Jeff Kao

Spectroscopic identification of tri-n-octylphosphine oxide (TOPO) impurities and elucidation of their roles in cadmium selenide quantum-wire growth.

Tri-n-octylphosphine oxide (TOPO) is the most commonly used solvent for the synthesis of colloidal nanocrystals. Here we show that the use of different batches of commercially obtained TOPO solvent introduces significant variability into the outcomes of CdSe quantum-wire syntheses. This irreproducibility is attributed to varying amounts of phosphorus-containing impurities in the different TOPO batches. We employ (31)P NMR to identify 10 of the common TOPO impurities. Their beneficial, harmful, or negligible effects on quantum-wire growth are determined. The impurity di-n-octylphosphinic acid (DOPA) is found to be the important beneficial TOPO impurity for the reproducible growth of high-quality CdSe quantum wires. DOPA is shown to beneficially modify precursor reactivity through ligand substitution. The other significant TOPO impurities are ranked according to their abilities to similarly influence precursor reactivity. The results are likely of general relevance to most nanocrystal syntheses conducted in TOPO.

Near-Infrared Fluorescent pH-Sensitive Probes via Unexpected Barbituric Acid Mediated Synthesis

Novel near-infrared pyrimidine-fused pH fluorescent probes were prepared by an unusual barbiturate-mediated debenzoindolation and subsequent heteroannulation. A plausible mechanistic pathway is proposed, and the final structures were further elucidated by 2D-NMR. All new compounds are highly fluorescent in the near-infrared region and possess excellent spectral sensitivities to environmental pH changes.

Inhibition of Heat Shock Induction of Heat Shock Protein 70 and Enhancement of Heat Shock Protein 27 Phosphorylation by Quercetin Derivatives

Inhibitors of heat-induced heat shock protein 70 (HSP70) expression have the potential to enhance the therapeutic effectiveness of heat-induced radiosensitization of tumors. Among known small molecule inhibitors, quercetin has the advantage of being easily modified for structure-activity studies. Herein, we report the ability of five monomethyl and five carbomethoxymethyl derivatives of quercetin to inhibit heat-induced HSP70 expression and enhance HSP27 phosphorylation in human cells. While quercetin and several derivatives inhibit HSP70 induction and enhance HSP27 phosphorylation at Ser78, other analogues selectively inhibit HSP70 induction without enhancing HSP27 phosphorylation that would otherwise aid in cell survival. We also show that good inhibitors of HSP70 induction are also good inhibitors of both CK2 and CamKII, kinases that are known to activate HSP70 expression by phosphorylation of heat shock transcription factor 1. Derivatives that show poor inhibition of either or both kinases are not good inhibitors of HSP70 induction, suggesting that quercetins effectiveness is due to its ability to inhibit both kinases.

Near-Infrared Dichromic Fluorescent Carbocyanine Molecules†

Central to major advances in biochemical assays, molecularsensor technologies, and molecular optical imaging arefluorescent materials that provide high detection sensitivityformolecularprocesses.Inbiologicalopticalimaging,thelowtissueautofluorescenceandthedeeppenetrationoflightintothetissuesobservedatwavelengthsbetween650 and900nmallow the use of near-infrared (NIR) fluorescent dyes ascontrast agents in heterogeneous systems.

Rapid screening and structural elucidation of a novel sibutramine analogue in a weight loss supplement: 11-Desisobutyl-11-benzylsibutramine☆

A novel analogue of sibutramine, 11-desisobutyl-11-benzylsibutramine, has been discovered. During routine ion mobility spectrometry (IMS) screening of a weight loss supplement collected at an US FDA import operation facility an unknown peak was observed. Further analysis of the supplement by liquid chromatography-mass spectrometry (LC-MS) and high resolution mass spectrometry revealed an unknown peak with a relative retention time of 1.04 with respect to sibutramine and a predicted formula of C20H24NCl. In order to elucidate the analogues structure, it was isolated from the supplement and characterized by tandem mass spectrometry and nuclear magnetic resonance (NMR), which revealed the analogue possessed a benzyl moiety at the 11 position in place of the isobutyl group associated with sibutramine.

