Hierarchical modeling N2 adsorption on the surface of and within a C60 crystal: from quantum mechanics to molecular simulation

The adsorption of N(2) on the surface of, and within, a C(60) face-centered cubic crystal has been studied using a hierarchical approach. First, an ab initio potential between N(2) and C(60) is obtained from a recently developed quantum mechanical hybrid method, and then the adsorption behavior is predicted using Monte Carlo simulation. On the crystal surface, N(2) adsorption isotherm at 77.3 K is of type II. The adsorption simulated with the ab initio potential is slightly greater than that with the empirical Steele potential derived from experimental N(2) adsorption on planar graphite, and both are in fairly good agreement with measured results. With increasing pressure, N(2) molecules are found to sequentially occupy three favorable sites: the octahedral sites, the tetrahedral sites, and the top of C(60) molecules. Finally multiple layers form and wetting occurs as the bulk N(2) saturation pressure is reached. The isosteric heat of adsorption exhibits two maxima and finally approaches the enthalpy of vaporization of bulk N(2). Within the crystal, the N(2) adsorption isotherm at 77.3 K is of type I, and the use of ab initio potential leads to significantly greater adsorption than the Steele potential. N(2) molecules are observed to intercalate only the octahedral sites, and the isosteric heat of adsorption is nearly a constant. As in our previous work of N(2) and O(2) adsorption in the C(168) schwarzite (Jiang et al. J. Phys. Chem. B 2004, 108, 9852), this work demonstrates the importance of an accurate adsorbate-adsorbent interaction potential in the determination of gas adsorption behavior.     

On the noise of high-transimpedance amplifiers for long-wavelength pulse OTDRs

Noise measurements on high-transimpedance amplifiers suitable for long-wavelength OTDRs give results higher than is predicted by normal noise models. Consequently, we have developed two useful techniques to measure independently the noise contribution of the JFET and the feedback resistor to the overall amplifier noise. p ]Our results show that the noise of the JFET is in accordance with an accurate theoretical model for such a device. In contrast, the noise from the feedback resistor is much higher than is predicted from the normal resistance-capacitance model for such a component. This increase results from the distributed nature of high-ohmic resistors. Our results indicate that both choice of resistor manufacturer and individual selection of a resistor from a specific manufacturer are warranted. By selecting a low-noise resistor we demonstrate a 500-M transimpedance amplifier with an input equivalent noise current of 13.8 pA. In comparison, the same amplifier with a noisy resistor had an input equivalent noise current of 23 pA. p ]We use our results to show that a reasonable value of the input equivalent noise current of a low-noise photodiode-amplifier combination is 20 pA.     

An ab initio study on the torsional surface of alkanes and its effect on molecular simulations of alkanes and a DPPC bilayer

Energies of 119 conformations of normal alkanes from butane to heptane were calculated at approximately the CCSD(T)/cc-pVQZ level. Energies of gauche (g) conformers relative to trans (t) decrease as chain length increases. In what is termed the "positive pentane effect", adjacent gauche conformers of the same sign are stabilized compared to nonadjacent conformers; e.g., for hexane the energies of tgt, tgg, and gtg are 0.600, 0.930, and 1.18 kcal/mol, respectively. Torsional terms in the CHARMM27 (C27) force field were fit to the calculated QM energies to yield a revised potential, C27r. Molecular dynamics simulations of normal alkanes (heptane, decane, tridecane, and pentadecane) with C27r yield higher populations of gauche states, increased transition rates, and improved agreement with experiment as compared to C27. In addition, C27r simulations of a hydrated DPPC lipid bilayer yield improved agreement with the experimental NMR deuterium order parameters for the aliphatic chain ends.     

Horner-wadsworth-emmons reaction: Use of lithium chloride and an amine for base-sensitive compounds

A mild olefination procedure, utilizing LiCl and an amine, has been developed for use with base-sensitive aldehydes and phosphonates.     

Bridged ring a steroids: total synthesis of (±)-14β-hydroxy-1β,4β-methano-5β,8α,9β-androstane-7,17-dione

A modification of the Torgov steroid synthesis in which a vinyl methyl ether group in ring B stabilizes the critical cationic intermediate has been used to synthesize the title compound (15), whose structure has been confirmed by X-ray crystallographic analysis.     

Computational de novo design and characterization of a four-helix bundle protein that selectively binds a nonbiological cofactor

We report the complete de novo design of a four-helix bundle protein that selectively binds the nonbiological DPP-Fe(III) metalloporphyrin cofactor (DPP-Fe(III) = 5, 15-Di[(4-carboxymethyleneoxy)phenyl]porphinato iron(III)). A tetrameric, D2-symmetric backbone scaffold was constructed to encapsulate two DPP-Fe(III) units through bis(His) coordination. The complete sequence was determined with the aid of the statistical computational design algorithm SCADS. The 34-residue peptide was chemically synthesized. UV-vis and CD spectroscopy, size-exclusion chromatography, and analytical ultracentrifugation indicated the peptide undergoes a transition from a predominantly random coil monomer to an alpha-helical tetramer upon binding DPP-Fe(III). EPR spectroscopy studies indicated the axial imidazole ligands were oriented in a perpendicular fashion, as defined by second-shell interactions that were included in the design. The 1-D 1H NMR spectrum of the assembled protein displayed features of a well-packed interior. The assembled protein possessed functional redox properties different from those of structurally similar systems containing the heme cofactor. The designed peptide demonstrated remarkable cofactor selectivity with a significantly weaker binding affinity for the natural heme cofactor. These findings open a path for the selective incorporation of more elaborate cofactors into designed scaffolds for constructing molecularly well-defined nanoscale materials.     

