Nucleotide-directed growth of semiconductor nanocrystals

We engineer colloidal quantum dot nanocrystals through the choice of biomolecular ligands responsible for nanoparticle nucleation, growth, stabilization, and passivation. We systematically vary the presence of, and thereby elucidate the role of, phosphate groups and a multiplicity of functionalities on the mononucleotides used as ligands. The results provide the basis for synthesis of nanoparticles using precisely controlled synthetic oligonucleotide sequences.     

Intramolecular electron transfer through the 20-position of a chlorophyll a derivative: an unexpectedly efficient conduit for charge transport

Suzuki cross-coupling reactions have afforded 20-phenyl-substituted Chlorophyll a derivatives (ZCPh) in good yields and significant quantities from readily available Chl a. A series of donor-acceptor dyads was synthesized in which naphthalene-1,8:4,5-bis(dicarboximide) or either of two perylene-3,4:9,10-bis(dicarboximide) electron acceptors is attached to the para position of the 20-phenyl group. Comparisons with the analogous dyads based on a zinc 5,10,15-tri(n-pentyl)-20-phenylporphyrin donor show that, for a given acceptor and solvent, the rates of photoinduced charge separation and recombination as well as the calculated electronic coupling matrix elements, V, for these reactions differ by less than a factor of 2. However, EPR and ENDOR spectroscopy corroborated by DFT calculations show that the highest occupied MO of ZCPh+* has little spin (charge) density at the 20-carbon atom, whereas Z3PnPh+* has significant spin (charge) density there, implying that V, and therefore the electron-transfer rates, should differ significantly for these two macrocyclic donors. DFT calculations on ZCPh+* and Z3PnPh+*, with two -0.5 charges located where the nearest carbonyl oxygen atoms of the acceptor would reside in the donor-acceptor dyads, show that the presence of the negative charges significantly shifts the charge density of both ZCPh+* and Z3PnPh+* from the macrocycle onto the phenyl rings. Thus, the presence of adjacent covalently linked radical anions at a fixed location relative to each of these radical cations results in nearly identical electronic coupling matrix elements for electron transfer and therefore very similar rates.     

An intercalator film as a DNA-electrode interface

DNA-surface conjugation is achieved through an intercalating molecular wire, resulting in more efficient electron transfer relative to systems utilizing conventional insulating tethers.     

Identification and characterization of homologues of vertebrate beta-thymosin in the marine mollusk Aplysia californica

The beta-thymosins have been known as actin-sequestering proteins, but now are recognized as molecules with multiple and diverse intracellular and extracellular functions. Two closely related proteins, beta-thymosin(His) and beta-thymosin(Gln), have been de novo sequenced by top-down mass spectrometry in the common neurobiology model, Aplysia californica. As determined by nanoelectrospray quadrupole-enhanced Fourier-Transform mass spectrometry with collisionally activated and electron-capture dissociations, both of these Aplysia beta-thymosins are acetylated and differ by a single residue in the central actin-binding domain. Profiling of individual cells and tissue by matrix-assisted laser desorption/ionization mass spectrometry reveals that these proteins are widely expressed in the Aplysia central nervous system, including in individual identified neurons, neuronal clusters, nerves and connective tissues. Newly identified beta-thymosin(His) and beta-thymosin(Gln) are also detected by mass spectrometry in hemolymph, and in releasates collected from whole ganglia. When applied exogenously, beta-thymosin proteins, purified from nerve cell extract, support the anchoring of neurons, and increase neurite sprouting and total neurite outgrowth in culture. These positive effects on neurite regeneration in cell culture suggest that the beta-thymosin proteins have an extracellular function in the central nervous system of Aplysia californica.     

Synthesis and Crystal Structure of the Coordination Polymer [Cu(ppca′)<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)(HgI<Subscript>2</Subscript>)](H<Subscript>2</Subscript>O) (ppca′ = 3,4′-bipyridine-6-carboxylic acid)

Abstract  

The coordination complex Cu(ppca′)₂(H₂O)₂ (1) was synthesized hydrothermally using the ligand 3,4′-bipyridine-6-carboxylic acid (ppca′). Complex 1 consists of pseudo-octahedral copper (II) units hydrogen bonded into a three-dimensional network that crystallizes in the monoclinic P2₁/n space group. In complex 1, a = 5.3429(4) ?, b = 29.343(2) ?, c = 6.7940(5) ?, and β = 110.635(2)°.     

Deconvolution of the cellular oxidative stress response with organelle-specific Peptide conjugates

Oxidative stress is a deleterious force that must be combated relentlessly by aerobic organisms and is known to underlie many human diseases including atherosclerosis, Parkinson's disease, and Alzheimer's disease. Information available about the oxidative stress response has come primarily from studies using reactive oxygen species (ROS) with ill-defined locations within the cell. Thus, existing models do not account for possible differences between stress originating within particular regions of the cell. Here, oxidative stress is studied at the subcellular level using ROS-generating compounds localizing within two different organelles: the nucleus and the mitochondrion. Differences in cytotoxicity, gene expression, and survival pathway activation are detected as a function of the subcellular origin of oxidative stress, indicating that independent mechanisms are used to cope with oxidative stress arising in different cellular compartments. These comparative studies, enabled by the development of organelle-specific oxidants, examine the cellular responses to site-specific oxidative stress with heightened precision.     

Enantioselective synthesis of N(alpha)-Fmoc protected (2S,3R)-3-phenylpipecolic acid. A constrained phenylalanine analogue suitably protected for solid-phase peptide synthesis

Reported herein is the first enantioselective preparation of (2S,3R)-3-phenylpipecolic acid as a conformationally constrained phenylalanine analogue bearing N(alpha)-protection suitable for solid-phase peptide synthesis. Stereochemistries at both the 2- and 3-positions are derived inductively from a single chiral center provided by the commercially available Evans chiral auxiliary, (4S)-4-benzyl-1,3-oxazolidin-2-one. By constraining phi and chi(1) torsion angles, this novel amino acid analogue can serve as a useful tool for the induction of defined geometry in phenylalanine-containing peptides.     

Nickel-mediated hydrogenolysis of C-O bonds of aryl ethers: what is the source of the hydrogen?

Mechanistic studies of the hydrogenolysis of aryl ethers by nickel were undertaken with (diphosphine)aryl methyl ethers. A Ni(0) complex containing Ni-arene interactions adjacent to the aryl-O bond was isolated. Heating led to aryl-O bond activation and generation of a nickel aryl methoxide complex. Formal β-H elimination from this species produced a nickel aryl hydride which can undergo reductive elimination in the presence of formaldehyde to generate a carbon monoxide adduct of Ni(0). The reported complexes map out a plausible mechanism of aryl ether hydrogenolysis catalyzed by nickel. Investigations of a previously reported catalytic system using isotopically labeled substrates are consistent with the mechanism proposed in the stoichiometric system, involving β-H elimination from a nickel alkoxide rather than cleavage of the Ni-O bond by H(2).