A Second Look at Mini-Protein Stability: Analysis of FSD-1 Using Circular Dichroism, Differential Scanning Calorimetry, and Simulations

Mini-proteins that contain <50 amino acids often serve as model systems for studying protein folding because their small size makes long timescale simulations possible. However, not all mini-proteins are created equal. The stability and structure of FSD-1, a 28-residue mini-protein that adopted the betabetaalpha zinc-finger motif independent of zinc binding, was investigated using circular dichroism, differential scanning calorimetry, and replica-exchange molecular dynamics. The broad melting transition of FSD-1, similar to that of a helix-to-coil transition, was observed by using circular dichroism, differential scanning calorimetry, and replica-exchange molecular dynamics. The N-terminal beta-hairpin was found to be flexible. The FSD-1 apparent melting temperature of 41 degrees C may be a reflection of the melting of its alpha-helical segment instead of the entire protein. Thus, despite its attractiveness due to small size and purposefully designed helix, sheet, and turn structures, the status of FSD-1 as a model system for studying protein folding should be reconsidered.

2,6-Dimethoxyphenylphosphirane Oxide and Sulfide and their Thermolysis to Phosphinidene Chalcogenides—Kinetic and Mechanistic Studies

Abstract 2,6-Dimethoxyphenylphosphirane is readily converted to its oxide and sulfide whose pyrolysis and (perhaps) photolysis lead to the generation of phosphinidene chalcogenides Ar–P Z (Z=O,S). Trapping experiments were carried out under conditions similar to the kinetic studies of the phosphirane chalcogenide pyrolyses that confirmed the formation of free Ar–P Z. The trapping experiments were in accord with carbene-like character for Ar–P Z, and the activation parameters suggest a non-least motion pathway for the addition of Ar–P Z to olefins. This represents quantitative evidence for the validity of the predictions of frontier-orbital theory for species that undergo reactions with small (or no) enthalpic barriers.

Novel trihydroxamate‐containing peptides: Design, synthesis, and metal coordination

Novel trihydroxamate‐containing peptides were designed to mimic desferrioxamine (Desferal®, DFO, a naturally occurring siderophore) but possess distinct conformational restrictions and varied lipophilicity to probe structure vs. metal coordination. The synthesis was performed via fragment condensation of hydroxamate‐containing oligopeptides such as Fmoc–Leu– Ψ[CON(OBz)]–Phe–Ala–Pro–OH and H–Leu–Ψ[CON(OBz)]–Phe–Ala–Pro–OBut (Fmoc: 9‐fluor enylmethoxycarbonyl; OBz: benzyl; OBut: tert‐butyl) either in solution or on a solid support. The metal‐binding properties were studied by electrospray ionization–mass spectroscopy (ESI‐MS), ultraviolet (UV)‐visible spectroscopy, and 1H nuclear magnetic resonance (NMR). Similar to the dihydroxamate analogs previously explored [Biopolymers (Peptide Science), 2003, Vol. 71, pp. 489–515], the compounds with three hydroxamates arrayed at 10‐atom intervals, i.e., H–[Leu–Ψ[CON(OH)]–Phe–Ala–Pro]3–OH (P1), cyclo[Leu–Ψ[CON(OH)]–Phe–Ala–Pro]3 (P2), and H–[Leu–Ψ(CONOH)–Phe–Ala–Pro]2–Leu–NHOH (P7), exhibited high affinities for intramolecular coordination with Fe(III) and Ga(III). As expected, both P1 and P2 showed higher relative Fe(III)‐binding affinities than the corresponding dihydroxamate‐containing peptide analogs (P11 and P12). Even though both P1 and P2 did not compete with DFO in the relative metal‐binding affinity in both solution and gas phases, P1, P2, and DFO exhibited similar relative binding selectivities to 11 different metal ions including Fe(III), Fe(II), Al(III), Ga(III), In(III), Zn(II), Cu(II), Co(II), Ni(II), Gd(III), and Mn(II). Compared to the other metal ions, they had higher relative binding affinities with Fe(III), Fe(II), Al(III), Ga(III), and In(III). The decreased metal‐binding affinities of P1 and P2 in comparison with DFO suggested the conformational restrictions of their backbones perturb their three hydroxamate groups from optimal hexadentate orientations for metal coordination. As detected by ESI‐MS, P2 was distinguished from both P1 and DFO by solvation of its Ga(III) and Fe(III) complexes (such as acetonitrile or water), thereby stabilizing the resulting complexes in the gas phase. Noteworthy, P2 led to 69% death rate in Hela cells at a concentration of 50 μM, exhibiting higher cytotoxicity than DFO in vitro despite its much lower affinity for iron. This enhanced toxicity may simply reflect the increased lipophilicity of the cyclic trihydroxamate (P2) together with the improvements in its cell penetration, and/or subsequent intracellular molecular recognition of both side chains and hydroxamate groups. The cytotoxicity was significantly suppressed by precoordination with Ga(III) or Fe(III), suggesting a mechanism of toxicity via sequestration of essential metal ions as well as the importance of curbing the metal coordination before targeting. The potential of such siderophore‐mimicking peptides in oncology needs further exploration.