Synthesis of the putative structure of tridachiahydropyrone

[reaction: see text] A short total synthesis of the putative structure of the marine natural product tridachiahydropyrone as a single enantiomer is described. Novel steps include a cuprate addition and cyclization to form a cyclohexanone ring and formation of the bicyclic pyrone with P(2)O(5) on Celite. The spectroscopic data obtained for compound 1 do not match those reported for tridachiahydropyrone; therefore, revision of the assigned natural product structure is warranted.     

Characterization of steric and electronic properties of NiN2S2 complexes as S-donor metallodithiolate ligands

The physical properties and structures of a series of six complexes of the type (NiN(2)S(2))W(CO)(4) have been used to establish electronic and steric parameters for square planar NiN(2)S(2) complexes as bidentate, S-donor ligands. According to the nu(CO) stretching frequencies and associated computed Cotton-Kraihanzel force constants of the tungsten carbonyl adducts, there is little difference in donor abilities of the five neutral NiN(2)S(2) metallodithiolate ligands in the series. The dianionic Ni(ema)(2)(-) (ema = N,N'-ethylenebis(2-mercaptoacetamide)) complex transfers more electron density onto the W(CO)(4) moiety. A ranking of donor abilities and a comparison with classical bidentate ligands is as follows: Ni(ema)(=) > {[NiN(2)S(2)](0)} > bipy approximately phen > Ph(2)PCH(2)CH(2)PPh(2) > Ph(2)PCH(2)PPh(2). Electrochemical data from cyclic voltammetry find that the reduction event in the (NiN(2)S(2))W(CO)(4) derivatives is shifted to more positive potentials by ca. 0.5 V compared to the ca. -2 V Ni(II/I) redox event in the free NiN(2)S(2) ligand, consistent with the electron drain from the nickel-dithiolate ligands by the W(CO)(4) acceptor. Differences in Ni(II/I) DeltaE(1/2) values appear to have a ligand dependence which is related to a structural feature of the hinge angle imposed by the (mu-SR)(2) bridges. Thus the angle formed by the intersection of NiN(2)S(2)/WS(2)C(2) planes has been established by X-ray diffraction analyses as a unique orientational feature of the nickel-dithiolate ligands in contrast to classical diphosphine or diimine ligands and ranges in value from 136 to 107 degrees . Variable-temperature (13)C NMR studies show that the spatial orientations of the ligands remained fixed with respect to the W(CO)(4) moiety to temperatures of 100 degrees C.     

Highly efficient enantiospecific synthesis of imidazoline-containing amino acids using bis(triphenyl)oxodiphosphonium trifluoromethanesulfonate

A highly efficient enantiospecific synthesis of imidazoline-based amino acids is reported from dipeptides composed of a C-terminal beta-amino-alpha-amino acid residue using bis(triphenyl) oxodiphosphonium trifluoromethanesulfonate. These imidazolines were easily converted to imidazoles and incorporated into macrolactam analogues of bistratamide H without loss of stereochemical integrity.     

Adsorption of beta-hairpin peptides on the surface of water: a neutron reflection study

Neutron reflectivity has been used to determine the thickness and surface coverage of monolayers of two 14-residue beta-hairpin peptides adsorbed at the air/water interface. The peptides differed only in that one was labeled with a fluorophore, while the other was not. The neutron reflection measurements were mainly made in null reflecting water, NRW, containing 8.1% D(2)O. Under this isotopic contrast the water is invisible to neutrons and the specular signal was then only from the peptide layer. At the highest concentration of ca. 4 microg/mL studied, the area per peptide molecule (A) was found to be 230 +/- 10 and 210 +/- 10 A(2) for the peptides with and without a BODIPY-based fluorophore, respectively. The thickness of the peptide layers was about 10 A for a Gaussian distribution. With decreasing bulk peptide concentration, both surface excess and layer thickness showed a steady trend of decrease. While the neutron results clearly indicate structural changes within the peptide monolayers with increasing bulk concentration, the outstanding structural feature is the formation of rather uniform peptide layers, consistent with the structural characteristics typical of beta-strand peptide conformations. These structural features are well supported by the parallel measurements of the adsorbed layers in D(2)O. With this isotopic contrast the neutron reflectivity provides an estimate about the extent of immersion of the peptide layers into water. The results strongly suggest that the 14-mer peptide monolayers were fully afloat on the surface of water, with only the carboxy groups on Glu residues hydrated.