane interaction and the implications of its interplay with other domains of gp41 on the membrane fusion cascade were discussed.

Syntheses of quaternary carbon-containing oxazatricycle and spiropyran libraries via multicomponent reactions and their molecular switching properties.

A practical protocol for the preparation of parallel solution-phase libraries of the quaternary carbon-containing oxazatricycles and spiropyrans is reported. Target compounds were obtained in moderate to excellent yields by an Et(3)N-mediated multicomponent reaction from various N-methylisoquinolium or flavylium salts and 4-hydroxycoumarins or dimedone in acetone. Purification of the final products by recrystallization in ethyl acetate/methylene chloride or by column chromatography allowed easy isolation of nine compounds of each array. Preliminary studies indicated that some of the prepared compounds exhibit redox switching, photochromic, and thermochromic properties.

Dual fluorescent photochromic colorants bearing pyrano[3,2-c]chromen-5-one moiety.

In this study, the photochromic processes of 8-N,N-dimethylamino-2,2-dimethyl-2H-pyrano[3,2-c]chromen-5-one (1) and its derivatives (2, 3) are investigated with steady-state, temperature-dependent and time-resolved absorption and emission spectroscopy. The differences among compounds 1-3 lie in their various substituents anchored at the pyran moiety that is subject to the photoinduced ring-opening reaction. Compounds 1 and 2 exhibit salient photochromism with a very unique phenomenon, in which fluorescence is observed in 1 for both the ring-closed form (1-CF, lambda(max) approximately 445 nm) and the ring-open form (1-OF, lambda(max) approximately 650 nm in CH2Cl2). The yields of forward and reverse photochromism processes were determined to be 0.40 and 1.0% for 1. Along with fluorescence quantum yields of 9.5 x 10(-2) and 5.8 x 10(-3) for 1-CF and 1-OF, respectively, 1 enables fluorescence detection while it exhibits photochromism in both directions, that is, a photoinduced on/off fluorescence switch. An increase in on/off ratiometric fluorescence between 1-OF and 1-CF can reach a factor of 4.0 upon excitation at the absorption isosbestic point. The activation energies for the ground-state OFtrans --> CF thermal bleaching processes were determined to be 58.2 and 54.8 kJ/mol, with frequency factors of 1.7 x 10(5) and 3.6 x 10(5) s(-1) for 1 and 2, respectively. Conversely, bromo-substituted 3 did not undergo photochromic reaction, as evidenced by the lack of changes in the absorption spectrum after a prolonged (2 h) 354 nm (0.2 W/cm2) photolysis, manifesting the fact that other relaxation processes, such as enhanced intersystem crossing, may govern the deactivation of 3 (3-CF) upon excitation.

One-pot synthesis of coumarin-based oxazabicyclic and oxazatricyclic compounds and their fluorescence redox switching properties.

An oxazabicycle and an oxazatricycle were efficiently synthesized by coupling of coumarins and N-alkylquinolium or isoquinolium salt to investigate their fluorescence redox-switching properties. Chemical reduction of the strongly fluorescent oxazabicycle and oxazatricycle results in the ring-opened products with a distinct decrease in emission intensity. Both resulting ring-opened species can be swiftly reverted to the original ring-closed forms by oxidation.

Recruitment of HIV-1 envelope occurs subsequent to lipid mixing: a fluorescence microscopic evidence.

Entry of the human immunodeficiency virus (HIV) into the target cell is initiated by fusion with the cell membrane, mediated through the envelope glycoproteins gp120 and gp41, following engagement to CD4 and the co-receptor. Previous fusion kinetics studies on the HXB2 envelope protein (Env) revealed that Env recruitment occurred at about 13 min concurrent with the lipid mixing. To resolve the temporal sequence of lipid mixing and recruitment, we employed an inhibitory assay monitored by fluorescence microscopy using a gp41 ectodomain (gp41e) fragment, which blocked Env recruitment in stark contrast to the lack of gp41e effect on the lipid mixing. In addition, to demonstrate the mode of action for the inhibition of gp41e, our results strongly suggested that lipid mixing precedes the Env recruitment because lipid mixing can proceed with Env recruitment inhibited by exogeneous gp41e molecules. Importantly, it was found that the random clustering of Env molecules on the membrane surface occurred at approximately 1 minute whereas the Env recruitment was observed at 13 minutes after the attachment of Env-expressing cell to the target cell. This > 10-fold temporal discrepancy highlights that the productive assembly of Env molecules leading to fusion requires spatio-temporal coordination of several adjacent Env trimers aggregated via directed movement.