c[D-pro-Pro-D-pro-N-methyl-Ala] adopts a rigid conformation that serves as a scaffold to mimic reverse-turns

Naturally occurring cyclic tetrapeptides (CTPs) such as tentoxin (Halloin et al., Plant Physiol 1970, 45, 310–314; Saad, Phytopathology 1970, 60, 415–418), ampicidin (Darkin‐Rattray, Proc Natl Acad Sci USA 1996, 93, 13143–13147), HC‐toxin (Walton, Proc Natl Acad Sci USA 1987, 84, 8444–8447), and trapoxin (Yoshida and Sugita, Jpn J Cancer Res 1992, 83, 324–328; Itazaki et al., J Antibiot (Tokyo) 1990, 43, 1524–1532) have a wide range of biological activity and potential use ranging from herbicides (Walton, Proc Natl Acad Sci USA 1987, 84, 8444–8447; Judson, J Agric Food Chem 1987, 35, 451–456) to therapeutics (Loiseau, Biopolymers 2003, 69, 363–385) for malaria (Darkin‐Rattray, Proc Natl Acad Sci USA 1996, 93, 13143–13147) and cancer (Yoshida and Sugita, Jpn J Cancer Res 1992, 83, 324–328). To elucidate scaffolds that have few low‐energy conformations and could serve as semirigid reverse‐turn mimetics, the flexibility of CTPs was determined computationally. Four analogs of cyclic tetraproline c[Pro‐pro‐Pro‐pro] with alternating L‐ and D‐prolines, namely c[pro‐Pro‐pro‐NMe‐Ala], c[pip‐Pro‐pip‐Pro], c[pro‐Pip‐pro‐Pro], and c[Ala‐Pro‐pip‐Pro] were synthesized and characterized by NOESY NMR. Both molecular mechanics and Density Functional Theory quantum calculations found these head‐to‐tail CTPs to be constrained to one or two relatively stable conformations. NMR structures, while not always yielding the same lowest energy conformation as expected by in silico predictions, confirmed only one or two highly populated solution conformations for all four peptides examined. c[pro‐Pro‐pro‐NMe‐Ala] was shown to have a single all trans‐amide bond conformation from both in silico predictions and NMR characterization, and to be a reverse‐turn mimetic by overlapping four Cα‐Cβ bonds with those for ∼6.5% (Tran, J Comput Aided Mol Des 2005, 19, 551–566) of reverse‐turns in the Protein Data Bank PDB with a RMSD of 0.57 Å.

A new nucleoside analogue with potent activity against mutant sr39 herpes simplex virus-1 (HSV-1) thymidine kinase (TK).

Nucleoside analogues, such as penciclovir, ganciclovir, acyclovir, and their fluoro-substituted derivatives, have wide utility as antivirals. Among these analogues, FHBG ((18)F-Fluorohydroxybutylguanine) is a well-validated PET (positron emission tomography) probe for monitoring reporter gene expression. To evaluate whether or not imposing rigidity into the flexible side chain of FHBG 4 could also impact its interaction, with amino acid residues within the binding site of HSV1-TK (Herpes Simplex Virus-1 Thymidine Kinase), thus influencing its cytotoxic activity. Herein, the synthesis of a new fluorinated nucleoside analogue 6 (conceived via ligand-docking studies) is reported. Agent 6 demonstrates selective activity against HeLa cells stably transfected with mutant HSV1-sr39TK and is also 47-fold more potent than FHBG.