Recoil in macromolecular solutions according to rigid dumbbell kinetic theory

A macromolecular solution is represented by the simple model of rigid dumbbells suspended in a Newtonian fluid with Brownian motion included. Hydrodynamic interaction is not taken into account. It is found that for this model there will be recoil after the cessation of steady shearing flow. The ultimate shear recovery S _∞ is developed as a power series in κ^?, the shear rate prior to the cessation of the steady shear flow: $$S_\infty = (\theta _0 /2\eta _0 ) \kappa ^\user1{ - } + O(\kappa ^\user1{ - } )^3$$ where η₀ and θ₀ values of the viscosity and primary normal stress functions respectively at zero-shear rate. The coefficient of the term in (κ^?)³ is calculated. In addition, the behavior of the normal stresses during the recoil process is found; during recoil τ₂₂?τ₃₃ has the opposite sign from τ₁₁?τ₂₂.     

CO dissociation mechanism in racemization of alcohols by a cyclopentadienyl ruthenium dicarbonyl catalyst

(13)CO exchange studies of racemization catalyst (η(5)-Ph(5)C(5))Ru(CO)(2)Cl and (η(5)-Ph(5)C(5))Ru(CO)(2)(Ot-Bu) by (13)C NMR spectroscopy are reported. CO exchange for the active catalyst form, (η(5)-Ph(5)C(5))Ru(CO)(2)(Ot-Bu) is approximately 20 times faster than that for the precatalyst (η(5)-Ph(5)C(5))Ru(CO)(2)Cl. An inhibition on the rate of racemization of (S)-1-phenylethanol was observed on addition of CO. These results support the hypothesis that CO dissociation is a key step in the racemization of sec-alcohols by (η(5)-Ph(5)C(5))Ru(CO)(2)Cl, as also predicted by DFT calculations.     

The development of an FIA-CD strategy for screening sulfated polysaccharides using antimalarial drugs and related species as probes

Heterogeneous sulfated polysaccharides have attracted significant attention in light of their various biological activities. However, recent events involving heparin have dramatically illustrated that several analytical challenges exist in accounting for such species. In this case, tainted heparin was associated with acute reactions that lead to numerous deaths. Researchers were forced to use time-consuming, sophisticated techniques (e.g., enzymatic digestion, NMR, CE, HPLC, MS, etc.) to identify the cause of these adverse effects. Extensive investigations ultimately showed oversulfated chondroitin sulfate, a semi-synthetic sulfated polysaccharide, to be present in the contaminated samples. These events highlighted the need for a new generation of screening techniques. In this work, we report the development of a screening strategy that exploits unique circular dichroism features observed as a function of association between investigated polymers and judiciously selected probe molecules (i.e., chloroquine, N1-(7-chloro-4-quinolinyl)-N3-methyl-1,3-propanediamine, quinacrine, and N2-9-acridinyl-N1,N1-dimethyl-1,2-ethanediamine). Application of obtained spectropolarimetry results to a flow injection analysis circular dichroism platform allowed for the establishment of linear polysaccharide response curves for dextran sulfate, heparin, and oversulfated chondroitin sulfate in the low micromolar range. Lastly, through additional work with heparin, the proposed method was shown to be capable of rapidly screening sulfated polysaccharide samples for closely related impurities.     

N-terminal protein modification using simple aminoacyl transferase substrates

Methods for synthetically manipulating protein structure enable greater flexibility in the study of protein function. Previous characterization of the Escherichia coli aminoacyl tRNA transferase (AaT) has shown that it can modify the N-terminus of a protein with an amino acid from a tRNA or a synthetic oligonucleotide donor. Here, we demonstrate that AaT can efficiently use a minimal adenosine substrate, which can be synthesized in one to two steps from readily available starting materials. We have characterized the enzymatic activity of AaT with aminoacyl adenosyl donors and found that reaction products do not inhibit AaT. The use of adenosyl donors removes the substrate limitations imposed by the use of synthetases for tRNA charging and avoids the complex synthesis of an oligonucleotide donor. Thus, our AaT donors increase the potential substrate scope and reaction scale for N-terminal protein modification under conditions that maintain folding.     

Noninvasive imaging of cell death using an Hsp90 ligand

Cell death plays a central role in normal physiology and in disease. Common to apoptotic and necrotic cell death is the eventual loss of plasma membrane integrity. We have produced a small organoarsenical compound, 4-(N-(S-glutathionylacetyl)amino)phenylarsonous acid, that rapidly accumulates in the cytosol of dying cells coincident with loss of plasma membrane integrity. The compound is retained in the cytosol predominantly by covalent reaction with the 90 kDa heat shock protein (Hsp90), the most abundant molecular chaperone of the eukaryotic cytoplasm. The organoarsenical was tagged with either optical or radioisotope reporting groups to image cell death in cultured cells and in murine tumors ex vivo and in situ. Tumor cell death in mice was noninvasively imaged by SPECT/CT using an (111)In-tagged compound. This versatile compound should enable the imaging of cell death in most experimental settings.     

The supersymmetry index and the construction of modular invariants

Relationships between the modular properties of affineG characters and the modular properties of the affine characters of regular subgroups ofG are derived by considering the branching functions that appear in calculation of the index of the Dirac-Ramond operator on super-coset models. Various applications of these relationships are described, and in particular a simple algorithm is given for generating modular invariant combinations of characters of affineG at any level by using the shift vector method on suitably chosen Lorentzian, self-dual lattices.Work supported in part by funds provided by the NSF under grant No. 87-08447 and by the U.S. Department of Energy (D.O.E) under contract # DE-AC02-76ER0306, and also by a fellowship from the Alfred P. Sloan foundation     

Spectroscopic study of a representative polar cap of buckminsterfullerene: Cyclopentacorannulene

The spectroscopic properties of corannulene and cyclopentacorannulene are examined by use of absorption and steady-state fluorescence measurements. A red shift in the emission maxima of cyclopentacorannulene is noted with respect to the emission maxima in the corannulene spectrum. Similar differences in the absorption spectra of both molecules are also observed. Reasons for the dissimilarities in the absorption and emission spectra of these molecules are discussed. The fluorescence quantum yields and lifetimes of the molecules measured in organic solvents are reported. The fluorescence quantum yield and lifetime of cyclopentacorannulene are lower than those of corannulene. This difference is attributed to the highly strained and nonplanar structure of cyclopentacorannulene. The effectiveness of nitromethane as a quencher of corannulene and cyclopentacorannulene fluorescence is examined. In contrast to previously reported results, the fluorescence of these molecules is quenched by nitromethane. Fluorescence quenching of the molecules has been attributed to complex formation in the ground state, i.e., static quenching.