The application of perfluorooctanoate to investigate trimerization of the human immunodeficiency virus-1 gp41 ectodomain by electrophoresis.

The transmembrane glycoprotein gp41 of human immunodeficiency virus has been proposed to form trimer-of-hairpin during virus-cell membrane fusion. To investigate its oligomerization propensity under soluble and membrane-mimic conditions, sodium salt of perfluorooctanoate (PFO) was applied. A recombinant gp41 ectodomain devoid of disulfide linkage was overexpressed in Escherichia coli and characterized by MS and circular dichroism spectropolarimetry in PFO solution in comparison to SDS. The helical content of this ectodomain in PFO is higher than that in SDS. Notably, PFO employed in PAGE clearly conduced to the formation of trimer under the optimized condition as visualized in the gel. In addition, the proteins expressed from the two mutants in the heptad repeat (HR) domains of gp41, I62P, and N126K, were also examined by the PFO-PAGE analysis for functional ramification of molecular organization. Remarkably, the I62P mutation completely abolished the gp41 trimer formation, whereas the N126K mutation resulted in a more stable trimer. The data suggested that PFO-PAGE analysis is appropriate for evaluating the effect of mutations on the trimerization of gp41 and other fusion proteins which may be implicated in the alteration of their fusogenicity.

Membrane interaction and structure of the transmembrane domain of influenza hemagglutinin and its fusion peptide complex.

BACKGROUND: To study the organization and interaction with the fusion domain (or fusion peptide, FP) of the transmembrane domain (TMD) of influenza virus envelope glycoprotein for its role in membrane fusion which is also essential in the cellular trafficking of biomolecules and sperm-egg fusion. RESULTS: The fluorescence and gel electrophoresis experiments revealed a tight self-assembly of TMD in the model membrane. A weak but non-random interaction between TMD and FP in the membrane was found. In the complex, the central TMD oligomer was packed by FP in an antiparallel fashion. FP insertion into the membrane was altered by binding to TMD. An infrared study exhibited an enhanced membrane perturbation by the complex formation. A model was built to illustrate the role of TMD in the late stages of influenza virus-mediated membrane fusion reaction. CONCLUSION: The TMD oligomer anchors the fusion protein in the membrane with minimal destabilization to the membrane. Upon associating with FP, the complex exerts a synergistic effect on the membrane perturbation. This effect is likely to contribute to the complete membrane fusion during the late phase of fusion protein-induced fusion cascade. The results presented in the work characterize the nature of the interaction of TMD with the membrane and TMD in a complex with FP in the steps leading to pore initiation and dilation during virus-induced fusion. Our data and proposed fusion model highlight the key role of TMD-FP interaction and have implications on the fusion reaction mediated by other type I viral fusion proteins. Understanding the molecular mechanism of membrane fusion may assist in the design of anti-viral drugs.

Self-association of glutamic acid-rich fusion peptide analogs of influenza hemagglutinin in the membrane-mimic environments: effects of positional difference of glutamic acids on side chain ionization constant and intra- and inter-peptide interactions deduced from NMR and gel electrophoresis measurements.

Two glutamic acid-rich fusion peptide analogs of influenza hemagglutinin were synthesized to study the organization of the charged peptides in the membranous media. Fluorescence and gel electrophoresis experiments suggested a loose association between the monomers in the vesicles. A model was built which showed that a positional difference of 3, 7 and 4, 8 results in the exposure of Glu3 and Glu7 side chains to the apolar lipidic core. Supportive results include: first, pK(a) values of two pH units higher than reference value in aqueous medium for Glu3 and Glu7 CgammaH, whereas the deviation of pK(a) from the reference value for Glu4 and Glu8 CgammaH is substantially smaller; second, Hill coefficients of titration shift of these protons indicate anti-cooperativity for Glu3 and Glu7 side chain protons but less so for Glu4 and Glu8, implying a strong electrostatic interaction between Glu3 and Glu7 possibly resulting from their localization in an apolar environment; third, positive and larger titration shift for NH of Glu3 is observed compared to that of Glu4, suggesting stronger hydrogen bond between the NH and the carboxylic group of Glu3 than that of Glu4, consistent with higher degree of exposure to hydrophobic medium for the side chain of Glu3.